The Siren Board

Discussion of Outdoor Warning Systems
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PostPosted: Fri Mar 27, 2020 4:28 am 
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I've synthesized some tones for DIY electronic sirens, which are as simple as lashing up some horn speakers to an amplifier and a computer's sound card. If you're in an area where you can make some noise, like out in the country, you can blast these out with impunity, but if you're in a city you need to be more circumspect to not cause a panic.

The limitation with many electronic sirens is that the common horn speaker drivers are not robust enough to produce low frequency tones through smaller horns of practical size. In some other posts, I've covered the type of compression drivers that are valuable for "playing with" using siren tones, and they are the drivers commonly used for vehicle sirens or voice-range outdoor pubic address systems. (Typically they have a phenolic diaphragm instead of a metallic one, because phenolic can stand up to more punishment without fracturing) and they have a more compliant suspension that helps them move more freely at lower frequencies, but the larger internal clearances also reduce their ability to produce high frequencies, so their bandwidth is normally between 400 Hz and 8kHz.

Modern electromechanical sirens typically peak around 400, 600, or 800 Hz, or in that general frequency range, and most electronic sirens sound higher pitched tones to reduce the amount of stress placed on the speaker drivers. So the challenge is producing tones that are able to produce a carrying siren sound, without operating at a low frequency that will stress siren drivers. For this purpose, I think dual-tone signals are ideal, because the two tones produce a "difference tone" that is lower in pitch and can carry farther, without requiring the speaker to be driven at that fundamental pitch, and this also produces the nice "buzzing" quality that is so distinctive, and which modern single-tone sirens lack, thus also lacking in sonic punch. It is fine to drive a speaker with two frequencies, because the force input from the speaker's voice coil will just be the superposition of those two tones, and this will actually decrease the RMS power of the signal relative to the peak power, which can be used to either reduce the thermal output from the voice coil, or drive the siren driver harder until the RMS power of the signal reaches the RMS power of the driver. It's all very technical.

These tones consist of sine waves which are slightly clipped by about 3dB, producing square waves with some more harmonics, but not yet enough clipping to produce a harsh quality. This will help decrease the crest factor somewhat (the ratio of peak power to RMS power) by feeding more RMS power to the siren driver. They are similar to the sound of a 5/6 port Thunderbolt siren on chopper level 6, with the resulting frequencies increased by one octave, so the interval is 1000 Hz/1200 Hz, and the windup begins one octave below that, at 500/600 Hz. I've also thrown in a couple of warble and chirp tones, including an LRAD-style acoustic denial tone (meant to sound harsh and unpleasant in the frequency range where human hearing is most sensitive), and warble and chirp tones that waver between 1000 Hz and 1200 Hz. You can hit conventional siren drivers pretty hard with these tones and not be concerned about failure as long as the RMS power of the speaker is not exceeded.

The durations of these tones are nominal values, including the steady-state portion of the tone and not the wind-up or wind-down; together the wind-up and wind-down add about 13 seconds to the length of each file, and some of the wail or whoop signal timing did not work out to exactly a minute or exactly 3 minutes in length, and so forth. The tones tend to be a little longer than the nominal length values. ... il_60s.mp3 - Short wail with no steady tone portion ... ck_60s.mp3 - Wail with 5 second steady, 5 second wind down ... il_60s.mp3 - Wail with long 10 second steady period, 5 second wind down ... op_60s.mp3 - Whoop with 5 second steady tone portion ... op_60s.mp3 - Whoop with no steady tone portion ... est_3s.mp3 - Growl test (single wind up only) ... le_60s.mp3 - Undulating pitch between 1000 Hz and 1200 Hz ... ps_60s.mp3 - Fast sweeping pitch between 1000 Hz and 1200 Hz ... rp_60s.mp3 - Rapid sweeps from 3kHz to 6kHz, LRAD-style tone ... dy_60s.mp3 - 1 minute steady tone ... ow_60s.mp3 - Hi-Low between 1200 Hz and 1000 Hz ... se_60s.mp3 - Pulsed steady dual tone ... dy_10s.mp3 - Short burst before a voice message, perhaps. ... t_180s.mp3 - 3 minute Alert signal ... k_180s.mp3 - 3 minute Attack signal - Zip file of all tones in WAV format (big file)

Please let me know your thoughts.

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