Time Alignment of Speaker Drivers

[KSTR]

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

Ah, you misunderstood what is meant by target function. The target function is the frequency response of SPL magnitude and phase we want to achieve. The speaker has its own response and thus the electrical filter needed has to reshape that response to come as close as possible to that acoustic target.

In simple words, the electrical filter is the acoustic target response divided by the transducer's raw response.

 

[Sal1950]

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.
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.
View attachment 502677View attachment 502678

Was the change to the rounded baffle corners in itself something measureable?

 

[Nopiano]

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?

View attachment 502640

I think their point is that every pair of ears isn’t at the same height, hence they make adjustments.

 

[Vladimir Filevski]

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.

Time alignment is critical at crossover frequency which is common to both woofer and tweeter, so no long wavelengths involved here.

 

[Palmer]

One of my all time favorites, the UREI Time-Align 813

Cheers...

 

[dualazmak]

Many active speakers with DSP now can compensate for delays in software without sacrificing horror story baffle steps on the tweeters like shown above.

Yes, fully agree with you!

I have been applying rather primitive but well validated and reproducible methods (independent from REW) for time alignment (and phase+gain continuity) measurements and tunings in my PC-DSP-based multichannel multi-SP-driver multi-amplifier full-active stereo audio setup all has been shared on my project thread;

- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-1_ Precision time-shifted pulse wave matching method: #493
- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-2_ Energy peak matching method: #494
- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-3_ Precision single sine wave matching method in 0.1 msec accuracy: #504, #507

- Measurement of transient characteristics of Yamaha 30 cm woofer JA-3058 in sealed cabinet and Yamaha active sub-woofer YST-SW1000: #495, #497, #503, #507

- Perfect (0.1 msec precision) time alignment of all the SP drivers greatly contributes to amazing disappearance of SPs, tightness and cleanliness of the sound, and superior 3D sound stage: #520

- Not only the precision (0.1 msec level) time alignment over all the SP drivers but also SP facing directions and sound-deadening space behind the SPs plus behind our listening position would be critically important for effective (perfect?) disappearance of speakers: #687

- Summary of critical factors in integration of subwoofer(s) and main woofers in our individual acoustic environments: #3(remote thread)

In case if you, including OP @RegularStereoGuy, would be seriously interested in using/applying the tone-burst test signals I prepared for the above measurements and tunings, please simply PM me writing your wish.

For the latest details of my multichannel multi-SP-driver audio rig (including the time alignment tunings in DSP), you would please refer to my post #931 and #1,009 on my project thread.

Edit:
For subjective listening evaluation (it is the "must"!) for such objective time alignment and phase continuity tunings, I highly recommend you establishing your own consistent "Audio Reference/Sampler Music Playlist" consists of various genres of music tracks fit for your personal preference of excellent recording quality.

You would please find my such "Reference Music Playlist" here #1 (many links to my playlist post series) on the independent thread.

 

[theREALdotnet]

But it should be taken into account that the minimum-phase behavior of every analogue filter does induce a group delay or phase distortion not dissimilar to a part of an allpass. That is particularly the case with high-order and/or high-Q low-pass filters, basically shifting the phase of all signals they let pass more or less. Inverted tweeter polarity compared to the midrange leading to perfect summation in amplitude on axis, is an indication of a +180deg phase shift of the lowpass, which is in theory the result of a textbook Butterworth-style lowpass 2nd order.

Geometric time alignment in passive loudspeakers basically makes sense only if all the latter effects are taken into account and confirmed by measurements. Achieving equal distance between drivers and the ears alone, would not solve all the problems.

Yes, it should be obvious that all these aspects need to be designed to work together. Line-of-sight distance means nothing.

A classic example is this:

Recessed mid-woofer (tweeter mounted to front of baffle, mid-woofer to the back) and inverted tweeter polarity, to work in unison with the crossover.

 

[RegularStereoGuy]

Unfortunately, the acoustic center of the woofer is not where the dust cap is. It is some distance in front of the driver and is typically outside the loudspeaker enclosure.

Then why are the tweeters recessed on Genelecs and some other speakers? They must have had a reason for doing so.

 

[RegularStereoGuy]

That is quite interesting! Never seen that before. But this is not quite what we're after, is it? This mostly shows the point in space at which the waves appear to originate. This is, however, as far as I can understand, separate from the timing aspect of it.

"Do diamonds shine on the dark side of the moon?"

Yes Syd does. Miss ya Barrett!

 

[Sal1950]

Yes, it should be obvious that all these aspects need to be designed to work together. Line-of-sight distance means nothing.

A classic example is this:

View attachment 502710

Recessed mid-woofer (tweeter mounted to front of baffle, mid-woofer to the back) and inverted tweeter polarity, to work in unison with the crossover.

Not sure I would then agree with the raised edges both around the drivers from the front baffle and from the cabinet?
I've been told that was a no-no for a long long time ?

Same reason I questioned @DanielT about the redesign of his DYI cabinet.
I'm so confused.

Was the change to the rounded baffle corners in itself something measureable?

 

[NTK]

You do understand that this is completely different acoustic center, not relevant for time alignment?

Time alignment is critical at crossover frequency which is common to both woofer and tweeter, so no long wavelengths involved here.

Should have look up Wikipedia before hitting the keyboard. Now I know I am totally clueless. Sorry for disturbing this thread. I'll see myself out

Loudspeaker time alignment - Wikipedia

en.wikipedia.org

 

[RegularStereoGuy]

Should have look up Wikipedia before hitting the keyboard. Now I know I am totally clueless. Sorry for disturbing this thread. I'll see myself out

Loudspeaker time alignment - Wikipedia

en.wikipedia.org

It's my fault also. What I illustrated in my topic-opening diagram is not time alignment. It's called something else, but I can't seem to think of it right now.

 

[theREALdotnet]

Not sure I would then agree with the raised edges both around the drivers from the front baffle and from the cabinet?
I've been told that was a no-no for a long long time ?

There are no raised edges around the drivers, just around the outside of the baffle (to be flush with the front grille). What you see around the tweeter are felt strips.

 

[Sal1950]

That's a cop-out. I got eyes and can see that just at the edge of the "felt" a 1/2 inch step starts, at what point didn't it matter any more?
Specially around the tweeters most modern serious designers are taking steps to avoid issues.
Witness the steps Revel has taken even around their waveguide on the M106. Way too many other examples to start posting more.

 

[Keith_W]

Then why are the tweeters recessed on Genelecs and some other speakers? They must have had a reason for doing so.

They are recessed because of the waveguide. They handle the time alignment via DSP, which is a vastly superior way of doing it.

 

[Waveform Fidelity]

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.

Exactly, with my KEF R900 UniQ driver - I equalize the phase of both drivers to be the same - around 120 degrees and then adjust the delay between MF and Tweeter using a pulse signal driving both drivers. You can observe, on an Oscilloscope, the 2 pulses from each driver and “tweak” the delay so that both pulses occur at exactly the same time. As a check I then reverse the phase of the tweeter and check that the 2 acoustic signals are cancelling each other - I get a 40dB null.That’s pretty much perfect and the power of using DSP and most of that perfection comes from the coincident UNIQ driver!

The practical problem is extracting the correct minimum phase - ie. removing the time of flight from driver to microphone, which is needed before any time alignment is done.

Check out the Soundeasy web site - Bohdan has developed an automated way of doing this using the Hilbert Bode Transform (HBT) and the Inverse transform (IHBT). This is the best method that I have seen and used so far. I have verified a few times that the time of flight obtained by his methods and a my pulse-propagation delay measurements are in very close agreement.

 

[Arindal]

Then why are the tweeters recessed on Genelecs and some other speakers?

Are the tweeters really recessed in Genelecs? In the 3-way coaxial models these seem to be pretty aligned with the midrange plane, and the waveguide of the 2-way models is also not very deep. In either case, it is not really of importance, as they are all active, and mostly DSP-controlled, so can be time-aligned by delay easily.

 

[Arindal]

Witness the steps Revel has taken even around their waveguide on the M106.

Interesting example in this thread, as they seem to have accidentally created a maximum of lobing and directivity error step with this particular combination of midwoofer and tweeter:

You can see the naturally steep increase in directivity index between 1K and 2K, that is to be expected from a stiff 6.5" driven too high in frequency, subsequently exhibiting a proper summation of the 2 drivers at crossover point on axis (indication of perfect time alignment). The lobing suckout in the transitional band (x-over point around 2.3K) comes early and hefty, as visible in the +20deg and +30deg vertical amplitude responses, which is creating a pretty pronounced step down in directivity index and vertical reflection tonality for the lowest octave the tweeter is on its own (2.7K to 4.4K), without being supported enough by the waveguide (which presumably is too small for such wavelengths).

What does this have to do with time alignment? It is a pretty good example of how optimizing time alignment and amplitude summation at one particular point (on axis), will lead lobing and cancellation under different angles.

 

[Digital_Thor]

Yes, it should be obvious that all these aspects need to be designed to work together. Line-of-sight distance means nothing.

A classic example is this:

View attachment 502710

Recessed mid-woofer (tweeter mounted to front of baffle, mid-woofer to the back) and inverted tweeter polarity, to work in unison with the crossover.

If everything drowns in diffraction in this'wonderful' design, then phase is probably less of a problem.
Many aspects of audio, is not always worth a lot of attention, if all the basics are not fully dealt with.

 

[DanielT]

Was the change to the rounded baffle corners in itself something measureable?

Didn't measure. It has a trapezoidal shape..
Original:

...that reduces baffle edge reflections so I don't think sharp corners or rounded ones play a big role. Especially not with a waveguide tweeter.

I actually rounded the corners mostly for aesthetic reasons when I painted them. I thought they looked better that way.

Edit
OT
What really makes a difference is removing the original passive crossover and powering up the drivers separately with the flexibility to select the crossover point and slope on the crossover digitally or via active crossover.
But the vintage old bass woofers are still the Achilles heel. I can't get around that. I'm going to fix a spacer ring and then put my SB15NBAC30 woofers in them.