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post #1 of 42 Old 08-09-2014, 10:21 PM - Thread Starter
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Who here can explain time alignment in detail?

So I've been getting into higher end audio in the last year. One thing I simply cannot fully understand is time alignment. I understand that it works to bring together the sound to make a image. What I do not understand is how beyond you change the delay of the driver in MS.

How does it help to make a proper center image? What exactly is happening to the sound waves when you reach that happy point?

How would you set proper time alignment manually. I know there are a few things you can play to get a proper center image but what about the rest of the width? How would you know?

If anyone has any insight on this or could point me toward an article/book that would be great. I can't really seem to find much on this subject.
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post #2 of 42 Old 08-10-2014, 05:51 AM
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I am, like you, eager to learn all of the basic and complicated aspects of designing and building speakers. I will share with you the little bit that I know regarding time alignment.

Basically, on the front baffle of any given speaker, the voice coil of the woofer, mid's, and tweeter or compression drivers are all at different distances from the front of the baffle, so when a signal is being fed through the crossover the drivers (ie:woofers, mid's, tweeters) are all emitting sound that is supposed to be in alignment in order to form a coherent sound stage and provide optimal imaging between the Left + Right channels.

Unfortunately the drivers are all emitting sound waves from different starting points. Again, optimal would be all of the drivers voice coils on the same plane and all the same depth so that when say...the woofer plays a bass note and the tweeter simultaneously plays a high frequency note from a guitar, the sound reaches the listener at the optimal point in time , versus the woofers bass note played on the songs bass guitar arriving after the note from the lead mans guitar as he plucks a string that is supposed to be heard in perfect timing and harmony with the bass note that was just played with a delay in time which caused it to smear the time and screw up the way the song is supposed to sound.

I have no idea how to change the time alignment in a passive crossover, BUT, I do know that this is where an active crossover is superior to a passive (well, active XO's are always superior to passive if you know what you are doing!) because with an active crossover you can play with the delay of the various drivers independently. In this case, if the woofers depth is 10" and the tweeters depth is 2", then you will possibly need to change the delay of these two drivers in order to achieve a coherent end result! Another way of combating time alignment issues is to have the front baffle of the speaker tilted in a way that makes the tweeter further away from you as the woofer & mid's. I believe that this is mostly not used as much today as it was 20 years ago by the likes if Theile and Dunlevy.

As far as how to measure for and adjusting delays to achieve perfect time alignment, this is beyond my skill set. I would like to hear more on this subject from folks with more experience, though!
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post #3 of 42 Old 08-10-2014, 06:06 AM
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Since you bring up imaging you seem to be asking about time align with respect to left/right speakers. Imaging requires that you hear the common channel information identically from both speakers, and that includes arrival times. In general that's something which will naturally occur unless you have one speaker physically closer to you than the other.

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post #4 of 42 Old 08-10-2014, 07:05 AM
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Quote:
Originally Posted by Michael_92 View Post
So I've been getting into higher end audio in the last year. One thing I simply cannot fully understand is time alignment. I understand that it works to bring together the sound to make a image. What I do not understand is how beyond you change the delay of the driver in MS.
Time alignment refers to the fact that every acoustical transducer has what is known as an "Acoustic Center" which is a point or a surface from which the sound the reproducer makes seems to emanate. In standard direct radiator speakers this usually coincides with the apex of the cone or the center of the voice coil.

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How does it help to make a proper center image? What exactly is happening to the sound waves when you reach that happy point?
Please step back one step. Time alignment has traditionally been a property of a speaker system. Signals that are out of time synch don't automatically add up to flat response. One way to obtain the best possible performance from a set of drivers is to arrange them and drive them in such a way that their acoustical outputs are in some sense time aligned.

If you have two speakers, the easiest way to create an acoustic image between them is for them to be as identical as possible and deliver signals that are synchronized with each other at your ears.

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How would you set proper time alignment manually. I know there are a few things you can play to get a proper center image but what about the rest of the width? How would you know?
A central image automagically happens if identical speakers being driven by identical electrical signals deliver identical sound to both of your ears. Interestingly enough the speakers don't have to be particularly good, just very similar.

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If anyone has any insight on this or could point me toward an article/book that would be great. I can't really seem to find much on this subject.
Here's a good starting point:

http://www.aes.org/e-lib/download.cf...86&name=harman

The topic is further expanded upon in this book by the same author:

http://www.amazon.com/Sound-Reproduc.../dp/0240520092

"Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms"

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post #5 of 42 Old 08-10-2014, 07:42 AM
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I use to have speakers from a company called Audio Physic. Mine where the Tempo 3's. These speakers had great sound and disappeared when playing in a room.That is what first impressed me about them. They were referred to as time aligned by physical design. They were beautiful speakers that were slightly angled back. His thoughts on time alignment were that the highs arrived at the ear slightly ahead of bass energy. By angling the face of the speaker by his 7 degree design it time aligned the drivers so that the sound arrived at ones ears at the same time. Do a search on Audio Physic Tempos 3's and you will see pics and info on this.
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post #6 of 42 Old 08-10-2014, 08:18 AM
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It looks like the responses you are getting assume that you understand time alignment and, of course, the definition is the focus of your post. The concept is quite simple. Higher frequencies travel faster in air than lower frequencies. What that means, obviously, is that higher frequencies arrive at your ears sooner from a speaker system. if you were a very long way from the speakers, it might matter but, in practice, it really doesn't matter because we can't sense the difference in arrival times in the few feet or yards from which we normally listen. We can measure the differences but not hear them.

Some speaker manufacturers have built "time aligned" speakers that use a cabinet that places the midrange further from from the listener than the woofer and the tweeter further than the midrange. The idea is to compensate for the time mismatch to some degree by having the various frequencies arrive at around the same time. You can see an image of such a speaker system below:

These time aligned speakers are fairly rare mostly because they address a measurable but not audible phenomenon and, as you might guess, they are usually expensive.

That is about all there is to it. It is certainly not something I would suggest you worry about.

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post #7 of 42 Old 08-10-2014, 08:28 AM
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Quote:
Originally Posted by Martycool007 View Post
I am, like you, eager to learn all of the basic and complicated aspects of designing and building speakers. I will share with you the little bit that I know regarding time alignment.

Basically, on the front baffle of any given speaker, the voice coil of the woofer, mid's, and tweeter or compression drivers are all at different distances from the front of the baffle, so when a signal is being fed through the crossover the drivers (ie:woofers, mid's, tweeters) are all emitting sound that is supposed to be in alignment in order to form a coherent sound stage and provide optimal imaging between the Left + Right channels.

Unfortunately the drivers are all emitting sound waves from different starting points. Again, optimal would be all of the drivers voice coils on the same plane and all the same depth so that when say...the woofer plays a bass note and the tweeter simultaneously plays a high frequency note from a guitar, the sound reaches the listener at the optimal point in time , versus the woofers bass note played on the songs bass guitar arriving after the note from the lead mans guitar as he plucks a string that is supposed to be heard in perfect timing and harmony with the bass note that was just played with a delay in time which caused it to smear the time and screw up the way the song is supposed to sound.

I have no idea how to change the time alignment in a passive crossover, BUT, I do know that this is where an active crossover is superior to a passive (well, active XO's are always superior to passive if you know what you are doing!) because with an active crossover you can play with the delay of the various drivers independently. In this case, if the woofers depth is 10" and the tweeters depth is 2", then you will possibly need to change the delay of these two drivers in order to achieve a coherent end result! Another way of combating time alignment issues is to have the front baffle of the speaker tilted in a way that makes the tweeter further away from you as the woofer & mid's. I believe that this is mostly not used as much today as it was 20 years ago by the likes if Theile and Dunlevy.

As far as how to measure for and adjusting delays to achieve perfect time alignment, this is beyond my skill set. I would like to hear more on this subject from folks with more experience, though!
This explains mechanical alignment, but isn't there also electrical alignments depending on how the AC signal's phase shifts as it passes through capacitors and inductors?
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post #8 of 42 Old 08-10-2014, 08:38 AM
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Quote:
Originally Posted by FMW View Post

...Higher frequencies travel faster in air than lower frequencies...
Do you have a reference to this?

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post #9 of 42 Old 08-10-2014, 08:56 AM
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Originally Posted by RayDunzl View Post
Do you have a reference to this?
News to me, too, though it appears to be true in non-ideal gases, e.g. air. Having said this, the chart shown here (bottom right) would seem to indicate that it's inconsequential when compared to physical distance and/or group delay.
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post #10 of 42 Old 08-10-2014, 09:21 AM
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Quote:
Originally Posted by FMW View Post
Higher frequencies travel faster in air than lower frequencies.

Some speaker manufacturers have built "time aligned" speakers that use a cabinet that places the midrange further from from the listener than the woofer and the tweeter further than the midrange. The idea is to compensate for the time mismatch to some degree by having the various frequencies arrive at around the same time. You can see an image of such a speaker system below:


That speaker is designed that way to time align each driver's acoustic center. Not because because there might be a very slight difference in the speed of different frequencies through air.


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post #11 of 42 Old 08-10-2014, 10:22 AM
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Driver time alignment as I understand it: the passive crossover inside the speaker (as it contains inductors and capacitors) introduces phase shifts for the different drivers at different frequencies. To compensate for the different acoustic centers AND the phase shifts created by the crossover, the mounting depth in the front baffle is adjusted to align the drivers.

The complication is that the phase shift is not the same for all frequencies, so like most things in engineering, there is a tradeoff on the mounting location because can only compensate for the time shift / phase shift at one frequency - so the mounting depth is usually selected to align the sound at the crossover frequencies.

And sometimes, it is the other way around - the crossover design actually intentonally introduces a phase shift to align the sound at the crossover frequency because it is physically impractical to do it via mounting location.

Those huge baffle steps on some "time aligned" speakers must be something else (probably marketing), because I don't think the wavelength at those frequencies of the high frequency drivers is even as big as the size of some of the steps. The wavelength of sound at 5KHz is only 2.5" so there should never be a need to adjust the mounting location more than that.

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post #12 of 42 Old 08-10-2014, 10:29 AM
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Quote:
Originally Posted by sivadselim View Post
That speaker is designed that way to time align each driver's acoustic center. Not because because there might be a very slight difference in the speed of different frequencies through air.
Right - and nice diagram!

Time alignment couldn't adjust for a difference in speed of different frequencies (I've never heard of this before!) because the distance to the listener is unknown and the arrival time at different frequencies would depend on the distance.

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post #13 of 42 Old 08-10-2014, 10:41 AM
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Quote:
Originally Posted by mtn-tech View Post
Those huge baffle steps on some "time aligned" speakers must be something else (probably marketing), because I don't think the wavelength at those frequencies of the high frequency drivers is even as big as the size of some of the steps. The wavelength of sound at 5KHz is only 2.5" so there should never be a need to adjust the mounting location more than that.
It isn't related to the frequency. It's simply related to the fact that (generally) smaller drivers are required to reproduce higher frequencies and a smaller driver's acoustic center is (generally) not as deep (as in, 'far back') as that of a larger driver.

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post #14 of 42 Old 08-10-2014, 10:52 AM
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From the graph provided on speed of sound at different frequencies:



343.4m/s at 10hz

343.5m/s above 100hz

0.1m/s slower at lowest frequency

At a 3 meter listening distance, 2.5 microsecond difference between low and high frequency wave arrival times at the listener.

Recommendation: Move the woofer back 0.8mm to compensate.
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post #15 of 42 Old 08-10-2014, 11:02 AM
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Quote:
Originally Posted by sivadselim View Post


That speaker is designed that way to time align each driver's acoustic center. Not because because there might be a very slight difference in the speed of different frequencies through air.


Your response and illustration agree with my understanding.

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post #16 of 42 Old 08-10-2014, 11:41 AM
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Originally Posted by shivaji View Post
I use to have speakers from a company called Audio Physic. Mine where the Tempo 3's. These speakers had great sound and disappeared when playing in a room.That is what first impressed me about them. They were referred to as time aligned by physical design.

They were beautiful speakers that were slightly angled back. His thoughts on time alignment were that the highs arrived at the ear slightly ahead of bass energy.

The acoustic center of most normal drivers is a function of the depth of the speaker driver. Dome tweeters would be an example of a speaker with an acoustic center that ends up close to the front panel. In contrast, the acoustic center of a woofer is some distance back. Longer distance = more delay. The bass energy naturally arrives late.


Quote:
By angling the face of the speaker by his 7 degree design it time aligned the drivers so that the sound arrived at ones ears at the same time. Do a search on Audio Physic Tempos 3's and you will see pics and info on this.

Angling baffles and stepped baffles were the hot thing for a time. However, it is also possible to electrically align a driver by means of crossover parameters. Angled baffles cost more to make, so the more common final answer was to work with the crossover's design.
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post #17 of 42 Old 08-10-2014, 11:50 AM
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Quote:
Originally Posted by RayDunzl View Post

343.4m/s at 10hz

343.5m/s above 100hz

0.1m/s slower at lowest frequency

At a 3 meter listening distance, 2.5 microsecond difference between low and high frequency wave arrival times at the listener.

Recommendation: Move the woofer back 0.8mm to compensate.
I think you mean forward, not back. Or tweeter, not woofer.

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post #18 of 42 Old 08-10-2014, 12:15 PM
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Oops...

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post #19 of 42 Old 08-10-2014, 12:25 PM
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Originally Posted by RayDunzl View Post
Oops...
math is correct, though

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post #20 of 42 Old 08-10-2014, 12:39 PM - Thread Starter
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Wow lots of information. Going to read through the linked stuff here in a few. TBH I didn't actually think anyone would reply. Guess I was wrong

I should have went into more detail in the OP. I run active crossovers in my truck and in my room I have active 2.0 with the other 3.1 using audyssey.

I was more looking for how the time delay per speaker can change stereo imaging/staging if that makes any sense. Not necessarily each individual driver.

Another question would be are there ways to place the speakers (I have bookshelf ones right now) to maximize and help control the staging? For example when I play a song in stereo it sounds like the sound is ALL around me but in my truck it sounds like everything is above the dash and is controlled. I have the speakers pointed at me if that makes a difference.
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post #21 of 42 Old 08-12-2014, 06:42 PM
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Sorry if this is over-simplistic, I'd rather appear patronising than not be understood at all lol.

Stereo imaging: Imagine your sitting in front of a full orchestra, say the violins are to the left and the trumpets to the right for example. Even if you close your eyes you should be able to pinpoint each instrument group within the orchestra (if you know what to listen for). The brain uses two mechanisms, delay and loudness difference. The frequencies from the right will arrive at the right ear about 1ms before the left ear (yes this time difference is detectable) and your brain uses the delay to localise the source of the sound. Also frequencies above about 1500Hz from the right will be much quieter at the left ear due to being blocked by your head.

(for more details on sound localisation see here: https://en.wikipedia.org/wiki/Sound_...uditory_system)

When a performance is recorded, mastered, and then played back on your speakers, ideally the delay information should be perfectly preserved so that if you close your eyes, you can still tell which side of the orchestra the violins are on just as easily as if you were listening to a live performance. The biggest barrier to achieving this is usually room acoustics, reflections, standing waves, resonances etc which will not only give you uneven frequency response (easily remedied with acoustic treatment and room EQ), but also completely screw up all the delay information between the L/R signals that your brain uses to localise sound, and that is much harder to fix.

Inside a space as confined (and with as many sound-reflective surfaces) as a car or truck, you have almost no hope of maintaining the delay information present in the original stereo signal. This doesn't mean it can't sound good, just that you won't get the correct stereo imaging (ability to pinpoint where each sound is coming from like a live performance). In this situation, just use some trial and error with delays (and placement if you can) till you get it sounding 'good' and leave it at that. You could spend $$$ acoustically treating the inside of your truck but that's up to you.

Inside an average room, you're still going to struggle to maintain really accurate stereo imaging without some acoustic treatment, absorption panels, diffusers, basstraps etc. In terms of speaker placement, the ideal would be to have both left and right be at equal distances and equal angles from centre at your listening position, while also having both speakers and your listening position be symmetrical with respect to the room (this also requires a symmetical room layout). If you cannot achieve this you can add some delay to either to left or right channel to compensate but it will not be quite the same as having the ideal layout. The exact amount of delay you should add obviously depends quite specifically on how your setup differs from the ideal layout I described.

It also comes down to personal preference, some people require very accurate stereo imaging or they cannot enjoy the music, some people don't care at all, most (outside audiophile circles) aren't even aware the issue exists. Personally I can appreciate when someone has put the time and effort into a system to create great stereo imaging (that is I have pretty good hearing and can easily tell the difference) but it doesn't factor at all into my enjoyment of the music, I don't find a lack of correct stereo image to be at all distracting like some do.

For surround sound (5.1/7.1) all the same principles apply but the situation is much more complex because you've got 5 or more channels interacting with each other not just 2.

One thing I've found helps bass integration in my 5.1 setup is to add some delay to all channels except the sub (so theoretically the sound from the subs arrives first). This can improve the sound in most small (or untreated) rooms because of the room-gain (cabin effect) at lower frequencies. The enclosed space 'amplifies' low frequencies because of standing waves, so Audyssey turns down the bass for a flat response. However, the room-gain is caused by resonance, which means it will take a few milliseconds (the duration of one or two cycles/waves) for the energy to build up at that frequency and the sound to reach the correct level. To compensate for this I have the subwoofers start making their sounds a few milliseconds early by adding delay to all the other channels (actually I just increase the sub's distance setting on the AVR but the effect is the same).
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post #22 of 42 Old 08-12-2014, 07:14 PM
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Quote:
Originally Posted by Michael_92 View Post
How would you set proper time alignment manually. I know there are a few things you can play to get a proper center image but what about the rest of the width? How would you know?
The procedure to do this is rather simple assuming you are talking about speaker position and time aliment between channels.

Many test discs have tracks that play a simple clicking sound. If the time alignment is correctly set the "click" should sound at an equidistant point between the two speakers.

Here is one.

http://www.chesky.com/various-artist...etup-di51.html

Maybe there are others perhaps from AIX, Joe Kane HD Video Essential etc....

After that, the next level of wide band is basically insuring that the chosen speaker has consistent on and off axis frequency response which is not an easy task.

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post #23 of 42 Old 08-12-2014, 08:23 PM
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Quote:
Originally Posted by FMW View Post
The concept is quite simple. Higher frequencies travel faster in air than lower frequencies.
No. NO NO NO. That's rarely the problem.

High frequency drivers, however, have less time delay (time delay is proportional to phase shift divided by frequency), so one must correct that in a proper system design.

With passive crossovers, this is less than trivial.

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post #24 of 42 Old 08-12-2014, 08:26 PM
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Also, there are two kinds of time alignment, one is interchannel, and one is intrachannel. Interchannel is easier, if the speakers match. Big 'if' in many cases.

Interchannel can be handled by distance from speaker.

Intrachannel, which does help with things like percussion, at least in some cases, must be handled either in the speaker, or via some DSP correction, and is not necessarily simple.

And, once more, the speed of sound is rarely the actual problem. 2.5 microseconds is not a big deal here.

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post #25 of 42 Old 08-13-2014, 04:39 AM
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Another question would be are there ways to place the speakers (I have bookshelf ones right now) to maximize and help control the staging? For example when I play a song in stereo it sounds like the sound is ALL around me but in my truck it sounds like everything is above the dash and is controlled. I have the speakers pointed at me if that makes a difference.
Vehicles tend to be acoustically dead - all those heavily padded seats in a small room. You are getting great localization of sound, but its not your preference. That "music all around me" perception usually happens when the sound is coming at you from all directions, which happens in a reverberent room.
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post #26 of 42 Old 08-13-2014, 04:49 AM
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No. NO NO NO. That's rarely the problem.

High frequency drivers, however, have less time delay (time delay is proportional to phase shift divided by frequency), so one must correct that in a proper system design.

With passive crossovers, this is less than trivial.
Yes, I understand. The concept is to get the sound from all the drivers to the ears at the same time. I misumderstood the reason for it.
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post #27 of 42 Old 08-13-2014, 12:03 PM
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From the graph provided on speed of sound at different frequencies:



343.4m/s at 10hz

343.5m/s above 100hz

0.1m/s slower at lowest frequency

At a 3 meter listening distance, 2.5 microsecond difference between low and high frequency wave arrival times at the listener.

Recommendation: Move the woofer back 0.8mm to compensate.
note that you are four or five octaves below the tweeter crossover when speed of sound becomes frequency invariant, for all practical purposes . . . IOW, the difference between speed at 10 versus 100 Hz doesn't have any effect WRT the tweeter if it's vanished by 2000 Hz.
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post #28 of 42 Old 08-13-2014, 12:16 PM
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My two way speakers cross at 180hz.

I'll be back later...


1.5RQ > digits > OpenDRC-DI > DEQ2496 > DAC2 > KCT > FPB 350mcx > reQuest
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post #29 of 42 Old 08-13-2014, 01:17 PM
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My two way speakers cross at 180hz.
You sure that's not 280Hz?

Either way, it IS low. Of course, you know that your speakers are.............. 'special', too.

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post #30 of 42 Old 08-13-2014, 01:47 PM
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Yes, I understand. The concept is to get the sound from all the drivers to the ears at the same time. I misumderstood the reason for it.
Well, I will say that if you're building a sound system for a stadium, you are not wrong. What doesn't matter in a 10' carry really matters a lot in a 150' carry.

And yes, that can rather affect the articulation of a PA system, too.

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