Most speaker/amp systems produce a high-pass filtered step response - i.e. an exponential decay curve - because there’s always at least one DC blocking capacitor between the DAC and the speaker. (Usually there are more.)
DC-coupled systems exist, but they’re not common and they’re certainly not essential. I think part of the confusion is that audio systems are inherently AC, because sound is inherently AC - i.e. a time-varying *relative* pressure variation above and below a mean value. It’s possible to drive a speaker coil with a DC offset to produce a non-decaying step, but your ears won’t hear it as a constant step. They’ll hear an initial thump or click followed by silence. Your ears hear pressure *variations* around a mean value, in a limited frequency range - not absolute pressure/displacement. Transducers mimic human hearing. Richard > On 1 Oct 2017, at 08:58, Renato Fabbri <renato.fab...@gmail.com> wrote: > > tx for the infos and thoughts. > > I am getting the same difficulty when I > look at texts in the subject. > > Making the question very objective, > is anyone able to state very briefly if > LPCM samples 1000, 5, 500, 70 > will be converted by a loudspeaker > into displacements proportional to such > values or to pressure (differentials)? > > I mean, if the samples are constant, > say 6000, the speaker surface is displaced > an amount proportional to 6000 and > stays there until the samples end. > That is displacement, right? > > Apologies if I am trying to hard to > keep things too simple, but > I believe that there might be > a simple answer. > (Although probably not if we > think also about what is input > by mics.) > > > >> On Sat, Sep 30, 2017 at 6:40 PM, Nigel Redmon <earle...@earlevel.com> wrote: >> > I'm pretty sure an instantaneous [audio] voltage (or a number in a >> > stream of PCM values) represents a pressure differential, and not >> > displacement. >> >> This is one of those point-of-view things…I used the “usually” caveat for >> that reason… >> >> You strike a drum, you displace the head, create a pressure wave because the >> room has air, the pressure wave displaces the conducer capsule membrane >> resulting in a change of capacitance, which is converted to a change in >> voltage, it’s amplifier and used to displace a speaker, which creates a >> pressure wave, which displaces your ear drum… >> >> So, whether the electrical signal is of the displacement of the capsule >> membrane, or the pressure differential over time hitting it…ultimately we’re >> normally recording sound as it exists in the air, so you could rightly say >> the electrical signal is an analog of the pressure changes over time—or you >> could look at it on the electro-mechanical level and say we use the pressure >> to displace and element and record that displacement. >> >> I guess how firmly you stick to one or the other depends on conventions >> you're used to. As an engineer, I see it as representing the displacement. >> The reason I view it that way is because I’m intimately aware of the masses >> involved with dynamic or condenser mics, and their shortcomings. So, I think >> of it as the mic diaphragm trying its best to approximate the changes in >> pressure, and we convert that displacement approximation to an electrical >> signal. >> >> It’s probably easier to view the flip side, the speaker—so many reasons for >> a bad approximation; you need a big surface to move a lot of air, >> particularly for low frequencies, but a big surface has enough mass that it >> sucks for quicker changes so we split up the audio band; all the while the >> surfaces are flexing and the cabinet and surface attachments are messing >> with the attempt, and you wonder how the heck we manage to put something out >> that’s listenable ;-) Anyway, that’s why I view it as displacement; we’re >> trying like heck to make the displacement true, and the pressure changes >> follow (for speakers—the other way with a mic). It may be a different story >> with “plasma” approaches, I’m talking about our usual practical transducers. >> >> >> > On Sep 30, 2017, at 1:37 PM, Ben Bradley <ben.pi.brad...@gmail.com> wrote: >> > >> > I'm pretty sure an instantaneous [audio] voltage (or a number in a >> > stream of PCM values) represents a pressure differential, and not >> > displacement. A loudspeaker driver in air (within its rated response) >> > is constrained by the air, above its resonant frequency (at the low >> > end of its frequency range - for a woofer, this would be near the >> > resonant frequency of a ported bass cabinet). Below its resonant >> > frequency the output is a position proportional to voltage or current, >> > but the coupling efficiency to the air goes down with frequency, so >> > this isn't a good operating range. A woofer with a 1Hz input is going >> > to have the same displacement as with a 0.1Hz input at the same >> > voltage, because it doesn't have good coupling to the air at such low >> > frequencies. >> > >> > A speaker in air (operating within its intended frequency range) is >> > like an oar in water. You can move it back and forth very far of >> > you're doing it at a slow enough rate (low enough frequency). If you >> > do it at a higher frequency, it takes more force to move it back and >> > forth the same distance. If you use the same force, you end up moving >> > back and forth a smaller distance due to the "strong coupling" of the >> > oar to the water. This is how a speaker cone sees the air, and shows >> > how cone displacement goes down as frequency goes up, even though the >> > acoustic energy is the same. The voltage is proportional to >> > [differential] pressure, and not (as one might easily believe, and >> > probably some books say!) displacement. >> > >> > Regarding phase, as displacement is the integral of pressure, >> > displacement goes down with an increase in frequency, and there's a >> > phase shift between pressure and displacement. I vaguely recall that >> > the integral of a cosine is a sine, so there's a 90 degree (or pi/2, >> > not pi/4 - you're perhaps thinking of 1/4 of a complete wave) phase >> > shift between these two. But a dynamic microphone does the exact >> > inverse of a speaker, so the sound-to-sound conversion actually works >> > out without a phase shift. Even presuming a condenser mic does this >> > phase shift (I can't quite visualize how or whether it does offhand), >> > human ears are almost completely insensitive to phase shift vs. >> > frequency, so in practice it doesn't matter. >> > >> > >> > On Sat, Sep 30, 2017 at 3:39 PM, Stefan Sullivan >> > <stefan.sulli...@gmail.com> wrote: >> >> >> >> so there might be a phase >> >> offset between the recorded >> >> and the reproduced sound. >> >> >> >> >> >> Ah, I think I might be understanding your question more intuitively. Is >> >> your >> >> question about positive voltages from microphones being represented as one >> >> direction of displacement, whereas the positive voltages from speakers >> >> being >> >> represented as the opposite displacement? To be honest I'm not sure what >> >> the >> >> convention is here, but there must be an industry-wide convention or even >> >> one speaker manufacturer to the next might be phase incoherent? I actually >> >> don't know the answer here, but maybe somebody else on the list does? >> >> >> >> It is worth pointing out that Nigel is right about phase being frequency >> >> dependent. Even the mechanical system has dynamic components that have a >> >> frequency response, which means their phase response could be nonlinear, >> >> which transducer engineers would either need to compensate for with other >> >> reactive mechanical components, or with the electrical components, or DSP. >> >> >> >> Interestingly, the acoustical and mechanical systems of transducers can be >> >> modeled as electrical circuit complements themselves. I assume that all >> >> speaker/microphone manufacturers model their systems this way, but again >> >> it's not actually my industry so I can't speak to what actually happens. >> >> Marshall Leach has a really good book on the subject: >> >> https://he.kendallhunt.com/product/introduction-electroacoustics-and-audio-amplifier-design >> >> >> >> Stefan >> >> >> >> >> >> _______________________________________________ >> >> dupswapdrop: music-dsp mailing list >> >> music-dsp@music.columbia.edu >> >> https://lists.columbia.edu/mailman/listinfo/music-dsp >> > _______________________________________________ >> > dupswapdrop: music-dsp mailing list >> > music-dsp@music.columbia.edu >> > https://lists.columbia.edu/mailman/listinfo/music-dsp >> > >> >> _______________________________________________ >> dupswapdrop: music-dsp mailing list >> music-dsp@music.columbia.edu >> https://lists.columbia.edu/mailman/listinfo/music-dsp > > > > -- > Renato Fabbri > GNU/Linux User #479299 > labmacambira.sourceforge.net > _______________________________________________ > dupswapdrop: music-dsp mailing list > music-dsp@music.columbia.edu > https://lists.columbia.edu/mailman/listinfo/music-dsp
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