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Months were spent poring over articles, spec sheets, product brochures and other sources of information on microphones, mic preamps, PC-based audio spectrum analysis software, sound cards, and SLMs. I consulted independent acoustics experts, company representatives and other individuals who had hands-on experience with the gear. To go over all the twists and turns in the road that led to the final equipment choices would be far too tedious for you to read... and for me to write. Suffice it to say that these were some of the brands still left on the list near the end of the search:
Brüel & Kjær - Still a top brand, still very pricey, still one of the very few SLM makers that reach below 20 dBA, still beyond the budget. Their top models easily exceed $10,000 depending on firmware.
LinearX Systems - Developer of the LEAP loudspeaker modeling software (an early DOS version of which I've happily used), their pcRTA is a precision Type-1 true RMS 1/3 octave real time analyzer for PC. The use of an ISA full length PC slot card was the deal breaker; I just could not justify reliance on ISA, which was abandoned by the mainstream PC industry many generations of hardware in the past.
DPA - Mic division spun off from Brüel & Kjær in 1992, now a major quality microphone manufacturer with a huge product range. Their Type 4041-S Large Diaphragm Microphone with Solid State, 130V preamp uses a 1" diaphragm much like that used on the old Brüel & Kjær 2203 SLM, and it is rated for just 7 dBA self noise. It would have been the perfect choice. The ~$3,500 price tag was just too much of a reach, however.
THE FINAL CHOICES
The final choices were made on the basis of suitability, price and availability, with roughly that order of priorities. It's very possible that there are better options at similar or even lower prices, but there was time pressure to get it done.
ACO Pacific PS9200KIT Type 1 Measurement Microphone System
Most of the acoustics experts and engineers who were consulted agreed that a 1" diaphragm mic was probably the only way to make recordings and measurements at the super low target levels. Several people pointed towards the ACO Pacific measurement mic system. It is a kit of mic components that comes housed in a custom carrying case. A 1/2" capsule is the standard option, but the quieter ACO Pacific 7022 1" titanium diaphragm mic capsule was chosen instead. With the preamp, it has a self-noise of under 10 dBA. Low noise is probably the single most important parameter in a microphone when trying to measure and record extremely low sound levels.
The kit came with an individual frequency response plot for the specific mic capsule provided. Interestingly, this 1" mic capsule is 100% interchangeable with the 1" capsule on the old Brüel & Kjær 2203 SLM. The cost was around $2,000. All the available information led me to believe this was the best, yet cost-effective choice for SPCR needs. The complete list of components:
- PS9200 - 2 Channel 200V Power Supply
- 4012 - 1/2 Inch Preamplifier w/CA4012-5 Preamp Cable
- PS9 - 110VAC Power Adaptor
- WS1 - 3 inch Windscreen
- 7022 - 1" titanium diaphragm mic capsule
- 1/2" to 1" adapter for 7022
ACO Pacific PS9200KIT
1" diaphragm capsule with adapter on 1/2" preamp.
Frequency response on axis is within about ±0.5 dB from below 10 Hz to ~18 kHz.
The significance of the second trace is not known.
(Click on image to enlarge.)
Note that the microphone runs on a 200V power supply. The power is fed through a 5-conductor cable/connector, not the usual 3-conductor XLR used with 48V phantom power. This power scheme is apparently one of the keys to the ACO Pacific mic's low noise.
SpectraPLUS - Audio Spectrum Real Time Analyzer
SpectraPLUS is a highly capable spectrum analyzer with a huge array of functions which have been developed continuously for more than a decade. At the same time, it's reasonably simple to learn and use, as the 30-day trial allowed me to discover. There are many audio RTA software packages for use with a PC. Some require dedicated hardware; SpectraPLUS can be used with any PC with a good quality sound card. The M-Audio FireWire 410 and M-Audio Tampa
digital mic preamp used for the last few years are well suited for SpectraPLUS. The complete $1295 package of the base analyzer with all options was chosen to ensure we'd never outgrow its capabilities.
A few of the features most important for SPCR:
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