Bat detectors are normally adjusted by the manufacturer to show the correct frequency on their dial. This is called calibration. However, except for very expensive models the detector can go out of calibration through changes in temperature, ageing, or the rough handling that they receive when used over rough ground in the dark.
What this means is that the frequency shown on the dial isn't a precise indication of the actual frequency you are picking up. It's very important to be able to rely on the calibration of your detector, so it should be checked now and again. You'd feel a right idiot reporting a Nathusius pip just because your detector was out of calibration!
Calibration involves checking your detector against at least two reliable standards. For example suppose you have a thermometer with no scale markings.
The scale is then divided up evenly between the two points to give the reading. The more standard points you can check, the more accurate will be the calibration.
What you need is an ultrasonic sound at a precisely known frequency.
Calibration standards aren't easy to find, but fortunately most of us already
have equipment that can do the job in our own homes! Turn on the telly. Turn the sound right down, so you can hear the detector.
The television uses a "line scan" frequency of 15.625 kHz. It generates
ultrasound signals at 15.625 kHz and also at 31kHz, 47, 62.5, 78 and 94 kHz.
Slowly increase the dial frequency from the minimum setting. You will hear a high pitched tone, decreasing to nothing, then increasing again. Tune your detector for the lowest tone. This is called a "null". Note where these nulls fall on the dial of your detector. Another way is to use broadcast radio signals. You may be lucky enough to have a detector that will pick up the time signals from the RUGBY transmitter at EXACTLY 60kHz. When you have marked where all of these points fall you will have a much better idea of the true frequencies that the dial on your detector shows.
The calibration will only be accurate for the temperature at which it was carried out - so if you are using your bat detector outside at night at a temperature of 10 degrees try and calibrate it when it is still cold.
Here's a great utility! USonic, a freeware MS-DOS based utility for generating ultrasonic tones using the PC's built-in speaker. Written by Joost Ruijsch (Thanks for spotting this Bertrik!)