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Sound Diffusion

Sound diffusion in small rooms can make your small room sound larger. It can also make your small room sound smaller. A diffuser is an acoustical tool that you must know its strengths and weaknesses before you consider its usage within your room. You must ask yourself many questions regarding sound diffusion within your small room. First, have you successfully managed all the low energy within your room? The low-frequency fundamental energy below 100 Hz. must be managed since these fundamentals account for the middle range harmonics that diffusion works within. Diffusion is not a technology for low-frequency energy. It is a technology for middle and high frequencies. Reducing the low-frequency pressure issues within your room will let diffusion separate and clarify the middle range frequencies so diffusion can do its job and provide the proper amount of sound diffusion in small rooms.

Reverberation Times (Rt-60)

Once we have the low-frequency issues managed through the proper type, amount, and positioning of low-frequency management technology, we can then focus on the reverberation times within our room. Reverberation times are the time code if you will give to all the reflections that occur off of our floor, ceiling, and walls. Reverberation times are the summation of all the reflections from these six surface areas. We call this Rt-60. It is the measurement of the decay rate of sound after it bounces off all of our room surfaces. It is the time it takes for energy to decay 60 dB from its original amplitude. We must manage the Rt-60 times within our rooms before we introduce sound diffusion in small rooms. Diffusion added to a small room with high reverberation times will make the room sound worse. It will increase the Rt-60 times when our goal is to lower them. Excess reverberation times are managed using the proper rates and levels of absorption

Small Room Diffusion 

Once we have the low-frequency energy managed correctly or as best we can with the available space, we have addressed the reverberation times and lowered them to match our usage, we can then begin to think about sound diffusion in small rooms. Now, we must decide on room usage. What will we be doing within our room? Will we be using it as a control room where we mix music and voice? Is it a “live’ room where we record instruments? Will it be just a vocal room? We must decide on usage so we know what surfaces to treat within our room. each room usage has different room surfaces that must be treated with either absorption or diffusion. In our control room, we are going to use diffusion on the rear wall. We may use it on the ceiling. That usage depends a lot on the engineer who will be using the room. Some engineers like diffusion, some like diffusion. If it’s a live room, we will look to diffusion for the ceiling and rear wall. Absorption will dominate the other surfaces within our live room to manage Rt-60 times. Vocal rooms can benefit from diffusion on the wall that the talent faces when recording. You must pay attention to the small distances found in small vocal rooms. Sound diffusion in small rooms is distance and frequency-dependent.
Quadratic Diffusion on ceiling for sound diffusion in small rooms

Diffuser Speakers

Diffusers operate a lot like speakers. They are not electrically powered diaphragms that produce energy for music and voice but they receive the energy given to it by the room and then redirect that energy back into the room with no reduction in its time signature or amplitude. We must use the same care and attention to the radiation of sound energy into our rooms as we do the powered drivers of speakers. Speakers have a frequency response. Diffusers have a frequency response. Speakers are usually full range. Diffusers are a more narrow band with frequencies starting around 200 Hz. and going up through 8,000 Hz. You must have a long enough distance between the diffuser and your ears for the lowest diffused octave to fully form correctly by the time it strikes your ears. If it does not fully form in the time domain you will get phase issues at the listening or monitoring position.

Quadratic Diffusion 

There is only one true diffusion technology. It is called quadratic diffusion. All other names and “technologies” are worthless. Those devices redirect sound energy. They do not diffuse anything. They simply send the sound energy that strikes them back in a 90 degree opposite direction. Quadratic diffusion is predictable and consistent in its performance parameters. The diffuser is divided into wells or troughs. The number of wells is determined by a prime number. The higher the prime number, the lower the frequency response range of the diffuser. The depth of each well or trough is calculated using quarter-wavelength rules and the trough (well) width is calculated using half-wavelength rules. One can increase the frequency response by making the wells smaller. One can lower the frequency response by choosing a prime lower number for the diffuser.

Two Types of Diffusion 

We have two main types of diffusion. We have one dimensional and two-dimensional diffusers. Diffusers are labeled by the radiation pattern of sound energy that they produce.  A one-dimensional diffuser is usually positioned vertically then we have a one-dimensional diffuser. A vertically positioned diffuser will spread sound energy out in a horizontal dispersion array. It is a 180-degree array that spreads the energy out within a horizontal domain. A horizontally positioned diffuser will spread sound energy out in a vertical array of 180 degrees in the vertical sound field. We have three sound fields within our rooms. We have the floor to ceiling sound field, the sidewall to sidewall sound field and finally the third sound field which is front wall to rear wall. All have certain dimensions and those dimensions must be considered when choosing sound diffusion in small rooms. If we combine both horizontal and vertical diffusion we can create both a vertical dispersion array and a horizontal diffusion array. We then have two dimensions of diffusion on the same room boundary surface.
Quadratic Diffusers as an example of sound diffusion in small rooms

One or Two Dimensions? 

Whether to use one dimensional or two-dimensional diffusion depends on two major variables. The choice depends on the usage of the room and what distance from the walls to our listening position. For home theater applications, it is good to place two-dimensional diffusion on the rear wall and ceiling. One dimensional diffusion will add “air” to your presentation. It will increase your sound stage width and height. A two-dimensional diffuser will minimize the impact of the reflection. It will not add any dimensional differences to your sonic presentation but it will make your ears think the wall surface is much farther away than it actually is. Our studio has two-dimensional diffusion on our 9′ ceiling height. Blindfolded clients consistently guess the ceiling height to be 12 – 13′. Two-dimensional diffusion makes you think the walls are farther away. One dimensional diffusion spreads the sound out in a 180-degree array that will add definition and clarity. Distance and frequency response must be considered with all diffusion choices.

Powerful Technology 

Diffusion is a powerful technology that few have used and less understand. Once you have used diffusion technology properly utilizing the proper type whether one dimensions or two, positioned it on the correct surface area to match usage, and listened to it over a period of time, two things will occur. You will have wondered why you haven’t had this wonderful technology before and where can I go to obtain quadratic diffusion. We have two options for you. You can buy it from us or build it yourself. Here is a link to our diffuser DIY product line: You can also purchase from us already built choosing your own wood type and finish. Here is link: Whatever your choice, we will assist you with the proper diffusion type, amount, and positioning with your room usage.
Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. 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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