An Anechoic Chamber for SPCR

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After more than two solid weeks of grunt work, the window had been blocked, the outer wall built up, the surface of the walls and ceiling covered with 600 lbs of blue fill, and a second door installed to improve the soundproofing around the entry: In essence, a small hemi-anechoic chamber. Total cost of all the materials ran around $4,000. It was time to assess what had been achieved acoustically.

The assessment process took some time, working with subjective perceptions as well as the new recording / acoustic measurement system purchased before the anechoic chamber was built. This system is fully described in New Audio Test Gear, SPCR 2008. It is a PC-based audio spectrum analyzer with calibrated SLM functions as well as a high resolution audio recorder based around the following core components:

Ultra low noise (less than 10 dBA)$2000 7022/4012 ACO Pacific measurement microphone with 1" diaphragm capsule and 200V power supply.

M-Audio Tampa (link to PDF manual) 24-bit/96-kHz digital mic preamp and M-Audio FireWire 410 4-In/10-Out 24-bit/96-kHz FireWire Mobile Recording Interface.

SpectraPLUS - Audio Spectrum Real Time Analyzer software running on a custom built, virtually silent AMD A64X2 system.

A forum thread I used as a daily blog during the planning and construction of the chamber contain notes upon which many of this assessment is based. As with all complex tools, which an anechoic chamber certainly is, it takes time to appreciate all the various qualities, both positive and negative. This is my assessment after using the chamber for about three months. It may change after a few more months.

1. General - The most striking aspect of the chamber is the absence of echoes. Obvious, right? It's the primary quality of an anechoic chamber, so what's the big deal? The big deal is that outside of an anechoic chamber, you will never experience this kind of acoustic space. It simply does not exist in nature or in normal buildings. A few visitors have become uncomfortable when asked to remain quiet for a minute in the chamber. Apparently, the loss of aural cues can make some people feel physically unbalanced. It can be a bit spooky. It's also very quiet. It typically measures 10~11 dBA through most of the day, although both ground and air traffic noise breaks through and occasionally registers over 20 dBA. When you become acclimatized to the ambient background, it becomes easier to hear the rumbling sound (mostly of traffic 2.5 blocks away) which registers constantly on the spectrum analyzer.

2. Sound pressure levels and Frequency response - The image below shows the frequency spectrum of the chamber when there is no obvious external noise.

Note that the ambient SPL is 10.21 decibels, A-weighted. That's an averaging of the curve, with the A-weighting filter applied. Note the position of 0 dB, which is defined as the threshold of human hearing sensitivity; above 200 Hz, most of the curve lies over 20 dB below 0.

3. Reverberation time - This is defined as the time for the sound to die away to a level 60 decibels below its original level. What's ideal depends on the type of room. A general purpose auditorium for speech and music should have a reverb time of 1.5 to 2.5 seconds, depending on its size. A classroom should have a much shorter reverb time, less than a second.

SpectraPLUS has a reverb time measurement function. It requires a wide-range loudspeaker capable of producing pink noise at least 60 dB above the noise floor of the room. First the software activates the noise source long enough to saturate the environment (that is, an equilibrium is reached between direct and reverberant sound throughout the room). Then the sound is turned off and the decay of the signal captured and the reverberation response analyzed in detail. This test was performed in the anechoic chamber using one of the AudioEngine A2 speakers that we reviewed in the spring.

The anechoic chamber's reverb time measured 50 to 100 ms through most of the frequency band. This confirms what can be heard by anyone; there is virtually no echo at all. It rose to a maximum of about 200 ms (1/5 of a second) in the range below 200 Hz. This increase indicates there are echoes in the low frequencies. It also explains the steep rise in level at lower frequencies; this rise is caused at least partly by "standing waves", not only breakthrough of external low frequency noise.

Is this level of low frequency echo acceptable? Yes, more or less. It would be nice to have less echo at lower frequencies, but the amount of damping material to make any significant improvement is vast. One series of calculations performed by an acoustics expert with anechoic chamber experience suggested that doubling the blue fill in the room might give us an extra 50 Hz before the bass boost in the curve above. The price is another $2,000 and a lot of grunt work. It would also mean losing another 16" in every room dimension. However, there's a limitation in the size of the room; it's really too small to push the low frequency resonances much lower. The cost / benefit ratio doesn't appeal, at least not now.

4. Isolation from external noise - It is much better than the original room used to be. Before any of the sound proofing work was done, any sound outside the window was easily audible. The neighbors use the walkway along that side of the house to gain access to their back yard and side door; they were always audible before. Now, they're never heard from inside the chamber. In fact, someone can be shouting loudly (as if in an emergency) just outside the window, and they might be very faintly audible from inside the chamber.

A test of sound isolation could be done with a loudspeaker, amplifier and signal source outside the window along with a SLM to measure the SPL, and a SLM to measure the level inside the chamber. This test has not been done, as the wiring, AC requirements, etc are tedious. It would not be surprising for the isolation to measure 70 dB or better through much of the frequency range.

The main noise sources that served as the impetus for this project are planes and cars. They haven't gone away, so the most pertinent question is: Does the anechoic chamber allow the noise of planes and cars to be ignored? No, not exactly.

A scientific test has not been done, but in general, the noise of most commercial planes intrudes for a shorter duration than in the past. That is, if a passenger plane taking off forced us to stop doing audio recording or measuring SPL for five minutes, now, in the chamber, it stops us for only two minutes. Most of the noise that comes through is in the low frequencies, and these, unfortunately are exacerbated somewhat by the relative silence in the rest of the frequency spectrum, and by the low frequency standing waves or echoes. The same comments apply to automobiles.

Is this level of isolation from external noise good enough? This is a completely subjective question. The answer is all about how patient we are in the lab and how much work there is to do. The point is that we cannot ignore external noise, but as long as we're willing to wait a couple of minutes, we can usually get the acoustic measurements and recordings done. There's no question that the original floating-room-in-a-room design would give us better isolation through most of the frequency range. Whether it would help much with the low frequency noise that intrudes most is difficult to assess. The acoustic experts consulted on this question did not come to any clear consensus. A further complication is that STC, which guides acoustic building material choices, only applies down to 125 Hz. It would cost at least another $5,000 and several weeks of physical labor to find out. At this point, I'm not ready for that discovery.

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