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Just wave!

Just need substantiation on a speaker building point:

With a TL speaker, one of the main reasons for the transmission line is to reverse the polarity of the wave, off the back of the speaker, so that it will be in phase with the front of the speaker cone, when it exits the port.  Knowing the resonant frequency of the speaker and its wavelength, we them determine the length of TL which will allow the inverse of the back wave to be 'happening',...when it leaves the port?  To put it another way:  we are bouncing the wave within the TL until we are, essentially, releasing it....while it is in the proper orientation.  Is that correct?
128x128sound22card
audiokinesis
1,872 posts
 

Thank you very much Martin for taking the time. Your last post makes sense to me.

I was having a hard time with the second half of this sentence:

"You cannot produce a half wave resonance and the output will never be in phase with the driver output. "

But this from your last post makes sense to me:

"At 100 Hz – the driver and terminus are now in phase."

So at 100 Hz (where the line length is equal to 1/2 wavelength) there is NO standing wave resonance, BUT the outputs from the driver and terminus are IN PHASE.

Have I finally got that right?

And "At 200 Hz – the driver and terminus are now out of phase."  (But there is no standing wave resonance.)  So IN THEORY at least, assuming a lightly-damped line, couldn't we get a cancellation notch in the summed response in that region?

Duke


mjking57
13 posts
 
You have it correct. And in theory, yes a notch could be produced. But the minute you add back in the damping produced by the acoustic impedance of the terminus, add fiber or foam damping in the line, place the driver along the line instead of at the closed end, and account for the offset positions of the driver and terminus on the baffle the sharp peaks and deep nulls start to smooth out. The terminus output between the 1/4 wave resonances starts to be attenuated and the phase shifts a few degrees so the reinforcement and cancelling is not as dramatic. You start to get the rippled rolling frequency response typically associated with TLs.

Thinking about the measurement plots you linked, if they have designed a TL that when damped still has a deep notch at a frequency around 200 Hz, between the 1/4 wave resonant frequencies, when measured in a chamber with no boundary reflections then the design needs further work in my opinion. The damping does not appear to be very effective below a few hundred Hz. In a correctly damped TL, tall peaks and deep sharp notches should not occur. Usually you are left battling the ripple and use driver offset to tame it.
audiokinesis
1,872 posts
 

Thank you once again Martin!  I REALLY appreciate your taking the time to go through and explain this to me and correct my misunderstandings.  Your example above of the 50 Hz line is extremely educational for me.  I had failed to appreciate that a standing wave occurs every time the line length puts its output at phase quadrature relative to the cone.  Imo this is valuable information. 

So now you have my wheels turning about making the enclosure deeper and putting the terminus on the back and using the path-length-difference (relative to the listening position) to help mitigate that one-wavelength cancellation notch.  What imo makes this approach promising is that, on either side of the one-wavelength frequency, we have standing wave resonances which shift the output primarily to the terminus, so there would be insufficient output from the cone itself for a cancellation notch.  I'd have to do some math to optimize it, but for now it looks like a possible window of opportunity.  Of course this isn't the only thing that would need to be juggled.

Duke

twoleftears's avatar
twoleftears
1,457 posts
 

https://pmc-speakers.com/technology/atl

The commentary says it's tapered, but the cut-away doesn't look tapered to me.  Mixed in with the advertising copy, there are little technical nuggets dotted throughout the 3 mins.

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