This month, we have another guest writer, Jeff Madison from RPG. Jeff is the senior product application consultant in RPG's Home Theater Division. He spent his first years at RPG developing its computer modeling and acoustic testing capabilities. With a background in music and engineering, he enjoys the challenges of both large and small spaces - and golf.
Rooms without Boundaries
The home theater experience is about being immersed in the audio/visual experience - maybe even letting go of reality for a few brief hours in an attempt to join the characters on the screen for a new adventure or experience. The technologies that are available now to capture and manipulate images and sounds are continually pushing the envelope of challenging the senses into believing what they are experiencing. So why is it that we sometimes cannot entirely let go? What keeps us from making the journey into the world that the DVD is spinning for us?
|To blame any one thing would be ridiculous. There are many factors which contribute to a successful or not so successful theater experience. For the purposes of this article, we are going to assume that the temperature, lighting and the manners of your fellow viewers are all contributing positively to your experience. We'll also assume that in the construction phase of your theater, attention was paid to external acoustic issues such as disruptive sound transmissions into the theater and interference from the mechanical, electrical and plumbing systems. While we're at it, let's assume|
|that the audio/video delivery system has been engineered and implemented appropriately. We want to focus on what is happening in your theater environment as speech, music and effects spill out of the 5-10 speakers placed around the room and what your brain must be thinking as all of those sources and their reflections from different wall and ceiling surfaces impinge on your ear canal in a symphony (or cacophony) of sounds.
The Missing Element:
The missing element in most home theaters is proper acoustical treatment. Why is the room so important? The reason is that sound must travel the acoustic path from the loudspeakers to our ears. The sound that we hear in a room is a combination of the direct sound from the speakers and the indirect reflections from the room surfaces and room contents. Indirect reflections can cause acoustic distortion so that you end up listening to the room and not the intended sound-track. Hence, reflection control is a central problem in creating rooms that allow a suspension of disbelief. In my experience, most soon-to-be home theater owners know that "acoustic" panels are necessary components. This is mostly due to the fact that we see different types of wall and ceiling surfaces when we go to the large movie theaters and other performance spaces.
We may not exactly be sure what they are but we do know that they are serving some purpose and that they should probably be there. Although it is beyond the scope of this article to go into detail, one thing we do know is that small rooms can cause a lot of problems when you force sounds into them (see Minimizing Acoustic Distortion in Home Theaters). One of the main issues in creating a natural, neutral listening environment in a small room is what to do about the proliferation of reflections caused by so many speakers firing into the space. We know that if you absorb all of them, the room sounds unnatural - what most people refer to as "dead". This tends to keep the brain very aware that it is indeed in a small room and not out on the ocean on a sailboat or in the mountains where we hear no boundaries. Unfortunately, because not everyone is familiar with the latest acoustical technology, absorption is often improperly used as though it were the only solution. We also know that if you do not treat the room at all and leave the wall surfaces reflective, things will be excessively reverberant. It's hard to enjoy sitting in such a room due to the harshness and poor intelligibility of the program material. Even if we use absorption in moderation, we still won't be able to create a balanced room acoustic. Where does that leave us? We have to utilize surfaces that neither directly reflect nor absorb sound!
Sound diffusion has been around for a long time. In classic architecture, statuary, columns, relief or a variety of other ornamentation broke up sound waves and contributed to the experience of enveloping the listener in rich, even sound fields. While beautiful, these surfaces operated only at high frequencies. In the early eighties, Dr. Peter D'Antonio successfully implemented a new type of surface utilizing the theoretic mathematics of Dr. Manfred Schroeder. This new surface neither absorbed nor reflected sound. It uniformly scattered sound around. Today, the research of Dr. D'Antonio and Dr. Trevor Cox has created a broad palette of sound diffusers that provide uniform scattering over any designable bandwidth. We are all familiar with the need to use air diffusers to
|provide uniform temperature or lighting diffusers to provide uniform luminosity. It should come as no surprise that we need a surface called a sound diffusor to provide uniform sound coverage.
Here is a breakdown of the diffusion tools that you now have available for your own Room without Boundaries. There are many new developments in diffuser technology coming down the pike, so for now, we'll stick with what you will be able to find readily available on the current market.
1/ Primitive Root Diffuser
Designed by utilizing modulated prime root number theory, the math shows us that the reflected energies will be equal in all the diffractional directions, yielding an even sound redistribution pattern as shown in the above right balloon plot. Its special performance feature is that it virtually eliminates the spectral component of the incident sound wave. This means that in terms of creating a sense of spaciousness while supporting imaging, there is no equal to this diffuser.
2/ Geometric Diffusers
Units like the one shown have been around for many years. Other forms that they may take are trapezoidal, barrel and globular in shape. Since these geometric forms are really re-directors of sound rather than diffusers, they tend to work better in larger spaces such as choral rooms, churches and band rehearsal rooms. As they are technically classified as sound redirectors rather than true sound diffusers, you do not often find them in small rooms.
3/ Quadratic Residue Diffuser
Designed utilizing modulated quadratic residue number theory, the performance of these units is similar to their #1 counterpart, albeit in 2D! The scattered energy is in the form of a hemisphere. Now available in a lightweight expanded polystyrene as well as molded gypsum and furniture grade wood units, this panel technology brings high performance to a low profile, great for applications where headroom is at a premium.
4/Quadratic Residue Diffusor
Designed utilizing quadratic residue number theory, the math shows us that there will be equal energy in the diffractional directions which depends on the number of wells and how wide they are. The bandwidth depends on the deepest and narrowest wells. The scattered energy assumes the form of a hemidisc. The photo shows an optimized quadratic residue diffusor with nested components for increased performance. One of the most effective diffusers across the largest range of frequencies, these are most often used on the rear wall of theaters and listening rooms when the listening position is not close to the surface.
5/ Binary Amplitude Grating
Designed utilizing optimal binary number sequences, this panel technology has revolutionized small room acoustic design. By placing a 5/32" diffusing template between the fabric and fiberglass of a common absorptive panel (an often overused and misused tool in many theaters and listening rooms), this hybrid panel diffuses high frequencies in 2 dimensions while simultaneously absorbing low frequencies. The low frequency absorption is determined by the thickness of the fiberglass layer behind the template and the depth of the air cavity behind the panel. When the surface is flat, these panels are typically 2-4" thick and work well in rooms that cannot afford to lose many inches in its longitudinal or lateral dimensions. Also, because it is a hybrid surface, the listener can sit much closer to these panel without experiencing nearfield phasing effects. The template that separates the fabric from the fiberglass provides a diaphragmatic mass, which allows it to absorb frequencies much lower than a common fabric wapped absorptive panel. The panels are now available in a curved profile to enhance diffusion and increase imaging and spatial performance.
Breaking it all down:
Whether it is a fictional, historic or comical interpretation of the world, we are trying to put behind us for a few hours the real world. It is hard to do that and let go when there are constant reminders that we have never left our room. One of my biggest complaints in an untreated theater or one overly treated with absorptive materials (and often low bandwidth absorptive material) is that despite the wonderful imagery on the screen and sonic perfection coming from the speakers, the sound of the room will not allow us to forget that it's small. If someone had several spotlights shining on you during the movie, wouldn't that detract from the visual experience? From
|every direction, an untreated room is basically "shining" sonic spotlights on us. In a poorly treated room with low-performance absorptive panels, we are simply dimming these spotlights ever so slightly. This makes things a little more tolerable but still not into the ideal environment that it should be. A properly treated room will remove the spotlights and allow the quality of the soundtrack, the speakers and the electronics to be fully realized. Without the boundaries, you could be anywhere.
For more information on RPG, please visit their website. There is an extensive library section with many technical articles on room acoustics if you care to delve more deeply into the subject.
Please also visit Rives Audio who have provided this series of articles on acoustics. Rives Audio is the leading consulting and design group for small room acoustics and has an extensive resource section where you can learn more on acoustics as well.
To have your question answered, click here to e-mail it to 6moons - we will forward it to Rives Audio on your behalf. Please remember to ask questions of a general nature as the professionals at Rives get paid to provide customized assistance. If the auto-launch doesn't properly open for you, e-mail to firstname.lastname@example.org, subject header RAM:EF. For our interview with Rives' president Richard Rives Bird which became the impetus for launching this feature, click here.