Acoustic Myth Busting – Why Sound Redirection is not Sound Diffusion

A guest post by Acoustic Engineer Dennis Foley

Acoustic Myth Busting – Why Sound Redirection is not Sound Diffusion

Sound redirection is not sound diffusion and sound diffusion is not sound redirection. These terms are misused all the time in acoustics. They are mainly misused by acoustical product manufacturers who want to tell people that they do offer a sound diffuser, when in essence it is a sound redirection device. Lets examine both technologies more closely, so you can really understand the difference and avoid a serious acoustical mistake in your room.

Sound Diffusion Follows 5 Strict Criteria

A sound diffusion device must diffuse sound and create a true sound field within the room that is diffused. In order to qualify as a true sound diffusion device that does diffuse sound and create a diffuse sound field within small room acoustics, there are 5 criteria that the device being labeled a sound diffuser must achieve.

– First, there must be no frequency or spatial irregularities in the room frequency response.

– Secondly, beats in the decay characteristic at most frequencies of the room must be negligible.

– Thirdly, these decays must be exponentially perfect. They must be straight lines on a log scale.

– Fourthly, the decay characteristic must be the same in all room positions and finally,

– Fifth, the reverberation time in the room must be the same reverb time everywhere within the room.

These are the five minimum conditions that must be achieved, if you are to have a true diffuse sound field within your room.

Why Sound Redirection Devices Fall Short Of These Criteria

Well, you can see that meeting these 5 criteria is no easy task for a single technology but true sound diffusion technology can achieve this. Sound redirection devices fall short in many areas. You have seen a sound redirection device. They are mostly hemispherical in shape or half round in appearance. They take sound energy that strikes the device and then sends it in the opposite direction in which it came in at. If it struck the half round at 30 degrees west, it leaves 30 degrees east. Angle of strike equals angle of reflection. Redirecting sound energy in this manner will not create a diffused sound field because it will fall short in meeting the requirements of a diffused sound field.

Quadratic Diffusion Is A True Sound Diffusion Device

A quadratic diffuser is an example of a true sound diffusing device. A quadratic diffuser has a series of wells or troughs that correspond to quarter wavelength rule and diffuse frequencies that are quarter wavelength at each well depth. A quadratic diffuser can be designed to achieve sound diffusion across a specific frequency range. Sound energy that enters a quadratic diffuser is spread out in a fan like array after it leaves the diffuser wells. A vertically positioned quadratic diffuser diffuses sound in a horizontal fan like array. A horizontally positioned diffuser spreads sound energy out into a vertical array, so with different positioning of the same unit, two dimensions (horizontal and vertical) diffusion can be achieved. No sound redirection device can achieve two dimensions of truly diffused sound energy.

A quadratic diffuser can assist with smoothing out the room frequency response along with evening out the beats in a decay pattern within our room. A sound redirection device can not do either because most sound redirection devices are not effective below 300 Hz. Remember, we can not have any frequency or spatial irregularities. if it does not begin to work until 300 Hz. think of all the missing frequencies. A quadratic diffuser can assist with smoothing out decay rates and assisting with keeping the reverberation times consistent throughout the room. In fact, this is one of their stronger points. A sound redirection device operating above 300 Hz. can not do this nor can a sound redirection device assist with reverb or decay time balancing throughout the room.

In Summary

A true sound diffusion device must be able to create and contribute to 5 minimal acoustic criteria that we must have in small room acoustics in order to achieve a properly diffuse sound field. Only a quadratic diffuser or its cousin, the two dimensional diffuser, can help achieve these 5 criteria. A sound redirection device does what the name implies, it redirects sound energy. Redirection of sound energy will not meet most of the criteria for a true diffused sound field.

About the Author
I am an acoustic engineer with over 30 yearsí experience in the business and owner of www.acousticfields.com. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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