Of course, some music types prefer reflections as part of the performance. Musical styles before the electronic age such as classical, old time and polka use the room reflections to broaden the sound, generally to the enjoyment of the patrons. Modern recorded music played back is best in “dead room” conditions, since the recording mix inserted the desired ambiance already. So somewhere in between live and dead room acoustics is where most modern music performances are done.

As direct or “incident” sound waves hit a surface, such as a wall, some will be absorbed, some will reflect and typically a small amount will pass through the surface. Table 1 shows a list of common surfaces in music venues and the absorption in percent at three nominal frequencies of 100Hz, 1,000Hz and 4,000Hz. The 1,000Hz and 4,000Hz numbers are very important, as they represent the midrange and presence bands of frequencies where the human voice produces consonant sounds critical for intelligibility. A good assumption is that the majority of sound not absorbed is reflected back away from the surface.

A sound person’s worst acoustic enemies are masonry such as concrete, brick and rock-based materials. Of course, the exception is if you desire the half-space efficiency of sub-woofer cabinets on a masonry floor to give you that +3dB boost in SPL. Note that carpet covered concrete does great things, since it absorbs mid and high frequencies well but still leaves the subwoofer efficiency intact. Without any fabric material covering concrete, brick or stonework, you have a nasty surface for walls and ceilings. You best mitigation technique in dealing with this dilemma is medium- to long-throw speaker cabinets and aiming them away from the surfaces and downwards to the audience areas.

I intentionally placed the empty seats and seated audience absorption in adjacent rows to show the vast difference an audience brings. The audience is composed of fabric covered, fleshy encased bags of saline, meat and lard, known as human beings, and we are really good sonic absorbers. An important point is that pre-show sound checks show just half the picture, without showing how the room will sound when occupied. This is the whole reasoning for asymmetric horns on stacked speakers and rigged speakers aimed into the seating. The only thing people absorb better is wireless radio frequencies (UHF), and thankfully, no high power transmitters are allowed in venues (-52dB/wavelength).

Wood-based surfaces are slightly nicer than masonry, but are still surfaces of high reflectivity to mid and high frequencies. One of the reasons I run around with garbage bags filled with folded sheets of black velour and commando cloth drape (flame retardant) is to cover an upstage wall of wood that creates plenty of stage wash reflections. Unfortunately, my favorite venues love knotty pine décor, and wood is part of the visual ambiance, even if the aural ambiance is less than desirable.

Glass is another modern problem in acoustic reflections in that it does absorb a little low frequency energy, but is even worse than wood in reflecting mid and high frequencies nearly perfectly. With many modern performance areas opening up to mother nature or looking like terrariums, these are trouble spots that are hard to avoid. Fabric is the solution, but that is not likely because of the visual aesthetics.

Nothing brings a smile to my face like a dark, intimate club that tends to be a quiet setting until the performance starts. Things like acoustic suspended ceilings, lots of floor and wall carpeting, upholstered seating and no nearby side-walls at the stage. Using the hand-clap test usually yields nil reflections. Big rooms of this category are live sound nirvana. If sound men were live music club owners, they would all purchase old bowling alleys and carpet the seating areas.