Josephson Microphones
Tech Note 8



Using Josephson Measurement Microphones with SYSid

The Bell Laboratories SYSid software package, available from Ariel Corporation for use with their
DSP16+ data acquisition and processing hardware, has several calibration features that allow
measurements to be displayed in terms of sound pressure level. 

Users should remember that decibels are relative units, therefore 0 dB cannot be truly a zero; in
the case of sound pressure level, 0 dB is equal to 0.0002 microbar, or 20 microPascals. Sound
pressure levels are often given as if they were absolute measurements, but in fact they are always
in decibels above or below an rms air pressure variation of 20 æPa. Negative SPL readings are
possible.

In the SYSid Screen setup menu, the D key toggles between display in volts (default) or dB. When
switching to dB, the user will be asked for a reference voltage. This voltage will become the zero
reference for the dB display. Note, the voltages as reported by SYSid are in
peak volts, while most audio measurements such as microphone calibration reference rms volts. This is another correction that must be entered into the equation.


Microphones are typically calibrated in voltage for 1 Pascal SPL, which is 94 dB SPL. The C550L,
typically used with SYSid, has a nominal output of 60 millivolts (rms) per Pascal. First, convert this to peak volts by multiplying the rms sensitivity by 1.414. Therefore the mic sensitivity is (60*1.414) or 85 mV/Pa (peak), in other words the mic produces 85 mV peak for a sound input of 1 Pa or 94 dB SPL. 94 dB must be
subtracted from this value to derive the 0 dB SPL reference level. 94 dB loss is equal to a
multiplier of 0.00002. 85 mV times 0.00002 equals 0.0000017 volts, which can be entered into
SYSid as 1.7e-6. Be sure to include the gain of any external preamplifier, if one is used.

To derive the SYSid reference voltage, we can multiply the sensitivity number given on the calibration
curve, in volts, by the two constants above, 0.00002 and 1.414 or 0.0000283. C550 microphones typically have the following outputs:

      Type             Sensitivity      SYSid Calibration

      C550 (old)             8 mV/Pa          2.24E-7
      C550A, D, H            10 mV/Pa         2.83E-7
      C550F		     0.3 mV/Pa        4.24E-8
      C550L                  50 mV/Pa         1.7E-6

Summary: to get the SYSid Reference Voltage for your microphone, multiply its 
rms sensitivity in volts by 0.0000283.

For more information on SYSid, contact SYSid Labs, 39 Menlo Place, Berkeley,
California 94702, phone (510) 528-1277, fax (510) 526-8087. You can also 
contact Ariel at ariel@ariel.com.