Timing in Main Speakers

[KEM]

I've been using the impulse response measurement provided by REW to align the timing of the left vs right speakers.
Below you'll see an impulse graph from REW, showing before and after "alignment". I can vary the overlap of the speakers by physically moving one or the other fwd or back. My speaker complement is two Revel f208s (4 drivers each) and a single Hsu ULS-15 sub (1 driver). Lots of drivers.
After alignment, the sound is more "centered", "stable", more "together". I wish I had better adverbs.

My questions are whether this adjustment is even necessary. Am I fooling myself with what I hear?. And where is the subwoofer on that Impulse Response graph? And what other information can I get from this type of graph?

 

[DVDdoug]

"Perfection" never hurts but it's probably not necessary. Sound travels at about 1-foot per millisecond (and a 1kHz soundwave has a wavelength of about 1 foot). Your left & right ears are in different locations and the distance between your ears is about 1/2 millisecond (500us). But the difference from the speakers to your ears isn't that much because it's triangles.

And your head isn't in a vice so you really can't get perfection with location. And there might be more than one person listening, obviously at different positions. If you play a constant tone at around 5kHz or higher and move your head around you'll hear it get louder and quieter as the left & right and reflected waves go in-and-out of phase. We usually don't notice these changes with music.

I was doing some high frequency experiments once and just moving around behind the SPL meter was making big variations in the measurements. I never noticed music or the TV sound changing with someone walking around behind me, but I'm sure it would show-up with measurements. (I might hear a change with high-frequency tones, but that's an experiment I haven't done.)

 

[DonH56]

If you think it helps, and the impulse response is better, then go for it. I have, and have not, aligned speakers over the years and generally felt it helped audibly as well as measurement-wise. For echolocation (think that's the word) we (humans) can resolve a difference of 5~10 us, or about 1/10th inch, a number much smaller than I had thought before learning the real number. In practice, with music, the number is much larger (at least for me) but I routinely (and anally) try to get my main speakers within a quarter inch or so.

Besides imaging/location, moving the speakers affects the way they interact with the room, and sometimes small changes can have outsized effect.

FWIW, I generally agree with @DVDdoug, although the distance between your ears isn't really applicable AFAIK.

As for the graphs, you can determine things like how well the speakers recover from the impulse, how well aligned are the various drivers in the system, absolute phase, and so forth.

 

[KEM]

"Perfection" never hurts but it's probably not necessary. Sound travels at about 1-foot per millisecond (and a 1kHz soundwave has a wavelength of about 1 foot). Your left & right ears are in different locations and the distance between your ears is about 1/2 millisecond (500us). But the difference from the speakers to your ears isn't that much because it's triangles.

And your head isn't in a vice so you really can't get perfection with location. And there might be more than one person listening, obviously at different positions. If you play a constant tone at around 5kHz or higher and move your head around you'll hear it get louder and quieter as the left & right and reflected waves go in-and-out of phase. We usually don't notice these changes with music.

I was doing some high frequency experiments once and just moving around behind the SPL meter was making big variations in the measurements. I never noticed music or the TV sound changing with someone walking around behind me, but I'm sure it would show-up with measurements. (I might hear a change with high-frequency tones, but that's an experiment I haven't done.)

I understand.

REW uses a swept tone measurement to come up with the results it calculates. All frequencies are involved, some are angle and position dependent, some are not. I use a single microphone at the listener's location, so I realize that there will be differences when I introduce something else - such as my ears.

I am curious about the detais of this result. What does the shape and duration say about the system, such as driver alignment. And where is the woofer? It would be nothing if not fortuitous that I happened to place the sub in an exact spot where its timing doesn't show on this impulse plot.

 

[mixsit]

I've been using the impulse response measurement provided by REW to align the timing of the left vs right speakers.
Below you'll see an impulse graph from REW, showing before and after "alignment". I can vary the overlap of the speakers by physically moving one or the other fwd or back. ...

Hi. If you don't mind I'd like to ask a few questions, to try to understand better.
I thought initially the two curves might be a left and right arrival, but you've stated it is 'before and after "alignment". (But still both speakers?)
What seems curious is the alignment of both plot's initial arrivals, and first 100us are spot on?
Last, more general.. Might we use careful distance measurement matching for our alignment?
I used to get into these aspects with the speakers and microphone placements in my home studio when it was active. It's very interesting stuff'.
Thank you.
Wayne Smith

 

[dualazmak]

I too occasionally use REW for time alignment tuning and room acoustic measurements (wavelet analysis), but I also perform time alignment tuning by my rather primitive straightforward "time-shifted multiple Fq rectangular-tone-burst sequence" methods (and compare the results given by the two methods).

If you would be interested, please visit my relevant posts 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

- Identification of sound reflecting plane/wall by strong excitation of SP unit and room acoustics: #498

Incase if you would be seriously interested in using/applying the test tone signals I prepared and used in the above measurements and tuning efforts, please simply PM me writing your wish.

These posts would be also of your reference and interest, I assume;

- 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

- Reproduction and listening/hearing/feeling sensations to 16 Hz (organ) sound with my DSP-based multichannel multi-SP-driver multi-amplifier fully active stereo audio system having big-heavy active L&R sub-woofers: #782

- A nice smooth-jazz album for bass (low Fq) and higher Fq tonality check and tuning: #910, #63(remote thread)


Just for your possible further interest, you can find here (#931) the details of my latest system setup.

 

[KEM]

Hi. If you don't mind I'd like to ask a few questions, to try to understand better.
I thought initially the two curves might be a left and right arrival, but you've stated it is 'before and after "alignment". (But still both speakers?)
What seems curious is the alignment of both plot's initial arrivals, and first 100us are spot on?
Last, more general.. Might we use careful distance measurement matching for our alignment?
I used to get into these aspects with the speakers and microphone placements in my home studio when it was active. It's very interesting stuff'.
Thank you.
Wayne Smith

Hi, thanks for your questions. The plot is the result of two measurements made.

The REW sweep generates a wealth of data. Among the data is the Impulse Response..

The first measurement was both speakers + sub at "nominal" separation. This is the green trace. I used a tape measure to allow me to set the distance from the listener's position (LP) to each speaker and between the speakers. I set all three distances (LP to L spkr, LP to R spkr, and spkr to spkr) as close to the same as possible. I concluded that the two humps are from the spkrs and that the time delay is the result of the sound from the L and R arriving at the LP at slightly different times. The delay is about 200usec, which corresponds to about 2.5" or 63.5mm. This isn't much.

For the second (red) trace, I physically moved the R spkr back and forth, keeping the L spkr and LP in the same spots. I kept moving the R spkr until the two peaks overlap. The impulse response does not tell me direction, so in order to create the red trace, I move the R spkr one way (back or forth), measure, observe whether the two peaks have gotten closer or farther away and then move the R spkr in the direction that moves the peaks closer together.

I remeasured the LP to spkrs distances afterward. The distance (as measured by a tape measure) to the L are about 0.5" longer than from the R spkr. I don't have an explanation for this result.

REW allows one to remove the impulse response delay. This allows the two plots to coincide at the start.

As pointed out in another post, I may be fooling myself by thinking this timing difference is audible. But since it has caused no harm, I will keep it that way.

I hope this explanation has been helpful. If not, please ask for further clarification.