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Art Noxon, PE Acoustical is ASC’s founder & inventor of the TubeTrap. This week Art continues with part 4 of trapping bass in our recording studios. Enjoy!

Boom Busters

Mr. Noxon explores what has been done to make bass a welcome guest in the studio.

There seems to be a popular misconception about the role of bass traps. The uninitiated often say, “I want to kill my resonances with some bass traps”. When absorption is added to any resonant circuit, be it electronic or acoustic, only the rate of energy drain from the system is increased. It must be stressed, that from a practical basis, absorption can never eliminate resonance: resonance exists because the room exists. Absorption can only reduce the strength and sharpness of the resonance, (its “Q”) but not eliminate it.

Sound will build in intensity until there is a balance between the power delivered into the room and the power absorbed or leaked out of it. Increased absorption means the room reaches its peak sound level more quickly. Why? Because the equilibrium sound level attained in the room is lower and not because the energy rise rate is any more abrupt. Adding absorption, however, increases the sound decay rate in the room.

Other benefits are noted at the cold spot. The resonant field strength is weaker overall due to the added bass absorption. The reverb field’s reverse phase cancelling effect of the direct wave from the speaker is less strong. As a result, the cold spot “warms” up and the pulses at turn-on and off are accordingly diminished.

As to the coloration effects, added absorption reduces the “Q” of room resonance, the sharpness of its response. Low “Q” rooms lose attack transient and sustain distortion. The beating effects have disappeared and the tone in the decay is the same as that of the driven frequency.

Absorptive damping of room resonances, as we have seen, will improve the dynamic response characteristics of the room. It is quite clear by now that it is the room that we listen to in the lower registers. Accordingly, the better behaved the room, the better the track and mix will sound.

A caution needs to be noted at this point. Nearly all recording engineers have access to an RTA, typically 1/3 octave bands. Their experience with electronic equalization, particularly parametric, leads to the desire to see a flat room acoustic response curve. Good luck! It is always a surprise to realize that dynamic transient stability in the room can be developed to satisfaction, and yet the 1/3 octave RTA shows less than 1 dB improvement. Just as it is impossible to fix room acoustics with an equalizer, it is likewise impossible to read room acoustics with an equalizer meter, the 1/3 octave RTA. The narrow band Modulation Transfer Function (MTF) type of test is how room acoustics must be evaluated in the low end.

Bass Traps

Many ingenious designs have been developed to provide low-frequency absorption. In the beginning, no doubt a bass trap probably was little more than “great balls of fuzz,” fiberglass insulation or batting stacked to the ceiling in the back of the room. Such a system was so ugly that it was covered over with “scrim cloth.” It did, however, provide absorption for frequencies whose wavelength is up to four times the fill depth. A 3 foot deep fuzz trap is effective to the 12 foot wavelength, about 94 Hz.

It is instructive to calculate how deep this trap would need to be to dampen the fundamental room mode now that digital tape can store such low frequencies.

1st Mode Depth = ¼wavelength = ¼(2L) = ½L = ½ Length

A 24 foot room would need a bass trap about 12 feet deep. Obviously, converting half the room into a bass trap is not an option for most people!

An alternative to filling the back of the room with fuzz is to remove the closet doors at the back of the room and fill them with fiberglass. The frequency response curve of the ¼ wavelength trap system shows strong absorption on the first, third and fifth harmonics, because the air friction occurs at the position of “sound cancellation” or maximum air motion, typically ¼ wavelength and ¾ wavelength from the trap’s wall.

Slat Bass Traps

The basic mechanism for sound absorption is the friction of air as it moves across a surface. The more surface and the more air motion, the better the absorption. But large scale bass traps are physically unacceptable in the smaller home recording studio. Another problem with giant absorption is that it makes for an uncomfortable and distracting listening environment, because it is anechoic or too dead sounding.

Consequently, wooden slats are added to most traps, somewhat like a fence. The frequency response for such a system is much more acceptable, since the mids and highs remain lively, yet the bass becomes damped. Larger wavelengths pass easily through the openings between the slats. But when the wavelength is less than four times the slat width, the sound is back scattered.

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