Schematic has been updated - see bottom of page
Since the sound quality of my camera is poor I made some wave-files too:
On the Police Siren I made an extra buffered output that could be used for a future Rumbler/Howler Add-on. This varying voltage is added to the VCO on this new board and that is basically it. The results are pretty good.
Referring to schematic diagram... U1 is the VCO which creates the low frequencies. I have just copied the VCO section in the siren and changed the capacitor C7 to get a lower frequency. By adjusting the VCO (on R8) two octaves below the police siren you get the Rumbler/Howler results. The Coast-down cutoff comparator has also been added so that it "follows" the police siren.
The Rumbler from Federal Signal and the Howler from Whelen both have timers build in. I guess the low frequency siren is not meant to operate all the time. Both sirens can be set to be active from 7.5 sec to 60 sec. I may have to change mine since it is only active for 5.5 sec. So U2b is set as a monostable with a pulse length of 5.5 sec. In other words by pushing S1 the siren is active for 5.5 sec. By increasing C10 to 22uF you will get around 11-13 sec.
Adjusting the VCO was the same as the Police Siren. By applying a constant 8VDC to the VCO input (U1 pin 9) adjust the trimmer to get the desired max frequency. In this case it was two octaves below the Police Sirens 1510Hz...so 377Hz.
Many people have asked me for a better (longer) timer. So thanks to The Art of Electronics I made the new op-amp timer which can be set for a maximum of 30 sec.