ASC Acoustic Tip of the Week: Room Boundary Materials

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Do your walls, ceiling, or floor thump like a gigantic bass drum? Does your basement ceiling leak movie sound effects into the bedrooms upstairs? Does your studio floor add a mid-range coloration you can’t quite put your finger on? Do your painted concrete block walls make your room sound like a gymnasium? Do your wooden walls sing warmly like a Stradivarius?

Room boundaries can be both detrimental and helpful to the sound of your room and your overall sonic goals.

Sound Transmission vs Containment

Most people generally do not want sound leaking into or out of their critical audio rooms.  Unfortunately, sound always has to go somewhere.  So it does one of three things:

  • leaks through the wall, ceiling or floor (bad)
  • ricochets around the room for a long time (bad)
  • is dissipated by structural damping or surface absorption (good)

The sound reflecting properties of a material are inversely related to the sound absorption characteristics.   Therefore a room constructed entirely of OC 703 may act like an anechoic chamber, but it will leak a lot of sound.  On the other hand, a poured and polished concrete basement wall will keep you from waking up the neighbors, but the bass boom and mud will be unbearable.

A happy compromise (really, the only high-performance option) is to construct room boundaries that are highly strong and massive, with structural damping applied at joints and between layers.   Strong walls include: concrete, plywood + 2×6, tongue-and-groove lumber, laminated timber, and masonry.  Massive walls include concrete, double gypsum, lath + plaster, mass-loaded vinyl, and metal.  Structural damping requires constraining viscoelastic damping compound between two strong yet rigid layers (commonly double gypsum), using adhesion and fasteners.

Sound Coloration due to Surface Stimulation.

High-energy sound waves cause surfaces to move; some more than others. As a generally accepted rule, those dabbling in precision audio do not want their room surfaces to contribute to the sound by resonating. Several negative outcomes result when the walls, ceiling or floor “sing”:

  • increased reverberation time
  • uneven frequency response
  • degraded musical articulation (transient response)
  • blurred imaging

These issues cannot truly be solved by DSP or equalization. Altering the mechanical characteristics of the problematic surface to minimize the effects is the appropriate solution. As with most solutions, the degree of commitment you are willing to make has a strong effect on the final outcome.  Like a well-designed loudspeaker, a good room surface does not possess large unsupported sections that are free to vibrate. Windows and plain drywall on studs are common examples.

Go around your room and find the most resonant surfaces by thumping with the bottom of a closed fist. You will probably hear a tone that aligns with one you always thought seemed a little louder than it should be in your music. After you find the offenders, you can do several things calm down the surface:

  • install rigid bracing across the unsupported section, anchoring to strong points and fastening to the section*
  • install an extra layer of sheet material (drywall, plywood, hardboard) with damping sandwiched
  • remove the bad surface and rebuild with better bracing or stronger material

One common exception to this general rule applies to solid natural wood room skins. Like a classic violin or guitar, the type, thickness, age, and finish of the wood have a strong influence on the final sound generated. And in a cheap violin or guitar, an average wooden body still sounds better than one made of cardboard or plastic. Perhaps it is purely psychoacoustic, but the sound of lightly resonating real wood boards seems to appeal to most humans’ sense of sound quality.  So if you find yourself surrounded by gorgeously stained maple or mahogany tongue-and-groove, consider yourself lucky, put on your favorite record, and enjoy!

*tip: for freely vibrating glass surfaces, use wood strips and a strong adhesive!

Surface Absorption of Sound.

Mid and high frequency sounds are quite easily converted to heat through frictional losses in a process called “kinetic sound absorption,” where the kinetic energy of the sound wave is directly proportional to the energy losses. How is this conversion achieved? Mainly through porosity and particle surface area/volume ratio. Here are a few rules of thumb:

  • shiny surfaces reflect mids and highs
  • bumpy surfaces reflect mids and diffuse (scatter) highs
  • porous materials absorb sound (thicker = lower frequency absorption)
  • curved surfaces weaken reflection strength through diffusion

A great example of confusion arises with concrete block basement walls. The unfinished blocks are clearly very hard and assumed to be reflective. However, high frequencies are readily absorbed because of tiny holes in the structure. Now, the interior decorator decides to paint over the blocks to improve the appearance-and seals up all the holes. All of a sudden, the highs are reflected instead of absorbed.

Home theaters and some studios are designed with thin 1″ fiberglass covering all (or most) of the walls. This is very effective at absorbing the upper mids and high frequencies-but does nothing to the low mids and bass. This is not too different than taking the “treble” knob on your old stereo receiver and cranking it down hard to the left. The room would sound “dead” and dull, with no ambience or crispness. But the bass might still boom like crazy! Here are some other factoids about surface absorption:

  • acoustical tile ceiling absorbs almost no bass (but it leaks bass quite well!)
  • weak walls leaking bass is good for reverberation (but other problems may arise…)
  • perforated materials (pegboard, perf steel, etc) can allow bass frequencies to pass through to be absorbed while the mids and highs are reflected
  • foam is generally not effective below about 250 Hz (unless VERY thick)
  • irregular surfaces cause diffractive diffusion and weaken direct reflections (record cabinets, book cases, knobby rack gear, etc)

It tends to be fairly common to overdo high frequency absorption because it is so easy! Keep your room alive and balanced by applying the above principles.

Many people don’t know that you can use damping materials to calm the outside sheathing on your dedicated listening room/ home theater, or studio.  The interior surfaces are not the only ones you should be considering.We hope you enjoyed this week’s installment, and that you listen to some amazing sounds this weekend!
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San Francisco Audiophile Society

San Francisco Audiophile Society