Our panels are designed to be very effective absorbers of unwanted air borne sounds. If they are applied in sufficient coverage, to most walls and/or ceilings, high noise levels can be reduced so that the clarity of speech can be vastly improved.

However, before procuring panels like ours for conference facilities, performing arts, sound studios, etc., it is recommended that these projects be best left for a qualified independent acoustical consultant's careful examination because they are usually far from simplistic, often requiring a range of complex acoustical issues such as sound transmission, frequency spectrum concerns, and the privacy of speech.

For example, suppose, at great expense, "sound proofing" walls enclosing a CEO's private office neglects cutting off its common air duct with the nearby mail room. Would it be any surprise that imminent news of an impending merger or a company "downsizing" goes galloping through this duct?

The National Council of Acoustical Consultants, an international professional organization whose membership includes many highly qualified architectural acoustical consultants, can be reached by clicking on their web site at: http://www.ncac.com

First, some fundamentals...

If we are to get into specific applications of acoustical materials, we should at least know what sound is first. You could say that sound is an oscillation or variation in pressure or a whole host of things that can occur in an elastic medium such as air, metals, water, etc. But we are normally accustomed to have sound propagated through the medium of air; however, denser mediums such as metals conduct sound easier.

The most important parameters that we normally deal with have to do with the pressure level of the energy in a sound wave. We call this sound level or sound pressure level (SPL) which relates to the loudness and is a subjective reaction because its intensity is based essentially on perception.

For human hearing sound is only audible if it is produced within a certain frequency range...generally 400 to 4,000 Hertz (or cycles per second). We hear best in the range of 2,000 to 5,000 Hertz provided the intensity or sound pressure level (SPL) is low. With high SPL's, low and high frequency sounds are very disturbing to us.

Pitch is interchangeable with frequency. The higher the pitch, the higher the frequency, and vice versa. Our "acoustical comfort" depends on our perception of the characteristics of the sounds we hear. Usually, high pitched sounds, such as those produced by loud sirens can produce more discomfort than those made at low frequencies.

For some, it is sometimes confusing how we measure sound energy. The unit of measurement is called decibel or dB. It is not an absolute unit of measurement such as an inch or mile because, as the following scale shows it is fundamentally based on our perception or hearing. But reference of 0 dB has been selected to correspond to the smallest sound that can be heard by a healthy human ear.

This decimal scale that measures sound level is logarithmic. This means that in order to get the same effective reaction we have to change the energy by larger and larger amounts but in the same proportion, otherwise the difference in going from 0 to 50 dB would be equivalent to going from 90 to 140 dB, but it's not!

It works out mathematically that, if we double the energy of sound, this represents a change of just 3 dBs! In terms of our hearing perception, this difference of just 3 dBs is enough for us to detect the difference between two sounds. So anything less than this is pretty insignificant. But in terms of energy this is a ratio of two to one and, thus, it is very significant.

For example, suppose someone wants to reduce the sound level of a noisy computer room by 10 dBs. This is a big job because just making a change of 3 dBs means cutting down the acoustic energy to one half so nobody would hardly notice the difference. This is why the importance of the fact that the decibel is measured on a logarithmic scale.

What is noise?

Simply put...it's unwanted sound. If somebody decides they don't want it, then it becomes noise, depending on what the application is and who is listening to it. We cannot measure noise, only sound. When people talk about "noise levels" they mean sound levels.

How is sound propagated?

If we have a loud speaker or some other small device which we can call a point source, it will radiate sound energy in all directions. If we are out in the open (called a free field), this energy will radiate away from the source and continue on. There will be no obstacles to reflect this energy back.

Now if we remain in a free field, we will also observe that if we compare points along a path away from the source, we will find that the sound level will decrease about six decibels every time we double the distance away from the source. For example, suppose we measure a certain sound level at ten feet from the sound source. If we go another twenty feet away, it will be reduced by 6 dB; then, you will have to go another forty feet for a reduction of another 6 dB.

What is reverberation?

An entirely different phenomenon occurs if we put our sound source in an enclosure, such as a room—particularly if it is a small room. The sound energy will undergo multiple reflections from the walls and ceiling and it will even persist after the sound source is terminated because it will continue to bounce off the surrounding surfaces. This effect is called reverberation.

Now the other effect is that of sound energy arriving at any point not just directly from the sound source but from other locations at slightly differing times causing our ears to add the reflections together...even though they don't occur exactly at the same time.

We get this amplification effect as we start moving away from the source because we are being exposed to more and more reflections. However, if we are close to the sound source, the sound energy remains essentially constant.

The problem in any space, with non-absorptive surfaces, that has a high level of sound reverberation is that it causes problems with the intelligibility of speech. This is because of the multiple reflections of sound energy bouncing off existing hard (reflective) surfaces causing a lack of clarity in the spoken word. This occurs because the first syllable, for example, is still being reflected around the space as the second syllable is spoken, and so on, causing a blurring effect, much like having glasses with a greasy film on the lenses.

A common (and expensive) mistake in such reverberant spaces is to summarily introduce electronic amplification which only serves to further degrade the acoustics because it intensifies the reflections. At this point it needs to be said that selective reflections can be useful provided that the undesired ones are minimized with sound absorbing materials.

Ideally, we want to achieve a controlled amount of reverberation for the usage of a particular space. For example, in a concert hall, we don't want it "dead" because people will complain that it's lifeless. However, in churches, often a compromise must be worked out because the organist may want the longest reverberation time conflicting with the minister's wish to have his words understood.

Another primary concern is to provide sound isolation between enclosed spaces. If, for example, you have a number of meeting rooms in a chain, they would not function well if everybody could hear everybody else.

A common misunderstanding about sound isolation is that it is often confused with the reduction of sound levels resulting from reverberations made within an enclosed space. This misconception stems from the notion that, if we reduce these reverberations, they will not travel through the walls, ceilings or air ducts into other areas.

So this brings up the question of what kinds of sounds do we deal with in buildings. One type is called airborne sound which is the sound produced when one is talking, radiating out into the air. The second type of sound is structural borne.

We have basically an airborne sound problem if we have to isolate one room from another. So, it is necessary to provide effective barriers which will resist the transmission of sound energy through them.

However, in many situations there is structural borne sound which can be generated by impacts. For example, in an apartment building with even a 12 inch thick concrete floor and no carpet, it is easy to hear someone in the apartment above you walking in hard heels. So concrete which is efficient in terms of blocking airborne sound, is not necessarily effective to isolate structural borne sound.

Vibration is another type of structural borne sound. For example, an office building having the mechanical room on the top floor housing 100 horsepower fans can pose a serious acoustical problem for someone occupying an office just below it.

So to provide a high level of isolation, whether it be to resist the transmission of sound propagated through the air, structure or vibration, it is necessary to consider a whole host of design criteria.

What about the matter of sound absorption?

While sound absorptive materials have a role in providing sound isolation, its major application and primary role is for helping to create a less reverberant environment within a space.

Unfortunately, however, the use of sound absorbing materials has been equated with providing something called "sound isolation" which, in my opinion can be misleading.

Maybe this misuse comes from the fact that a lot of materials that are excellent thermal insulators are also excellent acoustical absorbing materials but they don't function in the same way.

For example, suppose we built a room with two ½" thick layers of drywall on each side of the wall. We would probably have difficulty hearing the outside world through it. But if we removed the drywall from the studs and replaced it with 1" thick sound absorptive panels consisting of fiberglass (say 6 lbs per cubic foot density), we would then easily hear outside sounds through it. So what has happened to all this "acoustical insulation"?

So here it is where a lot of people get very confused because, if the sound is being absorbed, why is it coming through the walls?

So what really happens to the sound energy that is "absorbed" in a fiberglass panel? Well, some of it is transformed into heat through kinetic energy (i.e. friction) as it passes through the porous fibers. The remaining sound energy continues through to the space on the other side unless this path is blocked by some dense (non-absorptive) material. So sound absorptive material in itself is not the sole answer for "isolating" one space from another. It must also be used in conjunction with material that will help stop the transmission from going through the partition.

The point here is the terms "insulation" and "isolation" are very misleading contextually when discussing acoustics.

To be more specific about what constitutes sound absorbing material, it is a material that generally has to be porous so it can interact with the sound wave and convert its energy to heat. It must be able to breathe so as to permit the free flow of air through it. The effectiveness of a sound absorbing material is defined by its "absorption coefficient", as measured in a reverberation chamber. The absorption is simply that ratio of the energy that is absorbed to the energy that is incident or falls on the material.

So if we have a coefficient of say 0.50, this means that half of the energy that impinges on the material is absorbed.

The method by which this coefficient is determined is by comparing the time it takes a sound of a particular frequency to decay in a room with very hard surfaces (called a reverberation chamber) with the change in decay time after the sound absorbing material is placed in the room. As more absorptive material is added, this decay time will be faster.

With all sound absorbing materials, the absorption coefficient changes with the frequency of the sound that falls on it.

In terms of building problems, we normally are most concerned with frequencies between 125 and 8,000 Hertz (or cycles per second). But in so far as speech is concerned, typically the frequencies between 250 and 4,000 Hertz, and their related sound absorption coefficients will give us an accurate account of a particular material's acoustical performance.

If we take a one inch thick panel, having a fiberglass core density of say 6 to 7 lbs. per cubic foot, covered with an open weave fabric, its absorption performance is poor around the low frequency of 100 or 125 Hertz, whereas around 1,500 Hertz it's very high indeed!

Besides the core density, the thickness of a material also plays an important role in its ability to absorb sound. At low frequencies, for instance, the wave lengths are very long, so thicker materials are needed. However, if one needed absorption at 16,000 Hertz, a ¼" thick cellulose fiber would be quite adequate.

Another term that is very commonly used is "NRC" which stands for Noise Reduction Coefficient. It's a single number that allows you to make a quick general comparison between two different sound absorbing materials. It is in effect the average of the sound absorption coefficients of four frequencies: 250, 500, 1,000 & 2,000 Hertz.

When you have a frequency specific noise problem...such as a generator vibrating at 120 Hertz, you don't want to find a material to apply to the walls simply based on its NRC value. Rather, you need to look for a material that provides the most absorption at or very close to 120 Hertz.

So it's not fair to simply compare materials solely on their NRC values for specific applications especially when the noise generated falls outside of the NRC frequency average.

The application of sound absorbing materials is frequently used in the so-called general purpose room which we often find in schools. Sometimes it's a gym used also as an auditorium. Frequently, these large spaces exist without any or enough absorbing materials. Because these usually consist of hard interior surfaces, they are too reverberant for the clarity of speech. So one must be concerned with primarily two things: The distribution and placement of sound absorbing materials.

You don't want to add so much absorption that the room ends up "dead"...or almost dead as in some cases in broadcast studios.

Before going further, let's define a very useful acoustical term called reverberation time. It is defined as the length of time (in seconds) after a generated sound suddenly stops and for its lingering echo to decay one millionth of what it was after it was terminated...which is 60 decibels.

To come up with the right amount of absorptive material required for a space, one can have the reverberation time (in seconds) measured for it. Then, compare it with the so-called optimum reverberation time for similar space based on empirical studies. From the difference in time, we can calculate how much sound absorption must be added, keeping in mind that we have to also account for the average number of people present (who are also good absorbers) who will typically occupy the space.

It is rather obvious that the placement and distribution of the absorptive material is also about as critical as how much is needed. Now what does that involve?

Usually, the ceiling receives the absorptive material if it is not already treated. This is because the ceiling generally is the largest single area for the reflection of sound. However, it would be noted that in special circumstances such as theatres, the ceilings are sometimes stepped or angled with hard surfaces designed to reflect the sounds from the stage to the audience...particularly those occupying the last several rows.

Ceiling treatment alone is frequently not the complete solution because you still have four walls and sound can bounce back and forth between two of them or go around in a ring and bouncing if you have two parallel surfaces that are indeed parallel and make a sound in the middle you can get what are called flutter reflections where the sound instead of traveling around the room must go back and forth along the same path and that can occur, in fact you can probably demonstrate this at home in a reasonably sized room even though it's furnished if you make a sound and listen in an upper corner. You can usually get, if the walls are reasonably parallel, a flutter reflection going back and forth that you normally hear if you are moving about the room. If you move up higher you will hear this. It's hard to describe what it sounds like, but when you hear it you will know it.