Using Diffusion To Create Space In Your Listening Room

The size of your room affects how music or movies sound in ways most of us don’t often consider. With a basic understanding of the science and physics of sound we can make smart decisions on how best to set up our listening space.

First, Some Science

In the 18th Century, Italian scientist Giovanni Venturi began experimenting with sound localization, and a century later without being aware of Venturi’s work, John William Strutt performed almost the same experiments. He found that because the head casts a “sound shadow” a sound coming toward one side of the head produces a more intense (louder) sound in one ear than in the other ear. At lower frequencies this “shadow” effect is small because of the wavelength of the signal, but at higher frequencies the effect is more pronounced. At 1000Hz the difference could be as great as 3dB while at 10kHz the difference could be as great as 30dB. Our brains have developed the ability to process these differences in amplitude and translate them into differences in location.

That’s how you know the mountain lion is at the left front of your cave, saving you from wasting the one good club you have throwing it at your cave-teenager who forget to put the entrance boulder back in place. Luckily, all that time we spent in caves helped develop our hearing into a finely-tuned movie and music listening processor.

Putting This Science In Your Room

There are three separate and distinct sound fields in any listening environment:

Direct Sound (sound that moves directly from the source to your ears)

Early Reflections (sound that reaches your ears after bouncing off one surface (typically between 10-30ms))

Reverberant Sound (sound that has been reflected one or more times and arrives at your ears after the first (early) reflection

In a concert hall reverb times may be as long as three seconds, but in a home those time relationships pretty much disappear. The walls and ceilings are so close that first reflections may arrive within a few milliseconds of the direct sound and reverberant sound (if it exists at all) may arrive shortly thereafter. This is why achieving an intimate feeling in a home listening environment is easy but achieving a sense of space is quite difficult.

At home, unless you are building a dedicated home theater, you will most likely be between 2 and 3 meters (6 to 9 feet) from your source speakers. That close to the source, build-up and decay times are short because the sound gets trapped in the smaller space, whereas in a concert hall or auditorium those build-up and decay times can be quite large. Your ears and brain process those build-up and decay times as space, which gives you the sense of the size of the room. In modern recordings and soundtracks, reverberant information is included in the source program and assisted by the sound processor in your A/V receiver in the form of added delay.

Multiple speakers coupled with delay in the source recording are used to create an aural environment that tricks your brain into thinking your room is larger than it is, but there are other things you can do to enhance the perceived spaciousness as well.

Even without any additional treatment, most rooms in a home will naturally absorb high frequency energy without affecting low frequency energy very much. Your room will also diffuse sound, but the key is not just reflecting as much high frequency (>1kHz) energy as possible but scattering it. Flat surfaces like walls and ceilings tend to reflect sound energy in predictable and uniform ways, and it is exactly that predictability and uniformity that gives a small room its small signature. In a large room sound comes at you after being scattered by many surfaces with multiple constructions. Triangular and rectangular protrusions and shapes and semi-cylindrical surfaces scatter sound in many directions which gives you the sense of space due by creating a diffuse sound field.

You should achieve diffusion in as many different ways as are practicable for your space and budget. If you can’t afford specially manufactured diffusers, use a combination of diffusers and maybe a book shelf or media library loaded with different sized and shaped items for example.

However, to really get the most out of what space you have in your room, you should consider a purpose-built quadratic or phase-grating sound diffusor. You can have one custom designed for specific frequency enhancement/attenuation or you can grab an off-the-shelf piece from one of the many quality manufacturers in that space. Believe it or not, art can be a huge (and attractive help as well), especially wall-hung carvings made of wood and the like.

The figure below is a cross-section of a quadratic diffusor consisting of a series of wells designed to scatter energy within a certain frequency band. The maximum depth of the wells determines the low-frequency limit of the diffusor (the well depth is 1.5 times the wavelength of the lowest frequency to be diffused). The highest frequency of the band is determined by the well width which is one-half the wave length of the highest frequency. The actual sequence of the well is determined through the number theory which requires a lot more math than we’re going to cover here. 

By seeing how it’s done professionally you can get an idea of how diffusors are constructed and installed. There are several reputable manufacturers of diffusion panels who manufacture panels for a wide variety of room aesthetics and decorations but with a little background and information, the DIY option is often an excellent solution.

Diffusion is an extremely important and often overlooked component to room acoustics, and you’ll be amazed at how much life you can inject into your listening space with the proper installation of specially designed diffusion panels, the strategic use of what you already have in your room, or a smart combination of the two.

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