Acoustic centres of drivers/measuring

[RobWells]

Hello all,

I've got to the point in my speaker project where I've got all materials to begin construction, but still need to design the mid/tweeter enclosures. (BAF in the post mon/tues btw,Dave )

I know what they're gonna look like, but need to know how to find the acoustic centres of the drivers, so they can be physically time aligned accurately....(s-speak revelator tweeter/ focal audiom 6wm mid/ sspeak 6585-01 bass)

Does anyone know for sure how to measure this.?

I searched and got this thread:

Time alignment question

But they ended up deciding it was easier to use a co-ax than measure the centres....

I'm about to e-mail the manufacturers too, to see if they can give me a figure..

Thanx in advance,

Rob

[roddyama]

Hi Rob,

The "acoustic center" of a driver is not one of those hard numbers that you can measure and have it work for all situations and speaker configurations. The “acoustic center” of the driver depends on the shape of the polar dispersion pattern of the driver in the finished box. Actually, the polar dispersion pattern, and therefore the “acoustic center”, will almost certainly change with frequency, size and shape of the box, baffle arrangement, and driver type, along with a list of other factors. The polar dispersion pattern has to be measured in an anechoic chamber with the speaker in its final configuration. Even with this information, the “acoustic centers” will be an approximation that will change off axis because the shape isn’t a perfect circular arc.

What can you do?

First, don’t get fixated on the “acoustic centers”. This is just an approximate starting point. What you really need to do is to make the sound emanating from each driver reach the listener at the same time. This takes us to the following thread.
Can't Reproduce a Square Wave.

The easiest place to start is where a few post in your link said to start. Align the voice coils first. You can experiment by mounting the driver(s) for each range in separate boxes. It will make the task of making changes much easier. As you add x-overs or any other components that may have an effect on the alignment, you have the flexibility to make the necessary changes to give you the best sound. Once you got everything where you want it, you can build the final boxes incorporating what you’ve learned.

Hope this helps,
Rodd Yamas***a

[RobWells]

Hello Rodd ,and thanks for your input....

The speaker will be : 2 x scan speak 8565-01 in their own enclosure, with an audiom 6wm in its own enclosure above, and a scanspeak 9500-9900 revelator in its own enclosure above.

I have followed the thread you posted , - btw I believe that a conventional driver would produce a square wave if you super cooled the voice coil - it would also be more linear...... does'nt really mean much in the real world though.....

btw I will be using a 3 way active x- over (esp boards linkwitz-rieley)

The x-over will be phase aligned, which I am assuming means I only have to work out the offset to each driver from where I'm sitting, hence my question...)

you said "What you really need to do is to make the sound emanating from each driver reach the listener at the same time. "

This is what I'm trying to acheive

I thought if I got the mid point (zero bit, when no signal is present) of each driver the same distance from me, then the drivers would be phase aligned.....

Just trying to find out where on each driver the centre is?

Cheers

Rob

[RobWells]

Sorry Rodd,

should also of mentioned that the mid/ tweet will also have no baffle as such - the enclosures will be cylindrical wilth the driver at the end, and no baffle at all....

I hope this makes sense....

Rob

[planet10]

Originally posted by RobWells
the enclosures will be cylindrical wilth the driver at the end, and no baffle at all....

How big is the baffle, err, the faceplate on the revalator.

Acoustic centres are a moving target that as well as being somewhat nebulous move around as a function of frequency. Somepart of the voice coil is a very rough approximation. You really need at least an SPL meter and a sine wave generator ('puter into amp works fine) to get it right easily. Run a sin wave at exactly the XO frequency. Flip one of the drivers out of phase. Move a driver (top one cause it is easier?) until you get minimum output. The less reflections and/or deader the room the easier it is to do. I guess you could get closer than the voice coil guess with a sentitive ear.

dave

[RobWells]

Hello Dave,

I've borrowed the drawings for each driver from their web sites..

The 2x scanspeak 8565-01 per side are in approx 750 high x 280 wide baffle (thats the straight front before it goes into 55mm radius corners..)

Heres the tweeter one :

Rob

[RobWells]

And the mid range:

[RobWells]

And the bass.....

Cheers for helping...
Rob

[RobWells]

I've just got what you're on about with the centres..... Its the signal addition thing isnt it? ie, as the cone is moving out as part of a 400Hz signal, it's already going in and out from the higher frequencies......

Does this mean there is only phase coherance at the x-over frequencies?

Its looking like I've got to build my active x-over and buy the 3 amps before I finish constructing the cabs....

I'll do the top of the bass unit as a seperate piece, which will let me get started anyway..

Rob

[planet10]

Originally posted by RobWells

I've just got what you're on about with the centres..... Its the signal addition thing isnt it? ie, as the cone is moving out as part of a 400Hz signal, it's already going in and out from the higher frequencies......

That too. Even without signal addition the acoustic centre moves as the radiation frequency changes.

Does this mean there is only phase coherance at the x-over frequencies?

You want to get them coherent at the "join",

Its looking like I've got to build my active x-over and buy the 3 amps before I finish constructing the cabs....

Yes.

dave