Let’s face it, a lot of modern buildings and facilities face one potentially big design flaw: bad sound. With flat hard-surface walls and ceilings now all the craze, people are coming to realize that these high-fidelity spaces are feeling a little, well, echo-y. The solution to this, of course, is to incorporate acoustical treatments and sound absorption for office design.
But it isn’t as cut and dried as simply adding some fabric-based materials on walls, ceilings and floors.
In order to get the best result, you must first understand that there are two types of acoustic room treatments – absorbers and diffusers – that can help alleviate echo and reverberation in studios or theaters.
Absorbers reduce excess sound reflected in a room while diffusers spread out those reflections.
Combining office sound absorption and diffusion solutions results in clearer, more satisfying acoustic experiences because it maintains an amount of reverberant ambience that is key to our ability to form accurate sound images.
The most important factor for room acoustics is finding the best balance of absorption, diffusion, and placement that will improve the unique acoustic properties of a particular space.
Determining what, when and why to use acoustic absorbers and/or diffusers – and where they should be in the space – should play a big role in your design. There are many strategies to consider, but first you’ll need a basic understanding of factors that contribute to the overall sound of a room.
First Reflection Point
The areas from which sound reflects first, after originating from its source, are known as First Reflection Points, which are responsible for the first and worst effects on a room’s acoustics.
Hard flat surfaces strongly reflect “original” sound, and these strong reflections combine with sound from the source to create destructive interferences at the listening position.
In order to develop an acoustical treatment strategy, it’s best to locate the first reflection points in the room.
There are videos available — like the one below — that demonstrate a simple method for locating first reflection points in your room. To summarize:
- Working with an assistant, sit at at the “best” listening/viewing location, facing the speakers.
- The assistant should place a handheld mirror against one side wall at the height of the seated person’s ears and move it horizontally along the wall until the seated person can see the center of the speaker closest to that wall in the mirror.
- Mark the position of the mirror with painter’s tape – that’s the first reflection point for that speaker and wall.
- Repeat steps 1-3 for the other side wall of the room. You can also find and mark first reflection points for the “front” wall (the wall you face when listening).
If you choose to add acoustic treatments to the ceiling (and you should consider it – it’s a large flat surface that contributes to the number of strong reflections), you’ll need to find and mark those as well.
Once the first reflection points have been located, you can decide whether to treat them with absorbers, diffusers, or both, depending on desired results.
Sound Absorption, Diffusion… and Phase
When sound reflects off the walls of a room, it reaches the listener later because it’s travelling farther than the direct (original) sound.
Because the reflection is a delayed version of the original sound, it destructively combines with the original sound, interfering with a person’s ability to accurately perceive the direction and composition of the sound.
As sound reflections fill a room, they also generate interference with each other, and the result is poor acoustics: too much echo and reverberation, and bad sound imaging.
Think of it as sloshing water around in a bathtub.
When you introduce energy to the water, that energy propagates in the form of waves, generating peaks, valleys and dead zones as the waves amplify, diminish or cancel each other out.
For many first reflection points in most rooms, the best solution for this is often diffusers, which spread out sound energy, reducing the destructive interferences caused by untreated strong reflections.
While sound absorption panels will also reduce the energy level of reflections, they can be overused. This is a common problem in many critical listening environments.
Diffusers won’t diminish ambience or alter pitch perception the way using too many absorbers can.
However, some diffusers come with a separate set of issues.
One example is “Quadratic Residue” designs, which work by distorting the timing or phase relationships within the reflections, thereby damaging the sound image.
In these instances, the acoustic experience suffers because we rely on the timing, or phase, of sounds to construct accurate sound images in our minds.
Phase Coherence and Accurate Sound Imaging
Because human hearing has evolved as a survival tool, our brains are very sensitive to timing or phase information for the purpose of sound location. “Phase coherence,” or unaltered timing information, is a critical component of hearing sound accurately.
Think of how quickly you respond when you’re surprised by a sudden sharp or loud noise. You instinctively look in the direction from which the sound came as your nervous system primes you either to fight, turn and run or simply not worry. We may take this for granted, but it underscores the amazing nature of our brains.
In the same way that having two eyes enables our brain to construct three-dimensional representations of the world, having two ears also enables us to hear where a sound is originating in the same three-dimensional space.
This is similar to the highly refined echo-location system that bats use to “see” in the dark.
Though the difference may be extremely small, sound emanating from a source off to one side or another will reach our ears at slightly different times because of their physical separation on our heads.
Our brains then interpret those tiny timing differences almost instantaneously to determine the location of the sound source. But if the timing of a sound is somehow not what our brains expect it to be, our ability to construct an accurate sound image is thrown off. In a word, things just don’t “sound right” to our ears.
Curved-surface phase-coherent diffusers smoothly spread out reflections and reduce “hot spots” to better preserve the timing (phase), loudness (amplitude) and tone (harmonics) information in reflections.
Curved diffusers also decrease the need for sound absorption, thereby retaining a level of reverberation that’s agreeable to our ears.
This type of room treatment is not a new discovery – RCA Studios in New York, for example, used this method in the early 1940s.
Phase-coherent diffusion also helps preserve accurate sound imaging/location, providing a wider, “stereophonic” sense of space. Using phase-distorting diffusers, like Quadratic-Residue types, in the first-reflection points results in our brain/ear combo being less able to form an accurate sound image of the audio it’s hearing.
Timing accuracy, or “phase coherence” is essential for both recording and hearing sound accurately.
Treating a Room
WALLS & CORNERS
While the conventional wisdom has been to use absorber panels to treat first reflection points, curved-surface diffusers offer a more effective option in treating these highly problematic spots.
Stereo fields will sound wider, with clearer and more focused details because of the phase-coherence that results from the constant-radius cylindrical shape of the diffuser.
For home theater applications, place a vertical curved diffuser at the first reflection points on the side walls, and possibly the back wall, with a horizontal curved diffuser on the front wall beneath and parallel to the center speaker.
Since the corners of a room further exaggerate sound reflections, place a fabric-wrapped absorber panel on each side of the front corners of the room to reduce side-to-side repeating reflections – also known as “flutter echoes.”
Use two additional fabric-wrapped absorber panels in the corners of the back wall to reduce front-to-back reflections along the length of your room.
Though each room presents its own unique set of acoustical challenges, this combined sound absorption/diffusion design should improve the acoustic quality of nearly any room.
You can also add another curved-surface diffuser on each side wall at a right-angle to the primary seating position. This will eliminate the most annoying “flutter echoes” from the listener’s position.
Like walls, ceilings are large, flat surfaces that should be acoustically treated in any critical-listening space – rooms will greatly benefit from ceiling absorber panels, typically called “clouds.”
For especially challenging rooms, the wall treatments mentioned above (a combination of sound absorption and diffusion) will typically provide a similar result on the ceiling,
Cloud absorbers are lightweight and easy to install, with good acoustical control of echo and reverberation. For the best effect, place two clouds on the ceiling at the first reflection points, and two above your seating area.
You don’t need to treat every square-foot of your room to have great acoustics. A strategic combination of absorption and diffusion targeting the problem areas – the first reflection points, corners, ceilings, etc. – can transform a poor-sounding room into a fantastic one.
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