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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?
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audiokinesis
1,872 posts
 

Ime TL’s are very good in the midrange, as there is essentially zero reflection back into the cone. That may well be their biggest advantage.

As for where the terminus (line opening or vent) is located relative to the woofer I’m sure that makes a difference but haven’t really analyzed it. My instinct would be to spread them apart as far as is reasonably possible in as many planes as you can, in pursuit of modal smoothing. Like if the woofer is up high on the front, and the terminus is down low on the back, if you toe the speakers in, now the woofer and terminus are displaced relative to one another in all three dimensions.

But I cannot reliably say that’s the best strategy for choosing where the terminus winds up - other considerations that I’m not taking into account may dominate. For instance the internal geometry may matter more.

I remember delivering a pair of my rear-ported speakers to the home of a customer who was replacing transmission lines that had the terminus on the front at the bottom. One of the first things he commented on was that the bass was smoother with my fairly low-tuned rear-ported box. Whether that was because of the terminus location versus my port location, or my speaker’s freedom from the half-wavelength bump and/or one-wavelength notch, I do not know. I was surprised that my speaker was competitive with the transmission line in the bass region.

Duke

mjking57
13 posts
 
A TL that is closed at one end and open at the other always produces a quarter wave standing wave resonance. It does not matter where the driver is along the length or if the geometry is tapered, straight, or expanding. Always a quarter wave. This means that at frequencies where the TL enclosure is producing output from the open end the phase must be +/- 90 degrees with respect to the driver. No matter what the physical length, the resonant frequency will be produced by a quarter wave resonance. You cannot produce a half wave resonance and the output will never be in phase with the driver output. Unfortunately that violates the laws of physics.
audiokinesis
1,872 posts
 

@mjking57 -

Thank you Martin, apparently I have some misconceptions. I hope you don’t mind if I ask a question or two.

I recall many of my primitive transmission lines of yesteryear having a distinct notch in the response that seemed to correspond with a line length of one wavelength. You can see what I assume is the same sort of notch in these measurements:

https://www.soundstage.com/measurements/pmc_gb1/

https://www.soundstage.com/index.php?option=com_content&view=article&id=775:nrc-measurements...

What’s causing that notch?

Thanks,

Duke

mjking57
13 posts
 
I don't know anything about the speaker measured or the measurement set-up, so my response is speculation. I have seen a similar notch in some of my measurements and have investigated the cause.

I don't believe any of the measurements that you have linked are for TLs, they appear to be bass reflex enclosures. I draw this conclusion from the electrical impedance plots where there are two almost equal peaks, one above and one below the tuning frequency, and no peaks from higher harmonics. If it was a TL and was stuffed to remove the higher harmonics then the lower impedance peak would also be strongly suppressed. If these speakers are not TL's then the cause of the null is not a cabinet standing wave resonance.

I measured a TL that exhibited a deep notch at a similar frequency to the one in your linked plots. I modeled the design and was able to reproduce the null. My initial thoughts were that the notch was created when the open end and the woofer were not at the same location (assumed in almost all simulation software) or from a room boundary reflection. I simulated both conditions and found that the floor reflection was the cause as sound traveling from the woofer to the floor, reflected, and then traveling from the floor to the mic causing it to arrive out of phase and cancel most of the direct sound from the driver to the mic.

That is my speculation.
audiokinesis
1,872 posts
 

Thank you for replying, Martin.

Those measurements were made in the anechoic chamber at the Canadian NRC, so I don’t think the notches are floor-bounce cancellations.

Both of the speakers in my links above are marketed as "transmission lines", and both are floor-standing two-ways with a small mid-woofer near the top and the terminus on the front near the floor.

https://pmc-speakers.com/products/archive/archive/gb1

https://pmc-speakers.com/products/consumer/twenty/twenty24

Does that shed any light on the source of the notch?

Also, could you clarify something you said for me?  "At frequencies where the TL enclosure is producing output from the open end the phase must be +/- 90 degrees with respect to the driver."

So is the output from the open end ALWAYS in phase quadrature with the output from the cone, regardless of the frequency? 

Or is it ONLY in phase quadrature at the quarter-wave resonance frequency? 

My apologies if I'm asking something that should be obvious.

Thanks!

Duke

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