Transmission lines and phase

[JerryLove]

Am I correct in concluding that the sound added by a transmission line (that is to say: the sound coming out of the back of the drivers, moving through the cabinet, coming out the rear port, and eventually getting to my ear) will be significantly out of phase (like 360 deg +) with the same sound coming out of the front of the driver and heading "straight" to my ear?

In essence, will any sounds helped by a transmission line suffer "time smear" (is that the right word?) where the individual wave is longer than normal because it is emitted from the cabinet twice with a delay in between?

 

[Send Margaritas]

Jerry, I'm still learning here, and am not qualified to answer your question from a scientific, or theoretical perspective. However, I like woodworking, and am researching making some 'Mission Statement' speakers, that do have a ported (dipole?) design where the sound comes out the back for the midranges. I noted in the design specs (cross-over diagram) that the mids are to be wired out of phase. (Check out the wiring on the mids...it seems like some sort of consideration of this.) There was brief mention of this in the design specs. Link: Statements I'm not sure if the designers were authorities in the field of speaker design, but they look to measure well.

My understanding, as light as it may be, is that when a speaker cone moves, the sound inside the cabinet (coming from the back of the speaker cone) is out of phase with the sound coming out the front. So what you're saying about the sound coming out of a port or transmission line sounds plausible, to this noob. (Time for me to get some books!)

You're almost surely way ahead of me. I guess I added little of value , but your questions about phase and timing is similar to what I was curious about, and is interesting. Hopefully others with insight into the science or theory will weigh in. This sounds like a good thread to watch.

 

[Swerd]

Am I correct in concluding that the sound added by a transmission line (that is to say: the sound coming out of the back of the drivers, moving through the cabinet, coming out the rear port, and eventually getting to my ear) will be significantly out of phase (like 360 deg +) with the same sound coming out of the front of the driver and heading "straight" to my ear?

In essence, will any sounds helped by a transmission line suffer "time smear" (is that the right word?) where the individual wave is longer than normal because it is emitted from the cabinet twice with a delay in between?

I suspect that what you said oversimplifies things to the point where you may have wrong or irrelevant conclusions. Most of the theory behind transmission line cabinets is well above my pay grade, but as executed in the various speakers based on Martin J. King's design principles, it works well.

Read the various entries at MJK's web site. When you understand them, come back here and explain it to me .

Quarter Wavelength Loudspeaker Design

Transmission Line Theory

Does any of this relate to your SCSTs?

 

[ridikas]

Am I correct in concluding that the sound added by a transmission line (that is to say: the sound coming out of the back of the drivers, moving through the cabinet, coming out the rear port, and eventually getting to my ear) will be significantly out of phase (like 360 deg +) with the same sound coming out of the front of the driver and heading "straight" to my ear?

In essence, will any sounds helped by a transmission line suffer "time smear" (is that the right word?) where the individual wave is longer than normal because it is emitted from the cabinet twice with a delay in between?

Transmission line speakers will have a severe time delay in the bass output. Transmission lines are a band aid for garbage woofers with no low end extension. They are harder and more complex to build, more expensive, and are a complete waste of time. Why not just use a good quality woofer to begin with? There are hundreds of cheap options!

I never understood this low wattage, single full range driver, pseudo intellect nonsense. ALL of these speakers sound so bad, it's laughable.

 

[3db]

Can we have this troll blocked from entering Audioholics?

 

[JerryLove]

I suspect that what you said oversimplifies things to the point where you may have wrong or irrelevant conclusions.

Is there a specific conclusion you think is misguided?

Is it that the tube is just mass-load on the driver and not a path for audio to actually get to my ears?

Read the various entries at MJK's web site. When you understand them, come back here and explain it to me .

My brain just crawled out my ear, backhanded me, and went towards the pub.

Does any of this relate to your SCSTs?

Yep. They are here, and beautiful, and sound terrific... but you know what they are setup next to.

I'm attempting to understand some of the differences in what I hear.

 

[GranteedEV]

Am I correct in concluding that the sound added by a transmission line (that is to say: the sound coming out of the back of the drivers, moving through the cabinet, coming out the rear port, and eventually getting to my ear) will be significantly out of phase (like 360 deg +) with the same sound coming out of the front of the driver and heading "straight" to my ear?

It won't be significantly out of phase, because it's not the sound traveling through the cabinet. The frequency being produced by the rear wave of the driver is simply causing the large mass of air to produce sound in phase with the forward wave over the tuning passband.

It will be out of phase below tuning, which is what causes a rolloff in output.

In essence, will any sounds helped by a transmission line suffer "time smear" (is that the right word?) where the individual wave is longer than normal because it is emitted from the cabinet twice with a delay in between?

Not audibly, because the wavelengths are so large that they will have been "smeared" by the room many more times. Group delay is inaudible in a non-anechoic environment.

What differences you might be hearing, are attributed to the frequency response and the limitations of the drivers or ports.

The SCST are not really a speaker i would push too hard, as the tiny excel woofers don't have much travel and are tuned especially low.

Further, even at low levels, they're a pretty flat speaker.

Your best bet is to measure frequency response.

 

[JerryLove]

Your best bet is to measure frequency response.

It's not a difference in FR response (that my ears can tell); it's that some sounds seem to hang in the air a little longer on the SCST than the customs.

It's not unpleasant. It's not unique to the SCSTs ; as I have a similar (indeed: more exaggerated) behavior on my 801 Matrix IIs (for sale if anyone wants some), which are ported but not TL.

And of course, it's entirely possible that the "not right" one is the customs.

It's just "different" and I always see that as an opportunity to learn more.

 

[GranteedEV]

It's not a difference in FR response (that my ears can tell); it's that some sounds seem to hang in the air a little longer on the SCST than the customs.

I would still measure the frequency response. Not Inherent frequency response, but in-room frequency response.

 

[JerryLove]

OK. Will do. (BTW: The way the SCST's interact with the room is extremely impressive)

If it helps: remember that I stick my ears in odd places.

I have 2-way, first-order-crossover, time-coherent obsessed artifical marble GMA Europas (not in general use, but listened to again recently).

I have 4-way, 5-driver, sealed active-DSP-crossover (with extensive correction) customs that think boxed steel and concrete are good building materials (these former Infinity 360s now way 118lb each)

I have 801S2's, and 801N's.

And now I have the SCST's (2-way, TL speakers).

Mind you, some of the most unusual stuff (single-driver, mechanical crossover, omni-polars, Bi-polar ribbons, Concentric-driver speakers, etc) have long since moved on to their next owner... but my ears have been stuck odd places

 

[MJK]

GranteedEV,

I really like your explanation in post #7, but I want to expand on it a little.

It won't be significantly out of phase because it's not the sound wave traveling the length of the TL. The frequency being produced by the enclosure, driven by the rear of the driver, is simply caused by the large mass of air in the TL path vibrating in a standing quarter wavelength shape. The sound from the open end is +/- 90 degrees out of phase with the driver. When the air in the TL is excited by the driver at one of the quarter wave resonant frequencies, the back pressure created will significantly attenuate the driver's motion and SPL output. Look at the picture of the standing waves on the opening page of my site to see the open end of the TL responding at +/- 90 degrees relative to the closed end where the driver is located.

The same thing occurs with a bass reflex design when the frequency of the input signal sweeps through the cabinet's tuning point. The output from the port is 90 degrees out of phase with the driver and the back pressure in the cabinet almost stops the driver from moving, essentially all the SPL you hear is coming from the port.

Below the cabinet's tuning frequency, the driver and TL's open end output will be 180 degress out of phase, which is what causes a 24 dB/octave rolloff in the SPL plots just like a bass reflex design.

Martin

 

[TLS Guy]

Am I correct in concluding that the sound added by a transmission line (that is to say: the sound coming out of the back of the drivers, moving through the cabinet, coming out the rear port, and eventually getting to my ear) will be significantly out of phase (like 360 deg +) with the same sound coming out of the front of the driver and heading "straight" to my ear?

In essence, will any sounds helped by a transmission line suffer "time smear" (is that the right word?) where the individual wave is longer than normal because it is emitted from the cabinet twice with a delay in between?

Not correct. In a stopped pipe only a quarter wave length is in the pipe. Pressure is high at the closed end, so the driver is well controlled. There is large air displacement at the ope end. The speaker is placed about 1/3 the distance down the pipe from the closed end, to avoid stimulating the odd harmonics.

So the phase change and time delay is a quarter cycle 90 degrees. In my view this is not significant.

I reverse taper my pipes to broaden the bandwidth of driver support. The lines are critically damped and there is only one peak of impedance.

Here is the impedance curve of the bass lines with the two Seas 10" excel drivers.

I duplicated a trick of John Wright in his final design before his untimely death and wound two lines around each other tuned a half octave apart. This is the impedance curve of the line with the two 7" Excel drivers.

Note the excellent phase response. The crossover to the tweeter is passive and the crossover to the bass speakers active, with active feed forward BSC to the upper driver.

The bass is deep, well defined and delicate, without over hang. It has a unique real quality about it. It sounds to me like it does live. I have just got back from the opera house, an outing to celebrate our forty third wedding anniversary. It really does sound like my speakers, or to put it correctly the speakers have that just how it sounds in real life presentation.

I know if no other form of loading that gives that just how it really is reproduction of the bass.

The problem is TLs are actually tough and not easy to design well. I have had 57 years of experience with them now.

 

[JerryLove]

That's at the tuning frequency? What about much higher frequencies (say, near the crossover point); what happens to the rearward energy of those sounds?

 

[TLS Guy]

That's at the tuning frequency? What about much higher frequencies (say, near the crossover point); what happens to the rearward energy of those sounds?

If any pipe is to be used above the last couple of octaves or so, then the pipe is FOLDED as well as tapered. This prevents mid and HF radiation from the open end of the pipe.

In addition the pipe is stuffed throughout except towards the last part of the open end of the pipe. The stuffing density, which is best made uniform along the length of the pipe, is determined by the impedance curve. You use just use enough to suppress one of the impedance humps. This determines, that the pipe is properly aperiodically damped and reproduction is non resonant, even though there is still very useful output from the open end of the pipe.

If you over stuff and compress the Polyfill, then the pipe is over damped and you get no output. So there is plenty of Polyfill behind the drivers, which also absorbs the higher frequency sounds from behind the drivers.

View from the top of the enclosure, showing the open end of the bass line, before the top of the enclosure is fixed in place.

 

[Swerd]

That's at the tuning frequency? What about much higher frequencies (say, near the crossover point); what happens to the rearward energy of those sounds?

There shouldn’t be any sound from the rear port at the midwoofer-to-tweeter crossover point (about 2500-2600 Hz). The cabinet tuning and/or the internal stuffing is designed to absorb the rear wave from the drivers at frequencies well above the tuning point.

If I remember correctly, in the original SongTower, the port is tuned to about 42 Hz, and sound contributed by the rear port is down roughly 20 dB or more by 200 Hz. By 300 Hz it's more than 30 dB down. The tuning for the W15 drivers in your SCST is somewhat lower, and its contribution at 200-300 Hz probably isn't much different than in the original ST.

The contribution of sound from the port in Paul Kittinger's design using MJK's software is much wider than from a standard bass reflex port, but it isn't wide enough to affect the upper midrange.

As TLS explained above, the density of the stuffing is a critical part of the design.

Remember also that the tweeter has a closed back and does not radiate rearward sound.

Does that address your question?

 

[Dennis Murphy]

There shouldn’t be any sound from the rear port at the midwoofer-to-tweeter crossover point (about 2500-2600 Hz). The cabinet tuning and/or the internal stuffing is designed to absorb the rear wave from the drivers at frequencies well above the tuning point.

If I remember correctly, in the original SongTower, the port is tuned to about 42 Hz, and sound contributed by the rear port is down roughly 20 dB or more by 200 Hz. By 300 Hz it's more than 30 dB down. The tuning for the W15 drivers in your SCST is somewhat lower, and its contribution at 200-300 Hz probably isn't much different than in the original ST.

The contribution of sound from the port in Paul Kittinger's design using MJK's software is much wider than from a standard bass reflex port, but it isn't wide enough to affect the upper midrange.

As TLS explained above, the density of the stuffing is a critical part of the design.

Remember also that the tweeter has a closed back and does not radiate rearward sound.

Does that address your question?

It should. Paul Kittinger, the bass desinger of all the Salk TL's, reports that in Jerry's SCST, the port output is already 33 dB down at 400 Hz and falling fast. The crossover point is around 2000 Hz, so not much to worry about.

 

[JerryLove]

Does that address your question?

Yes. Quite well when combined with some of the other responses.

Thank you all.