4 ms·
Has the hypothesis that this is noise from a class-D amplifier (which I assume most commercial speakers use) been eliminated? Those generally switch at MHz freq
by tlb 2y ago
Has the hypothesis that this is noise from a class-D amplifier (which I assume most commercial speakers use) been eliminated? Those generally switch at MHz frequencies, but have subharmonics all over the place except the ones that have been carefully eliminated by filtering. I wouldn't be too surprised if there was a cheap brand of class-D amplifier that had a 20 kHz tone.
- kurthr 2y agoI'd also be suspicious of this. You can also get a lot of beating artifacts from having many amplifiers in close proximity. If the speakers were still larger "mid-range" horns as were seen in decades past, this also might be less of an issue (low efficiency above 10kHz), but for cost/aesthetic reasons the speakers have shrunk quite a bit so they no longer mechanically filter much. If the problem was something everyone could hear it would likely be fixed, but since it's a minority, it's likely to persist. One other possibility for active US is crowd tracking. I've seen solutions that monitor # people and walking speed at doorways using ultrasonics. A lot of "one way" doors also use simple US detectors.