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
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