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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My understanding is that the strongest reinforcement from a transmission line is when line is 1/2 wavelength long. At that frequency, the backwave emerges from the terminus in-phase with the front wave, which gives full reinforcement. Unfortunately 1/2 wavelength lines are impractically large (and no, you can’t shrink the cross-sectional area significantly and still get good performance).

The lowest frequency at which the line is effectively reinforcing the front wave is when the line is about 1/4 wavelength long. At that frequency the backwave emerges from the terminus 90 degrees apart from the front wave, in "phase quadrature", which gives partial reinforcement.

Unfortunately at the frequency where the line length is equal to 1 wavelength, the backwave emerges 180 degrees out-of-phase with the woofer and we have a cancellation notch. Fortunately this notch largely disappears in the farfield response, but ime it is often still noticeable.

Various techniques exist to mitigate the 1 wavelength cancellation notch, including: Offsetting the woofer from the closed end of the line; incorporating a Helmholtz absorber into the enclosure; using a LOT of stuffing to absorb as much of that 1-wavelength energy as possible (and maybe using a high-Qts woofer so that the low bass is still strong); and using two woofers at significantly different locations along the line so that they are not both notching at the same frequency.

Incidentally no stuffing material, not even the best New Zealand lamb’s wool, slows down the speed of sound in the line to any significant extent.

I am NOT a transmission line expert; just happened to learn a few of the pitfalls the hard way.

Duke

Thanks, Duke.  This helps a lot.

Would a rear-facing or top or bottom facing vent help, if it was expelling an opposite phase signal?  The room them comes into play to disperse that signal, correct?  Is it true that a TL is also meant to lessen the signal from the back of the driver cone, and not always to enhance bass?

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

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.

@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