OT: Calibrating a sound level pressure meter

Other than the B battery test mark on the scale for running off the 1960s B122 type 22.5V battery . But of course no standard for what the B equates to for light load and less than 22.5V , seems to work adequately on 15V , I intend using 2xPP3 , 18V

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With nominal batery voltage [22.5V] applied from a bench supply, where does the meter pointer fall relative to the 'battery' markings on the scale? If the pointer is in the 'good' area just above the minimum battery mark then the meter deflection is in the right ball park, not as far out as your first post would suggest. My B&K came with a calibrator source that fits over the mic and emits a tone of about 1kHz to tweak the calibration if needed.

Neil S.

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Do you own a loudspeaker?

Most speakers are rated for so many dBSPL with one watt input, at one meter. You will need as anechoic a room as possible, or just set it up outside at night (or whenever the ambient noise is minimum). I would use a pink noise source (constant energy per octave) -- .wav files seem to be on the web.

That's not even wrong.

Isaac

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Do you own a loudspeaker?

Most speakers are rated for so many dBSPL with one watt input, at one meter. You will need as anechoic a room as possible, or just set it up outside at night (or whenever the ambient noise is minimum). I would use a pink noise source (constant energy per octave) -- .wav files seem to be on the web.

I can see that being as accurate as a calibrator that hasn't been calibrated for 20 years say or manufacturing variabilities of ipod microphones. Assuming the chosen speaker magnet does not decrease in power too much over time. I assume that is for speaker driver on its own, in free space as, I assume the sound from the rear of the cone ,in theory ,will never reach the SPL meter.

Mulling it over I assume you mean using a complete production speaker cabinet with bass+mid+tweeter and cross-overs etc, of known make and model and spec

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I meant an array in an enclosure, sorry.

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With nominal batery voltage [22.5V] applied from a bench supply, where does the meter pointer fall relative to the 'battery' markings on the scale? If the pointer is in the 'good' area just above the minimum battery mark then the meter deflection is in the right ball park, not as far out as your first post would suggest. My B&K came with a calibrator source that fits over the mic and emits a tone of about 1kHz to tweak the calibration if needed.

Neil S.

I'll go with your suggestion and leave as I found ignoring the new suspension ribbon and a replaced intermittant B-E transistor Looks as though Dawe bought in these Sifam ribbon suspension meters , the original label saying 1.45V and mention of an external pcb. I suspect Dawe removed that pcb and the movement itself is 100mV. Battery test is simple chain of resistors , protect diode and rest switched out. So if 100mV fsd then for a new B122 battery 100 percent fsd would correxpond to 24.5V and the B marker correspond to 15.5V which seems reasonable. As the mic is capacitive , about 300 to 3KHz 3dB bandwidth , not electret, not worth going to any effort calibrating this SPL. B mark is 64 degrees of 90 degree arc of meter swing, 71 percent fsd

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To measure SPL, A-weighted, it would have to cover 200 to 20000 Hz. No acoustical standard covers 300 to 3000 Hz.

To measure SPL, A-weighted, it would have to cover 200 to 20000 Hz. No acoustical standard covers 300 to 3000 Hz.

It is very basic one probably intended for schools use, no A or C wightings just 40 to 120 dB att sw and slow/fast response sw (470uF across the meter) and battery sw, but there is a calibration pot externally accessible. I will use the calibrator preset via ps variation and the B mark to reset the meter to deemed 15.5V and 24.5V B and 100 percent of the meter

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I am not all that comfortable with your declaration of NO standard covers

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If the only sound sources were telephone receivers, then a sound level meter that covered only the voice channel range (originally limited by the frequency response of the carbon granule "transmitter") would have some value. But real sound sources cover more of the 20-20000 Hz audio range.

If I get back to this meter. I cannot make sense of the B indicator , the resistor chain suggests 100mV fsd but reassembled and largely functioning meter fsd is more of order 1V.

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Whoosh much? No other cases ever occurred, and were measured?

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