Room Treatment

By John Strohbeen • Ohm Speakers • Tuesday, March 14, 2017


If you are a music or audio professional, the chances are good that you will have a room dedicated to music and its performance or reproduction. For the vast majority of music lovers there is a nearly constant battle between the comfort and décor of a multi-use room and the demands of making the sound the “best possible.”

The interaction of your speaker and your room usually has a bigger impact on the quality of sound than all your choices in electronics. There are ways you can change that interaction to get better sound. Trying different locations for the speakers and listeners can often make a dramatic change at no cost. If you have placement limitations (and most of us do), considering room treatments is the next step.

Cosmetic considerations.

The lower frequency problems take much larger room treatments than higher frequency problems. While moving a subwoofer four feet can make a big change in standing waves in the room, it would take a substantial bass trap to stop the buildup of the same standing waves. In the treble, much smaller diffusors can do the job for most seating locations.

Some definitions of room treatment:

In general, acoustic “room treatments” fall into several basic categories: Absorbers, Diffusers, and Reflectors.

Bass traps are absorbers for the bass frequencies. The most effective tend to have the narrowest frequency range.

An absorber is what it does: it absorbs incoming sound energy. The frequencies that are absorbed vary with the material, the size, the shape and the location in the room. Stuffed furniture such as sofas and big chairs are pretty good absorbers over a wide frequency range. Cloth covered furniture absorbs into higher frequencies than leather (or other hard-surfaced material) covered furniture. Carpeting and area rugs can absorb midrange and treble. The thicker the stack, the better the absorption. Real felt rug pads can have a major effect. (Hint: To improve dialog intelligibility in your home theater room, put a thick felt pad under a heavy area rug at the point of the floor reflection from the center-channel speaker.) Absorbers can be important treatment; but don’t get carried away — particularly in the front of your room — or you may lose some of the information from your recordings.

A reflector is the opposite of an absorber. As sound energy bounces off reflectors, it continues on its way. Which frequencies are reflected varies with the material, the size, the shape and the location in the room. Flat, rigid reflectors act as mirrors for the sound; but only reflect effectively when they are about a wavelength in diameter or larger. The major application of reflectors is in concert halls to direct the sound of the performers to the audience and in big recording studios. The outdoor band-shell is a big, full frequency range reflector. Home use of flat reflectors is limited (because of size) except when you are designing a custom-built theater and/or sound room.

Diffusors (aka Dispersers) are a non-flat variation on reflectors. While flat reflectors generally reorient the direction of the sound path over most frequencies in a coherent wave-front, the reflection of a sound hitting a diffusor coming from one direction is spread in many directions. Again, which frequencies are diffused varies with the material, the size, the shape and the location in the room. Again, you need big devices to diffuse low frequencies. Many modern diffusors use fractal designs. With these designs if you look at a 6” x 6” section, it will have a similar shape to 96” x 96” section. This would allow the effective low frequency limit to go from about 2 kHz down to about 125 Hz while keeping the high frequency limit the same (over 20 kHz with the smallest details around 0.5”). Diffusion has the advantage of keeping the sound energy the same while eliminating standing waves and echoes in their useful frequency range. Some of the most natural sounding systems I have ever heard invested a lot of money and time in large diffusors.

My recommendation of room treatment.

  1. Put the furniture, rugs and window drapes in place before you start making changes. Try to have a conversation with another person while you sit in the proposed listening position and the other person sits where the speakers are expected. Also listen to them clap a few times. You should be able to understand them easily and the clap should die out without any ringing.
  2. Move the furniture, listening locations and speaker locations around to find a set-up that maximizes the intelligibility and minimizes the ringing.
  3. With the system set up, try adding the floor-bounce absorber (a layer of pillows is a good temporary test device) to see if voices are more intelligible on speech and small group vocals. Get a cosmetically acceptable sound absorber for the floor if the improvement is appreciable.
  4. Get a couple medium sized diffusors (2-3 foot squares are a good start) and try them at the other early reflection in the front and sides of the room. Start with one diffusor placed where it has the biggest effect. Usually, this is the one closest to the speakers as that is the position where they are the loudest. Place the second diffusor next (but somewhat further from the wall) to the first and listen to see if this makes as big an improvement as adding the second diffusor into another early reflection spot.
  5. If you find the room is too sharp in the treble and/or too heavy in the bass, add a large absorber on the rear wall where the first reflection on that wall is located. A big mattress works well as a test fixture before buying the final absorbers.
  6. Experiment.
  7. My rule of thumb is to use reflectors and diffusors in the front half of the room and absorbers in the rear half of the room. This goes against the traditional norm in the home stereo industry, but if you have speakers with correct dispersion characteristics, you’ll appreciate how this approach incorporates early reflections into the overall sound from your system.
  8. Take a few days off and experiment more.

You can get great improvements in sound quality with refined room placement and adding some room treatment.

Until next time,

Good Listening!

John

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