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RE: Clarifications on Time Coherence

Posted by RoyJ (M) on March 13, 2012 at 22:13:09

In Reply to: RE: Clarifications on Time Coherence posted by villastrangiato on March 12, 2012 at 21:48:25:

Hi Villa,
I think we agree more than disagree. And I probably could have been more clear in my response.

You wrote:
"Not sure what kind of first order speakers you've built [visit our website] but reverse null in speaker designing parlance means reversing the leads of one of the driver pairs and examining the drop off in amplitude across the entire crossover region. And believe it or not Roy, it happens with 1st order designs too. And if you just stop and think for a moment, your taking one vector out of top two quadrants and placing it in the bottom. Each leg is no longer operating in quadrature. [WRONG, sorry- see below, thanks] Yes, the null isn't as severe as with an even ordered network but the null DOES EXIST."

You claimed 30-40dB cancellation, Villa. This is not even remotely close to true on a well-done first-order speaker design. In Figure 13a of that Rane PDF of yours, please note how the addition of 180 degrees to one of its two 0.707-amplitude vectors (which how one is supposed to represent a polarity reversal of one driver), results in two vectors still separated by 90 degrees, not the 180 required for cancellation. Again, perhaps you were misinformed.

You wrote:
"Group delay, as noted in the link above is the rate of phase change per incremental change in frequency and is represented in units of time. However this is not a measure of time or phase separation between the two driver outputs but rather a total time/phase shift of their sum from one frequency to the next since both outputs experience phase shift that is equal to one another at each frequency."

And later, you added:
"What is not constant with the higher order networks is group delay. That's linear with the first order network but not linear with the others. This apparently is where you are getting confused."

No, I think I failed to communicate properly, as I do agree with these statements.

You wrote:
"If you want further clarification, please read the RANE link above. And again, more modern studies (within the last 10-15 years) have found that humans can reliably detect group delay on the order of a few milliseconds at around 1khz."

Here, our studies show we can detect frequency-varying time delay on the order of microseconds, not milliseconds, over most of the tone range.
Don't believe me, fine. But everyone we know hears what we do when the adjustable tweeter on our Eos two-way model is moved back less than an 1/8th of an inch. Which is less than 10 microseconds difference in arrival-time. Or move the mid-driver in our 3-way Calypso back and forth less than 1/4". But for one to hear those differences, many other variables and problems must first be fixed or avoided, topics which we thoroughly present on our speaker's design-concept papers, especially on our flagship model(s).

I probably could have expressed myself more clearly, as you are exactly right-- in higher-order crossovers, the delay in the highs is not the same as the delay in the lows, and in-between the delay is constantly changing. I hope that is now clear enough for both of us!

This difference in delay between 'when the lows arrive' and 'when the highs arrive', a delay that is also constantly changing as we move up or down the scale, has also been found important by other speaker companies including Thiel, Dunlavy, Vandersteen and Soundlabs. And of course, very many more have not.

Why is a time delay that varies from low to high audible?
Partly because the sound of any musical instrument or voice consists of a fundamental tone plus its harmonics which span many octaves. A varying delay between the lows and highs thus changes the sound of an instrument or voice, making it much less natural in timbre and in other qualities, at least when one knows the real sounds.

A varying time delay between lows and highs also affects the sound of transients from instruments and voices, because the sharp, leading edges of their complex musical waveforms arrives much sooner than their lower tones- sibilance and 'coarseness' are exaggerated in some speakers. Detail is exaggerated- too much 'picking' on the guitar is often heard. And we have heard many examples where it has yielded speaker that can't play rock music nor 'poor recordings'.

A varying delay we hear detach the lower-range 'pulse' of the rhythm from the drive of the mid-band instruments and voices. The beat seems sluggish from that low-frequency delay.

A varying time-delay also hides the depth of the soundfield-- dissociating instruments with different tone ranges from being 'in the same studio'. One example is hearing the sounds from the tweeter physically separated upwards from sounds in the middle range (the famous 'rising treble image' which many report).

You wrote:
"As to the issue of transient response, again - that's electrical engineering 101. As you increase the number of poles and zeros in a circuit, the number of energy storage points is increased and instability (harmonics) is introduced. You can call them "artifacts" but Linear Control theory very explicitly defines the transfer functions and the various levels of Q associated with the resulting resonances."

I am not sure why you are bringing this up, as this is not an issue in first-order speaker designs. The audible artifacts I am concerned with include the ones I list above.

I do think you would quite enjoy that AES paper if you read through it, as it does clearly show (mathematically and with `scope photos), how the varying time delays look exactly like ringing, warping, or tilting of a square wave. Now, that is not to say these are not 'handled' by many engineers as if they were actual ringing, etc. It is the most-clear paper on the subject I have seen.

Thank you for your input. I hope you see we do agree on many points. However, I find that the execution of design in most all first-order speakers is not very thorough-- one indication is their use of complex first-order crossover circuits.

Best regards,
Roy

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