Phase at frequency w/speaker emulator circuit (Juan?)

[Chuck H]

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!

Wow! I was eminently aware of this phenomenon (but didn't know such impressive figures). As is anyone that's ever played different venues with different cabinet designs. I figured from the beginning that a blunt recreation of a speakers measured frequency response may well require some tweaking.

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?

Indeed it is. A full amp with OT and power tubes distilled down to a preamp. It can be used as the 2W amp it is or played intio an inductive dummy load followed by a line level out, compensated for speaker EQ or not via a switch so it can be used for anything from direct recording, into a PA or as a standard guitar rig preamp into a clean power amp and guitar speaker cab. Such an amp can create everything from clean to high distortion tones relative to pot settings and OVER ALL amp response. The only drawback is that there can be no "channel switching" per se since the power amp is a shared part of the gain structure for all tonal textures. So I'm setting up to work with another guy on digital pots and presets as a solution.

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]

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]

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.

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.?.

[Chuck H]

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.

I think this is what Gnobuddy was getting at then. IMHO it's way too variable to distill into an emulation program (more than I would venture anyway!). But some degree of parametric notch filtering might well be useful for really nailing a room effect and making it tune-able for many circumstances That's as far as I think is practical though. Trying to pinpoint useful, preset EQ parameters for this affect would be a goose chase.

[Gnobuddy]

I think this is what Gnobuddy was getting at then.

Mainly I was trying to say that I think people often overestimate the importance of phase inside an audio circuit. Back in the days when there was serious study going on of the way we hear sound (a lot of it by Bell Labs), there were many experiments that all came to the conclusion that people really don't hear phase.

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.

-Gnobuddy

[Enzo]

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]

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.

When we deviate from a point source, the phase makes cancellations, in the same way standing wave patterns can make a difference between a 'good' sounding room and bad. But from a point source I agree with Gnobuddy that phase is inaudible, having learned of the same studies he mentions. Of course, moving phase (like from a phasor or flanger) is the same as walking through a static sound field and we hear the comb filtering.

[Mike Sulzer]

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.

What you are hearing is changes in the amplitude of the frequency response caused by frequency dependent cancellation and reinforcement of multiple sources. You are not directly hearing phase changes.

[Gnobuddy]

What you are hearing is changes in the amplitude of the frequency response caused by frequency dependent cancellation and reinforcement of multiple sources. You are not directly hearing phase changes.

Exactly. The key words are "multiple sources". I did actualy mention this in an earlier post in this thread (#12), where I said "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."

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"!

-Gnobuddy

[The Dude]

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.