ARCHITECTURAL ACOUSTICS

BEHAVIOR OF SOUND

A standard approach for understanding sound and sound control is to see it as a 3-part system: Source, Path, and Receiver.

Source:
-Generally, a source is any vibrating object that radiates sound waves by compressing and rarefacting air around it. The most common sound source in architectural spaces is ordinary conversation. Other sound sources include TV’s and radios, musical instruments, business machines, mechanical equipment, etc.

-Sound radiates spherically. Even if the sound source is facing one direction, the sound it produces will travel in all directions. dB level will vary based on directivity. It takes a certain amount of time for sound to get from the source to the receiver. Sound travels in air about 1130 feet per second, or 13.5 inches in 1/1000 second.

Path:
-Usually assumed to be air in common architectural spaces.

-Also, any other physical stuff such as walls, floors, ceilings, etc.

Receiver:
For our purposes, usually a person.

SOUND IN ROOMS

A standard approach for understanding how sound behaves in normal rooms is to see it as a 3-part system: Reflection, Absorption, and Transmission.
Reflection
-When sound strikes a room surface, some of it will be reflected, i.e., bounced off the surface and reflected back into the room. You might think of this as the bounce of a billiard ball or the reflection of a light ray: the angle of incidence equals the angle of reflection.
-To help understand sound reflection, we often imagine it to be similar to a beam of light. A hard sound-reflection surface such as concrete or a gypsum board wall (comparable to a mirror or shiny surface for light) will reflect most of the sound that hits it. A soft non-sound-reflecting surface such as carpet or a heavy velour drape (comparable to a dull or dark surface for light) will not reflect as much sound. (Note: Carpet is not a problem solver.)
Actually, almost all material will reflect some sound: concrete will reflect about 99% of the sound that hits it; gypsum partitions about 95%; a 1-inch thick acoustical wall panel will reflect about 20%.

Absorption
-You can think of sound absorption is the opposite of sound reflection. Materials that don’t reflect sound will, generally, absorb it.

-We usually think of soft “fuzzy” materials as being good sound absorbers. This includes carpet, fiberglass insulation batts, acoustical wall and ceiling panels, etc.

-Actually, almost all material will absorb some sound: concrete will absorb about 1% of the sound that hits it; gypsum partitions about 5%; a 1-inch thick acoustical wall panel about 80%.

-For those dealing with sound absorbing materials, remember that no sound is absorbed until the sound hits a sound-absorbing surface. (A little is absorbed by air, but not enough to worry about in common architectural spaces.)

Transmission
Some sound that strikes a surface passes right through the material and travels into the adjacent room.

General Considerations

-When sound strikes any surface, some sound energy is reflected, some absorbed, and some transmitted into the adjacent space.

-The sound that we hear in typical rooms is made up of two parts: Direct sound, the sound that arrives at our ears directly from the sound source; Reflected sound, the sound that has reflected from a room boundary surface.

HOW PEOPLE HEAR SOUND

There are three primary features of sound that we identify in most common situations: Loudness, Pitch (frequency), and Tone Quality.
Loudness
-Decibels. The most common “unit” of loudness, but not one that most people understand. Decibels are based on logarithms, and they don’t add like ordinary numbers.

-Human perception.

-We can usually judge sounds that are roughly twice as loud or half as sound on a purely subjective level. The relation between subjective impressions and decibels is not obvious.

-Using percentages can be misleading and is usually done incorrectly. A 50% reduction of sound is not half as many decibels. A 50% reduction of sound will not sound half as loud.

Pitch (frequency)
-The highness and lowness of a sound. Usually associated with common experience (women have higher voices than man) or size (large instruments such as tubas or basses make lower pitches than small ones such as piccolos or violins.

-The pitch of a sound is important for sound control: low-pitched sounds are much more difficult to control than high-pitched sounds.

-Human hearing is most sensitive to middle-pitched sounds: especially in the speech range. We are far less sensitive to extremely high and low pitches. 4000hz is the high end of the mid range.

Tone Quality

-We are able to distinguish between different sounds largely because of tone quality: we can tell a dog from a person or a male from a female. This is largely, but not entirely due to tone quality.

-We also distinguish different instruments: a piano from a guitar.

-Some difference in tone takes more discrimination: a sports car from a pickup truck, or an MGB from a Triumph TR-4. This sounds very sophisticated, but note that almost any one can distinguish between two different people (even of the same age and sex) within a few words on a telephone.

STANDARD ACOUSTICAL RATING FOR SOUND CONTROL: NRC AND STC

Concepts, principals, and limitations.

NRC: Noise Reduction Coefficient.
-A single-number (over-simplified) designation for rating the amount of sound absorbed by a material.

-NRC is generally used in terms of the sound within one room.

-The NRC value is simply (and very roughly) the percentage of sound absorbed by the material. An acoustical panel rated at NRC-0.65 will absorb 65% of the sound that hits the material. But this is not true for all frequencies.
-The material only absorbs sound if the sound hits the material.
-Be careful in trying to translate % to sound reduction.

STC: Sound Transmission Class.
-A single-number (over-simplified) designation for rating the amount of sound blocked by a material.

-STC is generally used in terms of the sound between two rooms.

-The STC value is simply (and very roughly) the amount of sound reduction when a sound travels through a wall, from one room to another. A partition rated at STC-35 will reduce a sound traveling through it, from one room to the next, by about 35 dB.

Both are applicable primarily to speech, not music or mechanical noise. (ASTM)

REAL-WORLD EXAMPLES

Suspended Acoustical Ceilings: NRC versus STC (Example 1)

-Suspended acoustical ceilings will absorb sound within room. They can help reduce noise within that room.

-They will not keep sound from traveling through the ceiling and over the top of ceiling-height walls between adjacent offices. However, there are special techniques for dealing with this. (High-STC ceiling tiles, insulation batts above the suspended ceiling, fuzz walls. The solutions are not very helpful.)

-Suspended acoustical ceilings are very useful (necessary) in open-plan offices. However, it is very important that the sound absorption rating be very high: NRC-0.90 or better.

Acoustical Wall Panels: NRC versus STC (Example 1)

-Acoustical wall panels will absorb sound within a room. They can help reduce noise within that room.

-They will not keep sound from traveling through wall and into an adjacent room.

Gyms: How much is enough fuzz? Where should you (or can you) put it? You must deal with the low-frequency problem because the volume (ceiling height) is so great.

Churches: Wow! Too many variables. You must understand the situation (problem) first. Is it a traditional church or modern non-liturgical church? Is there a pipe organ and 50-voice amplifier choir or is there a full contemporary band with guitars and amplifiers and a choir with many microphones and 10,000 watts of sound amplification.

Open-plan offices; telemarketing complexes.

Quieting a disturbing noise (such as noisy HVAC) with acoustical panels or ceiling. Can you do it? Probably not!

NOTES, AFTER-THOUGHTS AND TOPICS FOR FURTHER DISCUSSION

Just because something is called an “acoustical” material doesn’t mean it is applicable to all “acoustical” situations. Compare STC and NRC of a 12-inch concrete wall; a 3-inch batt of fiberglass insulation.

The suitability of an acoustical material, for a particular purpose, depends on how it is used. E.g., 3-inch fuzz attached to the surface of a wall won’t keep sound from passing through the wall to an adjacent room: you may get a 1 or 2-dB improvement in STC best. However, if you place the fuzz within the wall cavity, you may get a 6-8 dB improvement in STC.
-Note also that placing a 3-inch insulation batt in a stud wall cavity will help. It will not help between a double masonry wall. But, a good acoustician would call for fuzz between a double masonry wall. Why?

Acoustics is not rocket science, but it is also not easy nor intuitive. Many things in acoustics seem to go against common sense. Show the loss of STC by adding layers of gyp.

In using sound absorbing materials in real rooms, it is often important and useful to use reverberation time (RT) as a design goal. It is important to understand the difference between reverberation and echoes: Here is some background:
-Both reverb and echoes are caused by the reflection of sound from hard sound-reflective surfaces, usually walls.

-Reverberation is an enhancement of sound that is heard when hundreds (or thousands) of sound reflections occur immediately after the direct arrival of the original sound at our ears. These multiple reflections blend with the original sound and create an audible extension of the original sound with a gradual fading of sound. This fading of sound can take as little as 1/10 of a second (as in a small room or office; almost too short to hear) up to 6 seconds or more (as in a gym or large church).

-When not too long, reverberation is beneficial. It actually increases the loudness of the original sound. Further, reverb enhances the tone or quality of a sound. However, if reverb is too long, it will tend to make speech hard to understand. There is actually an appropriate length of reverberation: a length that is long enough to sustain and enhance a sound, but not so long that clarity becomes obscured.

-Echo is a single (sometimes several) sound reflection that can be heard individually after the original sound. To be heard as an echo, the reflection usually occurs about 0.04 of a second or more after the original sound and is almost as loud as the original sound. There is no fading of sound; just an audible repetition of the original sound.

-Echoes are interesting, but almost always undesirable in buildings. Echoes ruin speech intelligibility. In music rooms (performance halls, theaters, etc.) echoes ruin clarity of speech and music, and often give the impression that sounds are coming from a different direction.

-The most important reasons that acoustical suppliers should know the difference between echoes and reverb are the following: (1) appropriate reverb is desirable, but echoes are almost always undesirable; (2) reverb should be controlled and/or adjusted, but echoes should be prevented or eliminated; (3) acoustical treatments for reverb control and echo prevention usually call for sound absorbing materials, but the method of treatment (quantity or location of materials) is different.

-A major factor in providing sound-absorptive treatments for a particular room is knowing or deciding what the appropriate RT is. Different reverberation times are appropriate for different kinds of rooms and there are charts and tables available to guide you.

-In selecting an appropriate RT, it is important to know the situation, not just the type of room. For instance, churches have a great variation in RT depending on many different factors. The sound contractor will want to minimize RT to simplify design and assure maximum speech intelligibility. This might call for an RT of 1.5 to 2 seconds. However, if there is a strong music program, a major pipe organ, and large traditional choir, an RT of 3 seconds or more is needed.

-The location of sound absorbing materials is also very important. Don’t put fuzz around the major sound sources: none on the chancel floor or music center floor. If congregational singing is a high priority, there should be no carpet and pew cushions. The size of the audience will affect reverberation levels.

-Modern evangelical non-liturgical churches, etc., have strong, but amplified, music. Many are broadcast via radio or TV. All of these demanding low reverb: 1.5 seconds or less.

-The most interesting thing about this RT approach is that there is a standard method for calculating the amount of sound absorbing material needed.

RT= 0.049 x Room Volume
Surface Area x average absorption coefficient

-If you are uncomfortable with equations, don’t worry. There are things that are easy to understand that are direct results of this equation.

-The bigger the room volume, the greater the reverberation time.

-The greater the sound absorption (i.e., the larger the area covered by sound absorbing materials, or the higher the absorption rating of the material, or both) the lower the reverberation time.
-Volume is a bigger factor than sound absorption. The larger the room, the harder it is to reduce reverberation. That’s why it is so difficult to control reverberation and sound build-up in gyms and churches.

-It matters where sound absorbing materials are located: It matters a lot. But, if you look at the equation above, it doesn’t say anything about where the materials are located: it just tells how much material is needed.

-Remember that sound will not be absorbed until it hits a sound absorber. Therefor, sound absorbers are most effective and efficient when they are placed near a sound source. This also means that they are less effective on high ceilings (in general) such as gyms. But in a gym, where else can you put fuzz?

-One rule-of-thumb is that sound absorbing materials are most effective when they are uniformly distributed along surfaces. It is also recommended that you don’t treat only parallel opposing surfaces. For Example, it is not recommended to have a heavily carpeted floor and acoustical tile ceiling. But, this is often the only option available.

-Some of you may have occasion to install sound absorbing materials in high-tech manufacturing plants with high ceilings (the standard Butler building). However, if all the noise is produced by machines at floor level, and most employees are working close to these machines, placing sound absorbing materials on walls and ceilings will usually not help reduce employee noise levels. You need to locate sound absorbing materials close to the noisy machines and close to the noise-exposed workers, but this is often impossible.

-It takes a very great increase in sound absorption to reduce the sound level within a room.

-If you double the amount of sound absorption (either by doubling the area covered by sound absorbing panels or replace the existing sound absorbing panels with one that has an NRC that is twice as great), the maximum sound level reduction would be 3 dB. That is a just-noticeable difference.
If you increase the amount of sound absorption by ten times (either by increasing the area covered by sound absorbing panels by ten times, or replacing the existing sound absorbing panels with ones that have an NRC that is ten times as great), the maximum sound level reduction would be 10 dB. To the average person, that would sound like a noise level reduction of one half.

-For people who are close to the noise makers (for example a person operating a noisy machine or a student in a noisy grade-school cafeteria), the sound level reduction will be much less than what this equation predicts.

-It matters how thick a sound absorbing material is: It matters a lot!

-There are two common situations where thickness of material is a major factor:
-Music rooms and mechanical rooms where there is a lot of low-frequency sound. One-inch acoustical panels usually won’t be sufficient. Two-inch panels may do it, but you usually need panels that are at least 3” thick for music, 4” thick for mechanical noise.

-Rooms with large volumes such as gyms, convention center, churches, etc. The large volume almost always causes very strong low-frequencies: sometimes called “boomy.” Here (in general) you must use at least 2-inch thick acoustical panels. Often you will need specially designed bass traps.

-It matters how sound absorbing material is mounted: It matters a lot!

-When called on to evaluated a sound absorbing treatment for a school cafeteria.

They planned to carpet the walls. They had seen an ad from a carpet supplier claiming an NRC-0.95 for carpeting attached to walls. I looked at the ad and noticed the fine print: the NRC-0.95 was achieved by mounting the carpet over 2-inch furring strips, 16-inch on center, and filling the space between the furring with R-11 batt insulation. If the carpet was directly glued to wall surfaces, the sound absorption performance was only about NRC-0.15.

Without specific information about mounting, all those advertising claims are meaningless. Don’t be afraid to challenge the claims of other products if you are suspicious.

-Sound absorbing panels can be used to control echoes as opposed to reverberation:

-Again, it matters where you put it. To eliminate an echo, you only need to put sound
absorbing panels on the echo-producing surface or area. This may mean you can use
far less acoustical material: a “laser” approach as opposed to a “flame-thrower.” In
many cases, you want to eliminate the echo but not the reverberation (churches,
concert halls, etc.) By applying sound absorbing panels only on the echo-producing
surface, there may still be some reverberation reduction. If this is not desired, you
will need to use sound diffusion, not absorption.

Learn more about Acoustics and FabriTRAK® Specifications.