Term-LAB was designed to measure extremely high SPL's. Because of this, the lower limit for stable SPL measurements is around 110 dB. (Normally, Term-LAB is configured to "mute" SPL readings of 120 dB or lower.) This article describes the technique that should be used to troubleshoot SPL readings of 120 dB or more when no sound is present.
1. Is your Term-LAB powered by the USB port or does it use a wall transformer? If you are using an older system that uses a wall transformer please make sure you are using the transformer that came with the system. If you are using a voltage converter, that could be the problem. Also, make sure you do not have a car battery charger (or any other "noisy", high-current device) plugged in to the same receptacle as the wall transformer.
2. If this problem has recently started happening, I would suspect that one (or both) of the sensor cables are damaged.
3. Label both sensors (A & B). Label both sensor cables (A & B).
4. Disconnect both sensor cables from the interface box and test. Do you still see readings that are higher than 120 dB? If not, then the computer, software, interface box, and power supply (if this is an older unit) are probably okay.
5. Attach sensor A to cable A and plug the cable into the Channel A input jack on the interface box. Test the system. Do you still see a reading in excess of 120 dB on the Channel A readout? If so, there is probably a problem with the cable or the sensor. Let's keep testing.
5. Unplug the sensor A cable (with Sensor A still attached) from the Channel A jack and plug it in to Channel B. Test the system. Do you still see a reading in excess of 120 dB on the Channel B readout? If so, the chances are even greater that there is a problem with the sensor cable or the sensor.
6. Plug the sensor A cable back into Channel A. This time, swap out the sensors (Replace Sensor A with Sensor B.) Test the system. Does the problem remain? If not, then the problem is probably with Sensor A. If the problem is still there, then the problem is probably with the sensor A cable.
7. Swap out the sensor cable. (Continue using Sensor B.) Does the problem go away? If so, the problem is with the sensor A cable.
8. Swap out the sensors. (Attach Sensor A to Cable B.) Did the problem go away? If so, the problem is with Sensor B.
9. If you get here then you will need to try using a new sensor cable. (Cat 5E network cable.) Test the new cable. If the problem goes away then both sensor cables are bad. Test both sensors with the new cable to make sure the sensors are good.
10. If a new cable does not fix the problem then both sensors must be damaged.