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Part Four - Acoustical Treatment -- by Doug (D.W.) Fearn

Part Four - Acoustical Treatment -- by Doug (D.W.) Fearn

Block diffusers

Written by Doug Fearn of D.W. Fearn.  Read more about Doug's Tube Products here!

http://dwfearn.com/wp/

 

After we have isolated our recording space from the outside world, the next step is to optimize the room for recording. We have three basic tools for doing that: room proportions, absorption, and diffusion.

 

Proportions

The proportions of the room may seem like it would only affect the visual aesthetics of the space, but the ratio between the height, width, and depth of the studio can have a profound impact on how it sounds. This applies to all rooms where acoustics are important, including the recording control room, a listening room, a classroom, a church, or a concert hall.

Maybe it is because I have designed quite a few recording studios that I find that a room that sounds good also has proportions that are visually pleasing.

All enclosed spaces, not matter how large or small, will have certain audio frequencies that are emphasized, and other frequencies diminished, depending on the spacing between any two boundaries of the room. That is physics and cannot be substantially changed. However, if any of the three dimensions is an integral multiple of another room dimension, you can have certain frequencies “build up” disproportionally, and that is very audible. The worst case is a cube, which will have a characteristic sound that is very uneven in frequency response (perhaps 10-30dB of variation). It will sound unnatural and unpleasant.

This is particularly a problem in small rooms because the frequencies that are a problem, 300 to 700Hz, will fall in the mid-range of the audio spectrum. This makes the sound “muddy” and indistinct.

Also, many homes built in the last 70 years or so utilize standardized building materials, usually in multiples of four or eight feet. A room might be 12 by 16 feet, with an 8-foot ceiling. That room is not going to sound good.

Acoustical designers mitigate this problem by using non-related dimensions as the fundamental overall dimensions of the room. You can find on-line references, formulas, and calculators for determining what should be “good” proportions for a room.

Even a room with perfect proportions for audio is still going to sound bad with just bare walls. The walls might be made of drywall in residential or commercial construction, but studio walls are usually made of concrete block or equivalent. This type of wall is much more sound-isolating (“soundproof”) than drywall. This room will be highly reverberant, which might be great if it is very large and designed for classical music recording. A smaller room will sound more like a garage – not appropriate for sound recording, or listening.

 

Absorption

To reduce the amount of reverberation, we can add sound-absorbing materials to the space. Most people think of things like thick carpeting, acoustic tiles on the ceiling, heavy drapery, or household insulation (fiberglass or mineral wool) to absorb sound. (Remember, these do little to “soundproof” a room.)

These materials absorb frequencies very unevenly. The examples above are good at absorbing high frequencies (above about 2kHz), but are almost totally ineffective at lower frequencies. The lower the frequency, the less the absorption. Those mid-range frequencies so often emphasized in smaller rooms with less-than-ideal proportions are hardly absorbed at all. The result is a sound that can still be “boomy” or “muddy,” and sound very dull and “dead” due to the high-frequency absorption.

Absorbing low- and mid-frequencies is challenging in small rooms. The absorptive material must be quite thick, feet instead of inches, to have any effect on those lower frequencies. The larger the room, the easier it is to make it sound good, and the space available makes it practical to use large structures to absorb low and mid frequencies.

In most well-designed recording studios, the ceiling is at least 15 feet high and absorptive, since the ceiling is usually the easiest place to add a lot of deep absorptive material. And this is more comfortable for the performers, because our ears do not like reflections from above (hundreds of thousands of years of humans living mostly with nothing above them but sky probably has an impact on what seems natural to us). Floors are usually wood or other hard surface, although rugs can be deployed to control the floor reflections somewhat.

 

Diffusion

The last tool we will discuss on this topic is diffusion. This is simply the scattering of the sound as it hits a surface. If a painted concrete wall is the sound equivalent of a mirror, a wall with diffusion could be thought of as one with a very uneven surface, perhaps a wall made of stones of different sizes and depths. And that’s how devices called diffusors are made: an uneven surface with random high and low spots.

The result is that the sound impinging on a diffusor is not reflected back as a distinct echo, but the reflected sound is spread out in time. This is much more pleasing to our ears.

But to be effective, the diffusion surface has to be large, and the depth of the “holes” and the height of the “peaks” has to be significant. Differences measured in feet is better than a difference measured in inches. This can take up a lot of space in a small room.

But diffusion does not have to be purpose-built. Abbey Road Studio 2 (Beatles and many others) has no dedicated diffusion at all. However, the room is big (roughly 60 by 27, with a 28 foot ceiling), and filled with a lot of stuff: several pianos, organs; racks of headphones; carts full of folding chairs; many microphone stands, etc. All these things in the room also provide a lot of diffusion.

When I started doing some informal recording in our manufacturing area, the rows of metal shelving holding hundreds of cardboard bins full of parts provided a very pleasant diffusion for the “studio.”

Studio designers often incorporate diffusion into the design, with uneven walls, often made of wood, and sometimes specifying a non-rectangular room. Large cylindrical diffusors can be designed into the structure, which scatter the sound in all directions. (Note that these wood walls are inside the primary heavy sound-isolating walls.)

Using the tools of absorption and diffusion, along with optimal proportions, results in a recording space that sounds good for the purpose.

 

Next time we will study how microphones are used in the recording process.
 
 

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