Acoustic Reflections

Understanding acoustic sound reflections is important in the process of building a recording studio. How the engineer hears sound reflections influences the decisions during the mix of the recording. When an audio wave bounces around the surfaces in a room causing sound reflections, some will be partially absorbed, some will bounce a bit less, some may pass through or around barriers, and others may bump into other audio waves.

How sound reflections behave in a room depends on a number of factors – audio wave frequency; the shape and dimensions of the room; the number of doors and windows, the construction of the walls, floors and ceiling, and the contents of the room (furniture, equipment, humans, and accessories). Think in terms of a rubber ball being bounced. Throw the ball against a hard surface such as a concrete floor or a smooth wall and it will bounce off at an angle that corresponds to the angle at which it was thrown. If you throw the ball at an angle near a second surface, the ball will bounce off of both surfaces. However, if you throw the same rubber ball at a soft pillow, it will either stop dead or fall to the ground.

That is the basic idea of sound reflections, but with real audio waves in a studio, things get even more complex. Audio waves generated by an instrument, vocalist or studio monitor are dispersed in multiple directions, all at the same time. And, if you are sitting in front of a speaker, you will hear the audio waves coming directly at you from the speaker, but you will also hear other sound reflections off nearby surfaces. To further complicate things, you will hear the sound coming from the speaker before you hear the reflections. This time delay between the audio waves can result in phase differences in the waves, changing the tonality of what you are hearing (aka comb filtering).

In a sound room, First Reflections are the audio waves that come to your ear after one bounce, from any reflective surface within about ten feet of your listening position. However, the materials that control sound waves make it possible to reduce or eliminate first reflections, so that even smaller rooms can be designed to work just fine.

Flutter Echo can be a problem with upper-mid to high-frequency sound reflections bouncing around the room when the waves are reflected directly between two parallel surfaces such as: opposite walls in a room. You often hear a rattling noise which is the fast echo effect known as flutter echo. This effect can be a problem when you are sitting in front of your studio monitor, and, it can also be a major problem when you are using a microphone to record an instrument. Fortunately, the materials that control sound reflections will also deal with flutter echo.

Absorption of Sound Reflections at Various Frequencies
125 Hz 250 Hz
500 Hz 1 kHz 2 kHz 4 kHz
Brick 3% 3% 3% 4% 5% 7%
Carpet 8% 24% 57% 69% 71% 73%
Drapes 14% 35% 55% 72% 70% 65%
29% 10% 5% 4% 7% 9%
Linoleum 2% 3% 3% 3% 3% 2%
Paneling 28% 22% 17% 9% 10% 11%
Plaster 14% 10% 6% 5% 4% 9%
Windows 35% 25% 18% 12% 7% 4%
Wood 15% 11% 10% 7% 7% 4%

Reverberation is sound still reflecting in your room after the first reflections. These remaining reflected audio waves tend to wash together. That ringing in your ears left after the audio source stops is called reverberant decay. It is important to control the reverberant decay in a room, and it is also important to make sure that the reverberant decay across the frequency range is even. If the reverberant decay for high frequencies is different than for low frequencies, the characteristic sound reflections of the room probably won’t be desirable; possibly a bright ring, or an uneven mid-range or a boomy low end. Conversely, if the reverberant decay is too short, the room will have a dry, dead feeling.

Standing Waves, also known as room modes or simply modes, are the result of low-frequency audio waves reflecting between walls inside an enclosed space. A room’s mode is determined by its three dimensions; length, width, and height. No matter how big or small your room is, it will have modes, there is no way to eliminate them. Typically a small room has bigger problems than a larger room. However, changing a room’s dimensions in order to tame the sound reflections is not easy if you are working with an existing room.

Other than the dimensions of a space, there are two methods of controlling sound and reverberation; absorption and diffusion. Both methods are effective for solving certain problems related to reflections, and both methods overlap in their results.


About Albro

With 25 years in sales and management, Lynn Albro is focused on internet marketing for Realtors and small business owners. Specializing in SEO and Social Networking, she is a creative problem solver, and loves to help!
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