Side walls, and the associated side wall reflections, exist in nearly every room. That is one of the features that makes it a room, instead of a patio or park. Some listening rooms have open floor plans and are “missing” one wall, and some very long rooms played sideways can essentially avoid early side wall reflections. But here we are talking about “normal” rooms, the dedicated HiFi or Mastering rooms that are rectangular, with hard painted surfaces somewhere around 3-6 feet away from the outside edge of your top-notch loudspeakers. Let’s see what to do with the sounds that invariably want to reflect from these surfaces.
Setting the (Sound) Stage
Consider the walls of a HiFi/Mastering Setup. The first rule of thumb is to kill the side wall reflections to get better imaging. Let’s re-establish the terminology for the reflections we are talking about. “RR” comes from the right speaker, reflects off the right wall, and enters the right ear. “RL” comes from the right speaker, reflects off the left wall, and enters the left ear. Vice versa for the left side.
The common ways to kill the side wall reflections are:
Use a large absorber on a wide swath of side wall
Carefully place a small absorber away from the wall so the front side absorbs the RL/LR crosstalk signal while the RR/LL stage widening (remember from last week?) signal is weakened by the back side of the absorber.
For A, the large sound panel is used and it does work, but the room sounds like there are huge holes in the side walls. It is a “medicine” with a bad side effect (get it?).
For B, the sound panel is small and the room does not sound like the side walls have big holes in them. This is more natural, and much better sounding.
Is There Another Option?
If a piece of acoustic foam is glued to a flexible sound reflecting panel then any number of configurations of absorption and reflection/diffusion are possible. Use spray adhesive to attach some cheapo sound absorber (you’ll upgrade soon anyway) to a piece of 1/8″ plywood or pegboard. About 2 feet wide and 5 feet tall works for most rooms and speakers.
The Good Way to Do it
We have one side absorbing and the other side reflecting. Now we absorb crosstalk and back scatter the stage widening reflection. This keeps more ambience in the room while maintaining the wide sound stage.
The Better Way to Do it
The reflecting side is curved. Curved surface reflections weaken sound by scattering it in many directions. A curved reflector creates time-delayed, low level diffusive lateral ambience. This is the best type of diffusion for the built-in stereo detection devices located in our ears.
Can I Buy One?
Once you are tired of looking at the funny-shaped foam wedge monstrosity that keeps falling over at the side of your room, you can step up to a manufactured product that will provide exacting performance, unflappable stability, and rugged durability.
The classic broadband absorber/diffuser is the most effective device ever made to serve this function. With half of its perimeter covered by a treble diffusive membrane, all standard and Isothermal model TubeTraps provide the ideal combination of absorption and diffusion to clean up crosstalk and laterally scatter near wall reflections.
The premier professional grade absorber/diffuser swiss army knife of acoustics was developed with precisely this type of function in mind, built with a small form factor for easy adjustment, deployment, and stowage. One or two StudioTraps placed near the side walls, between the listening/mixing position and the loudspeakers, can take care of all your side wall reflection concerns and bring out natural ambience.
Once a Legacy product, the PlanarTrap entered into ASC’s stock product lineup due to its tremendous value per size and easy shipping. Each piece is 16″ wide and just over 5 feet tall, making one pair of PlanarTraps the perfect side wall “medicine” for most rooms. Now with good side effects!