acoustics in design phase

7/27/2019 Acoustics in Design Phase

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Welcome to ReverberationTime.com, a collaborative arm of Acoustics.com. This site alignswith our goals of promoting the importance of acoustics and acoustic-related issues across avariety of related industries.

In an enclosed environment sound can continue to reflect for a period of time after a source hasstopped emitting sound. This prolongation of sound is called reverberation. Reverberation time(RT60) is defined as the time required, in seconds, for the average sound in a room to decreaseby 60 decibels after a source stops generating sound.

Reverberation time is the primary descriptor of an acoustic environment. A space with a longreverberation time is referred to as a "live" environment. When sound dies out quickly within aspace it is referred to as being an acoustically "dead" environment. An optimum reverberationtime depends highly on the use of the space. For example, speech is best understood within a"dead" environment. Music can be enhanced within a "live" environment as the notes blendtogether. Different styles of music will also require different reverberation times.

Reverberation Time

.8 - 1.3 1.4 - 2.0 2.1 - 3.0 Optimum**

Speech Good Fair - Poor Unacceptable* 0.8 - 1.1

Contemporary music Fair - Good Fair Poor 1.2 - 1.4

Choral music Poor - Fair Fair - Good Good - Fair 1.8 - 2.0+

* With an adequately designed and installed sound system, speech intelligibili ty concerns can be mitigated.** The optimium reverberation time can be somewhat subjective and can shift based on numerous variables.

Reverberation time is affected by the size of the space and the amount of reflective or absorptivesurfaces within the space. A space with highly absorptive surfaces will absorb the sound and stopit from reflecting back into the space. This would yield a space with a short reverberation time.Reflective surfaces will reflect sound and will increase the reverberation time within a space. Ingeneral, larger spaces have longer reverberation times than smaller spaces. Therefore, a largespace will require more absorption to achieve the same reverberation time as a smaller space.

Reverberation time can be calculated in the preliminary design stage. This is very beneficial indetermining how well a space will function for its intended use and if more or less absorption isneeded within the space. There are several formulas for calculating reverberation time, the mostcommon formula is the Sabine Formula, created by Wallace Clement Sabine. The formula isbased on the volume of the space and the total amount of absorption within a space. The totalamount of absorption within a space is referred to as sabins. It is important to note that theabsorption and surface area must be considered for every material within a space in order tocalculate sabins.

Reverberation time can also be adjusted within an existing space. Tests can be performed in aspace to determine the existing reverberation time. Absorptive materials can then be added to orremoved from a space to achieve the desired reverberation time. Whenever possible it is highlyadvisable to consider reverberation time and other aspects of acoustics in the design stage.

Making revisions to a space after the fact can be more costly and compromise aesthetics.

Reverberation time is not the only descriptor of an acoustic environment. There are several otherprinciples to consider. A few of the more important considerations include: reflections, loudness(strength), clarity, warmth and intimacy. Questions to consider in each of these areas:Reflections: Does the reflection of sound within the space cause negative results such as anecho or a megaphone effect? Or are reflective surfaces helping to benefit sound distribution?

Loudness (strength): Is the volume of the sound loud enough? Is it too loud? Or does it seemlouder than it would at the same distance outdoors?
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Clarity: Can I hear each of the various instruments clearly? Can I understand what is being sungby a solo vocalist, or what is being said by a speaker?

Warmth: Is there a balance of sound throughout the various frequencies? Or is the soundoverpowered with too much bass or too much treble?

Intimacy: Do you feel like you are a part of the performance? Or do you feel like the music orspeech is taking place in a separate environment?It is highly advisable to hire an acoustical consultant to assist with reverberation time issues.Determining an optimum reverberation time can be difficult, particularly in multi-use facilities. Inmany cases the reverberation time must be balanced in order to provide an environment suitablefor both speech and music. NRC ratings can be misleading, and can vary depending on thespecifics of a particular material being used and how that material is being installed. If properNRC ratingsare not used within the reverberation time formula, the calculation will not yieldaccurate results. Locations of materials within the space should also be considered. Qualifiedacoustical consultants have experience with these issues and can perform tests and calculationsin order to determine the best possible solutions for your specific environment.

Sabine Formula for Reverberation Time:

RT60 = .049 V/a

Where:RT60 = Reverberation TimeV = volume of the space (ft3)a = sabins (total room absorption at given frequency)

Formula for Sabins:

a = S

Where: = sabins (total room absorption at given frequency)S = surface area of material (ft2) = sound absorption coefficient at given frequency or the NRC

A direct sound from a source travels to the receiver without striking any surface.Within an enclosed environment, the direct sound also travels toward the surfaces

and boundaries of the room. When the sound hits these surfaces it is sometimesreflected back into the space. These reflections can travel back toward the source or

to the receiver as an indirect sound. If the direct sound is followed by a series ofindirect reflected sounds, the space can become noisy and speech can become hard

to understand. In such a case, reflections can have unwanted consequences.

Reflections can also be used to benefit sound distribution within a space. Directsound diminishes in intensity as it travels. In large spaces, the sound can diminish

before it reaches all of the receivers, particularly those furthest away from thesource. Reflections can be used to help distribute and amplify the presentation to all

of the listeners. Of course, this requires specific characteristics and location ofreflective surfaces.

The characteristics of a surface greatly influence the way it reflects sound. Materialsthat reflect sound are those that are rigid, smooth and non-porous. Rigid materials
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cannot bend with a sound wave in order to decrease the intensity of the reflectedsound. Smooth materials do not have surface undulations that could scatter the

sound wave and diffuse it. Non-porous materials have very little air space wheresound could be absorbed.

Sound is reflected in different ways depending on the shape of the reflecting surface:

Flat Surfaces

A flat surfaceis effective in distributing sound. If the surface is large enough and positionedcorrectly, a flat surface can project sound toward the listeners. Flat surfaces can also causeproblems if placed incorrectly. For example, a flat, reflective rear wall in an auditorium willreflect soundback toward the speaker, this is called "slap-back". Parallel reflective walls cancreate a reflection between the two surfaces, this is referred to as "flutter echo" or "standingwave". Two flat surfaces coming together to form a peak can act as a megaphone and

amplify the reflected sound.

Concave surfaces

Concave surfaces cause reflections to be concentrated rather than dispersed. This causes anabundance of reflection to be heard by the listeners in the focal point, or the point at which allof the reflections are focused. Reflections can also travel along a concave surface brin

ging delayed reflections around the room.

Convex surfaces are the best surfaces for distributing sound. They provide a wide

spread of reflected sound.


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Reflections can be controlled through room shaping as described above, irregularities withinthe room such as columns, trusses, surface undulations, etc., or through the use of absorptivematerials (those with a high NRC.)For more information on acoustic principles, visit Acoustics.com.

Noise Reduction Coefficient (NRC):

The NRC is a single-number index determined in a lab test and used for rating how absorptivea particular material is. This industry standard ranges from zero (perfectly reflective) to 1*(perfectly absorptive). It is simply the average of the mid-frequency sound absorptioncoefficients (250, 500, 1000 and 2000 Hertz) rounded to the nearest 5%.

*(Based on the testing methodology, and depending upon the material's shape or surfacearea, some products can test at an NRC above 1.)

To learn more...What you need to knowNRC ratings of common building materials

While NRC is widely used and accepted, it can also be abused or misunderstood. Be aware ofthe following items before specifying a particular material based on NRC:

The NRC rating is an average of how absorptive a material is at four frequencies (250,

500, 1000 and 2000 Hz). This rating is appropriate for assessing how well a material

absorbs sound within the speech frequencies, but can be inadequate for soundgenerated by music, mechanical equipment or other low-frequency sounds.

Because this rating is an average, two materials with the same rating might not

perform the same in identical applications.

The NRC is based on lab tests. Because the lab is a near perfect environment that is

rarely duplicated in everyday applications, some products will not test the same in thefield. Certain factors, such as installation variables, are not accounted for in the lab. Aproduct that receives high ratings in the lab may not perform as well in the field.

Make sure the mounting procedure used in the tests is consistent with your intended

installation if you expect the same results.

NRC does not have anything to do with the material's barrier effect (STC). Click here

for our tutorial on the differences between NRC and STC.

Communication of NRC ratings by manufacturers can be misleading and sometimes

deceitful for the following reasons:o Some manufacturers will quote absorption at the more-desirable higher

frequencies. NRC should be based only on the absorptive characteristics at250, 500, 1000, and 2000 Hz. Make sure the product data you're reviewing isat these frequencies.

o A manufacturer of a wall carpet product could provide an NRC rating of .80,

which is extremely good. But, if there is fine print be sure to read it, you maysee that this rating was achieved while the carpet was installed overfiberglass. In this installation configuration, the fiberglass, not the carpet, actsas the sound absorber. Without the acoustic material behind, the wall carpet

will probably only achieve an NRC of .20.
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Convex surfaces


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Project Remedies :: Controlling Noise Within a Space

When controlling noise within a space, there are usually two mainproblems to remedy: a noisy space due to reverberation or a noisy space

due to equipment noise.General rules of thumb for controlling noise within a space:

You have to at least double theabsorptionin a space before there is a noticeable

difference. Every time you double the absorption, the reverberant noise field is

reduced by 3 dB, which is classified as just perceptible.

Adding absorption to a space can provide a clearly noticeable improvement if the

space is fairly reverberant to begin with. The practical limit for noise reduction

from absorption is 10 dB, which sounds half as loud.

The improvement will not be as noticeable as you get closer to the noise source.

Carpet is not a cure-all. In fact, it is typically only 15-20% absorptive. It would

take four times as much carpet to have the same impact as a typical acoustic

material, which is about 80% absorptive.

Case Study 1

Location: Retirement Village

Area of concern: Multi-purpose clubhouse

Additional information: The original thought was that the sound system needed to be

upgraded or fixed because it wasnt working properly. Further review showed that it

was the lack of absorption in the room, not the sound system that was causing theproblems.

Questions asked of client:

Please describe the problem.

What are the dimensions of the space?

What activities take place in this room?

Is there a noise issue? A sound system issue? A reverberation ("echo") problem?

When is it the loudest?

Is it difficult to hear someone speaking when there is no loud noise?

Do presenters on stage complain about reflections?

Please describe the ceiling. Is it domed? Peaked? Flat?

What materials are used in this room? Drywall? Wood? Carpet? Tile?

Client feedback:

The room is too loud whenever there is a group in it, especially during dinners.
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Its difficult to hear presenters and understand announcements. Small group

conversations are hindered by excessive surrounding noise.

The space is 65'L x 54'W x 18'H.

The room is used for large dinners, performances, presentations, and other group

activities.

The original assumption was that the problem was the sound system, but we donthave problems hearing announcements when the room is quiet. It must be a noiseissue within the room itself.

Its the loudest during dinner when everyone is talking at once.

It is not difficult to hear a presenter when there is no other noise.

Presenters on stage do complain about reflections.

The ceiling is flat drywall.

Drywall and carpet are used throughout the room. Draperies and curtains are used

on the stage.

Evaluation: After speaking with the client and visiting the site, it was obvious that a lack

of absorption was causing the excessive noise in the room. Frequently, in a situation suchas this, a reflective ceiling, which is a large area that will project noise back down to the

floor, causes a majority of problems.

Addressing the ceiling alone would improve the noise level, but would not protect

performers from the problematic reflections called slap-back*. There are a variety of

products available for such applications. The products you choose are dependant upon thelook and feel of the room and your budget. In this case, acoustics improved as a result of

adding material to the ceiling (to control the overall noise) and acoustic wall paneling to

the back wall (to control slap-back and the overall reverberation time).

*Slap-back = A reflective back wall will reflect, or slap, the noise back to the sourcecausing a delay.

Case Study 2

Location: Headquarters for a large credit card company

Area of concern: Credit card processing center

Additional information: The first step in solving a problem related to equipment noise is

to call the manufacturer. Sometimes there is a problem in the installation or in the

equipment operation. Certain pieces of equipment have a retrofit noise reduction kit thatcan be purchased to reduce problems.

Questions to ask client:


What are the dimensions of the space?

What activities take place in this room?


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Is there a noise issue? A sound system issue? A reverberation ("echo") problem?

When is it the loudest?

Is it difficult to hear someone speaking when there is no loud noise?

Please describe the ceiling. Is it domed? Peaked? Flat?

What materials are used in this room? Drywall? Wood? Carpet? Tile?

Client feedback:

The processing center houses equipment that generates noise at 85-90 dB.

Workers are annoyed by this noise and the company is on the borderline of an

OSHA violation. The space in question is 260'L x 90'W x 20'H.

This room facilitates automated printing and folding of statements and stuffing

envelopes.

Equipment noise is the primary problem.

It is the loudest when all of the equipment is operating, which is during business

hours. There are no communication issues when the equipment is not running.

Evaluation: It is always best to control noise at the source, which, in this case, is theequipment itself. The level of improvement is related to the reverberance of the space. Themore reverberant a space is, the more dramatic the possible improvement. For this project,the space was not too reverberant, so the improvement would not be remarkable, but it wouldbe noticeable.

Hanging vinyl-covered acoustic baffles from the ceiling, particularly the areas directly abovethe equipment, controlled the noise from emanating within the space, but did not reduce thenoise level for the equipment operator (though it did help the other operators).

If adding absorption does not provide enough noise control, it might be necessary to isolatethe noisy areas from the quieter areas. Doing so would result in the implementation of ahearing protection program for those employees working in the unavoidably louder areas. Inthis case, enclosing the equipment with an acoustic shield (of plexi-glass) reduced the noiselevel for the operator by about 10 dB. The combination of the absorptive material and theacoustic shield reduced the overall noise by about 4 dB for all employees in the area, whichmet the clients needs and brought them into OSHA compliance.

Project Remedies :: Controlling Noise Between Spaces

Controlling noise between spaces is frequently an issue in residential projectsand office spaces. Noise will travel between spaces at the weakest points, suchas through a door or outlet. There is no reason to spend money or effort toimprove the walls until all the weak points are controlled.

General rules of thumb for controlling noise between spaces:

A wall must extend to the structural deck in order to achieve optimal isolation. Walls

extending only to a dropped ceiling will result in inadequate isolation.

Sound will travel through the weakest structural elements, which, many times, are the

doors or electrical outlets.

When the mass of a barrier is doubled, the isolation quality (or STC rating) increases

by five, which is clearly noticeable.
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Installing insulation within a wall or floor/ceiling cavity will improve the STC

rating by about 4-6 dB, which is clearly noticeable.

Often times, specialty insulations do not perform any better than standard batt

insulation.

Metal studs perform better than wood studs. Staggering the studs or using dual

studs can provide a substantial increase in isolation. Increasing air space in a wall or window assembly will improve isolation.

Case Study

Location: Newspaper office building

Area of concern: Space between CEO office and boardroom

Additional information: Noise usually travels through spaces at several different points.

Controlling only one point is like trying to save a sinking boat by patching only one hole

when 10 holes exist. You must be thorough to ensure effective results.

Questions to ask client:


Does the wall go all the way up to the deck and is it sealed airtight? Does it just

go up to the dropped ceiling? Are there any penetrations through the wall?

Are there any penetrations through the wall?

Could the noise be going around the wall? Are there any air gaps? Under the

door? At the perimeter of the wall? At the window mullion? Etc?

What materials are used in the space(s)?

What are your confidentiality needs?

Client feedback:

The CEO is distracted by noise from the boardroom when there are meetings in

progress. There are also confidentiality issues.

The wall does not go up to the deck, it ends at the dropped ceiling.

There are no penetrations other than the door.

The noise could be going around the wall by means of the door.

The materials used in this space are carpet, painted drywall and acoustic tile on

the ceiling. There are two return air ducts about two feet apart, separated only by

the wall. Confidentiality is an issue to some degree, but not a security problem.

Evaluation: In this particular project, there was a door and a window between the two

spaces and the ceiling did not go up to the deck. To improve the acoustics, an upgradedsealer was added to the doors and a flexible, vinyl barrier was placed on top of the ceiling

above the two spaces (since the wall could not be extended to the deck). Creating a

completely confidential space is very difficult and extremely expensive. Since


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confidentiality was an issue, but not a security matter, this improvement proved

successful.

If further improvements were needed, the next step would be to install a sound masking

system.

Further comments: In another office space, where complete confidentiality was essential,

a very expensive door was installed. This door had an STC rating of 65, but the

surrounding walls had an STC rating of 50. In this case, the walls served as the weakestpoint, rather than the door. Its important to note that the isolation quality of an assembly

is dictated by the weakest element of the assembly.

acoustics in design phase

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