Sound Absorption / Diffusion

We have two major choices to make regarding the management of sound energy within our home theater, personal listening, and professional control rooms. We can absorb or diffuse sound energy in order to manage it correctly.  We can use absorption to manage low, middle, and high frequencies. We can use diffusion to manage middle and high frequencies.  It is a blend and combination of these two technologies that go into each of our sound rooms.

Acoustic Products Expensive

Quality room acoustic products are expensive. You also need numerous units to treat the surfaces of the any room. To have any sonic impact from room acoustic treatment, at least 25% of the surface area must be treated. In a control room, we have the rear wall which normally would be treated with diffusion. Both side walls must be splayed or angled to minimize side wall reflections. The surface area of each side wall is treated with absorption. Low frequency management is essential in any room whether a listening room, home theater room, or professional recording studio. With today’s smaller rooms, low frequency control must include numerous units.

Wood Shop

If you have access to a small wood shop with a saw, it can be table or hand held, and you have some wood working skills, you can build your own professional low frequency absorbing units along with middle and high frequency absorption and diffusion. There are no angles to cut, everything is a straight line. Build drawings are provided with assembly instructions and an actual step by step build with photos. A tool list, cut sheet, and material complete everything you need to build your own units.

Low Frequency Absorption Units

Membrane / Diaphragmatic Absorbers

There are many types of sound absorption devices available that claim to be bass traps or low frequency absorbing devices. There is the standard box filled with building insulation.  Foam wedges are another popular unit that is claimed to absorb low frequencies. None of these units are really low frequency absorbing devices. John Storyk, the principle of Walters Stork Design Group,  did a test on all of these devices in 2006 and published his results in an AES paper. His conclusion was that of the eight units he tested, only one actually did work  as claimed. It was a membrane absorber with  the most dense  front panel or another term could be diaphragmatic absorber.

Diaphragmatic Absorber

Diaphragmatic Absorber Construction

A diaphragmatic absorber is a cabinet that has a front wall that moves or vibrates in sympathy with the amount of sound pressure that is exerted upon it. The cabinet is more rigid than the front wall and is designed this way on purpose. The depth of the cabinet and the density of the materials used are critical in determining the frequency of resonance of that cabinet. One can design a diaphragmatic absorber to absorb any low frequency issues within a room. Foam wedges and boxes filled with building insulation can not.

Cabinet Fill Material

Inside the cabinet, there is an internal cabinet fill,  usually some type of building insulation material. One can take the internal fill material and expand upon it to increase its efficiency. The internal cabinet fill material must absorb internal cabinet resonances and also contribute to the cabinet’s overall performance. It is by far the only low frequency absorber that has the horsepower to actually tackle the energy generated from low frequency wavelengths. Diaphragmatic absorption is used extensively by the top room designers when they are designing and building multi- million dollar rooms.

BDA – Broadband Diaphragmatic Absorber -DIY

The BDA unit that we are offering is unique in many ways. First, the cabinet design is a tested unit that has undergone vibrational testing in our facility. It is designed to be as inert as possible using only one layer of material. At 4.4 lbs./ sq. ft. it is a heavy unit but mass is necessary if you are going to attempt to stop low frequency energy. The front wall has also been calculated to work with the chosen cabinet density and comes in at 2.4 lbs. / sq. ft.

Perforated Absorber Cabinet Fill

The internal cabinet fill is not just material inserted into the cabinet. The internal fill material is actually another unit itself. It is called a perforated absorber and is designed to absorb at a frequency that will minimize internal cabinet resonances and then compliment the total cabinet design to achieve absorption down into the 40 cycle range. A perforated absorber is a separate unit that has a front wall that has perforations of a certain diameter and certain number to produce a unit that has a specific resonant frequency.

FB – Foam Box – DIY

Foam Boxes

The FB or foam box unit is a cabinet that is designed to hold any type of acoustic foam that one desires. There are numerous open celled acoustic foams that are available in the marketplace and our foam box is a unit that one can build that can be finished to match any decor. The fabric face and frame assembly is the same process that we have used in our production units and we have many units that are now over six years old and holding up well. If the fabric is damaged, the face frame can be removed and a new piece put into its place.

QRD-11 Quadratic Residue Diffusor – DIY

The QDR-11 is an actual quadratic diffusor based on prime number 11. Each well depth and width has been calculated to produce a diffusion frequency range from 300 Hz.- 3,ooo Hz. Quadratic diffusion has been time tested and proven throughout the years and is used extensively by professional studio designers. Quadratic diffusors can be used to produce two dimensions of sound diffusion. A vertically positioned quadratic diffusor will diffuse energy in a horizontal dimension. A horizontally positioned diffusor will distribute sound energy in a vertical array.

Low, Middle, High Sound Management

Quadratic Diffusors

We need to have low, middle, and high frequency sound energy control in our home theater, listening rooms, and professional recording studios. To accomplish this, we need to use sound absorption and sound diffusion technologies. Unfortunately, room acoustic technologies that really work well are expensive and you need numerous units to meet your acoustic objectives. With DIY units that one can build themselves, the costs of treating your rooms can be minimized without sacrificing room sonic integrity.

What Are We Rehearsing?

If we are going to answer the question on how to soundproof a rehearsal room, we must first define what we are rehearsing. Is it vocals or instruments? Is it one vocal or many. Is it a single instrument or a small band. We need to know the amount of energy that will be created within the rehearsal room to plan accordingly for the correct soundproof method to employ both to the inside and outside of our rehearsal room.

Sound Absorbing Panels

Energy Assumptions

Lets first make some assumptions. Lets take a small choir, say 10 vocals and an 8 piece band. Lets use these two as our sound generating sources. These two sources will become the benchmark, so we can illustrate how to soundproof a rehearsal room. They both produce energy in similar but different parts of the frequency spectrum.

Concrete Barrier Molds

Concrete Shell

Once we know how much energy we are going to produce in the room, we can design the shell or barrier that will keep the sound generated from within our room, inside where it belongs and the noise generated from outside our rooms, outside where it belongs. A good start is concrete which will provide the barrier protection we need. We should make ours walls 8″ thick and poured concrete into molds is preferable to block. Block is a good second choice if poured concrete is not an option.

STC – Sound Transmission Loss

Sound transmission loss is the ability of a structure to reduce sound transmission from one side of the wall to the other A 8″ poured concrete wall will provide a sound class rating of 56. This means that if we have a sound source on one side of the wall that measures 90 dB, it will be 34 dB on the other side because the concrete barrier will reduce the sound pressure level by its sound transmission rating.

Concrete Barrier Finished Edge

Dual Wood Framed Walls

if block or poured concrete is not an option, one can use a wood frame structure or rather two wood framed structures. We construct a 2″ x 4″ wall and then another 2″ x 4″ wall. We leave an air space of 4″ – 6″ whatever our physical location will permit. We isolate each wall from each other with the air space and we mechanically decouple each wall from the existing structure. This dual wall structure will afford almost the same STC value as a poured concrete wall. The poured concrete wall has a STC rating of and the dual wall has a STC rating of.

Room Size Critical

Once we have chosen our barrier configuration, we must focus on the inside of our room. If our room has been “acoustically sized” then we can begin right away with the acoustical treatment. If no thought or consideration has been given to the rehearsal room’s size then we must address that issue before we go any further. If we are rehearsing a small band then we need the correct room volume to accommodate the band’s frequency range which is a larger requirement than a vocal rehearsal room.

Room Modes

Room modes or resonances build up inside a room that is not large enough to accommodate the complete frequency range that is produced by the rehearsing source. Resonances, especially those created by lower frequencies, have no place within our rehearsal room. A microphone or band member placed in one of these modes, may not be able to hear themselves and the microphone will not be able to record the correct information because the sound needed to be recorded will be smothered by the resonances. For a small band rehearsal room make sure you have at least 30′ in one direction of the room. For a vocal room, make sure you have at least 15′ in one direction. Always choose higher ceilings for both rehearsal room sources.

Sound Absorption

Acoustical room treatment for your rehearsal room can be absorption. Low frequency absorption must be used to manage low frequency modes. It can not be foam or panels filled with building insulation. One must use tuned low frequency absorbers that can handle the low frequency energy created within room locations. Middle and high frequency absorption can be used to tame rehearsal room reflections to manage reverberation times. Make sure you choose the correct rate and level of absorption that compliments your use.

Sound Diffusion

Diffusion can be an important tool in dealing with room boundary reflections. Diffusion can take the reflected energy from the wall surfaces and spread that energy out in a fan like array in two dimensions. This spreading out of energy allows for a smoother presentation of energy at the microphone position. Two dimensions of diffusion can be achieved within your rehearsal room by using quadratic diffusion.

Quadratic Diffusors

Variable Acoustics

Variable acoustics have gained popularity. One can have absorption panels that can be absorption on one side and diffusion on the other. An engineer can alternate between absorption and diffusion to suit the recording engineer’s acoustical palette. Portable low frequency absorbers can be rolled in to handle room resonances at certain places within the rehearsal room.

Follow Steps

When you are planning on how to soundproof a rehearsal room, you must first define what sound producing sources are going to be using the room. Once determined, you can assign the correct barrier technology to manage the sound pressure levels generated from the rehearsing source and keep wanted sound within the room and unwanted sound outside. The rehearsal room must have the correct volume to accommodate each source whether from a single vocal, a choir, or a small band. Proper room volume minimizes room resonances. A combination of absorption and diffusion technologies can be used inside the room.

STC Defined

STC or sound transmission class is a rating system used mostly in North America to measure or rather assign a number to the ability of a barrier or partition to inhibit sound energy from passing through it. Outside North America, they use a term called SRI or sound reduction index.  STC is an average of measurement numbers that use 16 different frequency bands that begin at 125 Hz. and go through 4,000 Hz. The numbers are then assigned their respective positions on a sound pressure level curve that is derived using a complex algorithm. This algorithm produces one number which we call the STC rating of the structure or barrier. Unfortunately, the nature of the frequency range covered does not tell the whole story.

How It Works

If we have a noise source that we are trying to isolate from entering our room, and we measure the noise source to be at 80dB, then we have to decide what dB level we want in our room. If we want a noise level within our rooms of 50dB, then we need a barrier that can reduce our pressure levels by 30dB and we would seek a barrier with an STC rating of at least 30. However, this number is frequency dependent. Our barrier may attenuate 30 dB at 3,000 cycles but only 15  dB at 125 cycles. STC is an accurate number when the sound energy we are trying to isolate from is spread out evenly across the frequency spectrum and does not go below 125 Hz.

Concrete Barrier Finished Edge

Whole Story

If we are concerned with the human vocal range which is from 1oo Hz. – 800 Hz., one can see that an STC measurement has value, since the STC measurement frequency bands fall within that range. If we are trying to isolate human vocals in an office setting then an STC value of a considered barrier, will have some validity. However, if we are dealing with frequencies that fall below 125 cycles, such as large trucks and explosions in our home theaters, then we need to be more careful and consider how our barrier will react to frequencies below 125 cycles.

Old School

The STC rating system was developed back in 1961 and has not been updated since that time period. During that time period we did not have the lower frequency energy issues we have today with home theaters and more people. Computer processing was almost non existent and if it was processing power was low. STC ratings that were assigned during that time period are still used today even though the products they are assigned to from then are no where close in composition to their original form.  A good rule of thumb is to not trust ratings that were assigned before 2,000 because testing equipment was not that sophisticated and the margin of error could be as high as plus or minus 6dB. It is best to use STC in combination with other measurements.

Newer Rating Systems

The American Society For Testing Materials. The ASTM measurement system has three basic divisions: STC,  CAC, and OITC . The CAC is the ceiling attenuation class which is for ceiling structures. The OITC is for outdoor / indoor transmission class that measures the sound transmission between outdoor and indoor structures. OITC uses a noise source spectrum that takes into considerations frequencies down to 80 cycles which is far more useful in today’s world. The IIC or Impact Isolation Class is a number that tells us how well a floor attenuates sounds from footsteps and dropped objects. Similar to STC, the IIC is formulated using frequencies from 100 Hz. – 3,150 Hz. The same lower frequency limitations apply as with the STC number especially if the stereo system on the floor above is full range.

Barrier Fence / Wall

Noise Criteria

NC or noise criteria number is a popular index. NC is a measure of just the noise itself and not the ability of a structure to inhibit it. It operates beginning at 63 cycles and moves up through 8,000 cycles.  To arrive at the NC number, we look at one third bands for a given spectrum of noise. The noise spectrum is specified as having a NC rating that is the same as the lowest NC curve that is not exceeded by the noise spectrum.

STC – A Mixed Blessing

An STC number or rating has to be examined closely. If we choose 125 Hz. as our frequency for discussion and one barrier allows more energy to pass through than the other barrier, the former will achieve a higher STC rating. This occurs because remember from our prior discussion that 125 Hz. is the lowest frequency examined for an STC rating.  Any amount of energy that passes below our lowest measured value will produce a higher STC rating and manufacturers have abused this simple issue.

Do Your Research

Most manufactures have data that indicates how their products perform below 125 Hz. It is just that an STC rating has been around for so long that there is no other standard present. One needs to look through the numbers to find the actual performance and isolation value. If a manufacturer does not have the supporting data below 125 cycles, one should look to ones that do.

New Standard Needed

STC or sound transmission class rating is a system for rating a structure’s ability to stop or hinder the transmission of sound through it. It is an old system established back in 1961 and is overdue for a change. A new system should be developed that will address frequencies below 125 cycles and will also be able to address spikes in sound pressure levels across the frequency range determined. One thing is for certain, whatever system is devised needs to go lower and include more current information.

 Two Channel Set Up Misunderstandings

I see and hear many things when it comes to the set up of two channel stereo that makes me wonder if individuals that own these systems have done any type of study at all on how to set up a system properly. I see speakers set up in corners, next to glass windows, and even  furniture in front of speakers . I have even seen the left channel set up in one room and the right channel set up in another. Lets examine some two channel set up requirements that are a must have if one is to realize true stereo playback fidelity.

Step One: Room Size

The dimensions and associated volume of a room must be given first consideration.  There is a ratio of room width, length, and height that is more favorable to minimizing low frequency resonances and low frequency resonances must be dealt with. Room modes can smother and blur the low middle and middle frequencies. This is the emotional connection range where our vocals are. Nothing can be allowed to interfere with the frequency range from 100 Hz. – 500 Hz. and low frequency room modes can impact this frequency range in a big way. Choose a rectangular room. They are more predictable for our room tuning process when low frequency room modes are an issue.

Room Size Don’ts

Stay away from rooms with concave and convex surfaces. Parallel walls are fine assuming they are not made of concrete or other hard surfaces. Do not use a room that has the same length, width, or height. This would be considered a cube and the detrimental acoustical issues this room size configuration produces are to numerous to even discuss.  Oval rooms are also sound prohibited. Follow the ratios given in the literature for room dimensions. They have been tested and will save you time,energy, and much frustration.

Step Two: Optimum W, H, L  Ratios

What is the correct room size.  There is no right or wrong answer to this question. There is a series of  room ratios that do allow for low frequencies to be present without causing too much trouble. The dimensions of the room allow for the low frequency room modes to be spaced far enough apart that their presence is more easily managed.  One may even elect to move a wall to make a room smaller or larger to fit a better dimensional ratio. A good starting height, width, length ratio to follow is 1 , 1:19, 1.34.

Building Insulation In A Corner Does Not A Bass Trap Make

Step Three: Low Frequency Management

Once you have chosen your room dimensions or they have chosen you because you must pick between two possible rooms or maybe even three in your home or studio, it is time to address the low frequency issues that you have. Take the time and measure your room to find out what the problem frequencies are. Software is readily available to assist you in this area. Most rooms will require some type of broadband and frequency specific low frequency absorption. Do your research and find the companies that build and design products that address your specific concerns.

Low Frequency Management Don’ts

Do not buy into manufacture’s claims that foam and boxes filled with building insulation will absorb low frequency energy. Low frequency wavelengths are long and powerful and  it takes a specially designed absorber to handle them. Membrane or diaphragmatic absorbers work best for this frequency range. Do not think that one or two units will solve all your issues. It may take 8-10 units correctly positioned to have the acoustical impact your desire. Stacking foam or building insulation in the room corners is nonsense when it comes to low frequency energy management.

Keep Speakers Away From Glass

Step Four: Speaker Positioning

There is a “correct”  position for your loudspeakers within any given room. This position produces the smoothest frequency response for the given room size. It takes time and measurements to find, but it is there. Start with the “rule of thirds”. Divide your room long way into thirds and start by positioning your speakers along the first third division line. Measure the frequency response at that position. Start with your speakers at least 4′ from the side walls. Move your speakers forward 6″ and measure again. Move your speakers back 6″ and remeasure. Look at the three measurements for patterns and adjust accordingly. Take your time and perform listening tests at each chosen position. Move the speakers inward towards the center 6″ and measure again. Move them out towards the side walls, remeasure.

Speaker Positioning Don’ts

Do not have unequal distances between the side walls and your speakers. Sound energy from your speakers is an electromechanical signal that travels through the air at a constant speed. If the distances from the side walls are unequal, those side wall reflections will arrive at unequal times at the listening position and mix with the direct sound from your speakers producing an image shift from the wanted center position. Make sure the side wall materials are the same composition. Never, and I repeat never, place your speakers next to a glass window, even though you will see this set up in magazine ads by speaker manufacturers. I guarantee none of the designers of those speakers wanted that position for their creations.

Step Five: Listening Position

Choose a chair that does not have a high back. Your ears receive information from front, rear, and side walls. With a high back chair, that information will be blocked and not heard. Measure the distance between your speakers and position the chair, so that it is the same distance from each speaker as the distance between your speakers. You are forming an equilateral triangle with your speakers and listening positions as the indices of that triangle. As you listen more to your system, you may want to move your chair back to widen the sound stage. This is personal preference, but using the triangle approach is a good start.

Listening Position Don’ts

The inside of the triangle that is formed with your speakers and listening chair is sacred ground. Do not have anything in this area. If you must place your equipment between your speakers on the floor, make sure it does not rise very high. Keep it as low as possible. Do not have a coffee table in front of the listening chair. Reflections from it and your equipment can be audible. Keep chairs and ottomans away from this area.

Side Walls Must Be Same

Step Six: Active Listening   

Since you are setting up a stereo system, you need to play a mono signal through your speakers first, to make sure you have a centered image. We know that a stereo signal in a true stereophonic sound system has two independent audio signal channels, and the signals that are reproduced have a specific level and phase relationship to each other, so that when played back through a suitable reproduction system, there will be an apparent image of the original sound source. Stereo will replicate the aural perspective and localization of instruments on a stage or platform.

Use Mono Test Signal

Therefore, to make sure we have the correct balance between listening position and speakers, we need a mono signal to use as our focusing energy to make sure our image between the speakers is centered and focused. Mono or monophonic describes a system where all the audio signals are mixed together and routed through a single audio channel. Mono systems can have multiple loudspeakers, and even multiple widely separated loudspeakers. The key is that the signal contains no level and arrival time/phase information that would replicate or simulate directional cues. A mono signal will better show any speaker set up issues.

Put on the mono recording and sit in the listening chair. Close your eyes and the image you hear should be about the size of a tennis ball centered directly in the middle of your speakers. If it is there, put on your favorite stereo recording. The image should fill the distance between each speaker and hopefully extend farther left and right of your physical speaker positions. If is not, we need to move our speakers and listening position.

Basics First

Setting up a two channel system requires many steps to insure that you realize the full potential of your stereo system. Start by choosing the proper room size to minimize low frequency issues, select and position the necessary low frequency absorption technology, and correctly position your loudspeakers and listening position for a smooth frequency response. As you spend time with your system, there will be other issues to address, but initially following the above steps is a good start.

Smaller Rooms / Low Frequency

In a lecture at Drexel University on acoustics, John Storyk of WSDG, the premier recording studio designer with offices across the globe, stated that the trend towards the future in the recording studio market is smaller rooms with creative low frequency management technologies to manage the room resonances that go with smaller room volumes. Low frequency management can take two basic forms. It can be active or passive. Active low frequency management is electronically based and passive technology involves the use of smaller, but more powerful traditional absorption which can be built into walls or freestanding.

Active Low Frequency Management

Active low frequency technology involves the use of noise cancellation technologies. Noise cancellation takes two signals that have equal amplitude and frequency and then focuses on their opposite polarity. When all of these variables are in line, we will have a complete cancellation of the chosen signal. This phenomenon is used when it comes to large amounts of industrial noise. Building barrier technology to isolate workers from this noise is expensive. Instead of shielding through barrier technology, we go the other way and amplify the signal. The problematic noise is then amplified and reproduced by speakers with inverted phase from the frequencies we wish to cancel. The result is greatly reduced sound pressure levels at the chosen low frequencies and the noise levels are reduced drastically without using cumbersome isolation technologies.

Passive Low Frequency Absorbers

Passive low frequency technologies can take two basic forms. One can use resonators or membrane absorbers. Resonating absorbers, such as the famed Helmholtz resonators, uses chambers of certain depths with slots cut into the top of the chamber to let the air in. It is the air inside the chamber that resonates at a certain frequency depending on the chamber’s designed volume and area of the opening. All frequencies above the chamber’s resonate frequency are absorbed. Frequencies below the chamber’s resonating frequency are not.

Membrane Absorber

Membrane Absorbers

Membrane absorbers have a membrane or wall that vibrates when sound pressure energy strikes it. The front wall membrane is surrounded by a cabinet that is rigid and has a certain type of cabinet fill to absorb internal cabinet resonances. The vibrating membrane slows the sound energy down and then it enters the inside cabinet fill material and is reduced in intensity. Some of the energy is absorbed, some is sent back through the front wall membrane, and some leaves through the rear of the cabinet. The sum of all of these variables determines the cabinet’s total absorption performance.

Diaphragmatic Absorbers

Membrane absorbers are also termed diaphragmatic absorbers. The front wall or membrane acts like a piston and vibrates in sympathy to the sound pressure exerted upon it. This front wall movement is said to be diaphragmatic because of this movement. The diaphragm movement is not visible movement like a speaker. It is microscopic movement that the sound pressure exerted upon it produces. This moving of the diaphragm, slows the sound energy down before it enters the inside of the cabinet.

Diaphragmatic Absorber Improvements

To maximize this movement/absorption process in a diaphragmatic absorber, we can do three major improvements to this time tested and proven technology in order to improve its overall performance. We can add an additional front wall or diaphragm, use a more sound absorbing internal cabinet fill, and make the cabinet more rigid to force the dual front wall diaphragms to move more to increase rate of absorption.

Diaphragmatic Absorber

Two Front Walls

When we add an additional front wall, our design goal is to get both walls working together to maximize efficiency in slowing down the energy that strikes them before it enters the inside of our absorber. This sympathetic movement is based on the density of each wall which must be different and the distance between each wall. Vibrational analysis of each wall will assist one in determining the appropriate wall density to be used and the air space required between them to produce maximum efficiency.

Internal Cabinet Fill

The internal cabinet fill with our new, dual wall, diaphragmatic absorber has traditionally been a type of building insulation. Fiberglass and mineral types have been commonly used. In fact, the interior walls of most rooms can be considered a diaphragmatic absorber but with limited low frequency attenuation. The cabinet is the drywall on each side of the wall and the insulation between the studs is the internal cabinet fill. To absorb more energy, the internal cabinet fill can be activated carbon or charcoal. Charcoal has numerous pores that absorb sound energy and each charcoal pellet has thousands of pores. Sound energy enters each pore and is absorbed. This large amount of pore absorption, lowers the cabinet’s internal Q value to a level never before achieved in the scientific literature.

Multiple Layered Cabinet

The cabinet construction must work together with the dual front wall construction. We want to encourage the dual front walls to move without the cabinet moving. The cabinet must be designed to be as inert as possible, similar to a speaker cabinet. One can achieve this cabinet rigidity by using multiple layers of materials that have different densities with each layer separated with vibrational damping compound. The key to the process of making the cabinet as inert as possible is to choose cabinet layers that have the correct densities that when married together, the vibrational energy transmission is reduced from layer to layer throughout the cabinet.

Smaller Rooms Require More Creative Approaches

Smaller room volumes in today’s recording studios produce larger low frequency resonances. This requires the need for either active or more powerful passive absorption technologies. Active noise cancellation technologies require an electronic approach where a signal is interjected into the room through a speaker. The energy interjected into the room is out of phase with the frequency that is chosen to be absorbed and this active process results in frequency cancellation and reduced sound pressure levels. Existing passive, low frequency, technologies can be improved upon by fortifying existing design criteria.

Professional Vs. Consumer

We work both in the professional and consumer markets. They are the same in some respects but radically different in others. Both seek “good” sound quality, but go about it in different ways. The professionals use their room and the sound in it to make a living through the recording process. The consumer is all about playback and taking the recorded sound and actively listening to it in their rooms. The rooms they use to accomplish both their purposes are radically different.

Different Functions

Professionals want to hear everything that is going on in the mix. Their rooms are set up and treated to avoid most of the issues that small rooms have on sound quality. Most professionals want less room sound and more of just the instrument or vocal. If the room is a vocal room, it is set up to minimize reflections at the microphone position, so there is less room sound. Consumers rooms are usually rooms that have multiple functions attached to them. They are usually living rooms where many individuals socialize and listening to music is a secondary function.

Different Room Surfaces

With a living room, we have numerous types of room surfaces that must be dealt with. We have drapes, fireplaces, and the dreaded windows which can run across a whole wall. Recently, we were asked to tune a room for a client that had one whole wall as a glass window, a large video screen on the front wall, and a alcove full of albums for the front wall. With our right channel speaker next to a large window of glass and our left channel speaker positioned next to an alcove full of albums, one can immediately see the acoustical issues that must be dealt with. Don’t forget about the large screen glass video display unit on the front wall.

Low Frequency Management

When tuning any room, one should start with managing the low frequency energy first. Without proper low frequency management, there is nothing for the middle and high frequencies to sit upon without getting smothered or blurred in an acoustical “mud”. Most listening rooms are not optimized in size to reduce the resonances created by low frequency issues, so one must use large amounts of low frequency absorption to assist with resonance control. with our room tuning project consisting of a 15′ width, low frequency resonances must be addressed.

Portable Acoustic Technology

In this client’s room, we needed to manage everything. The room was 15′ wide with a full length glass window as one wall. The left channel had to fire its reflected energy into a alcove that was full of albums. At least the albums provided some mass to work with compared to the full length window on the right channel side. Our solution was to make a mobile “wall” that could be moved into place when listening and then moved away for living. Living and listening are two different room functions.

Right Channel Glass Wall

Dual Acoustic Purpose

Since our first goal with our 15′ wide room is resonance control with frequencies from 30 Hz. – 80 Hz., we must have a low frequency absorber that is capable of handling this energy. We also need to handle the reflections from our speakers off of the glass. To accomplish both of these objectives, we built larger sizes of our ACDA-10 and ACDA-12 series diaphragmatic absorbers that would cover a larger surface area. They were 30″ wide and 60″ tall on casters since they had to be mobile. They also weighed in at 250 pounds each. You must have mass when dealing with low frequency energy. There is simply no substitute for mass.

Middle And High Frequencies

To deal with the middle and high frequency issues within the room, we attached our 2″ foam to the face of each unit, directly behind the fabric. The foam technology begins its absorption at 125 cycles and goes through 7,500. The ACDA-10 unit starts its absorption process at 30 Hz. and goes through 200 Hz. With the addition of our foam technology, we now have a unit that can absorb energy beginning at 30 cycles and advancing through 7,500 cycles.

Left Channel Units

Front Wall
Our next step is to work on the front wall where the center image of our sound stage resides. It resides there because we have used the proper amount of absorption for the side ‘walls” to lower the time of the primary reflection from the side walls. We must lower the time signature of both the side wall reflections so they arrive at the listening position after the wanted, direct, sound from our speakers.

Absorption Technology

Since we have a video screen that is mounted on the front wall and the glass surface of it must be covered, we can install a portable panel that can be hung over the video display screen when playing music and then removed for video viewing. This panel will contain the same sound absorption technology as the side walls. It must be light in weight so as to not add too much additional weight to the video screen mount system.

Listening / Living Rooms

Listening rooms must serve multiple functions in most consumer environments. Unfortunately, this does not allow for the proper room acoustic treatment to be installed in the correct locations to compliment our stereo presentation. Any acoustical technologies used must be mobile and removable. All frequencies must be addressed from low frequency room resonances to middle and high frequency reflections from side walls. Room treatment technologies can be developed that can be positioned in place when playing music and removed or repositioned when other room functions are desired.

MIM – Scottsdale, AZ.

MIM – Scottsdale, Arizona USA

The MIM is located in Scottsdale, Arizona and is exactly what the name implies. It is a museum of musical instruments from all of the world. We have musical instruments from Asia, Africa, and South America, to name but a few. The MIM also has a guitar section featuring guitars, banjos, and other stringed instruments from around the world. There is a special emphasize in display areas for American guitars and the famous artists that have used them. Audio is readily available.

Stage Right / Piano Center

Inside Music Theater

Inside the Mim is a 300 seat venue with a stage. On the stage, they have a single, Yamaha piano that plays by itself. It plays classical pieces that last about 8-10 minutes each. The sound energy coming from the piano appears to be amplified at the piano itself. Probably with some speakers under the piano bed where they can not be seen. There is no one in the audience except me. It is just the piano, the room, and me.  I have waited for this day.

Theater Dimensions

I asked for some specifications on the room such as dimensions and volume, but they did not have anything other than the standard marketing literature. I would estimate that the stage could fit 8 grand pianos, side by side, across it. I used the existing piano as my unit of measure.  Lets call it 50′. The stage was made of wood and clear coated finished and about 18″ tall. The room was about 100′ long and best guess would put the height at 20′. Above the stage, was a single speaker positioned horizontally at top stage center. I did not see any other electronic sound reinforcement.

Convex Vertical Wood Panels

Stage Wood Walls

The stage front wall was beautifully curved wood sound redirection panels that were vertically positioned. The convex shape would lend itself to sound redirection principles. We want to take the energy created from the stage side walls and redirect that energy at multiple angles back out into the audience area.

Layered Slats / Diffusion Surface

Wood Diffusion Surface

Directly above these wood sound redirection panels was a series of wood strips, maybe 3″ wide, that were in layered and positioned on top of each other. The strips wrapped around to form a wave of wood strips that was about 10′ tall. It wrapped around the room until about room center.

Stone Side Walls

Marble Side Walls

Continuing along the bottom half of the side walls, in the area where the seats are located is a stone plated wall. I am guessing that it is some type of marble or granite that has a semi gloss type of polish to its surface. It is light colored and its surface is a raised type of surface. each stone panel is either recessed or protrudes past the one in front of it and behind it. This uneven surface configuration is aesthetically pleasing but also aids in higher frequency diffusion.

Slotted Helmholtz Resonators

Sound Absorbing Technology

Above the stone surface area, which is located directly left and right of the seated area, is another series of slats that have fabric covered spaces between them. The slots or fabric covered spaces are all the same length, which leads me to believe that these slots form the opening to a Helmholtz resonator. They appear to be around 3′ long with a 2-3″ opening, which is then covered with black fabric. With this size opening, using quarter wavelength theory as our guide, we are looking at a absorber with a resonating start frequency of around 90 cycles.

Rear Wall With Control Room Window

Rear Wall

The rear wall is similar to the front wall directly behind the stage and off stage right and stage left. There are large, vertically placed, convex shaped, wood panels that go from the far left of the rear wall to the far right. There is a control room window that occupies the top right position of the rear wall area as you stand up in your seat and face the rear wall. The rear walls and front walls redirect the sound energy into the audience.

The Ceiling

The ceiling system consisted of rectangular shaped panels which appeared to be covered with some type of fabric. They averaged about 20′ in length,6″-8″ deep. They were probably 6′-8′ wide. They were placed next to each other from front of the room to the rear and each panels angle was adjusted, so that all reflections from the ceiling area were routed back to the rear of the room through a series of angles. With a 100′ long room, you have time to let the reflection die on its own volition by simply running out of another surface to strike again. With their varied thicknesses, the ceiling panels had to be broadband absorbers.

Ceiling Panels

The Sound

The sound from the piano that was playing was the only source that I used. I have yet to attend any concert or activity at the Mim, but I will, that you can be sure of.  In short, it was as natural as could be with no hint of room sound overall. The piano on stage and the sound that radiated from it stayed on stage with the piano. It was not thrown around the room to pick up additional room sound on its journey. Every note was separate and distinct, with excellent attack and decay without the hint of too much absorption or diffusion from the room treatments. I have never heard a room this size sound so good. I stayed and listened  for at least an hour. It was so visceral, I did not want to leave.

SBIR – Speaker Boundary Interference Reflection

There was one issue that I did hear from my center seat. I am still the only one in the theater. People come and go, but they never do stay very long. There was a speaker boundary interference effect going on between the piano bed and the stage. This is what lead me to believe that there was a speaker that was amplified under the piano pointing down. It is the same sound one gets when you place your speaker  close to the wall in your listening room. It is a blurring and smearing of individual notes with improper time signatures on the attack and decay of chords. After listening for awhile, I realized that it covered all frequencies. A broadband absorber from 30 cycles to 6,500 cycles would do the trick.

Free Space Listening

MIM or Musical Instrument Museum is located on the corner of Tatum and Mayo Blvds. in North Scottsdale, AZ. One must make three trips. The first two trips are just to see the museum exhibits. There are two floors of exhibits. The third trip is to attend a concert in the 300 seat theater. This is where you were be transcended into hearing music as it should be heard. There has been so much care and concern put into the room acoustics that the room in effect has been made to acoustically vanish. It is the closest thing I have ever heard to free space listening, which is listening to music with no ambient noise around and no walls or ceiling either. I will report back after my first concert to let you know if the room translates as well with multiple instruments and vocals as it does with the piano. I sure hope so.

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.