Hi all,
Sorry to have delayed this post-- unexpected duties arose.
I hope the majority will be served by some words below, along with a close study of the diagram I've posted at
http://s1374.photobucket.com/user/greenmountainaudio/media/TimeCoherenceDiagram_zpse8c92f2a.jpg.html?filters[user]=140737398&filters[recent]=1&sort=1&o=0
For whatever reason, I cannot get this to post as a clickable link here, sorry. Perhaps someone else can! At least Cut and Paste works, so please open this image in a new window/tab and magnify, as it illustrates much of what takes too many words to explain.
Here we go:
When a speaker spits out a brief piece of sound, making just a "Beep" then falling silent, what is moving towards us is a chunk of higherthenlower ("wavering") air pressure. The air itself is not going anywhere.
I encourage you to conceive of this as a traveling packet of sound, silent before and silent after, a packet that contains perhaps six wavelengths (six cycles) of a pure tone just like what is emitted from a tuning fork.
However, that one "Beep", high or low ("Boop"), is not a perfectly accurate example for a 'pure tone'. We must imagine instead that "Beep" stripped of its B and p, leaving only the eee or ooo.
That simple burst of an 'eee' or 'ooo' still conveys useful information- perhaps to warn of a car door ajar. Its message comes from its possession of just two characteristics:
1) It has a unique tone, high or low on the scale,
2) Lasting for a unique period of time.
Thus, to make and then hear any message takes both tone and time. Time is important to our sense of hearing.
Non-time-coherent speakers delay bass tones more than voice range tones, and those tones more than its highs. Another way to state this is their highs always come out first/too soon. This cannot be completely fixed by digital delays nor by stepping the tweeter back from the plane of the mid, because the amount of time delay is DIFFERENT at EACH frequency, which also leads to serious measurement difficulties for most designers.
Perfectly time-coherent speakers do not delay ANY tones whether bass, voice or treble.
Designers of non-time coherent speakers quote studies showing we cannot hear less than a 2 millisecond difference in arrival between the voice range and the high treble.
This means they believe it is OK for the mid's voice range to arrive up to ~60cm (two feet!) after the tweeter's highest tones. And even greater offsets/longer-delays are OK in the bass.
When those tests first came out, I saw they misled by only using tones that do not mimic the complex sounds of music, nor even resemble sounds anyone has grown up experiencing everyday naturally, so untrained listeners instinctively would know how 'it' is supposed to sound.
For myself, after years of being intimately near to the artistry of very many world-class musicians and singers, each for long hours, sometimes for days on end (rehearsals), I think "Who are engineers to say time delays are OK-- that somehow those don't screw up the music?"
I have no doubt everyone has watched even simple music bounce along on the computer screen. Know that we are observing only a miniscule faction of that music's waveform.
When a loudspeaker injects time delay, the shape of that waveform changes. The 'wave envelope' changes.
What is in that one shape, inside that one envelope? A zillion different sounds, each occurring with its own unique loudness, tonality, onset, duration, and decay. In that shape also lies the ebb and flow, the give and take, the emotion of the music, and the texture of each sound, its unique timbre ('tam-burr').
And we only get to see and record that one complex wave-shape per channel. So I think best to strive not to change it and hear what happens.
When this time-delay-as-we-go-lower is progressively removed from a speaker's design, along with the sonic reflections off its cabinet surfaces, then we always hear from any recording more and more the sound of people playing music 'over there'. There are no microphones. Our attention is no longer drawn to "the details" such as "the sound of the bass", "the airiness of the highs" or the sharpness of images.
Instead, even an inexperienced listener soon focuses instead on HOW the bass player is responding to the others (and thus WHY). All of the hi-fi 'details' are still there but now serve to shape the tones, to give each sound precision and purity (or perhaps a wandering pace and a fuzzy tone) RELATIVE to all the other musicians' sounds. Also, each musician remains distinctly separate in space, regardless of the music's complexity.
Ebb and flow, sudden changes- all are part of music, and hearing those makes sense to the ear. A much wider range of music is enjoyed, with little effort. Music FEELS good, just as if you were a teenager again, before becoming caught up in hearing all the very cool and entertaining 'hi-fi details'. You also do not need to turn it up.
By the way, speaker designs have become far more time-incoherent since the 1970's. Marketing pressures combined with the appeal to designers of 'new technology' has led to the use of many drivers that require high-order crossovers to operate.
This has given the majority of audiophiles, reviewers and designers loudspeakers that present "details" instead of music ("I can't stand loud rock recordings from these expensive speakers!"). They do not hear this as being a problem due to time-delays for any or all of several reasons:
a) They have not gone to enough intimate concerts, live theater or recitals, or sat in a living room for hours listening to singing or an acoustic guitar, clarinet, piano, a violin, played superbly. In that intimate environment, such sounds are to die for; far more breathtaking that us mere mortals can imagine- until we hear 'it'.
b) They play only a limited variety of recordings to evaluate gear, much of those recordings electronically manipulated, even of acoustic instruments (I read the reviews).
c) They have collected some non-musical gear and cables, as there is a lot of it out there, some very expensive. Very clean, but sterile, devoid of musical flow.
d) They have been told over and over again that time-coherence does not matter. No editor wants to piss off any large speaker manufacturer that uses only high-order crossovers.
e) They have lived with only very time-incoherent speakers, never with original Quads, nor electrostatic or planar headphones.
f) They do not know how to work the math of speaker design from a time domain perspective, and I don't like it either.
g) They have ears of cloth, for which no amount of exposure to live music can help. Fortunately, I "had to take piano lessons" as a youth, enjoying it and eventually playing much music well enough as an adult, from Mozart to Bartok to Joplin, to know how terribly bad I still was compared to any prodigy! Now I am long out of practice, as the piano needs new strings and new action ($$$)-- an Everett upright grand from 1887, weighing 600 pounds with a bronze frame holding a spruce sounding board. Sorry to digress.
So, we have that "eee" from the speaker coming towards us at 343 meters per second, no matter whether it's a bass tone or a treble tone. It will arrive at our chair in about 10 milliseconds, to begin to push or pull on our eardrums, because we are sitting about 3 meters away. Unless the speaker delayed when this sound came out.
A jet liner up high is cruising at ~85% of this speed. Sound is about ten times faster than cars on a distant highway. It is the speed of that blast of pressure coming at us from an atomic bomb. We could see ordinary sound traveling if the air weren't so clear.
Non-time-coherent speakers create time delays by their choice of drivers, those drivers' locations from your ear, and the type of crossover circuits used.
Drivers have both mechanical time delays and electrical delays, as they are Transducers, which operate in both domains.
A tweeter may be stepped back from the mid, to "put it perfectly in phase with the mid" at the crossover point only. This still does not make for time-coherent operation (study my diagram and my other Audiogon posts in the links others supplied above, thank you).
ALL crossover circuits introduce time delays, but only first-order crossover circuits create time delays that naturally offset each other, when crossing from the tweeter to the mid, and mid to woofer, thus producing no RELATIVE time delay between the drivers, which is the (my) goal.
I hope this helps! For those wondering what the step response or the impulse response indicate in Stereophile, I advise you to study John Atkinson's explanations of them and to remember that, in those tests, nothing nearly as low 'C' above middle 'C' is shown. That takes a large, expensive anechoic chamber or careful measurement outdoors with the speaker up on a pole, far away from the ground, using a very loud pulse, one that likely damages any tweeter.
The detractors of first-order speakers talk of power-handling issues caused by the slow rolloff of the crossover allowing bass to get into a driver, even a tweeter, making it distort or melt. Maybe the mid's cone would ring in the tweeter's range, because of its strong resonance at a high frequency. For us, there has been no problem because we use the best drivers, by anyone's standards.
Detractors also claim there will be off-axis cancellations between drivers, leading to a weird tone balance for a listener off-axis. Not true with proper crossover points, slender cabinets, and a lack of cabinet reflections. Never is the math behind those claims shown, as it never supports them.
To the original poster- thank you for this opportunity, and know that one reason a tweeter is placed behind the plane of a mid is because the sound from that mid emerges first from down deep in the center of its cone, no matter the crossover slopes used nor the mid driver's design.
Best regards,
Roy Johnson
Designer
Green Mountain Audio
