Los Senderos Studio
Acoustic Treatment
 Issue 68 May 2015 
Acoustic Treatment

Words from the Glossary

Over the past few months, we have discussed standing waves, reverberation, and echo, and how they can be detrimental to recording in a studio. This month we look at how to control these problems.

This month's terms: acoustics, room acoustics, architectural acoustics, room ratios, room sound, room tone, acoustic treatment, reflections, absorber, porous absorber, resonant absorber, Helmholtz resonator, panel absorber, diffuser, bass trap, reflection free zone, room equalization.
(Note: Click on the term to view its definition in the glossary.)

Acoustic Treatment

Acoustics is the science or study of the properties of sound and sound propagation. It is also the way in which sound behaves in an enclosed space, which is more precisely called room acoustics. Recording studios frequently have to deal with room acoustics, because they have to make an existing space fit their needs. However, if you are building a new studio from scratch, you have to deal with architectural acoustics, in which case sound is considered during the design phase.

As we touched on in the past few issues, the first thing to consider to correct sound problems is the shape and size of a room or to choose the best room ratios (RR). This is the room sound, the characteristic ambient sound of a space including reverberation and background noise. This should not be confused with room tone, which is the sound of the room with no sound present.

The worst possible shape for a studio room is a cube, because with three sets of equal-sized parallel walls, a standing wave can be established in three different directions. A rectangular room is much better, but the dimensions should not be even multiples of one another. For example, you would avoid a 12-foot by 16-foot room with an 8-foot ceiling. The reason is that a tone with a 4-foot wavelength would set up standing waves and harmonics of standing waves in each direction.

Absorption Panels
Absorption Panels

In Issue 65, we discussed the golden ratio, which is approximately equal to 1.618... A room with a ratio of height to width to length of 1:1.6:2.6 (or the ratio of 0.62:1:1.62 as used by sound engineers) is considered to have "perfect" acoustics. However, even such a room would need some treatment.

An even better solution is to use non-parallel walls. Many newer studios are built like that. In my studio, I have a sloping ceiling which rises from 12 feet to 16 feet. I would have angled the walls as well, except this would have added additional cost and reduced the working area of studio. One question frequently asked is why not use rounded walls or a rounded ceiling? While this may seem like a good solution, it is not, because the sound would be intensified like using a parabolic microphone.

Bass Trap
Bass Trap

When a sound that occurs in a room meets the wall, it does one of three things. It is reflected, absorbed, or diffused. One of these is a bad thing. The other two can be used to eliminate or control the other one, reflections.

Absorption is the process of a sound wave being taken in and reduced, thereby minimizing reflections. A material that does this is called an absorber. A porous absorber is a material with an open structure that allows sound to enter and be converted into thermal energy. Examples include foams, mineral fibers, and fabrics. Porous absorbers are effective on medium and high frequencies. A resonant absorber is an absorber that reduces the level of sound in a certain range of frequencies. It is basically a big box designed to trap sound waves. They are useful for absorbing sounds in the medium to low frequencies.

Based on this, you might think the best thing to do is cover all your walls with foam or carpet. The problem with that is that you wind up with a dead acoustic space, which would not sound good for a recording studio. You need to have some degree of liveliness. Walls are not carpeted because carpet is too thin. The thicker the absorber, the better it is at lower frequencies. Carpet would only be effective for very high frequencies.

Helmholtz Resonator
Helmholtz Resonator
For a Corner

A better solution is to place sound-absorbing panels on the walls at regular intervals. This procedure creates a space with controlled sound, but not a dead sound. You may have notices that the absorbers mentioned above were effective for certain wavelengths of sound. That is one of the disadvantages of sound treatment. The effect is only applicable to a range of frequencies. That is why you frequently see bass traps in the corners of studios. These are large chunks of absorbing foam (and not all foam is absorbing) that is more effective on bass frequencies.

Panel Absorber
Panel Absorber

While panels of porous absorbers are good for mid to upper frequencies, resonant absorbers are better for the lower frequencies. An example of a resonant absorber is the Helmholtz resonator. It is an enclosure in which the size of the opening and inside volume determine the frequency that is absorbed. The main advantage of the Helmholtz resonator is its simplicity, but it is effective over a relatively narrow frequency range. Therefore, in order to achieve significant attenuation it must be precisely tuned.

Another resonant absorber is the panel absorber. It is basically a frame to which a material such as plywood is attached. Sound waves hitting the surface are absorbed by the air inside the cavity. The size and material determine what frequency is absorbed. However, if some of the air space within is filled with absorption material, then the frequency is reduced, but a wider range of frequencies is absorbed.

Unfortunately, the large surface area on the front of the panel tends to reflect some sound waves. To counteract that, the front is sometimes rounded or angled. Another solution would be to place absorptive foam on the front. In that case, you have the best of both worlds—resonant absorption and porous absorption.

Quadratic Diffuser
Quadratic Diffuser

The other method of attenuating unwanted sounds is to use diffusion. This can be accomplished by using a surface of irregular shapes. The quadratic diffuser is one such device. When sound waves hit the diffuser, they are bounced in a variety of directions, which tends to dissipate the sound. You can also find foam absorbers with very jagged shapes. The idea is to use both absorption and diffusion. How effective this can be is debatable, as diffusion needs an irregular hard surface, and, of course, the foam is not.

So far we have been talking about taming acoustic deficiencies in the recording space. However, proper acoustics are probably more important in the control room where you listen, mix, and master the sound. While this may not seem obvious, think of it like this. If you have a big standing wave in the control room, the bass will sound louder than what it really is. So you turn the bass down. When you listen to the mix on your car or home stereo, the bass will be lacking. The same is true if there are echoes or reverberation in the listening space. You will have a tendency to under represent them if reverb is already present. For these reasons, a properly tuned control room is a must.

There are complete books written on the topic of acoustic treatment. So we have only scratched the surface on this subject here. So we will see you again next month.

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Music Trivia
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Music Trivia

"Stranger On The Shore" by Mr. Aker Bilk reached No. 1 in the US in May 1962. Bilk learned to play the clarinet while in prison, having been sentenced to three months for falling asleep while on guard duty in the British Army when he was stationed in Egypt.

In 1953, after winning a local singing contest, Pat Boone competed on the NBC television show Ted Mack and the Original Amateur Hour, where he became a semi-finalist. Before the finals, he appeared on a competing CBS talent show, Arthur Godfrey's Talent Scouts, for which he received a fee of $600. He was disqualified by Ted Mack because he was no longer an amateur, missing out on on a shot at a $6000 scholarship, which by all indications he would have won.

Bob Dylan's backup band had played with several artists before Dylan, including Ronnie Hawkins. Leon Helms said they were "the band" for several frontmen. Even after landing their own recording contract, record company executives pressed them for a name, but they insisted on being called The Band.

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The cool thing is I get paid for any sale they subsequently make, even if they don't buy the original item they clicked on, as long as it is in the same session. So if you need to order something from Amazon, next time consider clicking on one of these ads, even if you have no intention of buying the item listed. It will help with the cost of my website, and I will greatly appreciate it. Here's thanking you in advance.

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