I rescued a bunch of (unlabeled) electronic sirens. I'm using them in the house, primarily *as* "sirens". E.g., to notify occupants -- who may be asleep or otherwise distracted/engaged -- of "critical events" (like "fire", "burglary", etc.).
But, as those events are (hopefully) extremely low frequency, they represent underutilized resources. In addition to being "wasteful", it also means I have to take explicit steps to verify their continued functionality (wouldn't want to discover that they didn't work when a true need arose!)
So, I'm looking at other uses/applications that would let them add additional value as well as give me some routine/periodic confirmation of their continued functionality.
Each is basically a compression driver loaded with a horn and driven by a dedicated amplifier/oscillator. I.e., apply power and it screams (a warble tone).
I'd NOT always want to have to operate them at that fixed SPL as they make smoke detectors sound weak by comparison!
So, drive them through an adjustable gain amplifier using the same bits of electronics that I use for my "network speakers".
But, they aren't wide range speakers and would easily be damaged if driven with low frequencies at high power levels. Fine. Just make sure never to synthesize any sounds that have LF components!
This is A Good Thing because they want to be *annunciators* not "speakers". They want to attract attention instead of just being generic "sound sources". E.g., alert the last occupant to exit the house that the stovetop is still on! Or, that the back door is open... ("Hey! Don't leave, yet!") How those notifications are conveyed (intensity, duration and content) obviously varies as a function of the notification and the environment.
So, I need to come up with some constraints on the signal that I present to the compression driver in the siren (after gutting the electronics). Without any published specs to fall back on (and with no desire to toast the devices during empirical testing!
I figure the safest set of criteria are to limit power to not exceed the level that the amplifier/oscillator currently delivers *and* limit the range of frequencies to those spanned by the "warble" (this saves me the effort of mechanically characterizing the horn and the loading it places on the driver).
Any other "safe" schemes that would let me see how much I could extend, for example, the range of frequencies applied? And, how much lower a drive level might be tolerated as that range is extended?
Like the piezo screamer "black box" we used to get at Radio Shack?
Is the structure resonant at any component of warble? Can you mechanically modulate the resonance-determining part of the structure? Use a relay, LVDT, servo...
I presume some electronics in the driver generates the warble. Can you switch one or more components of that say with quad CMOS switches? What can get switched, center frequency/frequency span of warble/warble rate?
Going from "warble" to slower "wee-waa" could indicate diminishing urgency.
Does voltage or current input trigger the driver? What's its impedance in the audio band? Can you feed it pulse trains to get only the first part of the warble? Does the wider-pulses-make-the-LED-brighter trick work to make it seem louder?
Applied current modulo impedance -> rate of change of excursion -> damage threshold?
Use a current-waveform sig gen (not voltage-waveform!), current limited per above suggested feed/damage potential relationship.
Ramps will -> warble getting louder/softer? Former "come closer" latter "go away"?
sort of like the noisemaker found in most smoke detectors.
This is similar in function -- apply voltage, get (loud!) noise. There's an internal "driver" that applies signal to the membrane and that drive is largely independent of applied voltage. It's a "buzzer". But, at a *much* higher SPL (my sound meter shows 115dB at 10 ft -- you can't
*hear* the smoke alarms when this thing is screaming!)
[I think these were intended for use on a shop floor -- where folks are typically wearing ear protection and you STILL need to be able to alert them of a "major problem/hazzard/emergency]
But, AFAICT, sonalerts are inherently single frequency devices; tuned to the frequency of interest (that which is known to be most noticeable by humans) and then driven at that resonance (for maximum "signal"). Yeah, you can probably disassemble the device and drive it with a variable frequency signal -- but, with awful "fidelity".
My devices generate a sweeping range of frequencies. They are, in effect, loudspeakers that are driven with a swept frequency. That (range of) frequency being "canned" in the drive electronics.
Compression driver == "tweeter" that you'd encounter at a rock concert.
So, it can more faithfully represent a wider range of frequencies. You can "play music" through it -- though the low frequency response would suck.
You wouldn't drive the "tweeter" in your home 2/3-way loudspeaker with a wide-range signal source -- because it isn't designed with that range of frequencies in mind. You'd cook the driver (tweeter) in short order.
The horn loads the driver to form part of the mechanical circuit. Its dimensions give you a clue as to where (frequency) the assembly is expected to operate.
The driver itself isn't tuned to a particular frequency -- as evident by the range of frequencies that it produces as it "warbles". The horn obviously is only effective for a certain portion of the audio spectrum...
It's a loudspeaker. I can just remove the amplifier/oscillator and replace it with a real amplifier driven by a similar signal source. I.e., if I set the gain of the amplifier to match the peak voltage delivered to the driver by the existing "warble generator", you'd not know that I'd replaced that generator.
Having done that, I could reduce the gain to make the warble less loud. Or, vary the gain/volume to suit my needs. E.g., start at a low "volume" and gradually increase it -- to give occupants time to "cancel" the alarm before it becomes unbearably loud.
OTOH, a fire detected in the middle of the night would merit going flat out to get folks awake quickly and effectively.
Or, I could alter the signal to convey different types of information. "chirp chirp", "beep beep", "warble", "police siren", etc.
Again, I already have a design that can drive a loudspeaker. So, I can replace the electronics with my existing design -- and then just limit the ranges of frequencies that I generate to ensure I don't toast the "loudspeaker".
[The question in my post is determining how to identify the limits on the "loudspeaker portion" of the siren]
I haven't disassembled them, yet. I'm looking through a collection of "alarms" to see which are most useful in that function.
Voltage corresponds to loudness. I haven't experimented to see how "soft" it will operate -- it is REALLY annoying to listen to at ANY volume! (how long would you play with a sonalert before your nerves felt irrecoverably jangled??)
I'm trying to decide how effective and flexible they *might* be before I invest much effort into them. Obviously, the wider the range of operating conditions (frequency, SPL), the more versatile they will be in application.
E.g., walk out the front door and I can "chirp" to bring you back into the house to address something you've apparently overlooked. If you don't reappear in short order, I can become more insistent (to compensate for the fact that you're probably farther away from the door by that time).
Likewise, if you're in the car, engine running, music playing and garage door opening, I'd need to make a lot more noise to get your attention as you drive out.
I thought of driving it with a signal source and characterizing impedance and SPL vs. frequency. But, I'm not sure how this scales with power level AND how to recognize when I'm about to do physical damage.
That's why I figured if I operate within the existing range of "warble frequencies" and at or below the power level applied by the built-in warble generator I *should* be safe.
I think there is a risk of damage regardless. I recall when I used to triamp that I'd take special precautions to ensure I kept all low frequencies out of the "tweeter cabinets". This is a common practice in "concerts", etc.
In my case, as *I* am generating the signals that are passed through the amplifier (my "network speaker" design), I can make sure I just don't generate any signals with components outside the range I decide as "safe" for the siren (i.e., for the loudspeaker portion of the siren).
OTOH, just how many different sorts of "annunciators"/alarms/sirens are really necessary? I'm *not* using it as a loudspeaker to reproduce an infinite variety of *music*...
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