Oh, alright. According to tests I’ve done with Audacity however, the Gentex PLACE detector is around 535 Hz & the Simplex sounder base is around 525 Hz instead.
Maybe, yeah. Oh yeah, they did…(sounds nearly-identical (albeit even lower-pitched!) to their older octagonal units that used that lower-pitch horn actually)
Speaking of BRK/First Alert’s current detectors, I found this video while looking for an example of what you said about some models’ horns: note how the other detectors (which have standard around-3000-Hz horns) for some reason take their time in responding to the interconnect signal from the initiating unit: First Alert SMCO100V-AC Talking Smoke and CO Alarm Test
Yeah, the frequency tolerances on 520Hz notification aren’t (and don’t need to be) super tight – UL calls for 10% on the fundamental and first three odd harmonics, which means that 512Hz (watch crystal /64) complies AFAICT as well as R-C 520Hz generator circuits with decent part tolerances (5% or better)
There always needs to be some margin for error since it is essentially impossible to replicate 520 Hz with perfect precision. 10% is a relatively wide margin, but I imagine that is because the studies that originally concluded that 520 Hz is optimal did not notice any significant difference in that tolerance range, so it did not make sense to use a narrower tolerance.
Also, the frequency probably changes slightly when the tone is played and then recorded using a microphone due to factors such as background noise, and Audacity will only be able to give an estimate. The sounders might be closer to 520 Hz than Audacity reports.