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OT: Calibrating a sound level pressure meter

Discussion in 'Electronic Repair' started by N_Cook, Sep 27, 2011.

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  1. N_Cook

    N_Cook Guest

    Just for my own purposes so relative rather than absolute but if possible to
    calibrate then all the better. No access to a calibrated one to
    cross-calibrate, falling ball calibrator etc.
    An old Dawe 1405D that sems to have little info out there. Mic and
    electronics works and I've now repaired the suspension on the meter but fsd
    is now .586V but is marked as 1.45V .
    As the replacement phosphor bronze is probably more stiff than the original
    , if any thing , I would expect greater fsd . It may be .45V marked and a
    type number 1 or something ahead of the .45, there is quite a gap.
    Anyone experinece of the ipod app, toy? or worth borrowing someone's ipod
    and obtaining the app if any absolute use for cross-calibration
  2. N_Cook

    N_Cook Guest

    Other than the B battery test mark on the scale for running off the 1960s
    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
  3. N_Cook

    N_Cook Guest

    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
    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
  4. N_Cook

    N_Cook Guest

    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
  5. N_Cook

    N_Cook Guest

    With nominal batery voltage [22.5V] applied from a bench supply, where
    does the meter pointer fall relative to the 'battery' markings on the
    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
  6. N_Cook

    N_Cook Guest

    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
  7. josephkk

    josephkk Guest

    I am not all that comfortable with your declaration of NO standard covers
    300 to 3000 Hz as that range is very commonly used in radio and telephony

  8. N_Cook

    N_Cook Guest

    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.
    2 db ranges 60 and 70dB are stuck together . But for good posistions
    switching between ranges for differing 1KHz sine signal in, consistent "0"
    dB and +6.7 dB switching betweeen adjascent ranges. Don't know if that 6.7
    instead of 10 is due to sine rather than noise source, will have to try
    again with an attenuateable noise signal
  9. josephkk

    josephkk Guest

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

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