|Chuck H||11/13/2017 9:40 AM|
|Phase at frequency w/speaker emulator circuit (Juan?)|
I'm working on a passive circuit to emulate speaker frequency response. I'm pretty happy with the EQ but I'm concerned about the phase differential between LF and HF (almost 360*) and was wondering what sort of problems it might cause in actual listening perception or electronically at the input of a mixer/PA. I know that phase error is basically a time lag and if I interpret what I've read correctly I'm dealing with about a 1ms differential. I don't think that's going to be terribly audible, but I don't have much experience with this. Below is a graph of the frequency response of a G12H with the plot for my circuit overlaid on top. Phase is indicated by the dash. It's pretty rough because of the overlay, but I hope the info gets across.
EDIT: Also, does anyone know the real world phase error for an average speaker?
|Mike Sulzer||11/13/2017 1:11 PM|
|If the phase of the speaker response is similar, then I think there is no problem. Do you know the past of the speaker response? There might be no problem anyway; phase often does not matter, but it can.|
EDIt: Not sure what you mean by phase error of a speaker, but speakers have phase shifts just as other electronic components do. In fact the phase response of a speaker can be complicated. I think Eminence has phase plots for many of its speakers.
|Chuck H||11/13/2017 1:56 PM|
|Thank you Mike. By "error" I just meant any differential from ideal. I mean, you wouldn't want your LF appearing 300ms behind the HF. That's extreme and ridiculous of course. Just illustrating. I know Eminence has impedance plots. I'll check there and see if they have phase plots as well. This circuit is strictly for EQ. It won't be a load for anything and it won't be driving anything with current. I did manage to find some speaker phase plots for high end audio stuff and it looks like phase relative to frequency can shift up to 150*. I'll bet a guitar speaker, with it's harder roll off top and bottom is worse. I only wanted to know if there was some inherent problem with phase shifts approaching 180* with a HF/LF differential approaching 360* for EQ purposes.|
|J M Fahey||11/13/2017 3:20 PM|
|What Mike Sulzer said: speakers have terrible phase shift problems, in any case your electronic circuit will always be better than the mechanical version, worst case will approach it, so thatīs what we are used to hearing anyway.|
|Chuck H||11/13/2017 3:27 PM|
|glebert||11/13/2017 3:38 PM|
|Dumb thought: presumably at some point after this emulation circuit someone is going to actually listen to the signal (either directly or from a recording), and they will be doing that through a speaker. If you try to build in the absolute phase response of a particular speaker once it goes through the final speaker you may end up with "extra" phase impact.|
|catalin gramada||11/13/2017 3:51 PM|
|R.G.||11/14/2017 6:43 AM|
|There is a hidden truth lurking under this thread. That is, there can be no filtering without phase shift, at least in analog electronics. The fundamental way that filtering happens is with the interaction of resistive (i.e. no phase shift) elements and reactive elements, things which have a differing impedance with frequency, and that effect by its very nature causes phase shift. The same thing happens in mechanical, acoustic, etc. systems. |
It is probably possible to use DSP programming to affect amplitude as a function of frequency and then to correct signal phase back to no phase shift (or any arbitrary phase shift) but the normal sorts of digital filters also introduce phase shift with amplitude variations.
So if you want filtering, you get phase shift too.
|J M Fahey||11/14/2017 7:10 AM|
That said, using a speaker emulator , which to be more precise should be called a "guitar" speaker emulator , or it would not be needed to begin with, sort of implies that final sound will be played through a Hi Fi, Studio or, worst case, PA speaker ... all of which have (or try hard to) flattest response and minimal phase shifts.
And we add the speaker emulator to that (flat but unexciting) mix precisely to add that off taste flavour we like
|Mike Sulzer||11/14/2017 12:32 PM|
|Chuck H||11/30/2017 8:00 PM|
|Fantastic and thank you to all for continuing discussion on the topic. FWIW I absolutely do intend to do listening tests and the final circuit will allow switching the emulator circuit out for use with either sound reinforcement or a guitar speaker cabinet. My assumption being that sound reinforcement speakers are designed for a flat(ish) response. I could well find this to be false to a greater or lesser degree requiring circuit modification.|
|Gnobuddy||11/30/2017 9:00 PM|
I happen to be working on creating my own speaker simulation software at the moment, so I'll attach a screenshot showing this, for a fictional speaker with a resonance at 100 Hz.
If you had an ideal infinitely stiff speaker cone, that would be the whole story. In practice, as you go higher in frequency, eventually you get cone break-up modes, which are themselves mechanical resonances, each one accompanied by 180 degrees of phase *lag* as you sweep through it (from a frequency well below, to a frequency well above).
So if you were dealing with a nearly ideal speaker, with only its fundamental (bass) resonance, plus one single cone break up mode, you would already have 360 degrees of phase shift within the frequency spectrum - 180 degrees lead at very low frequencies, zero phase in the midrange, and 180 degrees lag at high frequencies well above that cone breakup frequency.
Real life is far worse, with additional breakup modes coming thick and fast as you go up in frequency...each one accompanied by yet another 180 degrees in phase.
And all this is if you were placing your ear right on top of the dust-cap. If you are at a normal distance from the speaker, there is additional phase shift as the sound travels through the air to your ears - three hundred and sixty degrees of phase shift for every wavelength travelled. The speed of sound in a home at normal temperature is around 340 metres/second, so if you were listening to a 3.4 kHz tone, and your ear was one metre away from the speaker, there would be ten wavelengths of sound between the speaker and your ear. This means three thousand, six hundred additional degrees of phase shift, on top of the 360+ in the speaker driver itself!
Note, by the way, that if you were listening to 34 Hz (from your 5-string bass guitar, say), you are only one-tenth of a wavelength away, so only 36 degrees of phase. In other words, very little phase shift at 34 Hz, but lots and lots of phase shift at 3.4 kHz...
In other words, at one metre distance from the speaker, there is more than three thousand degrees of phase shift between deep bass and mid-treble, just because of the way sound behaves when it travels through air!
(And, at a more realistic listening distance, there may be two or three or four times as much - literally, over ten thousand degrees of phase shift between bass and treble, even if you were in an anechoic chamber!)
All this is why I pay no attention to most of the Audiophool fussing over speaker phasing. Phase only really seems to matter when you have two or more drivers simultaneously emitting the same signal, with a phase shift between them. In that particular case, the multiple signals will interfere with each other, and cause peaks and dips in the frequency response.
This sort of thing happens during the crossover region in Hi-Fi speakers (where both woofer and tweeter are emitting the same sound), and it happens all over the spectrum if you stuff four Celestions in one cab and drive them all with a full-range guitar signal.
But one speaker (or one simulated speaker) by itself? Your ear doesn't care. The big bass drum in the marching band sounds the same whichever side of the road you happen to be standing on when the parade goes by. Clear proof that 180 degrees of phase-shift in the bass makes no difference whatsoever to the way it sounds!
And the million dollar question: this speaker emulator is for a project involving running a micro valve guitar amp direct into a P.A. system, perhaps? Any nifty stuff to share?
|glebert||11/30/2017 10:18 PM|
|Gnobuddy||12/1/2017 12:27 AM|
Put another way, there is relative phase shift (between, say, two different sine waves at the same frequency). There is also absolute phase - the "wt" in the equation Y = A sin(wt). It takes 360 degrees of phase to make one one full wave, i.e, wt increases by 360 degrees from its initial value to create one full wave. There will be 360 more degrees of phase for each subsequent wave.
Still another way to think of it: put one microphone a half-wavelength further from the source than another, and we agree that the two mics will put out signals 180 degrees apart, yes?
Now move the further microphone another quarter-wavelength away, and now there will be 270 degrees phase shift between the two signals, yes?
Keep moving the further microphone in little steps, say one-hundredth of a wavelength further each time. You get another 3.6 degrees of phase with each additional increment in distance.
So what happens when the second microphone is a full wavelength further than the first? We kept increasing the phase shift beyond 270 degrees in steps. Clearly, the two signals are now 360 degrees apart in phase.
360 degrees might look like zero degrees on an oscilloscope, but that's because oscilloscopes are not good tools for looking at total phase. Take Fourier transforms of those two time-delayed waveforms, and you will see the additional 360 degrees of phase in one of them.
|Mike Sulzer||12/1/2017 5:14 AM|
|If all the wave frequencies move at the same speed, then the shape of the total waveform does not change, and so the relative phases remain the same. This is what counts, and it is not the same as introducing frequency dependent phase shifts at the source.|
For example (https://brilliant.org/wiki/amplitude...r-phase-shift/), propagation of a simple wave can be described by
sin(k(x - vt)) where:
the phase is the argument of the sine,
x is the spatial coordinate,
t is the time coordinate,
and k = 2*pi/(wavelength),
v is the phase velocity.
If the phase velocity is the same for all frequencies, the phase us the same at all frequencies for each x. (Frequency is not in the equation!)
Then we have v = omega/k ,
where omega is 2*pi*f,
and the equation can be written:
sin(kx - omega*t)
This form contains the frequency explicitly.
If v is a function of frequency, then the relative phase does change as a function of frequency.
|Chuck H||12/1/2017 7:58 AM|
All I have right now is a pencil and graph paper sketch of the analog circuit. When I get that part of it drawn up on computer media I'll post it. The digital stuff is out of my wheelhouse, but if there's any continued interest I'll update as final designs come into focus.
|Chuck H||12/1/2017 8:12 AM|
|That was my perception of it too (minus the math ). But that would be relative to all things being perceived at different distances once phase relationships are settled acoustically, right? In other words, with an open back cabinet sitting some distance from the wall behind in a small room... I would think that phase @ frequencies would be different at various vantages within that room, right? Only from outside the room (for example) would the phase relationships remain analogous @ distance.?.|
|Mike Sulzer||12/1/2017 10:39 AM|
|Yes, in a room you have many paths between source and listener, and so the phase is very complicated when you add them all up. If the human ear-brain did have high sensitivity to phase, it would be very distracting when you move. |
|Chuck H||12/1/2017 9:13 PM|
|Gnobuddy||12/5/2017 10:43 AM|
For example, you can take a square wave signal, put it through an appropriate all-pass filter, and it will come out looking nothing like a square wave, because each of the many frequencies in it has been phase-shifted by a different amount. But an all-pass filter has a flat frequency response, so all the frequencies in the original square wave are still present at their original strengths.
And when listening tests were conducted, guess what? People couldn't hear the difference between the original square wave, and the filtered wave that looked nothing at all like a square (but contained all the same frequencies in the same proportions). They sounded identical.
This, and so many other experiments, all came to the same conclusion: it's really the frequency response that matters. Get that right, and the accompanying phase seems to be pretty much irrelevant. It might look different on a 'scope or in a Fourier transform, but it will sound the same.
Which is why I don't think you have any cause for worry about the phase changes that come along for the ride with your speaker-emulation filter. If you get the frequency response to sound the way you want it to sound, that's all that matters!
As far as I know (and I know very little), simulating room ambience is a very different and far more complex thing. That involves creating dozens or hundreds of fake wall reflections, by time-delaying and adding together dozens of audio signals, usually in the digital domain these days. The amounts of phase involved are huge, thousands of times bigger than the phase shifts that occur inside an amplifier or filter. As I mentioned earlier, 360 degrees worth of phase for every wavelength travelled, so if you take a high frequency and bounce it off a wall many feet away, you get tens of thousands of degrees of phase shift compared to the direct sound. And when you hear both direct and reflected sounds at the same time, then the phase and time differences between them matter, in the sense that your brain hears something going on.
The people who design studio-grade reverb units know all about this stuff - they've been fooling us into hearing ambient space around recording artists for decades now. I still remember listening to The Eagles for the first time on a Sony Walkman, and loving that huge beautiful airy space that seemed to surround them. That big airy ambience was such a big part of their sound!
Closer to the original topic of this thread, I have been trying to make a silk purse out of a sow's ear - the short version is that I'm trying to build a guitar amp, on a very tight budget, to give to a friend. The budget dictates that I use a pair of 6.5" woofers that came out of thrift-store boombox speaker cabs, $5 for the pair. They sounded absolutely nasty on my first attempt, but by tinkering with a graphic EQ pedal, I found that putting a notch in the frequency response at 800 Hz took most of the nastiness away. Adding a little gentle bass boost below the notch, and then rolling off the high treble above the guitar range, improved things quite a bit more.
The story isn't finished yet, and I'm still tweaking things, but essentially, I'm trying to make an emulation filter that makes a contemporary small speaker sound like a guitar speaker. Obviously, only at relatively low volumes, which is how this amp will be used.
So while Chuck is building a filter to conjure up the sound of a guitar speaker/cab that isn't actually there at all, I'm trying to build a filter that makes a 6.5" boombox woofer sound like a big floppy-coned guitar speaker with a stiff suspension, weeny magnet, limited treble response, and all the other bad design characteristics which happen to be the way we like our guitar speakers.
|Enzo||12/5/2017 12:37 PM|
|People don't hear absolute phase, but hit the invert switch on one band of your active crossover and see if you hear it. Invert the connections to the mids on your PA cap or even the horn, and see if you hear it.|
When I ran sound, I used to put up music on the mains and walk across the back of the floor and I could hear the phase cancellations as I walked across the pattern from the two speaker columns. The woofers and tweeters having different patterns.
Whether those matter to a speaker emulator is another matter.
|eschertron||12/5/2017 12:53 PM|
|Mike Sulzer||12/5/2017 1:16 PM|
|Gnobuddy||12/5/2017 6:02 PM|
Enzo is absolutely right about switching the phase of a tweeter wrt the woofer, of course, and that is a classic example of "two or more drivers simultaneously emitting the same signal, with a phase shift between them"!
|The Dude||12/5/2017 6:43 PM|
|I've been following without much to add. Interesting discussion. I will say that one of the ways the BBE devices work is to add correction for speaker phase "problems". If a person wanted to actually hear what's being talked about in this thread, a BBE unit would be a good place to start.|