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post #91 of 141 Old 10-11-2010, 04:38 PM
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Quote:
Originally Posted by villastrangiato View Post

(from Dennis Murphy)

Err, I'm not Dennis Murphy. My last name starts with H.

Quote:


That laughable quote, where a sound pressure wave exiting a tube (transmission line terminus) somehow reflects off of itself - "caused by a pressure decrease as the wave expands into open air so it has reverse polarity" - that is a thing of beauty . It actually has been framed and is sitting on the wall of my office for all to see. It has served as exhibit A proof that not everything you read on internet forums is useful or correct.

Which all goes to show you still don't get the basic physics, no matter how much you amuse yourself thinking you do. The wave doesn't reflect off itself. It's a diffraction effect off the end of the physical tube and it's well documented in countless physics textbooks, simulation software and real speaker measurements. And the "it" I was referring to with the inverted polarity is the reflected/diffracted wave traveling back down the tube (which should be intuitively obvious to the most casual of observers unless they happen to be named Courtney.)

Of course none of that matters to you -- everyone else in the world is wrong and you are right.

Dennis H
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post #92 of 141 Old 10-11-2010, 04:52 PM
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"Actually, I believe it is the lateral reflections with longer delay times that create the sense of spaciousness."

hi doug, aren't the lateral reflections largely killed in an open baffle arrangement? i thought the polar response was kind of a figure 8 on the low to mids and then directional on the high end (figure of 0).

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post #93 of 141 Old 10-11-2010, 05:11 PM
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Quote:
Originally Posted by LTD02 View Post

"Actually, I believe it is the lateral reflections with longer delay times that create the sense of spaciousness."

hi doug, aren't the lateral reflections largely killed in an open baffle arrangement? i thought the polar response was kind of a figure 8 on the low to mids and then directional on the high end (figure of 0).

Not really. Take a look at those polar plots a few pages back, and don't forget about the opposite side wall reflections, which result from the main forward lobe.
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post #94 of 141 Old 10-11-2010, 05:14 PM
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See Linkwitz's ray diagram:




Toole teaches (and Geddes affirms) that in small rooms, the delays can be long enough to enhance source width (soundstage), but spaciousness in terms of listener envelopment cannot be achieved but by other means....
LL

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post #95 of 141 Old 10-11-2010, 05:26 PM
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i drew this picture in my notes a long time ago. i thought that geddess et al would be in favor of a forward cross.

what is going on in this picture is the first reflection will have the same timbre as the direct sound, so the brain will ignore it. this is psychoacoustics in action.



zilch, that is a strange pic for linkwitz. i thought open baffle cancelled low to mid frequencies 90 degrees off axis. doug smith says i'm missing something.
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post #96 of 141 Old 10-11-2010, 05:37 PM
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Quote:
Originally Posted by LTD02 View Post

zilch, that is a strange pic for linkwitz. i thought open baffle canceled low to mid frequencies 90 degrees off axis.

He's just showing reflection angles in that pic and not including dipole (or horn) directivity.

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post #97 of 141 Old 10-11-2010, 05:40 PM
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Quote:
Originally Posted by LTD02 View Post

i drew this picture in my notes a long time ago. i thought that geddess et al would be in favor of a forward cross.

what is going on in this picture is the first reflection will have the same timbre as the direct sound, so the brain will ignore it. this is psychoacoustics in action.

zilch, that is a strange pic for linkwitz. i thought open baffle canceled low to mid frequencies 90 degrees off axis. doug smith says i'm missing something.

Correct on the first count, but the brain doesn't ignore it. The relatively long delay in arrival time creates the illusion spaciousness (i.e, reverb). The similarity of timber makes the effect sound more "real".

There is quite a lot of energy at 60 degrees and above from the Orion. This will still create near side wall reflections.
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post #98 of 141 Old 10-11-2010, 05:43 PM
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roger that catapult.

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post #99 of 141 Old 10-11-2010, 05:53 PM
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"the brain doesn't ignore it."

agree. "ignore" was a poor word choice. the brain recognizes the similarity in spectral content, and then blends it into the directly radiated sound and uses the differential between the two to determine what space is providing the sound.

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post #100 of 141 Old 10-11-2010, 05:59 PM
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Quote:
Originally Posted by LTD02 View Post

"the brain doesn't ignore it."

agree. "ignore" was a poor word choice. the brain recognizes the similarity in spectral content, and then blends it into the directly radiated sound and uses the differential between the two to determine what space is providing the sound.

Yes.
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post #101 of 141 Old 10-11-2010, 06:09 PM
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Quote:
Originally Posted by LTD02 View Post

zilch, that is a strange pic for linkwitz. i thought open baffle cancelled low to mid frequencies 90 degrees off axis. doug smith says i'm missing something.

Dennis clarified. In theory, the one directly bouncing directly off the right wall isn't there with a dipole. In practice, at least according to Geddes's directivity map of Orion, it doesn't quite work out that way across the passband.

The other early, short delay (bad) one is bouncing off the front wall to the left of the speaker. The one doing the double reflection to the right is better due to the longer delay, but the "good" one exhibiting both long delay and low IACC is that one coming from the opposite wall generating the spaciousness cues. The relative tradeoff between imaging and spaciousness (again, apparent source width only) may be varied by adjusting toe-in when the sources are constant directivity.

Remember, also, in drawing these ray diagrams, the angle of incidence must equal the angle of reflection. Looks like yours is not quite that....

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post #102 of 141 Old 10-11-2010, 06:23 PM
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"angle of incidence must equal the angle of reflection. Looks like yours is not quite that...."

agree. good point. i didn't mean to introduce confusion. this was just a quick drawing from my notes. :-)

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post #103 of 141 Old 10-11-2010, 06:31 PM
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It's fine, just that the direct and reflected are not actually coming quite so symmetrically off-axis from the source as shown. With a constant directivity source having uniform power response, not a major issue. Your drawing illustrates the larger point well....

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post #104 of 141 Old 10-11-2010, 06:52 PM
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thanks z.

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post #105 of 141 Old 10-11-2010, 11:18 PM
 
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Anyone in the Southeast with Geddes speakers?
Doug?
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post #106 of 141 Old 10-12-2010, 08:37 AM
 
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Quote:
Originally Posted by catapult View Post

Err, I'm not Dennis Murphy. My last name starts with H.

Which all goes to show you still don't get the basic physics, no matter how much you amuse yourself thinking you do. The wave doesn't reflect off itself. It's a diffraction effect off the end of the physical tube and it's well documented in countless physics textbooks, simulation software and real speaker measurements. And the "it" I was referring to with the inverted polarity is the reflected/diffracted wave traveling back down the tube (which should be intuitively obvious to the most casual of observers unless they happen to be named Courtney.)

Of course none of that matters to you -- everyone else in the world is wrong and you are right.

Because you don't fully understand cavity resonance does not mean the rest of the world has to bask in the same ignorance. Your understanding of standing wave theory and cavity resonance is so infantile, it does not merit further discussion in this forum. This perspective is brought to this particular forum for others who might be reading this. In such instances, I would strongly urge those who aren't familiar with the concepts of standing waves and cavitation to do their own research online or in published papers to see what the facts truly are.
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post #107 of 141 Old 10-12-2010, 09:03 AM
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Quote:
Originally Posted by villastrangiato View Post

Because you don't fully understand cavity resonance does not mean the rest of the world has to bask in the same ignorance. Your understanding of standing wave theory and cavity resonance is so infantile, it does not merit further discussion in this forum. This perspective is brought to this particular forum for others who might be reading this. In such instances, I would strongly urge those who aren't familiar with the concepts of standing waves and cavitation to do their own research online or in published papers to see what the facts truly are.

Infantile? How pleasant. Post links to your sources.

T6

Clearwave 4TSE and 4CC build thread
http://www.avsforum.com/avs-vb/showt...1#post19489740
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post #108 of 141 Old 10-12-2010, 09:34 AM
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Quote:
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i don't think they look any better than modern designs, i just like the fact that they are part of the heritage. something about that makes them very cool, but i don't know why. kind of like the lansing iconic. that may be the coolest speaker of all time.

Truth be told I only really like the look of the multicells. Not really a fan of how any of the Altec speakers look over all. The model 19 is one fugly speaker (but is supposed to sound great).

What is cool though, is that these multicells (circa 1940's) spent their career announcing train departures and arrivals in down town Denver. Now they get to live up in the Rockies and spend their retirement keeping the family happy playing music and movies.

Anyway I think this went a little OT, so apologies to the OP.
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post #109 of 141 Old 10-12-2010, 11:07 AM
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I'm not going to reply to Villa anymore but, for others interested in standing waves in tubes such as a horn or a TL, here's a link with animations.

http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html

Note what he says about the open end of a tube:

"So the result is that a pulse of high pressure air travelling down the tube is reflected as a pulse of low pressure air travelling up the tube. We say that the pressure wave has been reflected at the open end, with a change in phase of 180°."

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post #110 of 141 Old 10-12-2010, 03:20 PM
 
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Quote:
Originally Posted by catapult View Post

I'm not going to reply to Villa anymore but, for others interested in standing waves in tubes such as a horn or a TL, here's a link with animations.

http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html

Note what he says about the open end of a tube:

"So the result is that a pulse of high pressure air travelling down the tube is reflected as a pulse of low pressure air travelling up the tube. We say that the pressure wave has been reflected at the open end, with a change in phase of 180°."

And a transmission line loudspeaker acts like a pipe that is open at both ends??

I'll give you this Dennis, you never fail to amuse.
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post #111 of 141 Old 10-12-2010, 03:40 PM
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He never said a pipe opened at both ends.

The link discusses open and closed end cylinders. FWIW, a clarinet is a closed end pipe.
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post #112 of 141 Old 10-12-2010, 04:09 PM
 
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Quote:
Originally Posted by soho54 View Post

He never said a pipe opened at both ends.

The link discusses open and closed end cylinders. FWIW, a clarinet is a closed end pipe.

It helps to actually follow the link and read.....


"Let's send a pulse of air down a cylindrical pipe open at both ends (such as a flute, shakuhachi etc). It reaches the end of the tube and its momentum carries it out into the open air, where it spreads out in all directions. Now, because it spreads out in all directions its pressure falls very quickly to nearly atmospheric pressure (the air outside is at atmospheric pressure). However, it still has the momentum to travel away from the end of the pipe. Consequently, it creates a little suction: the air following behind it in the tube is sucked out (a little like the air that is sucked behind a speeding truck).

Now a suction at the end of the tube draws air from further up the tube, and that in turn draws air from further up the tube and so on. So the result is that a pulse of high pressure air travelling down the tube is reflected as a pulse of low pressure air travelling up the tube. We say that the pressure wave has been reflected at the open end, with a change in phase of 180°. In the open-open pipe, there is such a reflection at both ends."


Which is quoted from here:

http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html

- and contains the quote Dennis posted. Again, reading first can be a good thing....


Transmission line or vented loudspeakers are not the equivalent of Helmholtz resonators, pipe organs, reed pipe organs, or reed instruments. Unlike a vented loudspeaker, the other devices require essentially constant radial flow of air across an orifice or open end to induce the pressure pulses that result in resonance. These conditions are not present in a loudspeaker to any appreciable degree. This sorry ground has been tread on repeatedly elsewhere and it is sad that people continue to cling to such wild extrapolations as if they possessed meaning or value for those deeply involved in loudspeaker design.
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post #113 of 141 Old 10-12-2010, 04:27 PM
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And which then illustrates the same occurring at the open end of a single-end-closed pipe....

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post #114 of 141 Old 10-12-2010, 04:31 PM
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Quote:
Originally Posted by villastrangiato View Post

It helps to actually follow the link and read.....

- and contains the quote Dennis posted. Again, reading first can be a good thing....

Where did he say a TL acts like an open ended pipe, cause I don't see it.

Yes, your quote comes from the description of a flute, but did you notice the last sentence? Did you read the following short section about the clarinet? I'll paraphrase, one end reflects like the wall it is, for the open end see the previous section, only there is just one here.
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post #115 of 141 Old 10-12-2010, 04:31 PM
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Quote:
Originally Posted by AJinFLA View Post

Anyone in the Southeast with Geddes speakers?
Doug?

I'm in the Northeast.
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post #116 of 141 Old 10-12-2010, 04:35 PM
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Man, I don't know why high school physics is so hard for some people to understand. A wave can reflect from a closed end of a pipe and also from an open end.

Closed end reflection:
No reduction in pressure amplitude
No inversion of pressure phase
Inversion of displacement phase

Open end reflection:
Reduction in pressure amplitude
Inversion of pressure phase
No inversion of displacement phase

Most of the pipes we deal with are closed (by a driver) at one end and open at the other. Here's the impulse response of such a pipe with the mic outside the open end. It's modeled in Hornresp but I've measured similar behavior. Each reflection is inverted from the previous one, lower in amplitude, and has some HF stuff removed because the pipe acts as a lowpass filter.


LL

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post #117 of 141 Old 10-12-2010, 04:42 PM
 
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I'm not going to pursue this further - already well into thread crapping territory and this is not supposed to be the Acoustics Resonance Thread 101. For anyone who's still confused about these fundamental aspects of acoustics, do yourself a favor and research Helmholtz resonance, standing wave theory, and cavitation resonance and you might actually learn something. I'm not going to continue to beat this horse to death on someone else's thread.
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post #118 of 141 Old 10-12-2010, 04:44 PM
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Stop editing your posts to add things after the fact.
Quote:
Originally Posted by villastrangiato View Post

Transmission line or vented loudspeakers are not the equivalent of Helmholtz resonators, pipe organs, reed pipe organs, or reed instruments. Unlike a vented loudspeaker, the other devices require essentially constant radial flow of air across an orifice or open end to induce the pressure pulses that result in resonance. These conditions are not present in a loudspeaker to any appreciable degree. This sorry ground has been tread on repeatedly elsewhere and it is sad that people continue to cling to such wild extrapolations as if they possessed meaning or value for those deeply involved in loudspeaker design.

Haha.. are you serious? A vented loudspeaker is not a Helmholtz radiator, but it employs a Helmholtz resonance.

As for your reed instrument examples you are just showing that you have no clue how they work. The air flow has nothing to do with the sound. The air flow is used to modulate the reed, which acts as a diaphragm here. Ya know like a speaker diaphragm. If you had ever had anything to do with instruments you would know that just blowing through and instrument gets you no sound at all. Just an amplified blowing sound. It works like Tom Danley's sonic boom generator, or somewhat like the Rotary Sub.

I beat there is a link at that site that will go into the mechanics for you.
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post #119 of 141 Old 10-12-2010, 04:59 PM
 
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Thanks Doug.
One day I'll find someone in this hemisphere .

cheers,

AJ

p.s. I see Villastrauricle is here. Oh joy :-)
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post #120 of 141 Old 10-12-2010, 05:01 PM
 
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Quote:
Originally Posted by soho54 View Post

Stop editing your posts to add things after the fact.

Wait till you see the Wikipedia article. That ought to be a a dandy
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