Such calibration is required in standards laboratories so that the microphones in sound-level meters and hearing aid testers, for example, can be set-up at the other end of the calibration chain.
Traditional ‘comparison calibrations’ (see below) involve exposing an unknown microphone and a laboratory-standard microphone to identical acoustic fields, and comparing the outputs. Sound source and microphone are connected through a sealed hollow hydrogen-filled coupler.
As it happens, the source of sound is another laboratory-standard microphone, driven as a loudspeaker – theses techniques only work with microphones than can be back-driven, such as the LS1P types used at NIST.
The new method employs a laser Doppler vibrometer and, according to NIST, is around 30% faster than its current comparison method because it is done in the open air, with no need for the coupler or hydrogen filling.
“People have been looking for a highly accurate calibration method that uses lasers, and they haven’t found an approach that is competitive with the most accurate existing method,” said NIST scientist Richard Allen (pictured). “But now we’ve found a comparison calibration that is better than the ones used in common practice.”
The proof-of-concept technique only drives the microphone as a loudspeaker and measures microphone diaphragm movement with a laser spot – a velocity signal is derived by mixing a sample of the source laser with the Doppler-modified return laser.
The clever bit is not the use of laser measurements at the Institute, but research using thousands of such measurements under different conditions to find a repeatable technique that agrees well with the current gold-standard – uncertainty was eventually ±0.05dB.
Measurements were made on nine LS1P (18.6mm diameter diaphragm) microphones at 250Hz and 1kHz, at hundreds of points on each diaphragm.
“Ultimately, they discovered that the best approach was to use data from a small section at the centre of the diaphragms, taking up only 3% of the total surface area. Further and further toward the edges, measurements were not very repeatable,” according to NIST. “The idea of using just the central section came from a recent paper by a team of researchers from the Republic of Korea and Japan.”
Compared with the usual NIST technique, “the numbers agreed very well,” NIST researcher Randall Wagner. “They were statistically indistinguishable from each other.”
The ±0.05dB of the laser-based comparison method, said NIST, compares with ±0.03dB for the gold-standard ‘reciprocity method’, and ±0.08dB for the traditional ‘reciprocity-based comparison’ method.
What are the existing methods?
The high-end ‘reciprocity method’ starts with three standards-grade microphones, connected in mix-and-match pairs using a hydrogen-filled coupler where they take turns at being loudspeaker and microphone. After all six combinations have been measured several times “researchers can be sure of the sensitivity of each of the three microphones without the need for a previously calibrated microphone,” said NIST.
‘Reciprocity-based comparison’ calibration uses one of the above three newly-calibrated microphones as the signal source at one end of the coupler and the unknown microphone at the other.
The laser Doppler method performs well enough to be used in industry, said NIST. “There’s nothing like this on the market now, not that I’m aware of,” said Wagner. “It would be far in the future – a pie-in-the-sky kind of thing – but I see this work as opening the door to commercial applications.”
He and Allen have applied for a provisional patent. In the coming months they will use a more sensitive laser vibrometer to examine a wider variety of microphone types at more frequencies, and will try to push uncertainty down far enough to compete with the primary calibration technique.
“This first attempt was sort of an example of walking past the trees, seeing the really low-hanging fruit, and grabbing it,” said Allen.
The work has been published as ‘Laser-based comparison calibration of laboratory standard microphones‘ in JASA Express Letters -the whole, thorough, paper can be read without payment.