From memory, in the UK the frequency offset is 7.5 kHz. So, if three transmitters are required to give the required geographical coverage, one will be on the nominal frequency, one will be 7.5 kHz above nominal and the other 7.5 kHz below nominal. When the transmitters are keyed an audio howl (hetrodyne) will be produced, caused by simultaneous transmission on different frequencies. This is often heard when two aircraft transmit simultaneously on the same frequency. However, with frequency offset transmitters the howl is not heard by the aircraft as the frequencies produced (in this case 7.5 kHz and 15 kHz) are above the upper limit of the receiver's audio passband (normally about 2.7 kHz) and are thus filtered out.