Parts 1 – 3

In part one of this series, we discussed how important it was to choose the correct room size. In part two, we discussed speaker and listening position and its importance to room frequency response. Part three discussed low, middle, and high frequency room treatment. In this part four, we will focus on low frequency resonances and the importance of low frequency management through appropriate absorption technologies.

Low Frequency Definition

There is so much misunderstanding when it comes to low frequency energy in small room environments. There is also a large misunderstanding of what constitutes low frequency. Low frequency is any frequency below 100 Hz. Human hearing is broken up into five critical bands in our audio world. Low frequency is any frequency below 100 Hz. Lets keep this round number of 100 cycles in mind as we proceed forward with our discussion.

Five Frequency Bands

Low Frequency Pressure Plots

Low frequency energy into our rooms does not fit because of its long wavelengths. Since it doesn’t fit, it builds up and creates pressure areas within the room. These pressure areas occur at room boundary intersection such as the intersection of floor to side walls and side walls to ceilings. The corners of our room are also areas of higher pressure for low frequencies. The corners of our room are also havens for other room modes that represent higher frequencies. We must pay attention to these low pressure areas if we are to have any chance of an equal representation of all frequencies at our listening position which is our goal.

Low Frequency Management

There are three main ways to manage low frequency energy within our rooms. First, we can choose a room size that can handle the pressure that long low frequency energy creates. This is the best option, although the one not usually chosen. It is better to choose a room that has the proper ratio of width, height, and length to be able to handle low frequencies. There are ratios that work well with lower frequencies. One can spend the money up front and choose a room that has the proper size and volume or one can add sound absorption technology to existing rooms and spend more money. It is a case of pay me now or pay me later.

Tuned Absorbers

Tuned absorbers called Helmholtz resonators are another option. These are technologies that are cabinets with certain depths or lengths and have a tuned port or slat that air enters through. The air vibrates inside the chamber and is reduced in intensity. They can be built into walls within the room or be freestanding units located in areas of highest pressure. A glass coke bottle is an example of a Helmholtz resonator, with a resonant frequency of around 185 Hz. Frequencies above 185 Hz. are absorbed. The resonant frequency of the unit defines the lowest limit of the unit’s absorption.

Diaphragmatic Absorbers In Listening Room

Diaphragmatic Absorbers

Diaphragmatic absorbers are another option that is used extensively in the professional market but not readily known in the consumer market even though the interior walls of their listening room are probably a diaphragmatic absorber. A diaphragmatic absorber is a cabinet with a certain depth and a certain density. Both depth and density are critical components. One needs cabinet depth to absorb lower frequencies along with the cabinet density. Foam, tubes, and boxes filled with building insulation will not absorb frequencies below 100 cycles. They will above 100 Hz., but not below.

Resonance Locations

Low frequency resonances must be defined and located within the room. Once defined and located, one can design the particular low frequency technology that will absorb at the necessary rate and level to minimize the amplitude of the resonances within our rooms. We may need to design absorbers that can handle low frequency issues that go from say 30 Hz. – 50 Hz. These frequency specific absorbers must be designed with care and attention to cabinet size, depth, and density. Broadband absorbers covering a wider frequency range can also be designed. One must define the low frequency issues and then design the proper type of absorber and place it in the correct location.

Low Frequency Management

Managing low frequency energy within our smaller rooms is always an exercise in compromise. Without the correct ratio of width, height, and length, we are always climbing up hill. We will never be able to achieve a perfect absorption curve, but we can minimize resonances so that they do not pervade our sound stage presentation all the time. We can forget about achieving a flat response down to 20 cycles and use 30 cycles as our low end goal. Thirty cycles represents the lowest string on a five string bass and will also cover any low frequencies generated by a bass drum.

No More Mud

Without proper low frequency energy management, our sound stage presentation will be plagued with low frequency mud. We have all heard this. It is the blurring and smearing of everything we value, especially in the mid range area where our emotional connection to the music lies. Resonances can be at 30 cycles and all of its fundamentals. A microphone placed in a room mode will not hear certain frequencies and may hear too much of others. Resonances and their reduction through proper low frequency management technologies should be the first goal of any listening room build.

Part 1 – Part 2

In part one of this series on How To Build A Dedicated Listening Room, we focused on room size. Starting with the correct room size is critical and sets the stage for the type of acoustical issues you must deal with down the road. Part two focused on positioning of your speakers and listening position. With a two channel system, there is a position for your speakers that will compliment the room’s size and volume. In part three, we will focus on low, middle, and high frequency room acoustical treatment to minimize reflections at the listening position and to manage low frequency resonances.

Low Frequency Issues

Low frequency issues should have been minimized if we followed the steps listed in part one. However, real estate is expensive and we must sometimes compromise on the dimensions that we really need. To really go after low frequency issues and the resonances that low frequencies produce, we need to make the room larger. We need a larger width, length, and height. Since that is not usually an option, we must go the other way and make the room smaller. We must make it smaller by installing low frequency pressure producing technologies.

Axial Modes

Low frequency energy issues are found mainly in the axial modes. An axial mode is the energy that occurs between two parallel surfaces. It is the energy that resides between the floor and ceiling, the two side walls, and the two rear walls. It is the most powerful of all the modal resonances. This is where our low frequency absorption technology must be positioned. We must have it in every corner and along the two shortest wall dimensions that we have. If the room is 15′ wide and 19′ long, we must have the low frequency absorption placed along those two short walls. We can not use foam or tubes filled with building insulation. We must use diaphragmatic absorption.

Diaphragmatic Absorption

Diaphragmatic absorption is pound for pound, the most powerful and absorptive low frequency technology one can use. It can be made to absorb across a broadband of frequencies or it can be made to absorb at certain frequencies. A diaphragmatic absorber has a front wall or two front walls that slow the low frequency energy wave down. Once it is slowed down, it can enter the inside of the cabinet and be absorbed depending on the internal cabinet fill material. Mineral wool, building insulation, and fiberglass are traditional fills but activated carbon or charcoal gives you the best performance to size of cabinet ratio.

Activated Carbon (charcoal) Diaphragmatic Absorption

Quantity Vs. Quality

How much low frequency absorption do we need? This depends on room size and volume. The smaller the room, the more one needs. Only by taking frequency response measurements within the room after the low frequency diaphragmatic absorbers are in place with tell the story. To impact the frequency response curve of the room, will require at least both sides of our 15′ width to be treated with absorbers. From that start point, one can add or subtract units based on the room’s attack and decay times in frequencies below 100 cycles.

Room Surface Reflections

Reflections from our room wall surfaces intermix with the direct sound from our loudspeakers. The direct sound is the sound that travels in a straight line from our speakers to our ears. This wanted direct sound is the purest sound quality our system can produce. Once the room wall reflections enters the direct sound, we now have direct and reflected energy mixed together. This is not a bad thing. We want both direct and if you will indirect sound for more realism and lifelike presentation. The sonic goal is to find the correct balance between room sound and direct sound.

Reflections Produce Spaciousness

Our reflected energy from our side walls is critical for the impression of spaciousness in our sound stage presentation. We do not want the reflections from our side walls arriving at our listening position “ahead” of the direct sound. We want to delay the reflections from our side walls, so that they arrive after the direct sound by a time frame of 15 milliseconds. This small time delay will add spaciousness and assist us in creating our sound stage.

Sound Absorption Technologies

To achieve this small time delay, we can use absorption or diffusion technologies. For the sake of this discussion, we will use absorption. The market is full of sound absorbing panels and choosing the correct one should not just be a matter of price and convenience. One must choose a panel that has the proper rate and level of absorption that will treat all the important frequencies with equal respect. We need absorption technology that does not over absorb or under absorb at any frequency below 500 cycles. Why 500 Hz. or lower? Because this is the region where all our vocals occur and our vocals are what provides the emotional connection we all seek from our music and sound stage presentation.

Acoustic Foams

Acoustic foams are popular treatment options and most work well above 500 Hz. The real test of an acoustic foam is how well it works below 500 cycles. Most acoustic foams absorb very little at 125 Hz. and absorb a little more at 250 Hz. This frequency region is the true test of a foam. Do your research and find a foam that has a smooth response curve below 500 Hz. with a gradual but smooth absorption rate and level.

Part 4: Diffusion And Electronics

In part 4, of How To Build A Dedicated Listening Room, we will discuss the other room surfaces such as ceiling and front and rear wall of our listening room and their overall impact on our sound stage. We will also discuss the electrical requirements for our system so that we are not introducing more “noise” into the room. That we don’t need. We have enough “noise” just dealing with the room boundary surfaces with reflections and low frequency resonances.

Part One

In part one of this series, we focused strictly on room size and volume. It is the most single important variable when it comes to realizing the full potential of our music systems. Without the room to run free, low frequency energy builds up within our rooms and excites the air inside our room turning it into a resonate cavity. Resonances must then be managed through proper room absorption technology and dealing with resonances below 80 cycles takes special sound absorption technology. If we get the dimensions correct from the beginning, we will hear more music from our systems.

Speaker Set Up

Once we have determined the correct room size for our playback system by following the suggested height, width, and length ratios, we are ready to set our two channel system up. Finding the correct location for our speakers and listening position is our task and we must locate these three positions at proper distances to obtain the smoothest frequency response we can achieve within the room. There is a position that will give us the smoothest response where all frequencies are evenly represented and resonances are minimized.

Is This A Dedicated Listening Room ?

How To Begin

To start, lets divide our rectangular room into thirds. A rectangular room configuration is essential because it offers us predictability. With a rectangular room’s parallel surfaces, we have energy that is moving and striking surfaces that are predictable distances from each other. Yes, parallel surfaces are not wanted when we are dealing with middle and high frequencies, but this can be better managed in a rectangular room. It is also easier to deal with low frequency issues within a rectangular room due to the room’s shape consistency.

Measure, Measure, Measure

After we have divided the rectangular room into thirds, we need to position our speakers along the first third division line we have set up within our room. Position the speakers at least 4′ from the side walls. Make sure both side wall/speaker distances are equal. Now, take a frequency response measurement at the listening position. There are numerous software programs that will assist you with this measurement. Look at the curve and notice any peaks or dips in it. Divide the response curve up into two sections. look at the response curve above 100 cycles and then look at it below 100 cycles. Move your speakers 6″ forward from your start position and take another measurement. Repeat the same procedure by moving the speakers 6″ back from the start position and measure again.

Is This A Dedicated Listening Room ?

Look For Patterns

Now, lets examine the three response curves we have taken. We should begin to see some patterns developing. Lets look at the response curve below 100 Hz. What does the curve look like at the original start position. How about the one with speakers forward. Is it smoother or more exaggerated. Is the response curve below 100 Hz. better when the speakers are more forward into the room. It probably will be. Now, lets move the speakers another 6″ forward and take another measurement. Repeat this procedure focusing on all energy below 100 cycles. One will start to see a pattern developing and the smoothest curve will eventually show itself.

Listening Position Position

We want our new speaker locations to be in synch with the listening position. If our speakers end up being 8′ apart, then we should start with our listening position at 8′ so that the speakers and the listening position form the angles of an equilateral triangle. If more sound stage width is desired the listening position can be moved backwards. Move at small increments. Don’t forget about listening position height. One may find that elevating the listening position up a few inches may take the listening position out of the direct beaming of the tweeter and balance the tweeter and mid range out more evenly. Remember, it is the off axis energy that includes the room sound and moving listening position up may benefit.

Cat On Sub Woofer

Sub Woofer Position

Are we adding a sub woofer? If so, we need to treat that low frequency device as its own system and find the correct position for it. Do not and I repeat do not put it into a corner. The corner of our rooms is where all room modes end and accumulate. We do not want to place a low frequency energy device in the corner and excite all of these room modal issues. Lets leave sleeping modes lie. Place the sub woofer along the longest wall about 1/3 of the way down the wall. Measure the room response. Raise the sub woofer up 12″ in the same position and measure again. Change to the shortest wall width and repeat the procedure. You will quickly see that maybe two sub woofers and possibly three would be even better to equalize the pressure within the room.

Move And Measure

Speaker and listening position have to be located within the smoothest measured room frequency response curve location which is determined by moving speakers and then measuring. All of this moving and measuring must have an eye towards the best response curve. It will appear, just keep moving in small increments and then measuring. Once found, align the listening position from the speakers. Don’t forget to move up at the listening position a few inches to add more room sound. Off axis sound is different from on axis frequency response. Treat the sub woofer as its own speaker. Start 1/3 of the way down the long wall and measure. Raise the sub off the floor and measure. This up front time will be well spent, you will see in part three of this series on “How To Build A Dedicated Listening Room”.

Dedicated Listening Room

A dedicated listening room is just what the name implies. It is a room dedicated to listening to music in. It is usually a two channel system with left and right channel along with the listening chair. The dedicated listening room is not for any other use. It is not for playing pool or ping pong. It is not a home theater room with multiple mono sources. It is a room dedicated to the playback and enjoyment of only two channel audio in all its splendor. Don’t have one but want one? Lets set one up.

Ideal Room Size

Our first step is to choose the correct room size. We determine this by the music type we will be listening to. If we will be classical music focused then we need to plan for frequencies that are starting at 20 Hz. and moving up from there. If our music choice is Jazz then our room should handle low frequencies down around 30 cycles. With rock music, 40 Hz. is a good beginning point. For this exercise lets assume a classical taste and with that in mind plan for a room that can handle sound energy down into the 20 cycle region. With that parameter, we will be fine for all other music sources.

Is this a dedicated listening room?

Air In The Room Vibrates

Our room dimensions determine how much sound energy that particular room can handle. Think of our room as a large container that we are going to “pour” sound energy into. The size of our container determines how much energy the room will “hold” without splitting apart at the seams. It is the air that vibrates between our room walls that causes resonances.

Room Dimensions Critical

Our dedicated listening room must be designed with the proper dimensions and thus room volume to minimize resonances. Resonances are sound energies way of telling us that they do not like the fit of the room. It is like buying a medium shirt when you need a large. For women, it is like squeezing into a size 6 dress when you are really a size 10, eventually something must give. The clothes will tear. The room will create resonances that can smother or blur and even exaggerate certain frequency ranges associated with our music choices.

Is this a dedicated listening room?

Ideal Room Dimensions

To minimize resonances, we need to look at our chosen 20 cycle wave as our low frequency goal and allow for that size wavelength to run free. Our 20 cycle wavelength is roughly 56′ long. To calculate exactly, simply divide the speed of sound, 1130 by the frequency 20. For our room purposes, we need to have the space for one half of this wavelength, say 28′, to run uninhibited without slamming into a wall or structure. Therefore, we need at least one room dimension to be larger than 28′. Lets call it 30′ for discussion purposes.

A No Compromise Dimension

The wavelength travels down our 30′ length and then strikes the wall and begins its journey back to its source which is the opposite wall. It will fall short of the other wall because our 20 cycle wave is 56′ long and it is traveling 60′ total. It only strikes the one wall and not the other. This minimizes resonances. To complete this resonance free scenario, we need a room that is 30′ in all three dimensions. This is ideal acoustically with no compromises. Unfortunately, the real world and existing room sizes are much smaller and are nothing but a compromise that must be dealt with to minimize resonances.

Room Ratios

There are a set of ratios that various individuals have proposed that do their best to minimize resonances but remember these are all compromises from an ideal room size and volume. What compromises one is able to live with usually depends on budget and room location. Room size and the ratios of height, width, and length must be taken into consideration. We have seen from our example of a 20 Hz. wave that we need 30′ in an ideal situation to ensure that the air inside our rooms does not begin to vibrate and produce resonances because the air within the room is squeezed into a room dimension it does not like.

Is this a dedicated listening room?

Follow This Ratio

The literature is full of numerous room ratios of height, width, and length that will minimize those dreaded axial modes, resonances between two parallel surfaces. To simplify these ratios, one can use the ratio of 1:1.14:1.39. If we take these numbers in order of appearance, we have the ceiling height as our first number, the room length as the second and the room width as the third. If we put 10′ in for the room height in the first position, we come up with a room length of 13.9′. Next, we keep our 10′ room height constant and calculate the room width to be 11.4′. Proper room dimensions will go a long way down the road in minimizing the type of low frequency absorption we need to use.

In part two of How To Build A Dedicated Listening Room, we will discuss proper speaker and listening positions and low frequency management along with reverberation times.

High End Clients

We have the good fortune of working with very high end clients. These clients have the funds to see a project through from the design to completion without too much concern over costs. Even though money is no object, price is always a concern, but a few thousand here and there is not that big of a deal if the end result justifies the expense.

Barrier Technology

One of our clients, who we built a small project studio for, was tired of the traffic noise from the street they lived on. It was not that busy of a street but the client wanted to reduce the street noise in his outside yard area to as low as levels as possible. The outside patio and pool area were the primary social settings and less noise and more privacy were the objectives. Barrier technology needs to be employed.

STC – Sound Transmission Class

Sound transmission loss or STC rating is the number assigned to the structure we need to design and build for a building. This STC measurement really does not apply but we can use it as an example to illustrate what we are trying to achieve. Our measurements indicated that we needed to build a barrier that would have a STC rating of at least 65 to put the breaks on our 40 cycle sometimes energy from the garbage trucks and motorcycles. If we are trying to stop 100 SPL from the source, a barrier with a rating of 65 will reduce that pressure level down into the 30 – 40 dB range. This is acceptable for noises that appear infrequently. It is also very acceptable for noise levels that are consistent and steady.

OITC – Outdoor / Indoor Transmission Class

STC would be a good number if we are constructing a building, but this is an outside barrier configuration. For this measurement we need to use an OITC number. This is a number that measures the sound transmission between outdoor and indoor structures. OITC uses a noise source spectrum that takes into consideration frequencies below 80 Hz.

Full Frequency Range Design

Our goal for the structure was to be able to handle the full frequency range of sounds faced from the traffic on the street. Since it was a residential area, truck traffic was minimal but the garbage trucks in the area produced the lowest frequencies. You know the ones that set off all the car alarms as they go down the street. We had low measured frequencies in the forty cycle range and at 100 SPL at certain times of the day. If we could build a barrier for this frequency range, we were not that concerned about middle and high frequencies.

Concrete Barrier Molds

Big Brother

Obviously, we had to conform to existing city codes. Our goal was to build the barrier as high as possible. After checking with the city, we realized that building our fence/barrier could be no more than 8′ high. Height is critical with outside barriers to reflect as much sound back to its source as possible and out of our living area, but big brother would only permit us up to 8′.

Poured Concrete

Our design consisted of starting with a poured concrete barrier of 8″ thickness. Poured concrete is expensive, but it is the best barrier material we could use. We used the Fox Bloc molds for our initial design. With these molds, one can leave the mold on after the pour and wall is dry or you can remove the 2″ thick mold sides and attach anything else to the wall itself depending on your acoustical requirements.

Concrete Barrier

Arizona River Rock

We were fortunate that the client had a large supply of what we call river rock in Arizona. River rock is a solid rock material with average rock size of between 6″ – 8″ and 2″- 3″ deep. They are dense rocks that have have had their edges smoothed out from having water running over them for probably millions of years. The average size river rock weighs between 5 and 8 pounds.

River Rock Barrier

River Rock Attachment

After our initial concrete poured barrier wall was in place, we began the process of attaching the river rock to the wall. We tried numerous adhesives, but we wanted one that would be able to handle the hot summer days in the desert and the colder winter nights of the Sonora Desert. Temperature extreme ranges can go from a low of 20 degrees in the winter to a high of 120 degrees in the summer. This is a 100 degree temperature swing range that must be accounted for. We do not want 8 pound rocks falling on anything or anyone. We chose to “wrap” the structure with steel rebarb that would rust naturally and lend its appearance to the natural rock structure.

Barrier Fence / Wall

Since it was an outside structure and in view of everyone, we had to make sure the river rocks were placed together in a pattern that lent itself to complimenting our 8″ thick poured concrete wall. This was the time consuming part of the project and accounted for most of the project’s cost. We wanted each rock to inlay with the the one next to it, so there was no gaps showing that would show the solid concrete wall structure behind.

Live Room Wall ?

After the project was finished, I stepped back and looked at the structure. I could not help but think how great this build would be for a recording studio. Not only could the studio benefit from the barrier technology and its high STC rating, but I think it would be a unique design for the inside of a live room. The river rock and solid concrete wall would provide the necessary sound attenuation from both inside and outside sources but the river rock would provide a natural diffusion for middle and high frequencies. We would need to compliment the other live room surfaces with sound absorbing materials to keep the sonic balance of the room.

Recording / Monitoring

How to soundproof a recording studio can be broken down into two main parts. First, we have the recording environment. Is it for recording vocals, drums, choir, band? What will the sound producing sources be that will be recorded? The second part is the monitoring or playback environment. Is it the mixing, control, or editing room for film? What are we monitoring? Both of these areas have different soundproofing requirements.

Live Room

Live Room

The “live” room or room in which instruments will be recorded in needs to be able to perform two very different acoustical functions. First, it must minimize the sound transmission from the live space to the rest of the facility. It must also employ sound absorbing and sound diffusion technologies to manage all of that “live” energy.

Concrete 8″ Barrier Technology

Barrier Technology

In order to keep the sound energy within our room and to also keep sound energy out of our room, we need to construct a barrier between us and or our microphones and the outside room noise. This barrier does two things. It must reflect sound energy that occurs outside of our rooms, back towards the outside source and it must also reflect sound energy created within the room, back into the room itself. Yes, a barrier keeps sound out but also reflects sound back into it. The barrier works both ways and we must be aware and plan for this.

Concrete Barrier Molds

Poured Concrete

A good barrier is poured concrete. One can use molds and build a wall by pouring concrete in the molds and letting it dry solid. The outside foam of the mold also helps a little with some sound isolation. A solid concrete room also has vibrational reducing qualities over a room built of wood. A wood framed room is another option. One should use two 2′ x 4′ walls that are at least 6″ apart. Both walls must be mechanically decoupled from each other and the existing room ceiling and floor.

Cinder Block

Another good barrier technology is block. One can add different materials to the block center to lower the sound transmission rating even more. Block is economical and can even be purchased with a quadratic diffusor built into the actual block itself. Sand can be added to the block center to minimize sound transmission through the cinder block sides. Barriers are necessary for our “live” rooms and our monitoring or control rooms.

Low Frequency Management

Inside our “live” rooms, we need to manage both low, middle, and high frequencies. Our goal acoustically is to make the room as balanced as possible with a smooth frequency response curve. We must address any room modal resonances with powerful low frequency absorption. Only a tuned absorbing technology will work for low frequencies at the SPL levels generated within a “live” room. Foam and boxes filled with building insulation will not work.

Middle And High Frequencies

We must always manage middle and high frequencies throughout the room. This management of middle and high frequencies can be accomplished through the use of absorption and diffusion technologies. In our live rooms we use absorption to lower reverberation times by minimizing the strength of reflections from the room’s surfaces. We do not want to overload our microphones with too many reflections. We need to know what the room sound is at our microphone positions and manage it accordingly. I like live rooms that have variable acoustic technologies. They can move a panel in with middle and high frequency absorption and produce a whole new room sound.

Low Frequency Absorbers

In a live room, portable low frequency absorbers can be placed in the room where resonances will be lurking. One can also keep the low frequency absorption next to the source of the energy such as a bass drum or tom and absorb excess energy closest to the source that is producing it. Absorb the low frequency waveform as soon as it leaves the source, so that it is reduced in strength immediately. Control how fast the low frequency energy “grows” in the room.This process takes a special absorption technology termed diaphragmatic absorption.

Low Frequency Absorption

Diaphragmatic Absorption

Diaphragmatic absorption consists of a diaphragm that low frequency energy moves through. The diaphragm actually moves in response to the low frequency waveform. Two front walls moving in sympathy are even better. Once the waveform is slowed by the two moving diaphragms, it then enters the cabinet. Inside is a powerful sound absorbing material. Mineral wool is a possible cabinet fill material because of its higher density when compared with normal building insulation. Activated carbon or charcoal exceeds both of these materials by a factor of 10.

Monitor Room Barriers

The monitor room of our studio needs the same barrier technology as just described for the “live” room with the addition of a sound lock. A sound lock is two doors separated by a small hallway. This provides a triple barrier approach; the two doors one to the outside world and one to the control room and the air space between them in the form of a hallway , which is also acoustically treated.

Windows Are Barriers

Windows also come into play in the control room. They are another barrier technology that must be addressed in any how to soundproof a recording studio tutorial. They must be seen through but must stop sound. make sure the wall in which the window is installed is equal in acoustic strength to the window. If you are using two 1/2″ glass plates separated by air, then you have a very powerful barrier. Make sure the window’s wall that is supporting it is equal in sound transmission loss strength.

Rear Wall Diffusion

The rear wall of our monitoring room or control room should always be diffusion. The type of diffusion can be debated, but not the need for diffusion. The time delayed bounce off the rear wall at the monitoring position can not be. It is difficult enough to get a balanced mix with control room sound added in. We definitely do not want a large reflection that is severely time delayed interfering with the direct sound from our monitors.

Common Needs

Live rooms and control rooms require the same barrier technology. We want to keep energy out and energy in. Low, middle, and high frequency absorption technologies must be used in both. Diffusion can be optional in a live room of adequate size. It is mandatory in our control room. Quadratic diffusion can provide two dimensions of sound diffusion for our control room rear wall.

Energy Management

How to sound proof a room for drums is all about energy management. Drums produce a lot of energy at many different frequencies. Bass drums can produce energy at 30 Hz. Toms can produce energy starting at 80 cycles and snares fall in between. So, we have an energy producing device that is capable of large pressure levels (over 100 SPL) covering all frequencies and we are going to take this instrument and place it in a small room and expect the adjective soundproof to apply. Lets think through this step by step, starting with the largest offender first.

Low Frequencies First

Lets start with the lowest frequency producing instrument in our trio, the bass drum. The lowest note produced is 30 cycles. A 30 cycle wavelength is about 40′ long. We want this wave to behave itself in a room that has its largest dimension at say 12′. Well, it is not going to behave itself. It is going to want to escape from that room into the next rooms. The energy that does stay will create pressure nodes within the room and excite all resonances that are fundamentals of the 30 cycle wave and the room dimensions.

Drum Room

Tom And Snare

A tom will produce energy at 80 cycles. An 80 Hz. wave is 15′ long. In a 10′ wide room, the wave will travel 10′, strike a wall and the remaining 5′ will rebound back into the room and play havoc with mid room modal resonances. The middle of the room is not where you want resonances to occur, especially resonances introduced and magnified by a drum which can easily push pressure levels over 100 SPL. How do we manage all of this energy that is squeezed into a tiny box.

Pressure Levels

With a drum room, our first objective is to choose a room that has the necessary volume to support all wavelengths and the pressure levels they will be produced at. A room whose resonance signature can be manged with sound absorbing technology. We need a room that is at least half wavelength length or width of the lowest frequency that our bass drum is capable of producing. That would be at least 20′ in width and ideally 20′ in length. We know we are going to have resonances because we are in a box, but lets choose our battles wisely and choose a room dimension that can handle the pressure levels produced by drums and produce resonances that we can manage properly. If we choose a room to small, we are continually running uphill in our bass management.

Concrete Barrier

Barrier Technology

Barrier technology must be employed to minimize the drum energy from leaking into other rooms or structures. A double wall approach is best here because of the higher pressure levels, we need to provide a high level of sound transmission loss. A double wall constructed so that each wall is mechanically isolated from the other would be an ideal situation. An air space of at least 4″ between the two structures would be welcome. We must construct a barrier or shell to keep outside energy out of our drum room but more importantly keep the drum sound in the room.

Resonant Management

Inside our drum room is all about resonant management. The smaller the room, the more high power absorption is needed. We must find the locations within the room where resonances occur. Once found, we must treat with the proper type and amount of absorption. We must also try and keep microphones from being placed within these resonance pressure nodes. A microphone placed within a room node may receive an exaggerated amount of energy or none at all. We want to try and achieve a room with a flat frequency response below 400 cycles.

Quadratic Diffusors

Middle And High Frequencies

Middle and high frequency issues within our drum room can be managed using sound absorption or sound diffusion. Sound absorption should be used to control drum room reverberation times. There is much debate on what is a good reverberation time for a drum room. Find a live room that you like the sound of and measure the reverberation time of that room. This procedure will give you a good start point for your room. Another approach is to over absorb and take away absorption technology until you find the right number for you.

Quadratic Diffusion

Diffusion can be used to manage wall reflections within your drum room. Diffusion technology will spread the sound energy out in a fan like array across two dimensions of sound. Make sure and use quadratic diffusion to aid you in your efforts. Adjust the prime number that works best for your budget and room. A general rule of thumb is to use the highest prime number one has room for on the drum room’s surfaces.

Not An Easy Question To Answer

How do we soundproof a bedroom ? That’s not an easy question. A bedroom is surrounded by other rooms in most cases. All that separates a bedroom from the other rooms is an interior wall that is usually made from 2″ x 4″ or 2″ x 6″ wood frame interior walls. Wood framed, interior walls that are and were never designed for any type of real noise control. It is usually easy to hear through a bedroom wall.

Must Address Usage

What will we be doing in our bedroom that we need to isolate sound from getting in or out of. Are we going to convert it into a project studio to do what? Are we going to record vocals, maybe instruments? Where is the bedroom located within the house. Is it located next to other bedrooms or areas of necessary quiet or is it located next to a higher noise producing area. All of these questions will have to be answered. Lets start with least noise producing first.

Small Project Studio

If the bedroom is located away from the rest of the house or apartment and there will be no noise producing sources in joining rooms, we can focus on the main use of the bedroom conversion. If we are playing back and monitoring a recording through monitors, then we need to begin by addressing the noise source within the bedroom itself. We do not want to disturb other members of the household.

Must Deal With Low Frequency

Lets start by treating the room with the necessary low frequency control technologies to minimize the impact of our lower frequencies from the monitors. This will require additional low frequency absorbers that are tuned specifically to the low frequencies generated by the monitors in our small bedroom. We will need floor to ceiling absorbers in every corner and along all four room sides. We actually could not have too much low energy absorption in a small room. The smaller our room, the more low frequency issues we have and the need to make it smaller by adding low frequency absorption.

Reflections Must Be Minimized

Room wall and ceiling reflections in small rooms destroy our stereo image and mess with the wanted direct sound from our monitors. Acoustic foam will work well on the side and front walls directly behind our monitors. Make sure you choose an acoustic foam that has the necessary rates and levels of absorption to handle the vocal and mid range presentation in your bedroom. Not all acoustic foams are created equal and one needs to be aware of their sonic impact especially in small room environments.

Sound Absorbing Panels

Pay Strict Attention To Rear Wall

The rear wall is of special concern. A small room has many acoustical issues because of its size and one must address these issues at the start. The reflection from the rear wall at the monitoring position is a time delayed one and can really have an impact on your mix. It must be dealt with by using diffusion. In particular quadratic diffusion which will reduce the reflection from the rear wall and spread it out in a series of smaller “reflections”.

Quadratic Diffusors

Use Both Vertical And Horizontal Diffusors

Try to use the highest prime number for your sequence as you have room for and make sure to position your diffusors both vertically and horizontally to afford two dimensions of sound diffusion. Small bedrooms need all the diffusion they can get especially from the rear wall at the mix position. position vertical diffusors across the total width of the rear wall and then place horizontal diffusors across the top of the vertical ones you just positioned.

Still Not Enough ?

If we perform all of these treatments and sound energy is still migrating to other areas of the house, then we need to get out our tools and build some barrier technology. This may or may not be feasible in a bedroom situation. What we will need to do is build a barrier within our bedroom that keeps the energy we produce within the room, inside the room itself. To accomplish this, we must place a barrier between the sound or energy source and the rest of the building. This is the room within a room concept.

Build Your Footprint

We start by laying a 2″ x 4″ across the floor bordering all around the room. Make sure our border is 6″ away from the existing wall. This is the new footprint of your room. Complete the floor assembly with 16″ on center studs. Fill the interior space between the studs with mineral wool insulation. Do not use fiberglass. Mineral wool is denser and will assist us in our sound isolation efforts better.

Isolate Floor Assembly

Raise the fl0or assembly up and place it on rubber isolators. Many companies make these and follow their recommendations for how far apart on center they should be placed. Now, our floor is isolated mechanically from the rest of our bedroom. Build the walls with 16″ on center studs upon our isolated floor assembly. Fill with mineral wool as the floor assembly. To cover the studs on the floor, one must use floor covering materials. Consult your local building codes One can use drywall for the walls or a commercial grade multiple density fiberboard which will give you more density and a much smoother surface free from drywall flakes and chips.

Ceiling Assembly More Difficult

The ceiling assembly will be more difficult and depending upon the width and length of your bedroom, may need a rafter type assembly approach. If you do not have the skill set for this, hire a professional. You don’t want the ceiling to come crashing down on your board or monitors, not to mention your head.

Must Have Game Plan

How to soundproof a bedroom is not an easy question to answer. The answer depends on what usage, the location of the bedroom to the rest of the house or apartment and what recording or playback processes are involved. Start by treating the inside of the room with low, middle, and high frequency absorption technologies. If that is not enough, you must go to building barrier technologies to isolate the sound energy produced in the bedroom from the rest of the house. If barrier technology does not work, you must move.

How To Soundproof A Floor

If one was going to write a book on how to soundproof a floor, the first thing that comes to mind is floating. Our floor however, we construct it, must “float” or be mechanically decoupled from the existing structure. There must be an air space between our new floor and the existing floor.

Assess Noise Issue

First, before we begin any construction project, we must assess our noise problem. Why are we building a new floor? The answer is usually to isolate ourselves or others from noise. We must then ask ourselves, how much noise do we need to isolate from. Are we isolating a drum room ? Is it a vocal room ? Each will have its own set of noise issues that must be dealt with. Lets put a number to the noise.

Measure Noise

Take out that Radio Shack dB meter and measure the amount of sound pressure within your room during the times when the most energy is produced. Assign a number to this noise. Is it 85 dB, 90 dB, or maybe a 100 dB. Now, take your dB meter into the room where you want the sound to stay out of and measure what the levels are in that room when you are producing the most energy. Are they 75 dB, 80 dB, or 90 dB? The difference between these two sets of numbers is what we need to address.

How Much Bleeding?

If the room directly below our floor is a regular type of living space, then we want ambient noise levels around 60 dB. If we are producing 85 dB of noise and that is raising the ambient noise level in our normal living space to 80 dB, then we need to isolate at least 20 dB of sound pressure from leaking or bleeding into our normal living space. Each dB of isolation costs money and takes special planning to achieve.

Who Is At The Party?

We now have a measurement of the overall noise levels. Now, we must focus on what frequencies are involved in this noise party. Are most of the frequencies in the middle to high range? Are there low frequency issues that we must deal with? The frequencies that cause the noise level we are planning to reduce, must be addressed in the structure of our new floor. Take a frequency response reading of all noise and look at what frequencies are contributing the most to the noise levels.

Below 100 Cycles, Best To Move

If we are faced with noise that is mostly below 100 Hz., we should consider moving to another location. Stopping or minimizing low frequency energy is no easy task. it is also not very cost effective. Constructing a floor to stop drum bass noise is completely different than building a floor to stop office noise from bleeding from one office to the next. It is usually desirable to find another location when low frequency energy is an issue. This is why musicians practice in rooms that are removed from other structures.

Above 100 Hz. Good To Go.

If our noise levels are caused by energy above 100 cycles then we can build and design a floor system that will provide proper noise management for the structure below. Our numbers tell us that we need to stop around 30 dB of energy. What type of structure to we use that will reduce our sound pressure levels by 30 dB? We need to look at another number called STC or sound transmission class. This is a number that will tell us what structure types will hold back what amount of unwanted energy.

Exceed Goal By 50%.

Our goal is to hold back at least 25 – 30 dB of sound energy. If we build a single, 2″ x 4″ “wall” for our floor and fill the interior of the wall with building insulation, we can achieve a STC rating of 40. This is a good start, but we should try and overshoot our goal by 50 % to ensure that we meet all the requirements for our space. Lets add another few layers of thick carpeting and a thick padding to add an additional layer of protection.

Installation

Installing our new floor system is critical. We must mechanically decouple the floor from the existing structure. We do this by floating the floor on rubber isolators that are placed at supporting intervals throughout the floor layout. We also need to leave at least a 1″ air space between our new floor bottom and the existing structure. Make sure the existing structure can handle the additional weight requirements you are imposing upon it. It will not matter how well you did with your new floor, if it comes crashing down into the room below.

Professional Advice

Sound isolation involves both air borne and structural noise. Both types must be dealt with and measured for any project. Once you have determined the actual problem through measurement, you can decide on the appropriate structure that will meet the sound transmission loss objectives. Building and installing the correct structure is not an easy task. If you do not mechanically decouple the new floor from the existing floor, you could negate performance and isolation qualities. If you have any doubt about how to proceed seek additional knowledge or hire a consultant to guide you through the process.

Ceiling First

How to sound proof a basement must be approached from the top down. Our ceiling to our basement is really the floor of the next level above us. The walls are usually block or poured concrete which provides another barrier layer. To our sides, behind the side walls, we have lots of earth. Earth is an excellent barrier material, especially for lower frequencies.

What Is Our Basement’s Use?

We must first decide what our intended use of the basement is going to be. For the sake of discussion, lets make our basement usage a small project studio. No musical instrument or vocal playing, just monitoring through playback monitors. Lets make our top playback level around 85 SPL or so. Lets focus on our ceiling which is the floor for the room above which is the room we want to keep energy from transferring out of our basement into the floor above.

Barrier Technology

We will have to keep as much energy within the basement as we can. To do this, we must employ barrier technology for the ceiling. The best way to make a barrier is to laminate different layers of different density materials. One can use plywood, MDF, or other composites. Arrange the materials in a manner that starts with say a 1/2″ solid piece as the center and build a multi-layered sandwich of materials out from that core piece.

Barrier Sandwich

Take a 3/8″ of plywood and stack it on top of the 1/2″ solid MDF core. MDF stands for multiple density fiberboard. We need the commercial variety for this project. Before we attach them together, lets separate them by placing a layer of vibrational damping compound between the 1/2″ MDF core and the 3/8″ plywood. Next, lets use another piece of MDF at a 1/4″ thickness and place it on top of another layer of vibration damping compound. each layer of different densities is damped by compound. Our goal is to end up with our “barrier sandwich” at 6″ thick.

Vibration Reduction

As vibrations move through each layer of material they must go through different thicknesses and also must go through the vibration damping compound layers. We want to make the vibrational journey as difficult as possible. Vibrations equal noise upstairs. This is the reason for alternating different materials with different densities. We want to reduce the vibrations that strike one side of the barrier from reaching the other side.

Barrier Installation

Once we have our barrier built, we must install it. It will not be easy because we will have to lift it into place and secure it. It also must be damped or decoupled from the existing basement ceiling, so that once again we minimize vibrations. This should be left to a professional who has the tools and equipment to do that. A 4′ x 8′ barrier sandwich could easily weigh a 1,000 lbs. Don’t ask your buddies for help on this project.

Double Wall Approach

Another way we can achieve sound transmission loss is by building two walls, if you will, that do not come into contact with each other. We will be building two 2′ x 4′ walls with an air space between them of at least 4″. These two walls will be standard interior walls with 2″x 4″ lumber covered in drywall. It is an easier build and install but care must be taken that each wall built is mechanically isolated from the other. Once again, we want to throw as many barriers as we can against air borne energy and the air space between each wall is another barrier.

Inside Basement

The inside of our room or basement already has walls that are concrete reinforced with earth surrounding them. We are not going to improve upon the density or sound isolation characteristics of this system. We will need to focus on the dimensions inside those walls to discover any low frequency issues that will be produced by the basement’s dimensions. Once you have determined what the dimensions are that you will be using, we will then go to work on taming the low frequency resonances first and then dealing with middle and high frequency reflections off of the wall surfaces.

Low Frequency Sponges

Once you have determined what low frequencies are going to cause you problems, one can design and build low frequency diaphragmatic absorbers that will absorb at the lower frequencies one needs to deal with. If you are fortunate and do not have specific low frequency resonances to contend with and you would indeed be fortunate, you can build a broadband diaphragmatic absorber that will absorb energy over a broader frequency range.

Concrete Wall Reflections

Side wall reflections off of the basement walls may be wanted but most will not. Sound “borrows” from the surface that it strikes some of that surface sound. If sound strikes glass, one gets glass sound. We all know this sound. This is the sound we hear in our vehicles. If sound strikes concrete, we get concrete sound. It can be harsh and brittle and increase the reverberation times in our room.

Acoustic Foams

Side wall reflections can best be dealt with by reducing their strength through the use of acoustic foams. They are lightweight and relatively inexpensive. Make sure you use a foam that has a smooth absorption curve below 500 Hz. This is the frequency range for most vocals and you want absorption in that range to be as smooth as possible. Most foams currently in the marketplace show poor performance below 500 Hz.

Sound Compromise

Keeping sound from entering and leaving our rooms is not an easy task. The best we can usually hope for is a compromise. We reduce the energy within our room using absorption so that our neighbors do not hear too mush of it. If the noise is coming from the outside, we build barriers to try and stop most of it but we can not usually get it all. Our basements have walls that have good isolation but we need to address the ceiling which is the floor of the rooms above us.