The question is if you want to "time align" the drivers, then "where those drivers are acoustically located" is a pertinent question. The time alignment concept make sense only if you can approximate the driver (e.g. the woofer) as a point. And the answer to the question of where that point is is not obvious. It is almost certainly frequency dependent, and we may not even know how to define it.
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Time Alignment of Speaker Drivers
- Thread starter RegularStereoGuy
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KSTR
Major Contributor
The acoustic center delta between drivers is where the time-of-flight measurement says it is, the absolute position doesn't matter.
Key point is to make high resolution low noise impulse response measurement of the raw driver response, then zoom in and locate the point where the signal emerges rapidly from the noise floor. For crossover duty, we are interested in the acoustic center for the end of the passband of the lower driver, and the start of the passband of the upper driver, respectively. This suggests using impulse response for the lower driver (which discards any too early LF better) whereas the step response (integral of impulse response) is better for the upper driver to reveal the start of its LF signal range better.
Practically, one can use two amp channels in the same IR measurement, with the sweep delayed on one channel by a known amount (a few seconds), then one single run can be used, with the mic far enough away (like >10 times driver spacing).
Geometrical time alignment is not really critical if you take care that the woofer is never later than the tweeter because delaying the woofer is trivial, in the crossover targets.
Key point is to make high resolution low noise impulse response measurement of the raw driver response, then zoom in and locate the point where the signal emerges rapidly from the noise floor. For crossover duty, we are interested in the acoustic center for the end of the passband of the lower driver, and the start of the passband of the upper driver, respectively. This suggests using impulse response for the lower driver (which discards any too early LF better) whereas the step response (integral of impulse response) is better for the upper driver to reveal the start of its LF signal range better.
Practically, one can use two amp channels in the same IR measurement, with the sweep delayed on one channel by a known amount (a few seconds), then one single run can be used, with the mic far enough away (like >10 times driver spacing).
Geometrical time alignment is not really critical if you take care that the woofer is never later than the tweeter because delaying the woofer is trivial, in the crossover targets.
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I once witnessed a demo with wilson audio speakers, where they said they tweaked the distance of each driver to match the distance to the listening position. Now that is total bs? Bass doesn’t travel slower then treble?
Vladimir Filevski
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You are forgetting that the same physical laws equally applies to the tweeter also - according to the same definition from the video/presentation, its acoustic center is forward also. When combining woofer and tweeter together, for time aligning tweeter should go behind the woofer (or using the allpass filter).
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You are also forgetting that because of the much smaller size of the tweeter (and the much shorter wavelengths of the sound it produces), the distance of its acoustic center forward of the diaphragm surface will be much less than that of a woofer.
I guess I could have made my point clearer in my post #17. I cannot find any basis in acoustics that one will achieve "time alignment" of loudspeaker drivers by lining them up according to some geometrically averaged centers of their radiating surfaces, such as the example shown in the opening post.
a4eaudio
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Disregarding the picture in post #1, isn't the typical approach to actually MEASURE the drivers and determine the acoustic offset (which is not a simple physical measurement, and can differ by frequency)? And, isn't that acoustic offset - assuming a flat faceplate tweeter and typical woofer - such that you would need to move the tweeter backwards (away from the mic/listening position) if you wanted the acoustic centers (at the crossover frequency) to be aligned?
KSTR
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Digital_Thor
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Nice and easy with active systems. Just invert the phase on the tweeter and move the tweeter "in time"(delay in DSP) - according to measurements - until the deepest cancellation is obtained together with the midrange. Re-invert the tweeter, and we're in-phase. Fact-check with a final measurement.
KSTR
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KSTR
Major Contributor
Note that this phase-based alignment approach has multiple solutions but only one is the correct wrt true time-alignment. You get a deep null at any multiple of 1/2 the wavelength, with alternating polarity.
Sokel
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The traditional picture as in first post draws the line at the voice coils, not at an imaginary position like the one posted.
The reasoning behind it is just because they dictate the start and end of the movement.
Of course even the simplest x-over (analog or digital) introduces it's own delays, etc.
But the origin of the picture is just that, voice coils, right or wrong.
The reasoning behind it is just because they dictate the start and end of the movement.
Of course even the simplest x-over (analog or digital) introduces it's own delays, etc.
But the origin of the picture is just that, voice coils, right or wrong.
KSTR
Major Contributor
But the foundation of the dominant class of analog crossover target functions is that there is no time delay between drivers which spoils the phase relationships.
That's the theory whilst reality is not purely represented in this.
Like my now halfway upgraded QLN One vintage speakers (now with Dayton ND25FW-4 WG tweeter, rounded baffle corners and new crossover). They have a slanting baffle.You can delay passively as well using an all-pass filter, tough it’s rarely used because of it’s component count. You can also just add a waveguide or slant the baffle to improve alignment, no needs for ugly steps. Even the Egyptians figured that out early on
I measured the distances and in terms of time alignment it turned out really well. However, I need a new, better woofer than the original, but that will come in the future.
KSTR
Major Contributor
What do you mean? An important aspect of the art of loudspeaker design is first designing the target functions and then trying to implement them with the given drivers and that can be done great precision.
Of course that's where we want to go, but it's very hard to find a 'transducer' on the market that represents these target function (the ideal step with linear FR in combination with, at least, constant phase).
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