## How To Soundproof An Apartment

How To Not Soundproof An Apartment

How to soundproof an apartment sounds like a daunting task. It is not easy. With paper thin interior walls and noise generating people living above, below and to the sides of you, it is not easy. Our soundproofing must also be portable and mobile, so after we get kicked out, we can take our stuff with us. Lets back track a bit and see if we can figure out how to soundproof an apartment without getting evicted for excessive noise.

Identify And Measure

Fist, we must decide who or what is making the noise and how much of it we have to soundproof for. Is it our neighbor to the east that is playing his stereo at certain times of the day. Is it the family below with the home theater system that has more sub woofers than acoustic sense. Is it your electric guitar playing or better yet, how about drums? We need to identify what source and level we are dealing with no matter if it comes from inside our apartment or outside.

Putting A Number To The Noise

After identification we now need to determine what our sound pressure issues are. Place a Db meter within your room at the quietest part of the day. Do the same for the loudest part of the day. Record this information. Measure outside your room to find the level of the noise outside our room. Pick the loudest part of the day and what you think is the quietest part of the day or night. You will have to set your alarm clock for this test. Once we have both numbers or sets of numbers, we can decide what method in our bag of tools on how to soundproof an apartment.

What Do The Numbers Mean?

What is the difference in the two sets of numbers? Lets look at the outside low to high representing the nosiest and quietest times we measured. Was the pressure reading at 85 SPL during the loudest outside. Was it, say 60, during the quietest. Was the inside measurement, say 50, at the quietest which was probably 2-3 in the morning and maybe 80 at the loudest when you were playing guitar. If we want the quietest within our room, we must stop 35 decibels of sound energy from the outside during the loudest part of the day for the quietest part of the day inside our room. Each dB of energy costs money.

We Are The Noise

For illustration purposes, lets take the path of least resistance and the path that we have the most control of. Lets say we are making noise from whatever the source and we want to protect our neighbors from it, so we do not get thrown out. The very first thing we must do is turn down, talk lower, and play more softly.

Room Within A Room

Don’t like any of these options? That’s all good. You want to play louder passages and sing rock and roll. No worries. We will just build a new room within your apartment, just kidding. It is an ideal approach but it would have to be designed so that it was self sealing, think of sound like water, and it would have to be assembled and then dis-assembled when you moved. Don’t forget about the additional weight upon the apartment exterior and interior structure. A room within a room is ideal acoustically but not practical in our discussion of how to soundproof an apartment. If we turn down in all areas, we can focus on the inside surfaces of our room as a start.

Sound Absorption

Excess energy from within our room can be dealt with using sound absorption technologies. If we have a low end or low frequency generating device in our room, we can use low frequency sound absorbing devices. The smaller our apartment, the more we need to make it smaller by adding low frequency sound absorbers. With the wall surfaces of our apartments we could use acoustic foams to absorb excessive middle and high frequencies.

Furniture And Drapes

Reflections from our room wall surfaces can be treated with sound absorbing materials to reduce the strength of the reflection and help us to lower the overall room reverb times, so our vocals and instruments can be heard clearly without trying to pull them out of some sonic soup. Draperies can be used or open celled foam acoustic panels can be color coordinated into the decor of the room. Furniture can be placed at least the large cushion sixes for lower frequency control above 100 cycles.

Drums And Bass Must Move

If you are going to play sound sources with bass drums and guitars, you need to move. There is simply no way to treat your room to manage the noise to your neighbors without building another room which is mechanically decoupled from your apartment at every mechanical connection. The floor must also be separated from your existing apartment. It is best with these instruments to get a free standing structure away from other people.

All In Control

We can control the noise levels within our rooms and certain instruments are just not practical for apartments because so much new construction is needed. Work within yourself and the surfaces of your room to control reflections and either add lots of thick soft furniture or build low frequency absorbers that can be tuned to your room for maximum performance.

## How To Soundproof A Door

Doors Have Many Functions

Our doors provide many functions. We must open and close them to exit and enter the room and when we close them, we expect them to offer some control with the transmission of sound energy from one room to the other. Doors need to be viewed as a barrier between different sources and levels of sound energy. Lets us a control room that needs to have access to a busy street.

Put A Number To The Noise

First, we must measure, using the control room / busy street access example, the amount of energy that we need to reflect back to the source with our door barrier. What is the noise level in our control room going to be and how mush energy from the street are we going to have to reflect back into the live room. We must put a number to each room’s noise. That number tells us how thick of a door, barrier, we are going to need. Measurements and the numbers they produce are directly associated to construction costs. We must be accurate and take many measurements over numerous time periods when different pressure levels occur, so we can design the appropriate barrier technology.

Step by Step By Step Process

How to soundproof a door has to be a step by step process. First, we must define how large of a sound pressure issue we must be dealing with. Lets use a control room door as our example. Our goal will be for “quiet please”. If the control room door is next to a street access we must make sure that that energy from those two sources does not enter into our control room. Control rooms should have ambient noise levels no higher than 40. Thirty is much better. This is a large difference if street noise is say 80 – 85 dB. We need a barrier that can at least stop 40 dB of sound energy.

Door Sound Lock

Our first thought must go to a sound lock. It will be the most powerful way to isolate this sound noise. We must construct a little room between two doors that is the “lock”. A control room sound lock is similar to the air lock on a spaceship. One is placing a barrier between air or lets say no air in the case of space and one is placing an air barrier for sound from the control room and the live room. What we will have is two doors separated by a small room. We want a minimum distance for our airlock of 3-4′ between each door.

Door Lock Construction

The sound lock that will be the small room between two doors, must be treated as its own room and maximized in performance from an acoustical perspective. Its acoustical size must be taken into consideration and dealt with. The inside of our “room” must be treated with the correct acoustical treatment both for low, middle, and high frequencies. Obviously, the barrier side of the structure must also conform to sound transmission loss principles. Isolation technologies for the airlock shell and sound absorption technologies for inside the “lock”.

Door Construction

The doors on each end of the sound lock must be constructed of numerous layers of materials. We want to create a structure with our door that is vibrationally inert. It must not vibrate or vibrate very little when sound energy strikes it. We will use a technique termed constrained layer, mass damping. Multiple layers of materials will be assembled in such a manner that sound transmission loss is maximized at all frequencies.

Constrained Layer / Mass Damping

Constrained layer, mass damping means that we arrange different materials with different densities next to each other and create a “sandwich” using all of these materials. materials must be chosen that have specific densities and these materials must be arranged in a way and pattern that reduces vibrations. Vibrations are created when sound pressure energy strikes the door and causes it to vibrate. We must minimize vibrations to ensure adequate sound transmission loss from one side of our door to the other.

Many Layered Sandwich

To minimize our vibrations, we create an obstacle course that vibrations must travel through with our door construction. Our goal is to interrupt the vibration signal transmission as it moves through our from outside noise sources to our interior control room. If we arrange materials of different densities next to each other, like layers of meat in a sandwich, we then surround each layer with a vibrational damping compound which is designed to absorb structural vibrations. A layering of materials with different densities, layered with damping compounds, will provide us a solid, relatively inert structure for our door barrier. Make the door at least 4″ thick following this procedure.

Bank Vault Doors

Once you have built your sound lock and your constrained layer, mass damped door, you are ready to install one on each end. Place it upon two pressure bearings top and bottom of the door centered. Make each door 36″ wide. Install long handles so that the door moves easily on the bearings, just like a bank vault door. Place a portal or round window in the door during construction, if desired. Use two 1/2″ thick panes of safety glass separated by as much air space as your door thickness will allow. Make sure your bearings are rated at least twice the door weight capacity. Make sure all edges of the door seal upon closing. The smallest hole can ruin everything.

Synergy Of Technologies

Building a sound lock between two doors to isolate noise from higher sound pressure levels is the ultimate way to isolate. We have space, air, absorption, and barrier technologies all working together, each operating at their maximum performance for their respective technology. Oh, yes, don’t forget about how cool the bank vault door is with the round portal for a window. You will be able to move 1,000 lbs with one finger strength.

## Recording Studio Acoustic Treatment

Two Main Categories

Recording studio acoustic treatment falls into two major categories. We have sound absorption and sound diffusion technologies. Sound diffusion can be broken down into three sub categories: low, middle, and high frequency absorption. Sound diffusion is its own category but must not be confused with sound redirection. Lets exam low frequency absorption technology first.

Low Frequency Absorbers

Low frequency absorbers can be freestanding or built into a recording studio. If the studio is built new from the ground up, low frequency absorbers can be built into the structure in the places that they need to be placed to absorb unwanted low frequency energy. Two main types of absorbers used in this manner are slatted and membrane absorbers.

Low Frequency Absorption

Slatted Absorbers

Slatted absorbers have slats or openings that allow the low frequency energy to enter and then be absorbed into the inside of the slatted chamber. Slatted absorbers are really another name for Helmholtz resonators. Air enters through the openings and resonates inside the slatted chamber which is designed with a certain depth and cabinet fill to absorb the problematic low frequencies the studio designer is working on. A coke bottle is a classic example of a Helmholtz resonator. It resonates around 185 Hz. Frequencies above 185 Hz. are absorbed.

Membrane Absorbers

Membrane absorbers or diaphragmatic absorbers are the second main category of low frequency absorbers. With a diaphragmatic absorber, we have a diaphragm or front wall that vibrates when low frequency energy strikes it. This vibrating front wall slows down the long, low frequency wave before it enters the inside cabinet dimensions. Inside the cabinet is sound absorbing material to assist in absorption of the low frequency energy entering it. Diaphragmatic absorbers can be designed to absorb a large amount of energy in a small amount of space.

Middle/High Frequency Absorption Panels

Acoustic Foams

Middle and high frequency absorption is well known in the recording studio acoustic treatment genre. Acoustic foams receive the most attention. Acoustic foams, more specifically, open celled acoustic foams, absorb middle and high frequency energy by converting sound energy to heat. Sound energy enters each open cell of the foam and is converted to heat. This energy transformation process results in energy conversion from mechanical energy into heat and this energy conversion process creates sound absorption. Acoustic foams are popular because they are lightweight and relatively inexpensive to manufacture.

Mineral Wool / Fiberglass

Middle and high frequency absorption can also be accomplished using fiberglass or building insulation type materials. Mineral wools also fall into this category. These materials are inexpensive to manufacture and are readily available. However, they are difficult to work with and do present some environmental issues when handling. Fibers from the material can break off the insulation and be inhaled. Proper air filtration methods must be employed when handling this material type to avoid any chance of inhaling the fibers.

Sound Diffusion

Diffusion can take many forms within the genre of recording studio acoustic treatment. We must define sound diffusion vs. sound redirection, so we are all on the same page. Sound diffusion takes sound energy and spreads it out into both horizontal and vertical planes. Sound redirection is just what the name indicates. A sound redirection device takes sound energy that strikes it and sends that energy off in an opposite direction of the angle of strike. Sound redirection and sound diffusion are frequently used as synonyms but they describe different processes.

The most popular sound diffusion in use today is termed quadratic diffusion. Quadratic diffusors are built using a series of wells or troughs that have a specific width and a specific depth. The number of wells are determined using a prime number sequence. A prime number of 7 would have 6 wells of different depths, A prime number of 23 would have 22 depths. Each well depth diffuses energy at a quarter wavelength and well width is based on half wavelength. A vertically positioned quadratic diffusor will spread sound out in the horizontal plane. A horizontally positioned diffusor will spread sound out in a fan like array in the vertical dimension.

Sound Redirection

Sound redirection devices are different than quadratic diffusors. A common sound redirection device looks like half a circle with a 180 degree arc to it. Sound energy strikes the hemisphere and depending on the strike angle, will be redirected into the opposite direction. The physical law of angle of incident equals angle of refraction applies here. There is no spreading out of the sound in a fan like array which is the hallmark trait of quadratic diffusion. Sound is simply redirected from one direction into another.

Sound Absorption / Diffusion

Recording studio acoustic treatment comes in two basic types. We have sound absorption and sound diffusion technologies. Sound absorption is broken down into three sub groups: low, middle, and high frequency absorption. Low frequency energy is absorbed using slatted and membrane or diaphragmatic absorbers. Middle and high frequency absorption can be achieved using open celled acoustic foams. Sound diffusion is not to be confused with sound redirection. Quadratic diffusion is a time tested and proven method of diffusing sound energy into the vertical and horizontal dimensions.

## How To Sound Proof A Window

Window Is A Wall

What is a window? It is a wall that you must see through for various reasons to perform certain tasks necessary in your control rooms, project studios, or listening rooms. In order to sound proof it, you must treat it as any other structure within our room in terms of acoustical issues. You must address isolation issues especially with low frequencies and reflection issues with middle and high frequencies.

Step 1: Sound Measurement

The sound isolation of a window is necessary to keep unwanted sound energy from outside sources out. If a band is playing outside your control room, you definitely want to keep that energy out. You need to first decide how much energy we need to isolate, lets say, a control room environment from. Measure the energy within the band room with a simple SPL meter. Take measurements from the band using a ballad and then a full blown rocking out song. Measure the frequency response of all that pressure and you will see most of the problem causing energy is low frequency.

Step 2: Window Size

Once you have assigned a number to all noise and sound issues, you can begin to design your window. Line of site is critical and you must decide how large of a window you really require to achieve all of your visual objectives. Determine this by blocking the existing area for the window and reducing its size down to the minimum window size required to accomplish all of your objectives. The smaller the better.

A wall that is used for sound isolation is only as good as the weakest acoustical link in the wall system. The isolation of the window can not be greater than the sound isolation provided by the wall in which the window is installed. If you have designed a window with 60 dB of isolation and your wall you will install it in is only 40 dB, you are wasting your money and time. Noise or sound energy will flow like water through the weakest link in the acoustical chain. Make sure your wall is equal in isolation to your window.

Cost No Object

Isolation is difficult for most individuals to grasp. Sound isolation numbers are hard numbers and it does not matter to the wall or window, what device or devices you use to create that energy. I have heard people say that they only want to practice their drum kit, not play on a full set, so therefore they do not need to spend very much on noise isolation. There is no relationship to the cost of isolation and the devices that will be used to create the energy. Sound energy is sound energy and is measured by a number. It does not matter what produces it or how much that device costs that does.

Step 4: Window Plates

Once you have determined your minimum window size, you next need to determine what thickness and type of glass you need. If your low frequency issues are large, the thicker the window the better. Obviously a 1″ thick piece of glass will be more powerful in reflecting low frequency energy from its surface without moving and creating vibrations which will create sound. We do not need a single pane of glass that is very thick. We can use two thinner pieces. Laminated glass is preferable over regular plate because it has a thin sheet of plastic between glass sheets that will minimize our vibrations. Laminated glass is also called safety glass because if it breaks it will not splinter because of the layer of plastic between the glass sections. So if a angry guitarists throws his guitar against the control room window, all is good.

Step 5: Window Plate Separation

We know that air space can be another layer of material that we can use to isolate sound energy with. Finding the correct distance is the more difficult decision. As a general rule, the more the better but we must work within existing construction materials and techniques. If we use a plate glass that is 4mm thick and use two plates close together, we are worse off than using a single 4mm plate especially between 200 Hz. – 700 Hz. With thin glass plates and larger spacing we are much better off. A good start point is to increase glass plate thickness to 12 mm which is about 1/2″ and to make your air space a minimum of 4″. If you have the space, a 6″ air gap is best.

Step 6 : Reflection Control

Reflections off of our control room window are always unwanted. I can not think of a single situation in which they are desired. In order to minimize the amount of reflections at the monitor position, we need to angle or splay the window surface so that the reflected energy moves away into another room surface. We accomplish this by angling or splaying the window at a minimum of 15 degrees from center. Find the area most impacted by the window surface reflections and splay or angle the window away from that area.

Step 7: Sealing And Foam Lining

When we install our glass wall, we are creating a miniature room between our two glass plates. In that room, with its walls, floor, and ceiling, we have a small box that can resonate with energy at the dimensions of the “room”. We need to line the window inside edges with acoustic foam that is designed to handle the resonances that can be created inside our “window room”. Placing the acoustic foam around the window edges with a minimum thickness of 2″ will handle most resonances within our 4″-6″ window depth. Larger window depths will require larger thicknesses of foam. Plan for this early in the window design.

Fitting a window into an existing wall requires several calculations before one starts. You must first determine what noise levels you are trying to isolate from. Next, you need to determine glass plate thickness and the distance between the two plates that will be needed to isolate from all invading frequencies. Angle the window away from the listening or monitoring position. Seal each plate to the existing structure against dirt and insect penetration and don’t forget the foam window lining.

## How To Soundproof A Room

What Is Soundproofing?

Soundproofing a room means that people want to either keep sound energy from entering a room or keep the sound made in the room, inside the room where it belongs. Most of the time both are desired. Each approach requires different science to solve and before you start any project you have to keep these two sciences in mind. So first lets define each area of barrier and sound absorption technology before explaining how to approach the solution.

Barrier Technology

To keep outside sound energy or any sound that is generated from outside your room, outside where it belongs, you need to use barrier technology. You need to create a barrier between the sound on the outside and your room. Barrier technology is designed to reflect sound energy back to the source or the direction in which it came by creating a barrier or sound blocking structure between you or the room and the outside sound source.

Concrete Mold For Barrier Wall

Sound Absorption Technology

To manage sound that occurs within our rooms, you need to use sound absorption technologies. You must absorb the excess energy within your room, so that it does does not create issues within the room. You must manage the excess energy through sound absorption, so that the sound energy does not “bleed” into the rooms that adjoin our sound room. Everyone has had this issue.

Step # 1 Define The Noise Issue

You must first define what noise issues that you need your barrier technology to stop. What is the noise level you need to address that comes from sources outside your room? Is it traffic noise, people talking, manufacturing sounds? One must put a number to this noise. One can use a Radio Shack SPL meter (60 USD) or as they call it decibel meter. Go out in the morning, afternoon, and evening and take some readings over a 15 minute time span. Takes highs and lows and average, say 10 or so readings. Measure it over a one week time frame. Include hours of most noise and then measure the quietest times. Now, you must define what you have measured.

Step # 2 Measure The Noise Issues

What kind of noise is it? What part of the frequency spectrum does the most noise occur at? What part of the frequency spectrum does the smallest sound pressure level occur at? Is it low frequency, middle frequency, or high frequency noise we are dealing with? This is a little more difficult than using a SPL meter. One will need to take frequency response readings to match the pressure readings. This is best left to the professionals unless one is familiar with the process. All of this information is required in order to build the proper barrier technology to minimize your noise issues. If you have low frequency issues from garbage trucks going by, one must build a different barrier than if you are trying to keep the phone voice from the next office from disturbing your lunch.

Step # 3 Define Your Room’s Purpose

Defining what your room will be used for is critical. If you are recording within this room, we need a certain sound pressure level of quiet. Are we recording vocals or bands. Each has different “quiet” requirements. If you are using your room as an office, you need to be concerned with middle and higher frequencies from entering, so that vocal mid range frequencies will not be impacted. Intended use is critical for defining how much we have to spend with barrier technology to protect your room’s sound environment and lower the noise floor to acceptable minimums for the room’s use.

Step # 4 Barrier Build Materials

Once you have all your data about the outside created noise and have defined your intended room usage, you can choose the materials and wall construction method to address the level of frequencies of the noise issues we have coming in from the outside. If they are low frequency issues, it would be advisable to find a new location. It is usually more cost effective to find a new location than trying to stop low frequency energy.

Low frequency energy take thick barriers to isolate you from the long low frequency wavelengths. The more mass we use and use it we must, for low frequency energy isolation, the more our costs go up. Considering the cost of isolation from these long and powerful wavelengths, it is very difficult to build the proper wall to keep this energy at bay. Every low frequency Db costs lots of mass and money to isolate oneself from. It would be better cost wise to consider another location if you have to deal with too much low frequency energy in your room.

Middle/High Frequency Absorption Panels

Step # 5 Inside Our Room

If you are in an office environment and need to absorb excess voice and office equipment noise, you can use standard sound absorptive technologies. Sound absorbing foam can be used along with sound absorbing ceiling tiles. Drapes can cover office windows and special builds can be created to control and mange equipment noise. Even a couch can be an absorber.

Recording Studio

If you are recording a band within your room, you will need to mange and control the full frequency range of sounds. Drums produce low frequency energy and that energy must be managed, so that the microphone picks up and thus records the drum sound the engineer wants. Absorbing excess drum sound means less room sound or more depending on what the producer thinks the drum part calls for. Excess low frequency energy within a recording studio is usually unwanted and specially designed and positioned low frequency absorbers must be used.

Vocal Rooms

Our vocal rooms are usually located within our studio walls. There is a reason for this. Vocal rooms are a room within a room. This is an ideal method for achieving sound isolation. They are full of sound absorption and diffusion technologies to manage the sound energy within the vocal room itself. The studio provides the barrier or shell to keep outside noise outside. Usually a balance of sound absorption and diffusion technologies are used inside a vocal room.

How to sound proof a room must be approached in a series of steps that you must follow in the order listed above if success is to be achieved both inside and outside the room. You must employ barrier technology to keep unwanted noise out of Your room and then you must use sound absorption technology to soak up excess sound energy that is generated within Your rooms from sound sources such as instruments and vocals. One must measure how large of a noise problem, both inside and outside your room, and construct the proper barrier to mange this unwanted energy effectively. Once you have your numbers, you can design and build the proper structure that will accomplish your acoustical objectives without draining your financial objects.

## Mixing Issues And Solutions

Mixing Issues

I have spoken with many engineers lately about issues they confront during the mixing process. I asked them to talk about issues that they commonly face and what they do to resolve the issues. The discussions focused mainly on equalization, compression, and time based effects.

Equalization

All the engineers had issues with equalization. Balancing the frequencies using equalization a was a common concern. One of the main concerns was using automation to compensate for an improper equalization. With an improper frequency balance, you have some frequencies that are masking others. Using automation by turning up the volume of the track that has been smothered will be a temporary fix. When the content of smothered track comes down, you now realize that the track that was turned up is now too loud.

Get It Right From The Beginning

Automation

Most engineers agreed on the solution. The solution was to use equalization to balance the frequencies between the tracks that are in question. most agreed that this was not an easy solution or process but that the trade off in time and effort was worth the results. This process was viewed as a check against doing the wrong thing. One engineer said that if you think you need to automate every word in a vocal track, you should be looking at equalization to solve the issues instead of automation.

Old Mix Review

One engineer suggested pulling up your old mixes and get rid of all the automation. Spend time with equalization and you will quickly see that a well balanced mix can be achieved without using volume automation. All commented that automation has its place because you can not always use a compressor to level out vocals that are extreme in nature. Sometimes you want a vocal part a little louder than other parts and that automation should be used mostly to create certain sonic effects by changing levels, using panning, and with some plug ins.

Use Less To Create More

Reverberation Levels

Adding too much reverb or delay was another issue that seemed to be common among the engineers. Most made the comments that too much reverb was the indication of an unprofessional mix. Usually too much reverb is used to try and solve pitch issues or as some stated to make the vocalist sound better. Vocalist wanted more reverb, especially if they were the engineer.

Reverb Necessary Evil

All agreed that the best way to use reverb and everyone agreed it was necessary, was to find the correct amount of reverb that sounded good at loud volumes and then lower the volume until it sounded good all through the mix. Some suggested bringing the reverb down to the level of barely audible. Spend time listening at this low level with the send bypassed. Most believed that this method added thickness and a richness to the mix without adding any undesirable effects. Most said that this approach also worked with the delay.

Big Picture First

If you solo a track and use EQ on it it will sound one way. If you then put it into the mix, it can sound thick or muddled. All agreed that one needs to do the majority of your processing with the entire mix playing. Examining a solo track or a couple tracks is acceptable but keep the entire mix in your mind and focus on the total mix.

Middle Frequencies

When you ask someone to listen to your mix and they tell you to turn it up because it will sound better. Obviously, these comments lack the experience of a professional mix. Professional mixes sound good at soft and loud levels. This is the result of the engineer understanding that our ears can hear more of the middle frequencies than it can the low er and higher frequencies. This is probably due to our need to hear vocals for communication purposes as a species. It takes more energy to produce low and high frequencies than it does to equal the apparent loudness of our middle frequencies.

Focus On Vocals

As you turn down your mix, you will notice that the vocals jump out in the mix since they are focused in the middle frequencies. Pay attention to the lead vocal as you turn up the mix. The drums and bass energy will start to take over the vocals. If you mix loud, you can not balance the mid range correctly. If you listen at higher volume levels, everything seems to smooth out but more difficult to hear the mid range. When you turn the volume down and listen at lower levels, the issues in the mid range stand out.

Mix At lower Levels

Mixing at lower levels is the solution. If you need to check the low and high end, one can turn it up for this purpose but once checked and verified, lower the volume levels. Less is more and this philosophy should also be applied to the use of effects and processors. Use small amounts of EQ and small amounts of compression. Train your ear to hear the subtle differences in music and add or subtract small amounts of processing just enough to compliment and not dominate.

## All Acoustic Foams Are Not Created Equal

Open Celled Foams

Acoustic foams or foam that absorb energy usually above 125 cycles are technically called open celled foams. Open celled foams have just what the name implies, open cells. If you look at an open celled foam closely you would see each one of these cells arranged cell to cell with no cap on the end. The cell structure is open in order to allow air movement hopefully carrying sound to enter each cell. The cells in most open celled foams are irregularly shaped.

Lightweight Technology

The goal of acoustic foam is to provide for absorption in a portable and lightweight technology. It is also less expensive than building cabinets or boxes. Absorption is the goal and it begins for foams that are at least two inches thick around 125 cycles. The design goal after these variables are met is to provide as much absorption as one can within that 2 inches of foam and provide it quickly. Most acoustic foams begin absorbing around 100 Hz. and climb through 4,000 – 7,500 Hz.

Active Absorbers

There are two general types of sound absorbers. They are broken down into two main categories based on how they function. Our active absorbers is termed active because it will have a front panel or wall some call it a membrane. Air can also be part of an active absorber such as in the case of a Helmholtz resonator. An example of a Helmholtz resonator is a coke bottle. If you blow across the neck of the bottle you will recreate a resonating system that starts at 185 Hz.

Passive Absorbers

A passive absorber will have no “moving parts”. Well, maybe some kinetic energy working with normal air flow across the surface of our passive absorber material. Examples of porous, passive, absorbing materials would be drapes, couches, or acoustical foam. Air flow through the porous material causes friction between the fibers or cells and acoustic energy is converted heat and transformed forever.

Absorption Coefficient

The term sound absorption coefficient is a number we use to denote the amount of absorption a material exhibits. If 50% of the energy is absorbed by the material, then we would assign it an absorbing coefficient of .50. One square foot of the material would be assigned an absorption coefficient of .50 which would equate to .50 absorption units or sabins. An open window is a perfect absorber. Sound leaves the room through the window and never returns. Each square foot of the window would be assigned an absorption coefficient of 1.00.

Auralex

Auralex is a company that is well known. It offers many room acoustic products that range from low frequency absorbers through middle and high frequency absorption. Lets look at their two inch foam as our example:

TABLE HERE 125 at 11%, 250 at 30%, 500 at 91%, 1,000 at 100%, 2,000 at 100%, 4,000 at 100%.

Sonex

Sonex is a second well known company that also has many sound absorbing products. One of their foam technologies is termed Super Sonex. Here are the numbers:

TABLE HERE 5% at 125, 30% at 250, 80% at 500, 90% at 1,000 95% at 2,000 and 99% at 4,000.

Acoustic Fields

Acoustic Fields is a new company that also has its own foam. Here is the data:

TABLE HERE 125Hz.at 30% 250 at 64% 500 at 90%, 1,000 at 1,000Hz.

Three Foam Comparison

Lets look at all three sets of data. At 125 Hz. Aurelex is 11% and Sonex is at 5%. Acoustic Fields is at 30%. Both Auralex and Sonex are at 30% for 250 Hz. Acoustic Fields is at 64% more than twice as much absorption. All are around 90% at 500 cycles and close to 100 % after 1,000 Hz.

Acoustic Field’s Difference

Aurelex and Sonex perform basically the same from 125 Hz. – 1,000 Hz. but Acoustic Fields’s foam performs differently. It absorbs at higher rates starting at 125 Hz. At 125 Hz. Aurelex is at 11%, Sonex at 5% and Acoustic Fields is 30%. Acoustic Fields foam is 6 times more powerful at 125Hz. than Sonex and 3 times more powerful than Aurelex. At 250 Hz, the trend continues. Aurelex is at 30%, Sonex is at 30% and Acoustic Fields is at 64%. This is twice the absorption at 250 cycles than either Aurelex or Sonex.

ALL THREE IN ONE TABLE HERE

Critical Band: 125Hz.-250Hz.

The 125 Hz. – 250 Hz. band is a critical band for vocals and guitars. It is critical for piano and brass. Our mixes build up in this low middle frequency area and extra absorption is always welcome for clarity and definition. This frequency range is also very problematic in today’s smaller recording studios. Smaller room dimensions create unwanted resonances within the 125 Hz.- 250 Hz. ranges. A smooth even absorption rate and level goes along way to clarity and reduces mid range “muddiness” both in playback and recording environments.

New Foam On The Block

All foams are not created equal. Acoustic Field’s foam technology absorbs at higher rates than other open celled acoustic foams. This increased absorption is centered in the 125Hz. – 250Hz. which is a critical band for our middle frequencies that affect our vocals, guitars, and pianos. This 125 – 250 cycle range is also critical in small room acoustical environments. Our physically smaller project studios almost generate resonances within this area. We need all the absorption help we can get in this frequency range.

Acoustic Fields 1″ foam: 15 30 65 92 90

## HVAC System Noise In Our Sound Sensitive Rooms

Barrier / Sound Absorption Technology

In our professional recording studio and listening rooms, we now know the the two types of technology that we need to deal with the energy within our rooms and the energy that is generated from sources outside our rooms. We use barrier technology to reflect energy from outside sources back to the source and we use sound absorptive technologies to manage the sound energy we create within our rooms. We use a combination of both barrier and sound absorption technology when silencing our HVAC systems.

Air Movement

Air moves through our HVAC systems. This can be cool or warm air and all of this air movement against the duct work causes friction and with friction we have noise. We have this air movement through the duct work and we also have the noise from the machinery that produces this cool or warm air. The fan that moves the air and the grill work that the air flow exits through are all contributors to the background noise in our sound sensitive rooms.

ASHRAE Handbook

First, we must determine what noise levels we are willing to tolerate. The lower the noise level we require for our sound sensitive rooms, the more expense we must endure. Every dB of unwanted HVAC noise costs a certain amount of money. Finding the number that works within the intended use of the sound sensitive room is a balance between noise level and budget. If one has any doubts about how to establish the noise criteria, the best book to consult on the numbers is ASHRAE Handbook. It stands for The American Society Of Hearing, Refrigerating, and Air Conditioning Engineers.

Fan Noise

Fan noise is one of our unwanted contributors. There are two basic types of fans. There are pressure blowers and centrifugal fans. Pressure blowers are the nosiest of the two. The smaller the pressure blower the more noise created. It is the opposite with centrifugal fans which as their size increases, so does the amount of noise they create. We have the blades and the motor mechanism that drive the fans. Fans with less than 15 blades, produce relatively pure tones that are spread across the frequency spectrum. This is predictable noise. The air rushing through the duct work creates vortexes which are responsible for random noise.

Machinery Vibrations

All of this machinery noise is produced by numerous vibrations. These vibrations must be managed, so that they do not transmit into our sound sensitive rooms. Mounting the machinery on the roof is not to be considered. The equipment must be mounted and positioned on its own slab of concrete or other vibrational damping structure. The mounting slab must not contact the building in any form or manner. Keep the equipment as far away from the sound sensitive area as one can physically accommodate considering all connecting components. Vibrational mounts can be utilized as long as the vibrational energy spectrum of the equipment is dictated by the isolators.

Air Movement

As air moves from the machinery that creates it through the transport system or duct work, we have air flow. The speed or velocity of the air flow produces the noise levels. For every doubling of air speed, we have a 16 dB increase in noise. The quantity of air the room requires will determine how large the transport or duct work that will be needed. It is advisable to use larger duct work than is required by a normal room that is not devoted to sound. An air velocity of 500′ / min. is considered to be the maximum amount of air flow for rooms that must manage sound energy.

Air Flow

The air flow through our duct work must travel across a smooth and uniform surface. Any bumps or curves will increase friction and friction is the baby of noise. Air flow is similar to water flow. If you are moving water through a 2″ diameter pipe, then every 90 degree bend you make, the water traveling through and the additional friction created, will be the equivalent of adding an additional 10′ of pipe in the water’s path way. The intensity of the noise will increase 5 – 6 times with the power of the moving water.

Duck Lining

To keep all of this noise under control, there are classes or types of sound attenuating devices. We can line our duct work with an absorptive lining that will be able to absorb energy. Open celled foam would be a good choice here. One must pick the lining material that will absorb at the wanted frequencies. Lower frequency absorption may be needed at one section and middle and high frequency absorption needed at another.

Blocked Line Of Site

Another way to minimize duct work noise levels is to vary the shape of an attenuator that is placed within the duct work. Duct lining allows for the absorbing material to be placed on the duct work inside walls. If one looks down the duct work they will have a clean line of site using a regular duct lining approach. If we place a foam wedge in the duct work, we can alter the air flows straight pathway. This process acts as a ” air flow re-director” by spreading or redirecting the air flow across different surfaces instead of a straight line.

Plenum Chamber

A plenum chamber is a chamber that is built into the duct work to redirect energy from a straight line shot to flowing into a plenum chamber. It is an expansive chamber or to use a car analogy, we would have a muffler inserted into the duct line. This muffler would cause pressure fluctuations which will act as a sound attenuating system at certain frequencies.

Compressors, Fans, Duct Work

Our HVAC systems move our warming and cooling air through our sound sensitive rooms. Care must be taken in locating the equipment that produces the temperature changes to the air. The cooling mechanism and the fans that move it are to be located away from our sound sensitive environments. Once we have the air source treated, we must then focus on the duct work where all the air will move through. Reducing the friction this air flow causes within the duct work is accomplished using numerous options.

## Ways To Increase Existing Wall Sound Insulation

Barrier Technology

When we look at our existing walls in our sound rooms, whatever the type of room we are in, we need to keep in mind that we are dealing with two very different processes. We have to keep the sound that is coming from sources outside our room, outside where it belongs. We also need to keep the energy created within our room, inside our room where it belongs. We use barrier technology to keep energy out of our room. We use sound absorptive technology to manage the excess sound energy within our room.

Reflection Vs Absorption

Only three things can happen to sound. It can be absorbed, reflected, or diffused. It is sound reflection that we are concerned about with any barrier technology which is the term for the technology to keep sound out of our rooms. Mass in our structure reflects sound energy back to the source it came from with our barrier technology. Mass through weight increase will help us to produce a surface that will reflect sound back to its source, so that it will not enter our room where we only want to deal with the energy we produce within the room.

Mass Is Friendly

Mass is our best friend when it comes to barrier technology. Mass can take many forms. It can be drywall, plywood, concrete, or even lead. Density or the weight of any material per square foot is the number we need to look at. The higher the number or the greater the density will give us isolation properties against sound energy of all frequencies. The lower the frequency, the greater the mass we will need to stop it from entering our room. If your studio is next to a highway, drywall will not stop the traffic energy from entering your room.

Concrete Barrier Molds

Air Is Good Barrier

Air is also a good barrier element. If we take two walls of a certain density and add an air space between the two walls, we can keep more energy out. A 4″ air space is the minimum. A six inch air space between two walls is a better starting point. We also need to make the mass of each one of our two walls different in order to alter the vibrational signature of each wall to keep the vibration transmission of sound energy “confused”. Placing insulation type material in the air space between the walls will also increase the sound transmission loss by reducing internal wall resonances.

Staggered Studs

If we use staggered studs to support our walls, we can gain another advantage. Staggering the studs breaks up the vibrational paths that the energy can take. If sound energy from the outside of our room strikes our barrier, it is changed from acoustic energy to mechanical vibrational energy. By staggering the studs, we keep the vibrational path ways uneven in position and placement and this unevenness will disrupt the vibrational energy flow’s pathways.

Concrete Barrier

Resilient Strips

Resilient strips follow this similar thinking. A resilient strip is another way to alter the vibrational pathways and create a new barrier asset. If we use another layer of drywall to attach to an existing layer, we can use a resilient strip to place between each layer of drywall. A resilient strip will attach itself to one wall and then the other. The strip will also add a small layer of air between each layer of drywall. The combination of the resilient strip and the additional air space will establish another multiple layer of defense from vibrational assault.

Wall Cavity

Inside our dual wall cavity there will be an air space separating each wall. Each air space has a length, width, and height. It is a small room by itself. It will produce the same sonic issues as a regular size room only on a smaller proportion. We will have to deal with those issues accordingly. If we fill the space with sound absorbing technology, we can control those issues. Our goal is to create a wall that keeps unwanted sound energy out. Our goal is not to produce another sound generating source, especially one the size of a wall.

Concrete Barrier Finished Edge

Acoustical Sealant

Sound can leak through any opening in our structure. Sound energy is like water. It will find the smallest opening to enter and come through. All edges of our material that touch another surface must be caulked or sealed. The proper sealant must be used. It must dry when applied but not dry completely, It must be pliable so that expansion and contraction of our wall surfaces can be allowed to occur without breaking the seal surface to surface contact. There are many quality acoustical sealants available for this important task.

Doors And Windows

Doors and windows must be given the same care and attention as our walls. Just as we used double walls with airspace between them to increase our STC or sound transmission loss number, we must use double and triple pane windows to isolate our sound energy. Laminated layers of glass will go along way to reduce the vibrational energy that sound energy generates. Laminated glass is the same glass we have in our windshields. Each layer of glass is separated by a layer of plastic. Our doors must be solid and be sealed correctly, so that no sound energy can leak in or out of our room.

Barrier / Sound Absorbing Technology

Mass and air space can assist us in our sound isolation efforts. It is the two working together that creates a good barrier combination. In order to determine how much density and what type materials we need to use, we need to know how much noise we are dealing with from the outside and how much energy we will produce on the room inside. We need to also keep in mind that any energy generated within the room that leaves the room will be reflected back into the room by our barrier technology. Care must be taken inside the room with sound absorbing technologies to minimize any energy leaving the room and striking our barrier shell.

## Room – Resonanting Chamber

Room Is Chamber

What is a room? It has four walls, a floor and ceiling. It is a box or chamber. In this chamber, we introduce sound energy when we speak or play instruments. All of the sound energy sources within our room have a specific low frequency energy and a highest frequency energy that they produce. This is called their frequency response range. Once this energy is interjected into our room, it must all fit into the room or chamber. Lets look at each surface of our chamber.

Two Parallel Surfaces

The two side walls of our room or chamber are part of a resonating system. in fact, they are a resonating system all by themselves. The sound energy that enters our chamber strikes one wall and that energy then in turn, strikes the opposite wall. After the sound energy strikes the second wall, the process is repeated with third, fourth, and fifth order reflections. As these waves and rays of energy make this journey of reflection multiple times, they start to combine. If the wavelength of the sound is a factor of the physical distance between the walls, a resonance is created. This resonant is termed a standing wave.

Maximum / Minimum Pressure

Each standing wave and there as many of them as there are distances between room walls has a maximum sound pressure region and a minimum sound pressure region. The maximum region of pressure will be that area that is closest to the wall boundary surface. The area of lower pressure or null will be middle distance between the walls. If we double the chosen frequency, we will have another area of maximum pressure and another area of minimum pressure created just as in the first example. If we triple the example frequency, the same thing happens. Now, we have three areas of maximum pressure and three areas of minimum pressure.

More Modes

These examples are for one set of parallel surfaces and are termed axial modes. We have two other mode producing energy situations that we must take into consideration. The oblique and tangential resonating systems involves energy moving from different surfaces other than a single, parallel neighboring wall. These two new modes produce their node creating energy in the same way as axial modes but involve different surfaces.

Tangential / Oblique

A tangential mode occurs as a result of four different surfaces. We can have a tangential mode created with the two side and front and rear walls. The oblique modes occur when the energy travels between six different surfaces. All of these surface areas assist us in reducing the intensity of each reflection and thus mode. An axial mode only has to travel between two surfaces, so axial modes have the highest pressure of the three modes in our room or resonating chamber and are the most troublesome to deal with.

Low Frequency

Our low frequency energy waves must fit into our chamber. At 40 cycles, our low frequency wave is almost 30′ long. If we have a room that is 30″ long then we have a room that can receive the full length of the wave without it striking a room boundary surface. Most rooms do not have a 30′ dimension to work with. Most rooms are much smaller. Smaller room sizes forces the low frequency wave to “cramp up” and cause resonances within the parts of the room that the wavelength does not fit.

Vocals Fit Well

Our vocals have a less difficult time. Our vocal range is 75 Hz. – 800 Hz. With higher frequencies we have shorter wavelengths. Shorter wavelengths will fit more easily into smaller room dimensions. Vocal resonances within a room are rare unless one is recording a large number of individuals such as a choir or other larger mass amount of individuals and the room is not large enough. Vocals also do not produce the energy that say a kick drum would produce. Vocal cords are much smaller moving diaphragms than a kick drum head.

Pressure Piles

Pressure build up at certain positions within our room or chamber. The corners of our room or chamber are areas of higher pressure build ups. Along with the corners, we have any floor/wall or ceiling/wall intersection where energy likes to accumulate. Throughout the room’s center area these pressure piles build their way along and carry through the room or chamber center. Each resonance or pile of energy has its own bandwidth or frequency range.

Mix Colorations

All of these pressure piles produce colorations to our sound energy that enters into our chamber. Room modes produce resonances that can exaggerate certain frequency ranges that are part of the frequency response of the resonance. If your microphone is placed in the pressure maximum area of the mode, it will be smothered with resonances and you will be not recording certain frequencies at the microphone position. If that resonance’s frequency response is in the same range as an instrument or vocal you are trying to record, you will be fighting the resonance through your whole mix. If you place your microphone in a pressure minimum area, you will not hear everything that is produced by your chosen sound source.

No Cubism

A resonating chamber or room that has the length, width, and height all equal would be cube. With a cube all axial, tangential, and oblique modes will coincide along with their fundamental frequencies. Low frequency pressure build ups within the “cube” would be almost impossible to control. One could make the cube larger to increase pressure handling qualities or one could make one or two room length,width,or height dimensions even smaller provided the appropriate type of low frequency sound absorbing technology is used.

Resonating Chambers

Our rooms are resonating chambers. The amount of energy created from our instruments and vocals must be taken into consideration and the correct size of the room must be coordinated with the intended use. Too much of the wrong type of energy within the chamber sets off a series of resonances. Modal issues can impact the information received at the microphone position. Proper room acoustic treatment can assist in minimizing room resonances, but there is no substitute for getting the room size correct from the beginning.