MODULATED lobing and response

[Bill Fitzpatrick]

In another thread Guss make this post:

"With the woofers coil moving forth and back all the time is it really worth wasting some time to align the drivers to perfection???"

I thought this was a super question and it got me to thinking that the woofer's motion, particularly in a 2 way, is going to "modulate" the lobing pattern and frequency response of the system.

Whether this is audible is another question.

Anyone care to discuss this modultation and it's potential audibility?

[sreten]

It doesn't occur therefore its not audible.

Its the same arguement than for bass / mids, bass causes
doppler distortion of the midrange, which it doesn't.

And if accepted full range drivers wouldn't have a prayer.

A non-issue,

sreten.

[Guss]

What is doppler distortion ? Why doesn't it occur, I mean the coil is really moving lol ?

[Svante]

Originally posted by Bill Fitzpatrick
In another thread Guss make this post:

"With the woofers coil moving forth and back all the time is it really worth wasting some time to align the drivers to perfection???"

I thought this was a super question and it got me to thinking that the woofer's motion, particularly in a 2 way, is going to "modulate" the lobing pattern and frequency response of the system.

Whether this is audible is another question.

Anyone care to discuss this modultation and it's potential audibility?

To get an idea of the magnitude of the problem, let's assume that the cone moves back and forth 10 mm by the low frequencies, and that drivers are separated by 100 mm, and that the main lobe is straight forward, the angle of would vary arctan(10/100)=+/- 5 degrees. Assuming a frequency of 2 kHz (the crossover frequency where both drivers are of equal magnitude) the polar diagram at 5 degrees of two sources separated by 100mm has dropped by cos(pi*f/c*x) or 0.15dB. (x=cone displacement, c=sound vel., f=freq)
So, straight ahead it would result in an amplitude modulation of
+/- 0.08 dB.

Audible? Hardly. At least not using the assumptions above and that I did the math right .

[Svante]

Originally posted by sreten
It doesn't occur therefore its not audible.

Its the same arguement than for bass / mids, bass causes
doppler distortion of the midrange, which it doesn't.

And if accepted full range drivers wouldn't have a prayer.

A non-issue,

sreten.

Are you saying that there is *no* doppler effect, or that it is way too small to be audible?

[Bill Fitzpatrick]

OK, let's assume Svante got the math right on the lobing; I'm tending to think he's right.

Now, how about frequency response?

If you have response curve X with the woofer mounted on the baffle, you would have curve Y with the woofer mounted forward by, say, 5mm and curve Z with the woofer mounted rearward by the same amount.

So, with the woofer mounted flush and moving back and forth 10mm as a result of some low frequency signal, the response is continuously sweeping from Y to Z. True - Yes. Audible - ?

I thought Doppler might arise - so who says Doppler is not a problem? Klipsch thought it was.

[Guss]

lol will someone tell me what is doppler distortion !

[Bill Fitzpatrick]

The Doppler effect occurs when an object emitting a continuous tone moves either toward or away from you. If it moves toward you the tone sounds higher in frequency and if it moves away from you it sounds lower in frequency. Like the sound of an ambulance passing you.

[Svante]

Originally posted by Bill Fitzpatrick
OK, let's assume Svante got the math right on the lobing; I'm tending to think he's right.

Now, how about frequency response?

If you have response curve X with the woofer mounted on the baffle, you would have curve Y with the woofer mounted forward by, say, 5mm and curve Z with the woofer mounted rearward by the same amount.

So, with the woofer mounted flush and moving back and forth 10mm as a result of some low frequency signal, the response is continuously sweeping from Y to Z. True - Yes. Audible - ?

The +/-0.08 dB I calculated was the frequency response change at 2kHz. AFAIU the maximium effect should be seen at the crossover frequency, so for the speaker with 2kHz crossover frequency, the effect should be of that magnitude, and no more.

It would be worse at higher crossover frequencies, but IMO not very important anyway.

[Bill Fitzpatrick]

Yeah, I guess you're right. The cone doesn't move far enough.

But, wait . . .

With one of those new, super high 1.5" XMax, 4" woofers crossed first order at 4K . . .