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Generating a Probe Tube Correction Curve
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Probe Tube Correction Curve

Ravi Kochhar
Dept. of Neuroscience
University of Wisconsin - Madison
Feb. 1983
rev. 1.004, Oct. 22, 1996
Programming Note no. 13

Probe Tube Correction Curve

When NEUCAL is used to calibrate an acoustic system, the values (in millivolts) obtained with the A/D system must be converted to Sound Pressure Level (SPL) in dB re 0.0002 dyne/cm .

A reference, or correction, curve for the probe-tube microphone combination must be available to make the conversion and correct for the presence of the probe tube, filter, amplifier etc. In log units the correction and conversion to SPL is a simple matter of subtracting the correction curve for the probetube/microphone combination from the calibration curve obtained in mvolts.

Note that the correction curve is for other things in the calibration system besides the phone itself. Specifically, the probe tube, the 1/2" B&K microphone, the filters/amplifiers etc. are all corrected by this one correction curve. Thus if any of these are modified or replaced, the correction curve must be regenerated. Typically the amplifiers and the 1/2" B&K microphone are not changed, but there may be more than one probe tube in the lab. It is customary to have as many correction curves as there are probe tubes, and the correction curve is often referred to as the probe tube correction curve.

The correction curve corrects for both the amplitude and phase, but note that an absolute phase calibration is not possible. The correction curves for all probe tubes are stored in the Calibration Storage File (CSF) where they are available at all times during any experiment.

A procedure for obtaining the correction curve with the program NEUCAL is described below.

(1) Determine the primary standard, X dB/mV, using the piston phone. Figure 1 shows the block diagram of the equipment. (Click on the figure to enlarge)

Correct the output of the piston phone for current barometric conditions. For example, on Jan 31, 1978,

        the correction = -0.17 dB

The piston phone generates 123.9 dB SPL at 250 Hz for STP.

        123.9-0.17 = 123.73 dB SPL

The output of the 1/8" B&K microphone is next determined with a voltmeter or wave analyzer in rms volts.

For example, on Jan 31, 1978, the output of the 1/8" B&K was 3.04 volts, or 9.7 dB re 1 volt rms.

Therefore :

        1 Vrms = 123.73-9.7 = 114 dB SPL 
     or 1 mvrms = 114-60 = 54 DB SPL 
     or 1 mvrms at the B&K pre-amp = 94 dB SPL
         (after correcting for the gain of the amplifiers).

The following precautions should be observed during the above step :

(a) The signal should also be observed on an oscilloscope.

(b) Check the batteries of the 1/2" piston phone.

(c) Make sure there is a proper seal between the piston phone and the 1/8" microphone (but don't overtighten).

(d) All 1/8" microphones don't give the same output when connected to the 1/2". Expect about a 50% variation.

(2) Determine the response of the B&K 1/8" microphone.

The equipment setup for this step is shown in Figure 2. The 1/8" is placed where the animal would normally be. (Click on figure to enlarge).

(a) Use the program SETUP to create a data file of size 300 blocks. Give the file any name, for example, EDF.DAT.

(b) The filter is set to the "OUT" position in this step. The PAR amplifier provides a gain of 1000, or 60 dB.

(c) Run the program NEUCAL and calibrate from 60 Hz to 40000 Hz in steps of 20 Hz. Use the data set ID (for example) BK18.

Do not "Convert to SPL". Plotting or printing the data at this step is optional and does not affect any of the subsequent steps.

(3) Run the calibration editor as follows :

        $ ECAL

The program will present various options. Select the option "Convert Voltage to SPL" and the answer the queries regarding data file name (which is EDF.DAT in our example) and Data Set ID (which is BK18 in our example). The program will then ask for the conversion factor which was obtained in step (1) above.

The conversion factor is computed as follows in our example

Since 1 mV = 94 dB and a gain of 60 dB preceded the A/D, the conversion factor is 94-60 = 34 dB.

The program ECAL will convert the amplitudes to dB SPL and store the results back in the data file under the same ID (BK18 in our example).

(4) Obtain the response of the probe tube and 1/2" B&K microphone.

The equipment setup is shown in Figure 3. Note that in this step the amplifier and filter settings are exactly as will be used later during a regular calibration. (Click on figure to enlarge).

(a) The data file which was created in step (2) above should be used again during this step (i.e. EDF.DAT in our example).

(b) Set the 100 Hz high pass filter to the "IN" position to get rid of the low frequency room vibration noise which occasionally causes the PAR to saturate. The filter characteristic will be incorporated into the correction curve.

(c) Run the program NEUCAL and calibrate in the usual manner. Specify a frequency range of 60 Hz to 40000 Hz in steps of 20 Hz. Use the data set ID (for example) BK12.

(d) Do not "Convert to SPL". Plotting and printing the data at this step is optional and will not affect subsequent steps.

(5) Run the calibration editor ECAL, and select the option to subtract one calibration from another. Use this option to subtract the response of the 1/8" microphone (BK18 in our example) from the 1/2" microphone response (BK12 in our example). Store the difference of the two responses back into the data file (EDF.DAT in our example) and assign it a new data set ID (for example, PROBEL).

At this point the desired correction curve for the "probe tube" is located in the data file EDF.DAT with the data set ID PROBEL. It can be graphed using the display option in NEUCAL. The correction curve obtained in our test case is plotted in Figure 4 (amplitude only). (Click on figure to enlarge).

(6) Run the editor program ECAL again and transfer the data set PROBEL from the EDF to the CSF. At this stage the probe tube correction curve exists in the CSF (CALTAB.DAT) with the ID (for example) of PROBEL, and is available for use during subsequent calibrations. Delete the data file EDF.DAT.

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  1. "Calibration Storage File and Calibration Editor", R. Kochhar, Conversion Report # 12, Feb. 1983.
  2. "NEUCAL - Acoustic Calibration Program", R.Kochhar, Tech.Report # 4, Dept. of Neurophysiology.

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Please send feedback/suggestions/questions/complaints to the author via email, at

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This work was done at the University of Wisconsin - Madison under the direction of Dr. W.S. Rhode. Supported in part by a grant from NIH.

(This page last modified on May 21, 2015)

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