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Acoustics 101 - Chapter 1: Basics of Acoustics, Definitions, General Technical Info
Practical Guidelines for Constructing Accurate Acoustical Spaces

Edited by
Jeff D. Szymanski, PE
July 2004
Version 3.0

Copyright © 1993, 2004 by Auralex® Acoustics, Inc.

Basics of Acoustics

If you are reading this, you are very likely interested in improving your sound. The concepts put forth in these pages are not new. They are not revolutionary. You can find them in many other texts. Our hope is that our presentation and treatment of these topics will be "down to earth" and easier to understand, putting complex concepts into perspective.

Acoustics is not all common sense. Unfortunately, the subject can sometimes be quite confusing. However, we are confident that you can build a great room by following Acoustics 101. And there is nothing stopping you from taking these concepts and coming up with even better ideas than what we have presented herein. If you do, that's great! Fax or e-mail us your ideas so future Acoustics 101 readers can benefit from what you have developed. What you are reading right now is the newest incarnation of Acoustics 101. Many contributions from readers like you have been incorporated into this "new and improved" version. The only thing about making changes is to make sure you have really thought through the ramifications of what you are doing. Random substitutions could degrade everything you are trying to accomplish. If you are unsure, contact us.

Some of the basics of how sound behaves are implicit in Acoustics 101. Some examples of concepts we assume you have a basic understanding of include:

• When sound strikes a surface, some of it is absorbed, some of it is reflected and some of it is transmitted through the surface. Dense surfaces, for the most part, will isolate sound well, but reflect sound back into the room. Porous surfaces, for the most part, will absorb sound well, but will not isolate.
• The best way to stop sound transmission through a building structure is to isolate the sound source from the structure before the structure has a chance to vibrate.
• Walls need to be isolated from ceilings and floors, usually by means of dense, pliable rubber.
• The main ways to minimize sound transmission from one space to another are adding mass and decoupling.
• Limp mass is most often better than rigid mass (actually, a combination of the two is really what you are after).
• Every object, every construction material has a resonant frequency at which it is virtually an open window to sound - kind of like a tuning fork that "sings" at its particular resonant frequency.
• Different materials have different resonant frequencies.
• Trapped air (a.k.a., air spaces and air gaps) is a very good decoupler.
• Airtight construction is a key concept. Sound, like air and water, will get through any small gap. (Sound can leak through openings as small as 1/32" - in some cases even smaller.)
• Sound bounces back and forth between hard, parallel surfaces.

One of the single biggest concepts to understand and appreciate is that acoustic foam, one of our core products, is not going to "soundproof" your room. It is an extremely effective absorber of ambient, reflected sound and helps make rooms "sound better." Acoustic foam does contribute some sound isolating properties (mostly high frequencies), but is not sufficient by itself to keep sound in or out of a room. Thicker acoustic foam is better at absorbing low frequency sounds. Controlling reflected sound within a room is extremely important in producing good sounding recordings. When you hear Mike Wallace's voiceovers on 60 Minutes, you might be surprised to find out that they did not spend a million bucks on it. (It is amazing what some good 2" acoustic foam can do for a glorified, yet well-constructed closet!)

Isolation construction - the core concept in Acoustics 101 - is not inexpensive. Acoustics 101 carries with it an assumption that you have a few bucks to spend to make your studio the best it can be. For example, it is important to realize that empty egg cartons, cork squares and carpet scraps are not going to (a) keep sound from leaving or intruding upon your studio and (b) yield that pleasing, neutral, "Mike Wallace" sound within your studio.

If the guidelines, tips, techniques and advice in Acoustics 101 are improperly implemented, the desired results will not be achieved. Auralex cannot be held liable for the advice given because we are not going to be there watching you do the work or assisting with the construction. Please note that these tips are being provided on this website free of charge.

If you cannot handle a circular saw and other common power tools or you do not have the money to hire someone who does, then you should probably stop right here. It is going to be difficult to implement the advice given here if you or someone you hire cannot handle basic construction methods, such as applying drywall tape and mud, creating solid, airtight and level partitions and floors, "measuring twice; cutting once," etc.

There are myriad benefits to constructing your control room to be symmetrical geometrically and building using the best materials you can afford. Money well spent now will benefit you for a long time into the future.

One of the keys to getting good, clean sound on tape or hard disk is removing the sound of the room from the equation, to one degree or another. For a great example of this objective successfully implemented, listen to the Eagles' Hotel California or Pink Floyd's Dark Side Of The Moon.

Some of you will be able to grasp all this quicker than others. Please understand that any extra effort you expend implementing the tips contained in Acoustics 101 will pay you back sonically for a long time to come. Make no mistake: they are worth whatever work it takes to put them into practice.

Acoustical Definitions

For a complete treatment of acoustical terms defined, two additional sources are recommended (besides the overview of the most important terms discussed in Acoustics 101):

Rane's Pro Audio Reference (free web-based dictionary of audio and acoustical terms)

and

ANSI Standard S1.1-1994 ($150.00 - official, standardized acoustical definitions)

Acoustics 101 Definitions

Noise Reduction Coefficient (NRC)

NRC is a single-number rating representing and overview of how much sound is absorbed by a material. Example: ½" gypsum board ("drywall") on 2x4 studs has an NRC of 0.05.

Soft materials like acoustic foam, fiberglass, fabric, carpeting, etc. will have high NRCs; harder materials like brick, tile and drywall will have lower NRCs. A material's NRC is an average of its absorption coefficients at 250, 500, 1000 and 2000 Hz. In general, the higher the number, the better the absorption. NRC is useful for a general comparison of materials. However, for materials with very similar NRCs, it is more important to compare absorption coefficients.

Absorption Coefficient (?)

The actual absorption coefficients of a material are frequency dependent and represent how well sound is absorbed in a particular octave or one-third octave band. Example: ½" drywall on 2x4 studs has an absorption coefficient at 125 Hz of 0.29.

Comparing the absorption of materials should involve a comparison of their respective absorption coefficients in the different bands. Provided the materials are tested in a similar fashion, the material with a higher absorption coefficient in a particular band will absorb more sound in that band when you use it in your room. Be careful though: Materials are tested using different mounting methods. For example, if one material is tested by laying the materials out on a predetermined area of the floor - called A mounting - and another tests their materials by spacing them off the floor by several inches, then the comparisons are "apples and oranges." To truly compare, find numbers derived from tests that used the same layout of materials in the test chamber. Also, there are three main standard methods used to test materials for absorption. Two of them are reverberation chamber methods - ASTM C423 in the U.S.A. and ISO 354 in Europe. These two methods are quite similar, but the ISO method - in general - will produce slightly lower overall numbers than the ASTM method. The other method is the impedance tube method, or ASTM C384. This method places a small sample of the material under test at the end of a tube and measures the absorption. Again, the numbers from this test are usually lower since a different method of calculation is used. They are also not as representative of real-world applications of materials relative to the reverberation chamber methods.

Sound Transmission Class (STC)

STC is a single-number rating of how effective a material or partition is at isolating sound. Example: ½" drywall has an STC of 28.

Hard materials like rubberized sound barriers, concrete, brick and drywall will have high STCs. Softer materials like mineral fiber, acoustic foam and carpet will have much lower STCs. Virtually every material filters out some of the sound that travels through it, but dense materials are much better at this than are porous or fibrous materials. Like NRC, STC is useful to get an overview-type comparison of one material or partition to another. However, to truly compare performance, the transmission loss numbers should be reviewed.

Sound Transmission Loss (STL or TL)

STL represents the amount of sound, in decibels (dB), that is isolated by a material or partition in a particular octave or one-third octave frequency band. Example: ½" drywall has an STL at 125 Hz of 15 dB.

Comparing material or partition performances should involve comparing the STLs of each in the different bands. If both materials or partitions are measured in accordance with the STL/STC standard, ASTM E90, then the comparisons being made will be "apples to apples." It should be noted that real-world performance is not going to provide the same level of STL that is achievable in the laboratory. However, the relative performance of one material or partition versus another typically holds true in real-world construction. I.e., if the lab measures one partition better than another, it should hold true for a real partition built in your studio. Even though an actual field test of a concrete wall might reveal a field STC (FSTC) that is 5 points lower than the lab test, it is still better - relatively speaking - than a simple, single-leaf, uninsulated drywall partition in the same configuration.

Decoupling

This is the concept of detaching partitions from each other, or physically detaching layers in a partition in order to improve sound isolation.

The most common methods of decoupling are:

• Air gaps or air spaces between two partitions.
• Using resilient channels (RC8 from Auralex) between layers and
  structural framing members for walls and ceilings.
• "Floating" a floor using springs, rubber isolators (such as U-Boats from
  Auralex), or other decoupling layers.

Room Modes

A room mode is a low frequency standing wave in a room.

Normally, this is a small room phenomenon, though large rooms have (very, very low) modes as well. A mode is basically a "bump" or "dip" in a room's frequency response that is facilitated by the room's dimensions and the way those dimensions cause sound waves to interact with each other. There are three types of room modes

• Axial modes: Standing waves between two parallel surfaces.
• Tangential modes: Standing waves between four surfaces.
• Oblique modes: Standing waves between six surfaces. (Oblique modes are more complex, higher in frequency and decay faster. Therefore, they are not typically a big problem.)

For a complete treatment of modes, there are ample discussions in acoustic reference books. There are intricate formulas in these texts that can help you determine your room's modes. There is also software that can do the same. We have developed our own proprietary software and would be glad to work with you or your salesperson in figuring your room's modes to help steer you in the direction of the proper acoustical treatments. (Note that rectangular rooms are the easiest to predict. Our software is based on rectangular rooms. For non-rectangular spaces, we can assist to a degree, but the software required to actually predict the exact modes - which Auralex does not use - is much more complex.)

General Technical Information

STC

As mentioned before, mass and decoupling are the two components that are most effective at stopping the transmission of sound from one space to a neighboring space. This fact is plain to see when you examine the Sound Transmission Classes (STCs) of various types of walls. The following
illustrations of wall constructions represent a small sampling of the myriad possibilities:

Note: "Gypsum board" is a generic name. Brand names include "DrywallT" and "SheetRockT." Also, metal studs (instead of wood) will provide incrementally higher STC for each of the configurations above.

The following table gives a subjective equivalent for different STCs:

And finally, we would encourage the reader to review the STC FAQ for a more complete discussion.

Absorption Coefficients and NRC

The table below shows absorption coefficients and Noise Reduction Coefficients (NRCs) for some common building materials. They plainly illustrate the need for specialized acoustic treatments in studios that require well-controlled sound.

Some other useful links include:

• The Auralex Master Acoustical Data Table (PDF)
• The NRC FAQ for a more detailed discussion.

A point that is worth reiterating here is the fact that absorption coefficients and NRCs are not percentages. In other words, if a material has an NRC of 1.10, it simply means that more sound (on average) is absorbed than a material with, say, an NRC of 0.50. A few more facts about NRC that you may want to know when comparing acoustical materials:

NRCs can only be multiples of 0.05. For example, and a material that is reported to have an NRC of 0.72 was probably not tested in accordance with the standards.

Absorption coefficients and NRCs can only be reported for materials tested in accordance with very specific mounting methods. Beware of absorption coefficients and NRCs that were "calculated" using numbers that were only reported by the testing lab as "Sabins per unit." (One of our competitors is
notorious for this.) Since there was not standard area under test, converting to absorption coefficients and NRCs is forbidden per the ASTM standards.

A final thought

STCs and NRCs are both very useful numbers for comparisons. However, if two (or more) materials or constructions are being compared and their STCs or NRCs are very close, the octave band or 1/3- octave band data should be compared. This is discussed more thoroughly in the FAQs mentioned above. Should you be unsure of how to make certain comparisons, please contact us and we'll be happy to assist!