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Small Room Acoustic Size And Shape

By July 1, 2012No Comments

All Shapes And Sizes

Our small room acoustical environments seem to come in all sizes and shapes. Most of the rooms I see are small. I see a lot of rooms that were originally designed by the builder to be used for another purpose. Closets become vocal booths and a garage becomes a drum room. Most of the rooms also have unique shapes and layouts. Uneven or unbalanced room shapes cause many unwanted “noises”. Taming room acoustical issues is difficult but a rectangular room gives us a fighting chance.

Rectangular Room Shape

In our small rooms, it is imperative that we have a room that is rectangular shaped. This shape allows us to have a consistency and predictability to the sound energy’s behavior within the room. With a room that has a length and width, with the width always smaller in distance than the length, we can have a high degree of confidence in the way the room acoustics will behave. Both walls that make up the width and both walls that make up the depth or length face each other at a given distance. If we have rooms that have alcoves, closets, or other “holes” for sound energy to enter,we create more acoustical problems for ourselves.

No Closets!

A closet or a hallway can be turned into a “speaker” with a small amount of sound energy. Energy into a closet can cause the closet to resonate at the frequency its dimensions tell it to. Now, not only do we have the room to deal with, we have another room attached to our room that wants to speak for itself. Acoustically, we must now treat this resonance and make sure it doesn’t interfere with the rest of the room sound.

Sound Stage

Side wall reflections and the correct acoustic management of these reflections in a music play back environment are critical in developing a sound stage. Most of our time is spent minimizing the time signature of the side wall reflection, so that it is slower than the direct sound. With a parallel side wall surface and structure with given distances, it is easier to manage both left and right channel side wall reflections with predictability and consistency. If you do one thing to one side wall, you must do it to the other.

Reflection Treatment

Side wall reflections are usually treated with some type of sound absorption technology. To slow down the primary side wall reflection, individuals use some type of absorption material which is usually acoustic foam. With a parallel side wall surface is is easier to hear subtle differences in room treatment techniques because one has to treat both sides the same. Parallel side wall surfaces give us signal predictability.

Rear Wall Distances

We must be careful and pay attention to the distance from the listening or monitoring position and the rear wall of our rectangular room. Sound energy reflected off of the rear wall at the listening position causes, in essence, a comb filter effect if this distance is not far enough. Now, one is sitting in this zone of reflections off of the rear wall. This reflection not free zone interferes with the sound from our speakers. We will lose definition and separation at our sound stage.

Rear Wall Treatment

Using sound diffusion technology to treat the “rear wall” is popular. With true sound diffusion and not just sound redirection, we need distance. Distance is required because of the way a quadratic diffusor works. A diffused waveform needs room to grow to its mature length in order to sound good. It sounds good by not being heard. Once it achieves its full length, then it can arrive at the listening position hopefully, with out being noticed. A properly diffused rear wall surface makes the whole room sound larger than its physical size.

No Triangles

Unparalleled wall surfaces such as half rounds, triangles or any other non parallel shape in our rooms will only compound our acoustical issues when we are dealing with the time signature of a reflection in order to keep it in proper line behind the direct sound from our loudspeakers. These shapes in the room can also redistribute middle and high frequency energy back into our rooms in uneven and unpredictable ways at our listening or monitoring position. Irregular shapes of wall structures can cause sound pressure energy to move to areas that it would not go to in a rectangular room, maybe even back to the listening or monitoring position.

Room Size And Volume

Room size and volume are very important. Their importance can not be stressed enough. There are optimum sizes and minimum sizes that one must choose. Choosing the proper length, width, and height of the room will assist us in dealing with resonances by “structurally” minimizing them. If one is considering using a given room within their structure for sound use, measure the physical dimensions of the room and then research sizes that may be acoustically better ratios. Obviously, we can not make our room larger, but we can make our room smaller, if it gets us a better frequency response curve, especially with low frequencies.

The shape of our rooms is critical if we are going to put together a room that will sound good for its intended purpose. We have to deal with low frequency sound energy pressure causing resonances and middle and high frequency reflections interfering with wanted sound at the listening position. Closets and hallways can become their own speakers, with the closet and hallways sides wall and ceilings become the surfaces of the “speaker” cabinet. They will resonate or talk at whatever frequency their size will allow for. Rear wall distances from the listening position to the rear wall must be at minimum distances to allow for the time delayed reflection off of that rear wall to be managed acoustically. A rectangular shaped room gives us predictability in treatment options and gives us a fighting acoustical chance at good sound.


I am a structural engineer as well as a master furniture maker. I design cabinets for low frequency, activated carbon absorbers. Connect with me on Google+

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