Coherence

Anglo

Member
2008-03-12 4:38 pm
Boys!

This forum seems to be the right place for me to share my thoughts on coherence.

So, to the point of the matter. Is coherence a subjective audiophile artifact?

I think it is an audiophile artifact that musicians do not talk about or see here an inexistant problem in LIVE unamplified concerts. To me the absolute coherence is one of the Fullrange driver. To me, and this is where the discussion lies, a multi-way system is never coherent like a fullrange driver. I can add here that the Acoustats are also absolutely coherent but I`d like to stay with fullrange drivers to simplify the matter.

This all said, what I am saying is, while wanting and achieving full frequency response, coherence takes a beating because in order to achieve the full 20 to 20, one must use a different approach than just one single driver. This in turn is choosing one`s poison.

I am not saying that multi way system can't produce a cohesif message, but rather, I am saying that as coherent as a patched togther sound can be "cohesif" and one could say it is indeed coherent, one listening session on a Fullrange driver will show the listener how incoherent a multi-way truly is.

Is this subjective or can we say it is objective and in fact can be measured?
 
Hi Anglo,

Most Fullrange (really only widerange) drivers are 'coherent' over a limited range only, that range being size dependent, and typically good at voice frequencies.

I feel that the main difference is that the phase changes wrt frquency are not so sudden as when a crossover is used with multiple drivers - until - the FR driver hits a natural HF limit where its phase change can be so much worse than when compared to a small separate tweeter.
When this HF limit is reached the driver does not suddenly stop transducing, it does so out of phase and reproduction becomes dissonant as energies fail to arrive coherently. This being a reason for adding a supertweeter to take over if a FR does not (cannot) cope as accurately as we would wish.

Cheers .......... Graham.
 

Anglo

Member
2008-03-12 4:38 pm
hi Graham

Man, you guys are quite the bunch. I love your response!

O.K., I have to break this down a bit.

>Most Fullrange (really only widerange) drivers are 'coherent' over a limited range only, that range being size dependent, and typically good at voice frequencies.<

I agree, that the fullrange drivers are limited in frequency response. I eluded to that in my initial post; where I lose you is when you say coherent depending on the frequency range. To me, I never hear a tweeter playing the highs (as limited and somewhat compressed as they are)

>I feel that the main difference is that the phase changes wrt frquency are not so sudden as when a crossover is used with multiple drivers - until - the FR driver hits a natural HF limit where its phase change can be so much worse than when compared to a small separate tweeter. <

Yes sudden phase shifts are very obvious on coherence i.e B&W speakers.


>When this HF limit is reached the driver does not suddenly stop transducing, it does so out of phase and reproduction becomes dissonant as energies fail to arrive coherently. <

Go a little further here, if I may sk you to do so.

>This being a reason for adding a supertweeter to take over if a FR does not (cannot) cope as accurately as we would wish.<
...because of the phase shift only?
 
Anglo said:
>A time- and phase-coherent one of course<

So, if coherence is all about time and phase then it can be measured...right?

It is unfortunate use of the term.

There actually is a technical definition of Coherence. It is related to the correlation of two power spectra as a function of frequency. It varies from 0 to one and is approximately similar to the notion of "variance accounted for" from your statistics class.

Two channel spectrum analyzers will typical have this function.

-Tom
 
Anglo said:


This forum seems to be the right place for me to share my thoughts on coherence.

So, to the point of the matter. Is coherence a subjective audiophile artifact?


When reading your first post "homogeneity" was the
term which came to my mind. I understand this as a
subjective concept and maybe it is worth to discuss which
attributes of a loudspeaker contribute to homogeneity.

tinitus: I am convinced, that abrupt changes in polar response
with frequency destroy homogeneity.

I think there was a discussion on that issue in "the problem with
hifi" thread some days ago.

IMO using a fullrange driver is no guarantee for
homogeneity. Most FR drivers using a whizzer can be seen
as a 2 way system comprising a mechanical crossover between
main cone and the whizzer cone. The crossover consists
of the mass of the main cone and the compliance of the
bonding between whizzer and main cone.

Some people think, there is no crossover, because you don't see
any inductors and caps. You just don't see them, that's all ...
There are lots of FR drivers which to my ears do NOT sound
homogenous, most of them using a whizzer.

We have to face the fact, that a FR driver acts as a bending wave
transducer above the frequency range of pistonic motion.
And i think it was the ability to act like that, which made the
success of the dynamic loudspeaker in the early radio days.

If designed properly the radiating surface of an FR driver gets
smaller with increasing frequency whithout excessive peaks
in frequency response.
Quad ESL 63 introduces this behaviour with its ring shaped
segments, the Manger transducer is a pure bending wave
transducer without pistonic motion even at low frequencies.

All these transducers "shrink" continuously with frequency and
- to me- are able to sound "homogeneous".

Some constructors, like Dr. Geddes, prefer small
transducers and a wave guide or CD horn to achieve
smooth polar response.

One could also build multiway systems with time aligned
XO and ideally each driver acting in its pistonic frequency range.

My current approach is a shaded line array of small FR drivers.
I am very statisfied with the result, but i would not say its
the only way to do it ...
 

Anglo

Member
2008-03-12 4:38 pm
...just seen your response Oliver.

Now, you (Linearray) are the reference in lack of coherence for me. By no means do I want to insult the or your approach but when loistening to a Line Array, I hear - well - an array of drivers. They all respond at a fraction of a difference in time, differently. I have many different approaches and yes, when the distance allows the sound can be cohesif, but not with Near Field listening.

Curious of your thoughts on this matter.

As for the fullrange driver; It has issues yes, big ones too! Even though a mechanical crossover is used or they compress in the higher frequencies, they remain very coherent. By coherent I mean, seamless integration and not hearing a tweeter, a mid, an a woofer. You can't hear what isn't there I guess!
 
Anglo said:
...just seen your response Oliver.

when loistening to a Line Array, I hear - well - an array of drivers. They all respond at a fraction of a difference in time, differently. I have many different approaches and yes, when the distance allows the sound can be cohesif, but not with Near Field listening.

Curious of your thoughts on this matter.


A line array has to be designed for listening at living room
distances. There was a very good article about line arrays
by
James R. Griffin, Ph.D.
"Design Guidelines for Practical Near Field Line Arrays"

which shows some constraints of line arrays and techniques
to overcome them.

Unfortunately i am not able to find the .pdf on the web anymore.

ah ... got a new site providing it
http://diy-audio.narod.ru/litr/nflawp.pdf

maybe you know that article already ...

My own line array shown at

www.dipol-audio.de

uses vertical power tapering of two kinds:

1. The lower 3 drivers are electrically rolled off above 2 Khz
very slowly < 6db / Octave

2. The grouping of all 6 drivers has been massively optimized
to produce low ripple at listening distances >2m over the
whole hearing range. The grouping of the upper 3 drivers
has been optmized especially for low ripple above 2Khz.

The grouping is also a kind of power tapering which
complements the power tapering introduced by the
low pass filter.

If you omit the electrical power tapering with given driver
grouping, there are some of the effects audible you
mentioned: If you listen to piano, the attacks can
be smeared vertically in space, soundstage has no definite
vertical extension. This happens, when listening at distances
< 3m.

With the combined effect of tapering by grouping and by low
pass filtering this effect vanishes completely. You can listen
as close as 1,5 m when your ears are approx in "sitting height".
At 3m distance any listening position (sitting on a chair, standing)
is OK without deficiencies in imaging.

The system sounds subjectively like a fullrange located approx.
coincidently with the 2nd driver from above.
Unlike Magneplanars or similar line like sources, there is
a stable vertikal extension of sound stage.

The subjective coherence of the system is simply
extraordinary, not only felt by its constructor ...

That subjective coherence is stable over a wide range
of listening distances from 1,5 to 7 meters, larger distances
have not been tested due to room limitation.

Even though frequency response is not very smooth under
certain off axis angles, impulse response is very good at
most relevant near field positions.

As you can see i use fullrange drivers ... even though the drivers
are suboptimal in acting as tweeters (in a technical sence) to me
it is not interesting to add a front radiating tweeter because it
would spoil the concept and you loose that coherence.