Calibrate White Noise Startle Probes for ER-4
Calibrating the ER-4S Ear Insert Earphones
The ER-4S has a known voltage input to sound intensity output function. This function can be used to calibrate the intensity of the startle probe, etc. This function is:
Intensity (dB) = 108 + 20 log10(voltage [in V])
Voltage should be measured in AC V using RMS (see note below on modifications present in Curtin Lab)
To measure this voltage you will need to build an adapter that will allow you to insert volt meter test point probes (black and red probes attached to meter) into a wire connected between your sound card and the ER-4S earphones. It is important that the earphones are connected when making this measurement b/c their presence in the circuit will affect the voltage measured. The easiest option is to splice two wires into a cable with male and female mini (1/8 inch) phono plugs. Splice one wire onto the ground/shield and the second to either of the audio channels (left and right ear should contain the same signal). Plug the male connector into the sound card, plug the earphones into the female end and alligator clip the meter probes onto the two wires you spliced in.
You should produce the sound with the software and wav file you intend to use for sound presentation b/c they both will affect the intensity of the sound. I have created two calibration scripts for DMDX:
The script uses a short (50ms) and long (32 s) wav file for white noise. These must be in the same folder as the script. The long file was constructed with the same parameters as the short file and presents at the same intensity. If you calibrate with the long file, you should obviously use this short wav file in your experiments.
Here is a quick reference table for typical values to convert from observed voltage to dB. Intermediate values can be determined using the formula provided above.
|Voltage Input (V)||Sound Intensity Output (dB)|
*IMPORTANT NOTE FOR CURTIN LAB. In our lab, our voltmeter is not RMS. 102DB should measure .5V RMS but on our meter it measures .35V. Calibrate to .35V using the 2V range setting