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Sep042008

What is Time-coherent Sound?

Home > About > Our Difference: What is Time-coherent Sound? - How We Engineer It - AIG Article

 

Timing matters

Timing is a natural part of life. We can see the importance of timing. We hear it, too. Once you hear proper timing in speakers, you will wonder how you lived without it.

What is time-coherent sound?

It is sound remaining undistorted as time passes. 

Any sound worth hearing is never a simple, pure tone such as what comes from a tuning fork. We listen to 'complex sounds'. Each of those is audibly unique, and has its unique waveform as shown at the right:

The repeating pieces of these waveforms are radically different. What makes each pattern unique, makes the sound of each instrument unique, is that this pattern, 'this sound', is made up of a unique collection of those pure tuning-fork tones, something which is not intuitive. Fortunately, each tone in a collection has but three parameters: its pitch at any moment, its loudness at any moment, and its initial polarity. The infinite variety of combinations produces infinite uniqueness— more on this below.

Seems obvious we should preserve those wave shapes seen at the right, for the simple reason they visually represent what we audibly enjoy, from hearing when any change occurs to any tone in that collection, and how much it changes and how quickly too, and to its polarity.

These changing pure tones put together create the timbre of an instrument, and is what this particular illustration really shows— which is 'how' instruments sound different while playing the same note. In this figure, you can see each one repeating 'some feature' at the same instant another repeats itself. This 'same width' feature among them is what proves each is playing 'the same note', such as when tuning to A440.

Changes in the constiutent tones inside any complex waveshape can not only alter the timbre of the instrument, those can distrub anything else— the sweetness in a voice, the anger in another. The temporal arrangement of these constituent tones shapes all musical phrasing, makes rhythms move around, and determines the dynamic contrasts of all sonic events, from the deepest sonority to utter bombast.

These simple tones put together create all musical inflections small and large, which go on to create drama and emotion. Together in our minds, all these changes create the 'message' , the 'experience'.

A time-coherent speaker does not delay when any tone reaches your ear.
Neither does it delay any change in any tone, nor how quickly that change occurred.
It does not invert the polarity of any tone.

That's it. Seems sensible, logical— why would we ever want to delay any tone, delay any dynamic change, invert any tone's polarity? Yet, most speakers allow the high-treble tones to come out first, by delaying tones longer and longer as we go down the scale into the bass. Very many in the industry state, "Well, fortunately we cannot hear these delays." Which is true— they cannot. Music lovers can. Allowing these timie delays saves companies a lot of time and money, and makes designing speakers far easier.

To be very specific for you, in this figure we add just three tones together to create a complex waveform— one neither smooth nor perhaps exactly repetitive. That 'sound' is visually unique because of this pattern. That 'sound' is audibly unique because, "That's how it sounds!"

Each of these three constituent tones has its own pitch. Any pitch can change at any moment, perhaps from a finger sliding up a string.

Each tone has its own loudness— here two have the same loudness. Any tone can change in its loudness at any moment.

Each tone can come and go at its own time. Maybe any change to one tone happens in synchrony with changes to other tones.

Any change of any kind may happen quickly or slowly.

Each tone has its own polarity.

 

The problem

A speaker's crossover circuit separates complex sounds into their constituent tones, sending the high tones to its tweeter and low tones to its woofer. Most all circuits delay the low tones compared to when the high tones emerge. Each type of circuit design injects a different amount of time delay at each frequency. This is the main reason all speakers sound different and certainly why none ever sound like 'the real thing'. Many speakers also flip the polarity of an entire range of tones 'upside down'.

In the illustration below, its left column shows three bursts of sound pressure arising from the woofer, mid, and tweeter cones/dome moving forwards as quickly as they can. Each is then allowed to return to rest.

No ongoing waves are produced, just three pulses of increasing air pressure, then a return to normal air pressure. Those travel across the room, pushing eardrums inwards, then allowing them to return to rest.

If we had shown any negative-going pulses, those are air pressures lower than 'normal' air pressure. They allow eardrums to be pushed outwards by the 'normal' pressure behind them— hence the need to 'pop' our ears. We also show each pulse starting at the same moment— from each cone/dome to beginning to move at the same moment. Each pulse reaches the same peak loudness, just to keep things simple.

Each pulse rises to its maximum pressure as quickly the speaker's crossover circuit allows the cones of woofer and mid to move outwards. The tweeter pulse's 'rise time' is limited by the mass and stiffness of the tweeter's dome. Each pressure pulse drops back to normal as quickly as the speaker's crossover circuit allows the cones of the woofer and mid to move back to their rest positions, and by the lack of pressure behind them, including behind the tweeter's dome.

Now, in the middle column, the time-coherent speaker lets them start at the same moment, at Time = Zero. The non-time-coherent speaker delays the start of the mid and woofer pulses. That is not quite right. These starting points are actually blurred/made vague by that speaker.

In the right column, these three pulses are simply added together, exactly as in the real world. The resulting pulse shapes are completely different. Their sound is completely different. 

These time delays can be called 'phase shifts' but that is inaccurate and misleading. While you don't want to see the math proving this, we can tell you this is because 'phase' is a relative term, and seldom will you be told "Relative to what?".

To minimize time-delay distortions, a designer must first completely understand what happens to all parts of a speaker dynamically on music at every instant, both electrically and mechanically. He must also know how and why, psychoacoustically, a listener experiences any type of sound.

The time delays are very short, because the speed of sound is high, part of why people think they can be ignored. In non-time-coherent speakers, compared to the high-treble, tones in the voice range arrive at least a thousandth of a second (1ms) late. In the low bass, the delay can be any amount from at least 5 to even 25ms. These delays are completely equivalent to moving a midrange driver back by 12 inches (30cm) behind the tweeter, and a woofer back by at least five feet, perhaps by more than 20 feet (7m). 

Current Green Mountain speakers are accurate within a few millionths of a second in their upper ranges and within one thousandth of a second in their lower ranges. No one has ever achieved these values, a level of time-coherence that makes music come truly alive, 'musical' in every sense of the word, and highly emotional even playing at a whisper. Once you hear proper timing in a speaker, you will wonder how you lived without it.

Here is less than one half second of Ray Charles singing the letter "a" in the word, "say" from his song, "What'd I Say." Since only Ray Charles sounds like Ray, that comes from the rasp of his voice, the hiss from his breath, subtle changes in his loudness and vibrato, the shape of his vocal tract, nasal cavity, teeth, tongue, damage to his vocal cords, and much, much more. And all of that is hidden in this one complex waveform, with no piano or band- only his echo/reverberation added by his sound engineer.

In this image, when you squint your eyes, the overall ups and downs of the larger waveform can more easily be seen. Those represent the main tone of his 'a.' The small wiggles riding on top of that main wave are unique to him. No one else can produce those. To re-iterate: Change the timing of any of the individual sounds —the when of any wiggles, large or small— and that new waveform could well be of your cousin singing in the shower.

Does timing matter? You bet. And our speakers preserve the original timings of your music better, incomparably better, than any other speakers on Earth. That's why we've been around for now more than twenty years, why our Owners keep their speakers forever, why they no longer visit the speaker forums, and why we offer them our Happy Ears For Life Warranty™. It also explains our numerous industry awards!

Gallery: A Nanosecond in Time

Gallery: Matters of Timing

 

 

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