Adobe fixes Premiere Pro bug that blew out some MacBook Pro speakers

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  • Reply 21 of 22
    MarvinMarvin Posts: 15,309moderator
    elijahg said:
    [...] Integrated units such as Bluetooth speakers, the HomePod, iPhone speaker etc are pretty much always protected from damage by limiting the amplifier output, or simply having an amplifier that is a lower wattage than the maximum the speakers can handle. Units that aren't integrated such as old 1990s Hi-Fis or discreet amplifiers tend not to be limited as it's not known what speakers you would use with them.

    This was obviously an oversight by Apple, they put an amplifier in the Mac which was more powerful than the speakers could handle, and they didn't limit its output in either hardware or software. 
    This is not completely accurate.

    While it's certainly possible to blow a speaker by overpowering it, the vast majority of speaker failures are caused by inadequate amplifiers. Pushing an amp to its limit causes clipping, which is a distortion of the waveform that makes it resemble a square wave. That can destroy a transducer even when delivered at less than the speaker's rated power handling.

    On the other hand, most speakers can handle more than their rated power when it's only in short bursts. Thus a few moments of too much power is generally less likely to damage a speaker than running an amp too hard for a more prolonged period.

    That means a power limiting circuit will not prevent the most common form of speaker failure. In the '90s there was a running joke about how a fast acting tweeter will protect the fuse every time. That's because what caused the tweeter to fail was not overpowering, so the fuse was pointless.

    What's happening in the Premiere case is probably a full-scale square wave. The actual culprit is the square wave, not the amount of power per se, but by delivering that square wave at full power the failure is accelerated.

    That's why, despite the Premiere issue, I don't want a power-limiting circuit in my audio path. I want the amp to pass transients unaffected, rather than "pumping" the volume on every harmless short-term peak. Since a limiter circuit wouldn't do much good anyway, the trade-off in intrusive, audible effects isn't worth it.
    That's interesting, this square wave/underpowered amp issue is described in the following document on page 2:

    https://www.electrovoice.com/binary/EV_PABible-01-1979.pdf

    Isn't there a way to prevent this square wave either from the amp or between the amp and speaker to prevent damaging the speakers? I assume the audio industry must have tried to come up with a solution to avoid easily damaging expensive equipment and not found one but it surely must be possible to put a component between the amp and speaker than can detect a prolonged square wave and shut down the signal for a period of time to avoid blowing the speaker.

    I can understand for audio production not wanting limiters in the audio path but it wouldn't matter for a laptop's internal speakers, as long as it didn't limit the output to external audio equipment that could handle the signal. Having said that, is there a need to ever hear a square wave? If the amp is clipping then why would anyone ever need to have that output reproduced at the speaker? It's just noise.

    Not having any software or hardware protection against this means someone could write malware to blow people's expensive laptop speakers and nothing would prevent it happening again and again. I would at least hope Apple offers an OS option that could be disabled if necessary by audio professionals and a hardware solution in future models, if only to protect the internal speakers.
  • Reply 22 of 22
    Marvin said:
    [...] is there a need to ever hear a square wave?
    Only if you listen to modern popular music.

    That's half joke, half truth. Most modern music exhibits characteristics that resemble clipping.

    All else being equal, humans tend to perceive a louder sound as being better than a quieter one (that's why it's so important to match levels when making comparisons). Somehow, a decade or two ago, that phenomena was twisted into what we refer to as the Loudness Wars. The reasoning is if louder is better, then making your song louder than everyone else's will give it an edge in the marketplace. Since there's no way to go higher than a full-scale digital signal, producers limit the dynamic range of their recordings to increase the average level. The result is a waveform that's squared off top and bottom. It's unfortunate because it ignores the "all else being equal" part of the formula. It achieves the goal of seeming louder, but at the expense of sound quality.

    I don't have any data on whether or not this kind of squared-off waveform contributes to speaker failure, but it is an example of needing to "hear a square wave." I suspect that any effort to impede transmission of square waves would adversely affect playback of pop music. I could be wrong though. It's been known to happen from time to all the time.
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