time alignment/acoustical center jbl 2426 and jbl 2206

[panduro]

Hi all, im building a 2-way speaker but the project is a bit stuck and i need a bit of help :o:

the speakers consists of

hf - jbl 2426 with a 3,54inch/9cm deep waveguide
lf- jbl 2206.

I think that Time alignment in my application(listening distance 7feet) is even more critical than if listening distance were larger.

My baffel is 1,8 inch thick and i would like to use mounting dept as a variable to realign the acoustical centeres.

but i dont know where the acoustical centeres of the drivers are? can anybody tell me some generel rules, directions or measurements?



best regards

bent

[Robh3606]

I tend to use the voice coils to align the centers, not entirely correct but I have gotten good results. You may want to try a free set-up so you can move them and look at the impulse response and use that to get the actual spacing at your crossover point. Then just work the spacing info back into the baffle spacing.

Rob:)

[Lee in Montreal]

On the compression driver, isn't the source at the diaphragm, and on the woofer at the cone's dome? :D

[4313B]

On the compression driver, isn't the source at the diaphragm, and on the woofer at the cone's dome?

AES E-Library » On the Movement of a Horn's Acoustic Center (http://www.aes.org/e-lib/browse.cfm?elib=8194)

Subject: The Acoustic Center: How it applies to Loudspeaker ... (http://www.audioheritage.org/url?sa=t&rct=j&q=horn acoustic center&source=web&cd=3&ved=0CDwQFjAC&url=http%3A%2F%2Faudioroundtable.com%2Fforum%2Fpdf .php%3Fth%3D16472%26&ei=wR9BT7X4OpP4gAfE-MitCA&usg=AFQjCNGqiB-VG0Ia0WQBXQM-7lJSSC83nw)

[maxwedge]

On the compression driver, isn't the source at the diaphragm, and on the woofer at the cone's dome? :D
That makes logical sense but most everything I have read on the subject, it isn't the case. Voice coil alignment.

Anyway it's an easy thing to hear with your ear if you have the gear. Basically run a test tone of your x-over freq and then move a driver forward or rearward. When it's loudest your pretty close.

[maxserg]

AES E-Library » On the Movement of a Horn's Acoustic Center (http://www.aes.org/e-lib/browse.cfm?elib=8194)

Subject: The Acoustic Center: How it applies to Loudspeaker ... (http://www.audioheritage.org/url?sa=t&rct=j&q=horn%20acoustic%20center&source=web&cd=3&ved=0CDwQFjAC&url=http%3A%2F%2Faudioroundtable.com%2Fforum%2Fpdf .php%3Fth%3D16472%26&ei=wR9BT7X4OpP4gAfE-MitCA&usg=AFQjCNGqiB-VG0Ia0WQBXQM-7lJSSC83nw)

The last link is dead...

[4313B]

Fixed

http://audioroundtable.com/forum/pdf.php?th=16472&

[Lee in Montreal]

That makes logical sense but most everything I have read on the subject, it isn't the case. Voice coil alignment.

Considering the depth of a 2441 or 2405, where would you align? The front, the middle, the rear? In the case of a 2441 that has the diaphragm in the back, and a 2405 with the diaphragm at the front. Where do you align? I have learned not to always believe what's on the internet and use logic instead. Maybe I am wrong, but I prefer to use logic as at least I will have arguments. ;)

Edit - BTW The voil coil is basicaly the diaphragm. 1/4" away ;-0

[Robh3606]

All you do to align the voice coils is use the Top Plate ring on the top of the magnet. The voice coil will be centered there.

Rob:)

[maxwedge]

On the compression driver, isn't the source at the diaphragm, and on the woofer at the cone's dome? :D


That makes logical sense but most everything I have read on the subject, it isn't the case. Voice coil alignment.

Anyway it's an easy thing to hear with your ear if you have the gear. Basically run a test tone of your x-over freq and then move a driver forward or rearward. When it's loudest your pretty close.



Considering the depth of a 2441 or 2405, where would you align? The front, the middle, the rear? In the case of a 2441 that has the diaphragm in the back, and a 2405 with the diaphragm at the front. Where do you align? I have learned not to always believe what's on the internet and use logic instead. Maybe I am wrong, but I prefer to use logic as at least I will have arguments. ;)

Edit - BTW The voil coil is basicaly the diaphragm. 1/4" away ;-0
I said: "what I read", in general, not what I do. Logic is fine with me but I don't like to argue, usually.:)

[Mr. Widget]

...but I don't like to argue, usually.:)And you've managed to hang around here for all these years? ;)


Widget

[grumpy]

LOL! ... that was my first thought. :D

Didn't Pi Speakers or someone else have a
method to align speakers that included a crossover
topology? ... More of an end-result measure than raw
input for a crossover design program?

[maxwedge]

And you've managed to hang around here for all these years? ;)


Widget
And why not? :dont-know:It's a good place, though I tend to get lost from time to time. Been hanging in the Les Paul forums for awhile.
Btw, the Marantz is still doing good.

Scott

[more10]

Get Holmimpulse, an interface with mic phantom power, and a measuring mic. Holmimpulse is not simple, but you can actually measure the "distance" to the diaphragm. If you measure at your listening position you will get perfect alignment there.

[panduro]

thanks for all your answers, looks like ill have to wait til i have all the drivers and go from there.

best regards


bent

[ivica]

Fixed

http://audioroundtable.com/forum/pdf.php?th=16472&

An interesting presentation. But for me one "question" :

..."..One of the definitions of the acoustic center is "the point where pressure attenuation from the
inverse-square law appears to originate." That is be the exact point where sound is the loudest..."

As I understand upper words, that means that this is the place from where sound starts to 'radiate' in spherical shape,
and 'acoustic center' is the center of the 'sphere'.

So, if that is the case, I think that acoustic center "Xo" (measured from the baffle) can be calculate in the
following manner.
X1- distances from the baffle for the first measurements
X2- distances from the baffle for the second measurements
n - differences [dB] in the measurements between the first and the second level of signal
Dx= X2 - X1

Xo=[ Dx/(1.122^n - 1) -X1 ]

But mentioned acoustic center is one thing , but just an other one is 'time alignment', that would include all time delays
mechanical, acoustic and electrical (crossover network). Even though that impulse response and frequency response are
correlated (frequency response is Fourier transform of impulse response), I have no ideas which of them is easier to apply in order to get easy and fast alignment.
In multiple drivers-box the listener position (or measuring microphone position) would make great influence to final alignment especially if the distance from the speakers-box is not so far.

Ivica

[4313B]

I measure the impulse response and align the z axis that way. Without measurement gear aligning the top plates is a reasonable solution, especially with cone transducers (Baffle tilts, tilting stands, etc. Arguably better with higher order networks as opposed to systems with first order networks such as the L110 for example).
Anyway it's an easy thing to hear with your ear if you have the gear. Basically run a test tone of your x-over freq and then move a driver forward or rearward. When it's loudest your pretty close.Yep. Being able to physically move one of the drivers (horn/c.d.) in the z-axis also works.