Instructions for Calibrating Term-LAB Classic Microphones

for dB Drag Racing Multi-Point Events

 

This article is obsolete as it no longer applies to products we currently sell. It is included here for archival purposes only.


This document pertains to the original, Term-LAB Classic system which uses a soundcard and microphones. The newer, sensor-based Term-LAB systems do NOT require calibrating.

 

System Preparation

 

  1. Verify that the Term-LAB Classic hardware is connected properly.
  2. Verify that all of the LED lights on the interface box are illuminated.

 

A/D Offset Error Compensation Procedure

 

  1. Unplug both XLR microphone cables from the interface box.
  2. Run Term-LAB.
  3. Select the Hardware Panel in the software.
  4. Find the Offset button in the Soundcard panel. Press this button to begin the offset calibration process. Click on Next and then Start. After the calibration is complete, press the Save button.

 

Verify the Existing Configuration Files

 

  1. Find the Edit button immediately to the right of the Line-In button in the Soundcard panel. Press this button. Verify that all of the numbers in the displayed window are set to 0. If they are not 0, set them to 0 at this time by pressing the Reset Button. When all of the fields are 0, click on Save.
  2. Repeat this procedure for the Line-In, Mic-In, and HV-In Edit buttons in the Interface Box panel.
  3. Click on the Edit button directly to the right of the Mic L serial number. Verify that all of the numbers (with the exception of K-Factor, Min-SPL, and Max-SPL) are set to 0. Note the K-factor as you will be editing this number during the mic calibration process.
  4. Repeat this procedure for Mic-R.

 

Attaching the Microphones

 

  1. Plug the XLR microphone cables back into the interface box.
  2. Verify that the microphone attached to the Channel A input on the interface box corresponds to the serial number that is selected in the Mic-L selection in the software. (The Interface Box panel under the hardware tab in Term-LAB.) This is your Lane 1 microphone.
  3. Repeat this procedure for the other microphone. This is your Lane 2 microphone.

 

Calibration

 

  1. Carefully insert the Lane 1 microphone (plugged in to the Chan A input on the interface box whose serial number corresponds to the Mic-L selection on the hardware panel) into the microphone calibrator. You may need to rotate the mic gently in a CLOCKWISE direction to facilitate insertion. Verify the microphone is inserted fully into the opening.
  2. Click on the SPL Tab in Term-LAB. Next, click on the SPL button on the toolbar at the bottom of the screen.
  3. While firmly pressing the microphone into the calibrator, press and hold the rocker switch on the calibrator. DO NOT LET THE CALIBRATOR RUN FOR MORE THAN 10 SECONDS! A stable SPL should be displayed in the LEFT SPL readout on the screen. Note this SPL value. It should be exactly what is written as the output level of the calibrator. (Typically, the calibrators put out 163.8 – 164.1 dB.)
  4. If the SPL displayed on the screen is not exactly the same as the output level of the calibrator, you will need to adjust the K-factor for this microphone. If the displayed reading is HIGHER than the output level of the calibrator, you will need to SUBTRACT the difference from the microphone’s K-factor. If the displayed reading is LOWER, you will need to ADD the difference to the microphone’s K-factor. (For example: If the calibrator is producing 163.8 dB, and the readout on the screen is 164.0 dB, you will need to SUBTRACT 0.2 from the microphone’s K-factor.)
  5. To edit the microphone’s K-factor, click on the Hardware tab. Next, click on the Edit button directly to the right of the microphone’s serial number. Note the current K-factor setting. Next, add or subtract the error from this K-factor and enter the NEW K-factor into the K-factor text box. When finished, click on Save.
  6. Re-verify the calibration of the microphone. Click on the SPL tab and then repeat steps 1-5 above until the SPL in the readout on the screen exactly matches the output level of the calibrator.
  7. Repeat this procedure for the other microphone.

 

When to Calibrate?

 

You should always calibrate the microphones before the start of qualifying and then verify the calibration every 2 hours or so. You should also check calibration if a microphone gets dropped or banged up against something. It is quite normal for the meter to vary a few tenths of a dB as the contest progresses. Environmental conditions, such as temperature, humidity, barometric pressure, and elevation will all affect the readings to a certain degree. It has been our experience that the biggest changes occur when there is a significant temperature change, or when a weather system blows in.

 

During the elimination rounds, it is best to verify the calibration of the equipment between each class of eliminations.

 

Important Notes

 

The shield of the XLR cable, the microphone housing, and the faceplates on the Term-LAB interface box are all attached to a “virtual” ground that is resting at approximately 6 volts DC. Therefore, it is important that you never touch these objects to another “grounded” object. For example, the sheet-metal in the calibrator and the aluminum “trim” on the calibrator’s case are connected to a hard ground (the 3rd pin on the electrical plug). It is attractive to lay one mic in the storage compartment of the mic calibrator housing while you verify the calibration of the other mic. YOU CAN NOT DO THIS! When you lay one mic against the grounded metal in the calibrator, it SHORTS OUT the virtual ground in the interface box and all of your readings will be erroneous.

 

You should also ensure that the interface box is not located adjacent to any source of strong EMF emissions (such as a computer monitor or power supply or audio amplifier). Keep the interface box away from other electronics if possible.

 

You should ensure that “noisy” electrical components (such as battery chargers) are not sharing the same 110 VAC electrical outlet as Term-LAB.