Acoustics: Basic Concepts, Guidelines and Materials

Acoustics Defined

Every room has a unique sound. This quality (acoustic) comes from reflections and resonance inherent to room dimensions and construction materials. Contents of the room also affect its characteristics.

Perceived acoustics are a result of the decay of frequencies over time or reverb. It is coloration of uneven decay times that must be addressed in order to improve accuracy. Time is the challenge. Too often acoustics are approached solely from a frequency perspective. It is important to understand that acoustics are a combination of time and frequency.

All critical listening rooms require some reverb to keep from sounding ‘dead’. Acoustic design must address size and energy of wavelengths in order to reduce this energy smoothly over time.

Higher frequencies have shorter wavelength. Shorter wavelengths move to positive and negative poles faster. More cycles use energy faster, so high frequencies decay faster than low frequencies.

Longer (bass) waves contain more energy, so they are harder to absorb evenly. Bass waves bounce from boundary to boundary, seeking the path of least resistance, a full cycle or half cycle. Consequently, the length of travel determines frequency. Waves traveling in one direction interact with those traveling in the opposite direction. This phenomenon is referred to as a standing wave. When mid-range frequencies interact with those traveling in the opposite direction, the effect is known as flutter.

How to know it when you hear it

A simple way to measure control room quality is an easy test.

• Monitor in mono, the same signal at the same volume from both speakers.
• Set volume at a comfortable level, not too loud, not too soft.
• Play a familiar piece of music.
• At the sweet spot, close your eyes and listen for more than a minute.
• Focus on how wide and how far away the music sounds.

In an accurate environment, the music will be a small dot at the end of your nose. The further away it sounds the more reflection problems. The wider it sounds the more frequency problems.

The most common test for flutter is to clap your hands. Flutter will appear as a ringing sound caused by parallel reflections.

Low frequency standing waves are easy to hear between 150 – 300 Hz. For example, your voice sounds bassy or boomy. More difficult is lower than 150 hz appearing as a false sense of extra bass; or a lack of bass and definition. Playing a known, familiar source best discovers this. If the bottom is out of balance, you are experiencing standing waves or boundary phenomena.

Speaking in a room is a terrific indicator as to the mid-range characteristic. Your ears are most sensitive to the frequency content of speech.

 Acoustics Tools

There are many tools available for acoustic design. Some are ergonomic in approach and encompass items as speaker placement, orientation, symmetry, use of furniture, position in the room, etc. As this is about acoustic materials I will not address these areas. I make mention so these tools won’t go unnoticed and their importance is duly noted. No amount of treatment will make up for neglecting these issues.

Acoustic treatment materials fit into four basic categories: Absorbers, Diffusors, Resonators, and Free Standing Devices.

Absorbers: Any material that allows air to enter or pass through will act as an absorber. Most common absorbers include open cell foam, fiberglass, mineral fiberwool, acoustic ceiling tiles, Tectum, just to name a few. The friction of sound energy moving through an absorber reduces sound by transfer to heat energy. Thicker materials offer greater resistance therefore are more efficient in reducing sound and will have better low-end characteristics.

Beware of thin absorbers: Carpet on walls sounds terrible and cannot be an effective acoustic treatment. I almost never use 1” foam. I would rather use half the amount of 2” foam than large areas of 1”. Soundboard is another mistaken material. It is intended for use inside of wall systems and can sound awful when used on the surface of critical listening areas. Applied acoustic wall coverings, like those used in hallways of movie theaters, are designed for just that and should not be used in critical music areas.

Diffusors: I am going to group a large class of acoustic devices as diffusors. Simply stated, a diffusor is a device that reflects sounds more than it absorbs and can change the direction and quality of the reflected sound. For more serious study, I recommend learning the differences between diffusion, diffraction, refraction, and reflection.

Resonators are most effective at low frequencies. They are primarily rigid or semi-porous panels (such as pegboard) with an enclosed air space, behind which may contain absorptive materials. Additionally diaphragmatic panels, such as 1/8” to 1/4” plywood, are often added to resonator designs to increase low-end absorption. There are few commercially available resonators as applied treatments. Most are manufactured on the job. There are plans and formulas for resonator construction in many acoustic manuals including ‘The Master Handbook of Acoustics’ by F. Alton Everest.

Free Standing Devices are growing in choice and popularity. I often recommend these devices, which offer ability to easily alter acoustics. They can be moved, do not require installation; are available as absorbers, isolators, diffusors, and can provide great low frequency effects. As absorbers, it is important to understand that air around and behind an absorber reacts, increasing its usefulness and frequency response. This is an inexpensive way to improve performance.

Thick curtains are often an asset. Although not free standing, they are moveable, which create a variety of acoustics and fit well into a home environment.

General Guidelines

Proper acoustic treatment is both art and science. All rooms present unique challenges. Placing too much or the wrong type of material can have disastrous effects. With acoustic design, less can be more. There is a point where unevenly or overly treated rooms expose greater acoustic anomalies. This can be worse than no treatment at all. In some circumstances rooms with carpet and a few pieces of furniture already are close to sounding good.

It is important to vary materials, both in thickness and performance. Applying too much of one material will slant response to create a colored and artificial quality. Opposing materials of different types are more efficient as the impedance of materials differs, creating greater resistance.

Often we place materials with small spaces between. There is no loss of absorption as long as the space does not exceed the combined thickness of the absorbers, 2” thickness plus 2” thickness = 4” space. Be careful not to exceed four inches if parallel flutter problems are a concern.

Read through all the areas even though it may not be applicable to your situation.

Performance Areas : A performance area is the easiest space to treat. Accuracy is not as important here as in control rooms. A studio just needs to sound good. Often we try to have one area drier than another. Vocals require very little ambience, yet acoustic instruments don’t behave well in overly quiet areas. Gobos and freestanding devices are great tools here.

Control Rooms: Symmetry is critical. Do everything possible to make left and right side walls equal in space and quality.

Avoid placing your speakers in corners or too close to walls.

Listening close to walls or exact center of the room will have more bass errors than anywhere else.

The wall directly behind the speakers needs to be an area of absorption. Generally, I want a minimum 60% covered, starting at the ceiling with the most efficient absorber you can afford. Do this first, as this will give great benefits.

Bass energy collects in corners. Corner treatments allow for thicker materials without giving up valuable space.

Sidewalls in front and adjacent to the mix position require 50% absorption. Place 4’ of absorber one foot below the ceiling on an 8’ wall. If the room has splayed walls, added shape, or generally needs to be quieter, use 5’ of treatment starting at the ceiling. Continue this treatment at least 6’ behind the mix position. At this point, you can begin to stagger materials. Absorbers should be opposite flat walls or diffusors. (DVD audio, 5.1 rooms need to be symmetrical in all dimensions.) The sidewalls can have less absorption value than front walls to vary the materials. For example, if the front wall is 4” foam the sides may be 2” foam.

On the back wall do not use diffusion unless it is a minimum of 8’ away from the mix position. The type and amount of diffusion will vary greatly due to the following factors: the size of speakers, dimensions, distance, aesthetics, and budget. The back wall is an ideal place for resonators. Again, it is important to have a different type of absorber than the one used on front or sidewalls. In homes, the back wall is a good place for shelves and storage. Keep space between items on the shelves and vary their depth.

As a concept, I prefer hard floors and soft ceilings.

If there is carpet, a sparse array of absorption on the ceiling will help.

If the floor is hard, treat 80% of the ceiling with absorbers from the front wall to 4’ behind the listener, then begin to stagger the treatment.

Diffusion can be applied to the ceiling, no less than 5’ behind the mix position.

I am a great believer in acoustic drop ceilings. Given sufficient space, this is a most effective, practical design. The variety of tiles available today, both absorptive and diffusive, affords opportunity to tune easily and create dramatic visual effects. Space behind the tiles adds bass absorption, and the mass effect of a membrane in air, is another great benefit.

Vocal Booths: Whether it is commercial voice-over or the most important element of a song, vocal recording requires proper acoustics. The environment around the microphone is part of the sound. Listeners are most sensitive to voice quality. Further demands are made on acoustics as compressors commonly used in voice recording increase ambience.

A much drier and even type of absorption is required in vocal booths. Small or large rooms often make this difficult. Let’s address small rooms first.

I have many famous V.O. clients literally working in small closets. Everybody knows the sound of his or her shower will not work. As absorption is placed, top end quickly disappears. This results in standing waves that create a false bass and/or boomy quality, occurring in the lower spectrum of vocal range. It is very important to put broadband absorbers in the corners to help this low-end reverberation. Often it is not possible at the walls because of space consideration. If this is the case, place treatments at the corner of the wall and ceiling. On the remainder of the ceiling, place as efficient an absorber as possible. 4” foam is good here. At least one wall needs to be treated from floor to ceiling, with the most efficient material. Treat remaining walls 5’ down from the ceiling with absorbers of different thickness and density. Carpet on the floor is preferred.

In large rooms, it is necessary to create an absorptive surround for the artist and microphone. Fortunately, there are many different products that can do this.

I hope that this information is useful to you. If you are considering acoustic treatment, you may consider utilizing our Consulting Services. Please browse through our Online Store for different product ideas. There is additional information, as well as recommended reading and links to other interesting articles, on the Plans & Info and Photo Gallery tabs. Please Contact Us – We are glad to help.

Thank you for your interest,
Steven Klein