Acoustics


Introduction to Acoustics

"Why would an architect make it look so nice, but then you can't use it", owner of a newly constructed Reception Hall referring to her acoustically unusable space.

Interior acoustics is vitally important to the functionality of a given space. Interior space has a multitude of uses and applications. The results of good acoustical design can be vary rewarding. For example, some concert halls, churches and theaters are admired for their marvelous acoustics. Even small projects without world recognition can function much more effectively with the proper acoustical care. On the other hand, overlooking important acoustical design considerations can cause many problems. Users can become so dissatisfied with a space, it can be classified as unusable. For example, when the German Government moved to the new parliament building in Bonn, officials found themselves in the middle of an embarrassing and costly mistake. The acoustics were so poor and the announcements so garbled, they were forced to move out the next day. If a space is not effective for its intended uses, the consequences are not only annoying, but very costly.

The following is a guide to help understand and enhance proper acoustical design, thereby eliminating expensive mistakes. This is addressed in the following three sections: Acoustical Principles, Acoustical Design Considerations and Acoustical Checklist by Project Type.

ACOUSTICAL PRINCIPLES

SOUND
Sound is produced by a vibration through a medium such as air. Sound travels in all directions from the source as a pressure wave, the same way waves travel through water when a pebble is dropped into a lake.

- Frequency (Hertz-Hz) the number of times per second that a sound wave repeats its cycle. A low frequency (large sound wave) repeats its cycle less frequently than a high frequency (small sound wave).

The hearing range of the human ear is from 20 to 20,000 Hz, with the upper range decreasing with age. The frequency range of speech is 125 to 8000 Hz. The frequency of a sound wave is dependent on its length. The lower or deeper frequency of 125 Hz has a wavelength of 9 feet. Conversely, a higher frequency of 8000 Hz has a wavelength of 1 3/4 inches.

Lower frequency sounds are more difficult to control because of their longer wavelength. This is why, when loud music is played in an adjacent room, the high frequencies are blocked, and you only hear the thumping of the bass. Although controlling low frequencies is more challenging, it cannot be overlooked.

- Decibel (dB) A unit for measuring relative loudness of a sound. The theoretical threshold of human hearing is zero decibels. The human ear, an amazingly sensitive instrument, is able to hear from 0 to 130 decibels (the average pain threshold). This range of sensitivity is comparable to a bathroom scale capable of measuring from 1 ounce to 300 million tons. This helps demonstrate the challenge involved in effective noise control and acoustics.

NOISE

Noise is unwanted sound that interferes with the function of a given space. Noise is subjective because what is disturbing and unacceptable to one room application, may be acceptable to another.

DIRECT SOUND

Direct Sound is sound traveling from the source to the receiver without striking any surface. Direct sound diminishes in intensity as the distance between source and receiver increases. Acoustical planning would be an easier process if all sound was direct, but it's not.

REFLECTIONS

Reflection is the occurrence of sound striking one or more surfaces before reaching its receiver. These reflections can have unwanted or even disastrous consequences. Reflections are attributed to the shape of the space, as well as the material on the surfaces within the space.

Domes and concave surfaces cause reflections to be focused rather than dispersed. This causes annoying sound reflections, reduced speech intelligibility and adverse effects on music.

Reflective parallel surfaces, such as conference and board rooms, lend themselves to a unique acoustical problem called flutter echo. This is when sound ricochets back and forth between the same spots on parallel surfaces. This problem should be avoided, and can be eliminated, by using absorption surface treatments such as FABRITRAK®.

See Design Considerations for more information.

REVERBERATION

Reverberation is the prolongation of a sound in a room, and is caused by continued multiple reflections. In an enclosed space, when a sound source stops emitting sound, it takes some time for the sound to become inaudible. The length of the reverberation needs to be addressed to ensure proper acoustics.

- Reverberation Time (RT60) is the time required, in seconds, for the average sound pressure level in a room to decrease 60 decibels after a source stops generating sound. More simply, this is the time it takes for the reflections to go away.

Reverberation time plays a crucial role on the quality of music, and the ability to understand speech in a given space. When room surfaces are highly reflective, sound continues to reflect or reverberate. The effect of this condition is described as a live space with a long reverberation time. For example, a long reverberation time in a conference or board room causes syllables, if not words, to overlap. This in turn degrades the ability to understand the presentation.

See Design Considerations for additional information.

- Reverberant Field The multiple reflections in reverberant (live) spaces build upon themselves, causing the noise level to increase. The more absorption in a room, the more unwanted reverberant noise levels can be controlled. A project that has a significant noise source may require other noise control procedures.

ABSORPTION

Absorption is a quality in materials that combats unwanted reflections. When sound waves strike a material, part of the sound energy is absorbed and part is reflected. All materials absorb some amount of sound energy. This amount is expressed in terms of a sound absorption coefficient.

- Sound Absorption Coefficient The percentage of sound absorbed by a material at a particular frequency. If a material has a sound absorption coefficient of 0.90 at 1000 Hz, it means that 90 percent of the 1000 Hz sound striking that material is absorbed, and only 10 percent is reflected.

- Noise Reduction Coefficient (NRC) The average of Sound Absorption Coefficients (at 250, 500, 1000, and 2000 Hz rounded to the nearest 5%) for a given material. The NRC is a single number index of sound-absorbing efficiency. For example, drywall has an NRC of 0.5 and 1" FABRITRAK® has an NRC of .80. The drywall is 5% absorption and the 1" FABRITRAK® is 80% absorptive.

SOUND TRANSMISSION

Reflection and absorption are properties that directly affect the interior of a space. Oftentimes, it is equally important to address the amount of sound passing through a partition into adjacent spaces.

- Transmission Loss (TL) A measurement expressed in decibels, indicating the ability of a material or system to block or attenuate the transmission of sound, at a particular frequency, from one area to another.

- Sound Transmission Class (STC) A single-number rating of a construction's airborne sound transmission performance at a range of frequencies from 125 Hz to 4000 Hz. Higher STC values are more efficient for reducing sound transmission. For example, loud speech can be understood fairly well enough through a wall with an STC of 30. On the other hand, loud speech should not be audible through a wall with an STC of 60.

ACOUSTICAL DESIGN CONSIDERATIONS

The wait-and-see approach to acoustics can be detrimental. It will save time, money, and many headaches to consider the acoustical needs of a project during the design phase. Initially, the intended use of the space should be defined.

Acoustical materials perform one of two acoustical functions. They either absorb (or diffuse) sound, or they block its transmission. Therefore, one must determine: is the goal to control noise and the acoustical environment within a room, or to reduce the transmission of sound from one room to another? Quite often both objectives need to be met. For example, speech intelligibility (sound absorption) is very important in a boardroom. Equally important, is blocking the information in the board room from being heard outside, and vice versa (blocking sound transmission).

Be careful when selecting a product based on its NRC alone. The NRC does not include the values below 250 Hz or above 2000 Hz. Two materials may have identical NRC ratings, but very different absorption characteristics. FABRITRAK® is a product relied on for its flexibility. Unlike pre-finished panels, the absorption product used in the FABRITRAK® system can be custom made to address the specific needs of a particular space.

SOUND ABSORBERS

Acoustical materials are used to reduce the noise level, eliminate unwanted reflections and control the room's reverberation time. This is typically measured by the NRC of the material used.

- Noise Control Within A Room - Increasing the absorption is one way to reduce sound levels in a room, but it does not affect the source itself. When multiple reflections in a room are reduced through absorption, the noise level of the room is also reduced. The degree of reduction is relative to the amount of absorption material used and its NRC.

- Reflections - Depending upon the use of the room, reflective parallel surfaces can cause flutter echo. This ricocheting effect of sound must be considered and treated by either changing the shape of the room or adding absorption.

Sound reflections caused by domed or cylindrical spaces are the most acoustically challenging. Although reflections themselves are often desired in musical spaces or to add in voice projection, concave surfaces (especially domes) focus the sound, resulting in an area with an annoying over-abundance of reflections. This leaves the remaining spaces without enough reflection to enhance the music. If a concave space cannot be avoided, a surface treatment such as FABRITRAK® is one of the few acoustical products than can be installed to conform to a desired shape, maintaining the aesthetic integrity of a feature.

- Reverberation - The effects of reverberation time on a given space are crucial to the clarity of music and speech. A reverberation time that is optimum for a music program could be disastrous to the intelligibility of the spoken word. Conversely, a reverberation time that is excellent for speech will cause music to sound "dry" and "flat". The optimum reverberation time is dependent on the use of the space. Reverberation time can be determined through testing in existing spaces or through calculations of a planned space. Once again, the amount of absorption material required will depend on its respective NRC as specified by an acoustical consultant.

BLOCKING SOUND TRANSMISSION

Sound Transmission Class (STC) has limitations and should be accepted with caution. It is roughly the combination of the transmission loss values at the various frequencies. However, it ignores frequencies at the extremes, resulting in some walls with high STC ratings that do not block low frequencies well at all. Sound transmission Class values are assigned based on transmission loss data collected in laboratory tests. Construction procedures greatly affect a system's ability to block sound, almost always resulting in lower STC values in the field than in the lab. Even a small air gap can significantly degrade the acoustical integrity of a wall. More complex systems tend to have higher STC ratings. Thus, when properly installed, they do a better job at attenuating sound between spaces. A single material may block only a specific range of frequencies, while allowing others to pass through virtually unobstructed. A wall construction composed of several different materials may have the ability to block a larger range of frequencies.

Reducing the noise levels within a space, through the use of absorption material, reduces the noise level that can be transmitted through the wall.

COMMON MISTAKES

The following is a list of common errors made in acoustical design:

- Acoustical ceiling tiles are designed to be absorptive. When painted without proper acoustical considerations, sound reflects off the paint, degrading the tiles' ability to absorb sound.

- It is a common myth that carpet is highly absorptive. Carpet can do an excellent job of reducing impact noise caused by footsteps, but it has a low absorption coefficient compared to other acoustical materials. For example, a newly constructed Performing Center was designed with a dome. During the construction, the owners were assured by the architect that "when the carpet is installed the acoustical problem will be solved." The carpet installation did not solve the problem and the architect was fired from the remaining five projects.

- Fabric by itself is not an effective absorber.

- The difference between Noise Reduction Coefficient (NRC) and Sound Transmission Class (STC) is often confused. Just because a material has a high NRC rating does not mean it will have a high STC rating. Usually the opposite is true.

- Be careful when using manufacturers' published NRC ratings. Be sure that frequency ranges and mounting procedures are applicable, for accurate comparisons.

ACOUSTICAL CHECKLIST BY PROJECT TYPE

BALLROOMS

Goal: To provide a multi-purpose space that can successfully control noise appropriate for meetings and receptions, both large and small.
Recommended Reverberation Time: 1 - 1.2 seconds

- Absorptive materials are needed to reduce the reverberation time, and are most appropriate when applied to the ceiling. FABRITRAK® works well as an acoustical ceiling treatment.

- Be cautious when designing ceiling intricacies as they may cause unwanted reflections.

- Movable partition walls covered with fabric alone do not guarantee absorption.

- Once divided by movable partitions, the acoustics in the new space can change.

- Control exterior and background noise levels from HVAC.

- Maintain an elegant appearance by addressing acoustical issues during the design phase.

CINEMAS

Goal: To properly use absorption, working with the sound system, to provide the best acoustical environment for every moviegoer.
Recommended Reverberation Time: 0.8 - 1.2 seconds*

- Background noise levels should be kept to a minimum. Two primary potential noise sources are mechanical equipment (HVAC) and outdoor noise. *

- Excessive room length should be avoided.

- Walls, except those close to the screen, should be absorptive.

*THX has published its own criteria for reverberation time ranges, background noise levels and room isolation levels.

CONFERENCE/BOARD ROOMS

Goal: To provide a space where a variety of communications styles can be effectively used.
Recommended Reverberation Time: 1 second

- Limit the amount of reflections to keep speech intelligibility at a maximum.

- Absorptive materials are needed to reduce the reverberation time.

- Avoid reflective parallel surfaces.

- Control exterior and background noise level.

- Ensure flexibility for future multi-media advancements.

- A sound system may be necessary.

CORRIDORS

Goal: To eliminate excessive traveling of noise which is common in hallways, particularly in sensitive or confidential offices such as attorneys, psychiatrists, and personnel managers.
Recommended Reverberation Time: 0.8 - 1.2 seconds.

- Be cautious with curved surfaces as they lead to the undesirable acoustical condition known as creep.

- If surfaces are left untreated, a corridor can act as a megaphone, transmitting conversations into nearby offices.

- Surfaces should be absorptive to maintain a low reverberation time. Horizontal FABRI TRAK® panels are an excellent solution for this application.

HOME THEATERS

Goal: To adapt an existing space in a typical home, to provide the best acoustical environment possible for speech and music.
Recommended Reverberation Time: 0.8 - 1.2 seconds

- Verify that ratio of room dimensions will not cause unwanted reflections.

- Surfaces should be absorptive to maintain a low reverberation time. FABRITRAK® works well on both wall and ceiling surfaces.

- Please see Checklist for Cinemas.

LECTURE HALLS

Goal: To allow all audience members to easily hear and understand the presenter.
Recommended Reverberation Time: 1 Second

- The front wall and ceilings can be reflective aiding sound in reaching everyone. FABRITRAK® can be used with a reflective core material.

- Absorptive material on the back and side walls will help reduce the reverberation time and unwanted reflections. Absorptive FABRITRAK® can be used to visually match the reflective FABRITRAK®.

- Excessive background noise levels caused by HVAC systems can be greatly degrade speech intelligibility.

- Splay the side walls to eliminate flutter echoes.

LIBRARIES & MUSEUMS

Goal: To limit noise levels, allowing users to read and contemplate without disturbances.
Recommended Reverberation Time: 0.8 - 1 second

- If domes or other concave surfaces are desired, they must be treated with absorptive material to reduce unwanted reflections. FABRITRAK® works well on curved surfaces.

- Absorptive materials are need to reduce the reverberation time.

- Books are not very absorptive.

- If possible, place noisy equipment and activities in remote areas.

OPEN OFFICE PLANS

Goal: To Provide an environment free from distractions, by reducing noise levels and the understanding of overheard, nearby conversations.
Recommended Reverberation Time: 0.75 seconds

- Avoid direct sound pathways between cubicles, by proper placement of wall partitions.

- Without controlling reflections off the ceiling and perimeter walls, partitions can be ineffective.

- Absorptive partitions, ceiling tile, and wall treatments will be necessary to control the reverberation time. FABRITRAK® can be used on all three surfaces.

- Reasonable precautions should be taken to insulate against noise from adjacent rooms, machinery, ducts, and the outside.

RECORDING STUDIOS

Goal: To have complete control over all acoustical aspects of a given space.
Recommended Reverberation Time: 0.5 second

- Absorptive materials must be used to control the reverberation time over a wide range of frequencies. FABRITRAK®, with the appropriate core material, will solve this problem.

- Sound must be both absorbed and diffused.

- HVAC noise must be reduced to an absolute minimum.

- An unusually high degree of isolation from extraneous noise vibrations is need.

THEATERS

Goal: To properly balance absorption and reflection to provide a favorable acoustical environment, similar to worship centers. One must address both the need to hear and understand speech, and the desire to have a pleasant space for music.
Recommended Reverberation Time: 1.0 - 1.3 seconds

- Control the reverberation time in the room by adding absorptive material.

- Control the reverberation time on the stage.

- Splay or use irregular surfaces on the side walls to avoid flutter echoes.

- Don't forget the ceiling. It should be faceted for better dispersion of sound.

- Remember the space will be less absorptive when only half full, since the audience itself is absorptive. By using absorptive seating areas, the reverberation time will remain consistent regardless of the audience size.

- Be sure openings, such as doorways, are properly sealed.

- The balcony should be no deeper than twice its height.

WORSHIP CENTERS

Goal: Consider and address both the need to hear and understand speech and the desire to have a pleasant space for music.
Recommended Reverberation Time: 1.2 - 3.5 seconds

- Address lower frequency or bass sound.

- Background noise levels should be kept to a minimum. Two primary potential noise sources are mechanical equipment (HVAC) and outdoor noise.

- Reflections should be carefully monitored, especially if domes or concave surfaces are incorporated in the design.

- The seating area should be absorptive. This allows the acoustical environment to remain constant regardless of the number of attendees.

- A sound-amplification system may be necessary.

Ranges dramatically for different worship center projects. The recommendation will be influenced by the musical program type and if congregational singing is desired. Highly reverberant spaces are desirable for some type of musical programs. However, reverberation time of 1.2 - 1.5 seconds is appropriate for a number of worship centers. This allows an adequate enhancement of the music program, and can allow adequate understanding of speech with an appropriate sound system.

FABRITRAK® is an excellent solution, for all applications, when absorptive material is required.

ABOUT THE AUTHOR

Tony Sola instructs Acoustics for Interior Design in the College of Architecture and Environmental Design at Arizona State University. He is a full member of both the Acoustical Society of America and The Institute of Noise Control Engineering. He is a principal with Acoustical Consulting Services (ACS) in Arizona and can be reached for additional assistance at ACS, P.O Box 41182, Mesa, AZ 85274 or 480-827-1007.

 

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