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FAQ's

ARCHITECTURAL

What does a sound absorbing material do?

How do walls block sound?

Why is my church fellowship hall so noisy?

Why do the footsteps of my neighbor upstairs sound like an elephant?

Why is my child's classroom so noisy they cannot hear the teacher?

My child's classroom is quiet, but they still cannot understand the teacher. Why?

We just had a new sound system installed, but there are dead spots in the room, and places where words are not understood. What is happening?

FAQs

What does a sound absorbing material do?
The term "acoustical material" is usually applied to a material that absorbs a majority of the sound impinging upon it. These are used to prevent undesirable sound reflections. Such reflections could result in clearly perceived echoes, slightly delayed echoes that reduce the clarity of speech, increased noise levels, or excessively long reverberation.
Sound absorbers are usually porous materials that turn sound into heat. These absorb primarily the higher frequencies unless the material is very thick. At thickness of one to two inches is practical to absorb mid-frequencies well. Examples of these materials are fiberglass, mineral fiber, open-cell foams (not Styrofoam), shredded wood fiber, un-painted porous concrete block, heavy drapery in folds, and carpet. Vibrating panels and tuned cavities are also used to absorb low frequency or bass sound. A gypsum or wood-panel wall with an air cavity, or a suspended ceiling can be a good bass absorber. Some fiberglass wall panels have high-density surface layers that improve bass absorption. Special concrete blocks with slots to their cavities are the most common cavity absorber.
Our ears expect most rooms will have more absorption at higher frequencies. However, if the difference between the absorption at high and low frequencies is too much, the room will sound "boomy." This often occurs with thin porous materials applied to hard surfaces. One must carefully select materials to balance the absorption. This is especially important in rooms that will be used for music, or where very natural sounding speech is desired. Most, but not all, suspended acoustical ceilings are better balanced than most wall treatments. They achieve good bass absorption acting as a panel with an airspace above. One way of balancing a room is to use a combination of materials with opposite characteristics.
The performance of an acoustical material is measured by the Sabine absorption coefficient. This will vary with frequency and with the thickness and mounting of the material. Thicker porous materials and those mounted with an airspace behind them will have more absorption at low frequencies. A widely publicized rating is the mis-named "Noise Reduction Coefficient" or NRC. This is the average absorption at 250, 500, 1000, and 2000 Hz. It can be very misleading. Two materials can have very different characteristics producing very different results but a similar NRC rating. NRC results also can be influenced by tuning systems to be absorptive only at the frequencies in the average, and not in the broad range in between.
Most sound absorbing materials are not good sound blockers. However, the addition of a sound absorbing material inside a cavity of a sound-blocking wall can improve its sound blockage ability. Back to Top

How do walls block sound?
Most solid walls with no open leaks will provide 40 to 50 dB of blockage for middle frequency sound. This is somewhat amazing if you consider that 40 dB amounts to blocking 99.99% of the sound, and 50 dB amounts to blocking 99.999%. With special care, blockage up to 60 dB or more can be achieved. However, success beyond 50 dB depends on careful construction, and is often limited by flanking or sound leakage by paths around the wall, door or window. A small leak can have a major effect on a wall with otherwise high blockage.
Higher frequency sound is easier to block than low frequency or bass sound. The bass sound has longer wavelengths. The wall looks thinner to the low-frequency sound. Walls are rated in their ability to block speech or speech-like sound by the Sound Transmission Class (STC). Their blockage of bass sound will be less than the STC. The wall with the higher STC may actually be a poorer blocker of bass sound.
The major factor that influences the blockage of the structure is its weight. However, the efficiency of the material used can be improved by separating it into isolated layers. The more separation space provided, the greater the increase in blockage. For instance, suppose you have a layer of material that by itself blocks 20 dB. If you double the thickness of the layer, the blockage is improved by about 5 dB at most frequencies. However, if you use two layers well separated from each other, the blockage can be increased 20 dB or more at some frequencies. The problem is in establishing good separation, especially for lower frequencies. The improvement for bass sound may be only about 5 dB. To achieve the best improvement with isolated layers, the cavity between layers must contain sound absorptive material, and the connections between layers should be flexible. Light gauge steel studs provide better isolation than wood studs. Resilient channels can be added to one side of wood studs to increase the flexibility. Two sets of studs can provide even better results even if wood.
All solid materials will have a weak point at a "critical" frequency. This frequency is lower and thus more of a problem for thick, stiff materials. The critical frequency of a thin, limp sheet of lead is very high. The critical frequency of many common materials is in the frequency range of speech. The effect of the critical frequency can be reduced by adding a vibration damping effect. An example is the laminating layer in safety glass. Another way is to use several layers of thinner material to raise the critical frequency. Using layers of different thicknesses assures that one layer will be strong at the critical frequency of another.
Structureborne sound, such as footsteps on a floor above, constitutes a more difficult problem. The structure itself becomes the noise source, rather than a blocker of the noise. Other examples of structureborne sound are plumbing and mechanical equipment that is not isolated from the structure. Sometimes very loud airborne sound can excite a structure (such as a side wall or roof) and become structureborne around a good wall.
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Why do the footsteps of my neighbor upstairs sound like an elephant?
Many if not most new apartments and condominiums are built using wood frame construction. Wood structures are inherently flexible. Floors can easily flex and vibrate at low frequencies. There is no known way to eliminate the characteristic "thump" of such floors. Carpet or special materials under hard floor surfaces can eliminate "tapping" sounds. Careful construction can eliminate squeaks and pops. However, a degree of the "Thump" will always be with us. Back to Top

Why is my child's classroom so noisy they cannot hear the teacher?
A number of factors have influenced the recent design of school ventilation systems to make them very noisy. Fans should never be located in a classroom or over the ceiling of a classroom, but they commonly are. Many classrooms are too noisy even for adults with good hearing and language comprehension. Young children, anyone with hearing difficulty, and people who are listening to a second language need quieter conditions. The key is a well-designed, quiet ventilation system. Back to Top

My child's classroom is quiet, but they still cannot understand the teacher. Why?
Even with quiet ventilation systems, it is difficult to understand in many classrooms because they are too reverberant. The extremes are rooms with high and hard ceilings. These were common many years ago. Unfortunately, there has been a recent return to this practice by some who believe that high ceilings with skylights are the key to a good classroom. Classrooms should be designed with low, highly absorptive ceilings. Otherwise, significant amounts of absorptive wall treatment should be used. Recent research indicates that young children, people with hearing difficulties, and those listening to a second language need less reverberation than has traditionally been considered acceptable. Back to Top

We just had a new sound system installed, but there are dead spots in the room, and places where words are not understood. What is happening?
Unfortunately, many firms that install sound systems do not fully understand how the system can interact with the room, or even how multiple loudspeakers can interact with one another. Sound from the loudspeakers could be reflecting from the walls and reaching your ears with a long delay after the initial sound has reached you directly from the loudspeaker. If you have multiple loudspeakers that are not close together, you could be hearing the sound from the different loudspeakers with a time delay. Sound travels faster as an electronic signal in a wire than as in a wire than it does through the air. Thus, if there are extra speakers near the rear of the room, the sound from these must be delayed electronically so you hear it at the same time you hear sound from the speakers near the front of the room. Another problem can occur because of interaction between the sound coming from two speakers very close together as in a cluster. At certain locations, sounds at certain frequencies from the two speakers will combine in a way that they cancel each other. We commonly find these problems and recommend that clients choose sound-system contractors and designers very carefully Back to Top

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