Speaker Calibration

Speaker calibration is for the purposes of speaker frequency roll-off documentation and compensation. It also sets the reference intensity to determine the necessary signal amplitude to correctly deliver stimuli in dB SPL.

  1. First set up the hardware.

    • From the AO channel you are using on the ADC/DAC BNC router, route the output signal to an amplifier, then to the attenuator, and then to the speaker.

    • Place the microphone exactly 10cm away from the speaker. The microphone is highly directional, it is important that it is directly in front of, and facing the middle of the speaker, with the shield off. Connect the microphone preamplifier (the wand that the little microphone is screwed on to), to the measuring amplifier or conditioning amplifier (which ever is present). Then route the signal through the Bessel Filter, and then to an AI channel on the ADC/DAC BNC router.

    • For the measuring amplifier: Make sure the Input Section Gain and Output Section Gain are set to 0.

    • For the conditioning amplifier:

      • Transducer supply -> 200V polarized
      • Transducer set-up -> 4.07mV per channel (actually should be microphone calibrated sensitivity)

  2. Navigate to the calibration tab.

  3. For saving a calibration, most of the input values are fixed, so all you can change is the duration and number of repetitions. The duration of the stimulus is fixed to the window size, so you change the recording window to change the stimulus duration.

  4. The microphone is calibrated first. Use the microphone calibrator thing that plays the tone (1kHz at 95dB SPL). Place the end of the microphone into the equipment. Press the calibrate button on the line that says stuff about microphone calibration. When it has finished recording, remove the device, and replace the microphone at it’s proper place w.r.t the speaker.

  5. Press the start button, the plot display will update with the data from the ouput and recorded signals. A tone will be played first to attain the reference intensity, then followed by an upsweep to get the speaker transfer function. After recording, the program will take several seconds to calculate the filter it will be using. Finally a frequency roll-off plot of the speaker will appear. Note that the microphone is HIGHLY DIRECTIONAL, the exact angle it is placed to the speaker can make a significant difference in the calibration of the speaker.

  6. Get a look at the efficacy of the calibration, by choosing the Test Calibration button and selecting one of the options from the drop down menu that is enabled. The closer to a flat spectrum recorded, the better. This step will not save anything.

  • When you run a calibration it will be automatically set as the active calibration for the program.
  • The program will automatically save the calibration to you data file under calibration_# where number is an increasing counter for each time a calibration is run.
  • You can switch the calibration, or set the calibration to none under Options/Calibration Parameters...
  • To change the effective frequency range of a calibration, also see the Options/Calibration Parameters... menu.
  • If you wish to change the calibration frequency or voltage, you will need to edit the Advanced Settings Dialog.

When a calibration is set, it will alter all outgoing (analog output) signals with a filter to compensate for the speaker roll-off. The exception to this is if the only component in the stimulus is a square wave; it is then assumed that output is not going to a speaker.

Calibration Tests

You can evaluate the calibration that was run by selecting the Test Calibration option at the top of the tab. Currently 3 different stimli are available:

  • White noise, a stimulus with equal intensity all frequencies

  • FM sweep (aka chirp) a stimulus with linear variation in frequency

  • Tone curve, which loops though a series of tones to record their intensity.

    • The input fields are the same as for a Tuning Curve
    • A live plot of the recorded dB SPL values is generated in place of where the PSTH plot otherwise resides.