An unfortunate listener a few feet in front of a police siren hears a sound of about 120 decibels. Allowing 20 to 30 decibels for directivity leaves 90 to 100 decibels of sound behind the siren.
A motorcycle officer listening to an earpiece would raise the volume of their receiver to match the level of the siren. However, close talking their microphone with a loud voice at 70 to 80 decibels would be no match for the siren that is a hundred times more powerful.
Further problems arise because regulations of the Federal Communications Commission specify background noise, such as a siren, must be more than 20 decibels below the transmitted voice.
In other words, physical functionality of the police radio requires reduction of the siren signal by 20 decibels. Legal operation demands attenuation of the undesired signal by 40 decibels.
Engineers solve this problem by using notch filters to sharply reduce the frequencies of the siren. These filters attenuate siren frequencies by 40 decibels without affecting the lower vocal frequencies. This is how dispatchers hear motorcycle officers over the sounds of sirens.
The Channel-II recording contains further evidence of the notch filters. Chief Curry keyed on the channel and paused before shouting into his microphone. The dispatcher heard the volume of the background sirens decrease as the excessively loud voice activated the AGC in the transmitter.
Notch filters have significant consequences for evaluation of the acoustic evidence. When comparing the loudness of sirens with speech or engine sounds one should account for about 40 decibels of attenuation by the filters. This consideration places the sirens much nearer the open microphone and explains their earlier absence and premature silence.
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