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In Reply to: RE: Phase Response Effects. . . what? posted by hahax@verizon.net on January 28, 2016 at 17:33:45
Hi Guys,I wish to correct the most common misunderstandings about phase shift and time delays for you. Here are what the mathematics show:
To get any kind of handle on a speaker's phase shift or time-delays, we must first set a proper Phase or Time = Zero point, for when the first 'sound' will arrive at our mic, at our ear, from each driver, BEFORE their signals are altered by any crossover circuits.
According to Dr. Richard Heyser's mathematics, which have been proven correct, 'the first sound to arrive' that we should use as our reference is the beginning of a tone burst (a 'beep') at a driver's highest pistonic frequency, with no crossover used. This would be a burst of a 10-12kHz tone for a good soft dome tweeter and maybe 5kHz on a great mid.
The statements made by Presto that there will be 360, 180, or even 90 degrees (a 1st-order filter) of phase shift at the crossover frequency is the common presentation in EE classes and quickie speaker-design texts. This is not quite right, but I've determined I cannot easily explain here why, because I'd also need to show the derivation of various Phase = Zero reference points along the way... I tried several times, by editing this post, and finally decided to parry it down to the following:
When the crossover-frequency tone, say 3kHz, comes out of both that good mid and good tweeter, through a first-order crossover circuit on each, then their signals will combine at the mic or ear showing NO phase shift, no time delay injected. This is peculiar to the use of first-order circuits. High-order circuits introduce phase shifts (time delays) that change with frequency.
To turn phase shifts into time advances or delays is simple:360 degrees of phase shift, at a certain frequency, means a timing difference of one period OF THAT FREQUENCY. If the frequency is 3000Hz, that has a period of 1/3000th of a second, or 0.33 milliseconds (ms). 360 degrees of phase shift is then 0.33ms time shift at 3kHz (you must specify the frequency). 180 degrees of phase shift is 0.165ms of time shift at 3kHz.
Down at 300Hz, a common crossover point between a woofer and a mid, these numbers would be ten times larger, as the wave period down there is ten times longer, at 3.3ms. For a speaker using high-order crossovers, the time delay imposed on that 300Hz can effectively place the location of 'whatever we think is making that 300Hz tone' nearly three feet (0.8 meters) BEHIND 'wherever the treble tones appear to emanate'.
Phase shifts/time delays in speakers are mostly responsible for what people discuss about imaging differences, transient differences, laid back or forward presentations, of 'apparent detail' (on those speakers, the sound of the 'pick' on a guitar string arrives at your ear well before the string's actual tone develops).
Phase shifts/time delays make speakers sound too bright, even though they measure flat. Why? Because the highs got to you first.
Phase shift/time delay can make the bass sound flabby and the rhythm sound lame. Why? Because the bass got to you late.
'Classic speakers' from the 40's to early 70's have less phase shift/less time delays because they used lower-order crossover circuits than do modern speakers.
One-way speakers have no crossovers, but still have severe phase shifts from cone breakups and whizzer-cone resonances, and from the time-delays caused by any extreme cabinet bass-tuning.
Phase shifts/time delays reduce the apparent efficiency of the speaker, since the peaks of music are not as 'peaky'. The dynamic envelope is reduced since the drivers are not pushing and pulling together from the first dynamic jump.
Phase shifts/time delays psycho-acoustically create image heights (note that we cannot record 'height' for 2-channel). They cause an image to wander at different frequencies.
Phase shifts collapses the depth of any image, and make musicians playing tonally different instruments, such as violins and cellos, sound as if recorded in different halls, having different echoes.
Phase shifts make poor recordings, loud string passages, loud horns, and loud rock music unbearable, especially to women. Phase shifts make some sounds or notes suddenly appear 'far too loud', as when a single note just drills right into the side of one's head. That's why engineers overly-compress and EQ horns, and de-ess singers.
Finally, phase shifts/time delays greatly affect our opinions of everything else- the cables, the gear, recording engineers, the speaker placement, and even the sound of our rooms! Makes us dislike digital more than we need to.
Fix these phase shift/time delay errors and the music is whole again, emotional not mechanical. One's attention remains on the music, as we are never distracted by 'the image' or 'the bass' or 'the airiness', since no part of the sonic spectrum is literally 'sticking out' out in time to attract our attention. But, experience shows one must be a music lover to appreciate that. Those with a limited musical palette seldom 'get this'.
Best regards,
Roy Johnson
Designer
Green Mountain Audio
Edits: 01/29/16 01/29/16 01/29/16 01/30/16Follow Ups:
Roy,How have you been?
Yes, it's really hard to try and explain the concept of group delay in laymans terms. It's really a derivative, a rate of change thing. The big deal is that if the group delay is non-constant, the original signal is not preserved. It actually sounds amazingly good despite that that poor waveform looks like!
A true first order *acoustic* design has constant group delay. A capacitor thrown on a tweeter is a 1st order crossover, BUT as you know this does not a 1st order ACOUSTIC design make. It's simply a first order ELECTRIC crossover. Obviously, 4th order LR acoustic targets are going to be far far easier to hit because of the rapid roll-off rate. And, you enjoy quite high power handling even with band-limited drivers and you don't need a tweeter with special power handling considerations and ultra-low Fs.
There are only two ways to go about it really: 1st order acoustic design or some form of trickery, and by trickery I mean passive methods like subtractive delay or "filler drivers" or DSP-based trickery like FIR filters or phase-corrected IIR filters. I've worked with the latter and really it was a lot of fun to be able to compare the pre and post correction impulse response.
A few big myths about group delay.
1. You can "fix" group delay and obtain time/phase coherence by flipping a driver's polarity, as in the case of the 2nd order LR crossover.
2. You can "fix" group delay and obtain time/phase coherence by ONLY time-aligning the drivers using a simple delay.
3. LR4 is time/phase coherent because all drivers are in phase electrically.The last one is where people don't get that although the LR4 is in phase at all frequencies due to the 360 degree phase shift, the group delay is actually worse that 2nd and 3rd order designs. Yep, even with it's inverted mid-range a properly executed BW3 design will have better pulse response than an LR4.
Cheers,
Presto
Edits: 01/30/16
Good heavens. Roy, you've confused nearly everybody with this rambling mess.Phase shift in filter circuits is basic electronics. Let's not make it more complicated than it really is. I believe most users understand the aspects of acoustic crossovers, electrical crossovers, time-coherence, etc.
I suggest to read Presto's post again....accurately this time. You've mischaracterized his statements and also Linkwitz.
Presto's post is absolutely correct in all aspects and much clearer in explanation than yours.Cheers,
Dave.
Edits: 01/29/16 01/29/16
Rambling mess? Really?
Well, he's edited it and now it reads better.IMO, phase distortion is one of those topics that can do without the rhetoric and should be described in stark terms.
It seems most audiophiles have an opinion on the audibility of phase distortion, but very few have evaluated it properly...by removing all other variables in/for an A/B comparison.
As I mentioned in another thread awhile back, "coherence" is a label bandied about and misunderstood by many in the audio industry. I think most audiophiles think of this in the subjective context.Regardless, in my opinion, nearly all conventional speaker systems are not capable of time-coherent reproduction. The physical configuration renders this impossible.
Just my two cents.......worth what you paid for it.
Cheers,
Dave.
Edits: 01/31/16
So sorry to cause any confusion, Dave! Although I still don't believe I was rambling, I did come to the conclusion that I needed to edit my post down to some very basic information. Thanks for poking me about this!Best always,
Roy
Edits: 01/30/16 01/30/16
... overly-compress horns, and de-ess the singers." Etc, etc...Assuming that the above statement accurately reflects the manner in which a majority of available recordings are made...
Do phase/time perfect loudspeakers somehow correct for these types of *mistakes* made by recording engineers? Or, do phase/time perfect speakers effectively exacerbate this dilemna?
In other words, are perfectly made recordings required in order to fully reap the benefits of the phase/time perfect loudspeaker design?
Until the day when perfectly made recordings become the standard, could we be better off using time/phase *imperfect* speakers that would seem to be (or actually are, in a twisted sort of way) a better match for all of the equally imperfect recordings that we might listen to?
Thanks in advance for taking the time to answer questions that you may deem silly. I'm no Einstein.
Edits: 01/29/16 01/29/16 01/29/16
Good question.
When coherence is achieved, such a speaker distorts any and every signal less, including the distortions of a poor recording. So poor recordings sound better, as their distortions are not being further distorted.
The goal of reproduction is accuracy to the RECORDED waveform, not to any REAL waveform. That real waveform is long gone, altered by the microphone, as it does not hear the way we do.
I agree that fidelity to the signal is the goal. And that includes ultimately to include phase distortion. But at the current state of the art linear phase speakers are difficult to do and there is a cost in other fidelity aspects. So given that the audibility of poor phase response seems to be way below the audibility of the more recognized distortions I would opt for solving those problems first. Hopefully in the not too distant future we'll be able to do better.
Allow me to make a correction to the above, thanks.
A 'linear phase speaker' is not a time-coherent speaker.
This is technically-nice-sounding shorthand for "an acoustic phase response that changes constantly and smoothly (i.e., linearly) with frequency." In other words, it has more and more time delay the lower we go down the scale.
When a linear-phase speaker's 'rotates', its phase spins through multiples of 360 degrees (from thinking about phase as if degrees on a compass). Every linear-phase speaker rotates through multiples of 360 degrees as we proceed down the scale. When it is a linear phase design, that rate of rotation is smooth with change in frequency. This linear rate of change produces angled-but-straight lines (i.e., 'linear') for its phase-response, plotted on a lin/log graph.
'Linear Phase' of course also means that this speaker is 'in phase' at any crossover point, i.e., exhibiting no cancellation/no suckout around any crossover frequency. So, a decent speaker has linear phase shift along with a smooth-looking frequency response curve. Its impedance curve is usually not flat.
Linear-phase speakers are the MOST easily designed, because a designer thinks nothing of the time domain, only of the tone balance, a smooth frequency response, with no cancellations. Finito. That's it. Ship it.
FYI, Tannoy, Dali, Dahlquist DQ-10, B&W's, MBL, the new Thiels, M-L, and many more, are linear-phase designs. None is time coherent.
On the other hand, a time-coherent speaker exhibits the SAME TIME DELAY at EVERY frequency. It is not changing the acoustic phase at ANY frequency. These speakers are very difficult to design, but only when a designer lacks the education.
Plenty of good woofers and tweeters now exist for making time-coherent speakers, at no increase in cost, no difficulties with power handling, nor dispersion, nor efficiency.
However, time-coherent speakers take MUCH longer to design, because of the additional time-domain variables. Once a speaker design becomes time-coherent, it turns out that making tweaks to it every six months will not substantially change what is heard. Therefore, marketing is not about offering a 'new and improved' model every year.
This is because, going to your last point above, designing a speaker to be time-coherent is actually MORE important to address BEFORE any other design variable. To get the darn thing to be time-coherent, you must automatically avoid poor drivers with cones that break up or ring (metal), drivers with high distortions (from poor magnets and suspensions), strange cabinet-tunings for bass, and varying-length horns, among others.
Best regards,
Roy
Roy,
I think it might be good to elaborate a bit more on your definition of the term "linear-phase." (I noticed you don't use this term at all on your website.)
Most (I think) audiophiles believe "linear-phase" denotes a system that indeed exhibits a flat phase-response and a constant group-delay through the entire audio range. One comes with the other....so to speak.
I'm not aware of any commercial speakers that are linear-phase but also exhibit multiple phase wraps in the audio range and non-constant group-delay. You might mention a few of those.
Cheers,
Dave.
Hi Dave,
The best I can do, without having you and I working out of a common textbook in a classroom situation, is to reiterate from my previous post that:
"Every linear-phase speaker rotates through multiples of 360 degrees as we proceed down the scale. When it is a linear phase design, that rate of rotation is smooth with change in frequency. This linear rate of change produces angled-but-straight lines (i.e., 'linear') for its phase-response, plotted on a lin/log graph."
What is 'going wrong' is that my use of the word "straight" here is often confused with the word "flat", as in your post just above. What you wrote actually describes a time-coherent speaker, not a "linear phase" one:
"... a system that indeed exhibits a flat phase-response and a constant group-delay through the entire audio range. One comes with the other....so to speak."
Best,
Roy
Thanks for the clarification, Roy.
What about Vandersteen?
To my knowledge, the only speakers that are time-coherent across most of their frequency range are/were Dunlavy, Thiel, Vandersteen and Green Mtn.
The first three have very complicated crossovers, which allows certain driver problems to be 'fixed' for the measurement microphone. However, such complexity reduces dynamic contrasts and blocks the transmission of small, subtle sounds, which together are responsible for music's emotional content.
Best,
Roy
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