gedlee said:
I just wanted to jump over to your thread but as you already got involved into my question - may I ask you how *you* would answer my question, Earl :
Would you agree that W.M. Hall has gotten confused between the wave *front* versus the sound field / pressure distribution inside the horn ?
Michael
mige0 said:
I don't think so. All "duct" problems are a combination of two types of "waves" - standing waves and propagating waves. It IS NOT possible to tell from his plot which is which we only know what the sum of the two waves is. Many years ago I did a paper with a grad student of mine on this topic for precisely an OS waveguide. With a dual mic technique you can sort out the propagating wave from the reflected waves, but a single mic scan cannot do that. His examples look like a strong standing wave content at those frequencies, which would not be unusual in devices like he shows. Other frequencies may be total propagating waves. Any number of things can happen.
gedlee said:
Thanks for your point of view Earl.
As often, I find your answer more confusing than clarifying – or better put - you haven't answered my question.
1.) I assume you basically agree to my statement (that W.M. Hall has gotten confused in the description what is seen on his pictures) – no?
W.M. Hall plots clearly show gradients that ain't follow: “wavefronts defined as isophase / parallel (= equidistant one from the other)”
Meaning, this gradients can't be titled as wave *fronts* as “wave fronts” *have to be* equidistant along its propagation.
2.) It shouldn't be such hard to distinguish between the term “wave front” and the term “sound field” – no?
3.)is there agreement that a “wave front” is whats created by (for example) a Dirac impulse spreading out in space, whereas a “sound field” describes an established pressure distribution at certain frequencies?
Michael
mige0 said:
Michael
And as usual, I find your statements just as confusing.
1) I said "No" and I meant it. I don't think that he got confused.
2) I think that the distinction you are trying to make is one that only you understand. From my previous post, it is not only hard to distinguish the propagating "wavefront" from the standing wave pattern (what you call "sound field" I think) it si impossible from those plots. It is difficult under any circumstances and I gave you a reference to a paper that describes how it is done.
3)No, I don't agree to your simplification of the situation. The "Impulse Response" is just as valid a description of a sound field as a "steady state" response. They are just different signals that's all. The impulse response does have an advantage in that it can show what happens at ALL frequencies at the same time, but only a single time (unless we make a movie) while the steady state response can only be shown at a single frequency, but for all time. I am sure that steady state measurements were all that Hall was capable of at the time, so thats all he could do. It is not a complete picture of what happens in a device like that as the impulse response is (if you have it for all time). But impulse response measurements and calculations were not possible at that time.
Bjorn if you are out there - may I ask how *you* look at fig 14-16
http://www.audioxpress.com/magsdirx...kolbrek2884.pdf
Woldn't you agree that W.M. Hall has gotten confused between the wave *front* versus the sound field / pressure distribution inside the horn ?
Michael
http://www.audioxpress.com/magsdirx...kolbrek2884.pdf
Woldn't you agree that W.M. Hall has gotten confused between the wave *front* versus the sound field / pressure distribution inside the horn ?
Michael
gedlee said:
gedlee said:
I didn't question if “impulse response” is a valid description of the sound field at all.
It seems there is something unclear in the definition of the term “wave front” between us and I've seen that in many of our question and answer gamblings – so its either you or me or its unclear in general.
Maybe we put it into another frame and look what happens with lightning and thunder or a distant explosion (if we don't care for the DC part).
1.)wouldn't you agree that the “wave front” is defined as what is heard first – roughly containing all frequencies with a subsequent steep rise in SPL?
2.)Wouldn't you agree that what comes *after* – the rolling, roaring tail – is what is created by whatever irregularities in the surrounding (mountains, clouds of different density, wind …)
Michael
mige0 said:
Thats certainly true.
mige0 said:
1) Not necessarily. Yes, one could define the wavefront as the steepest rise in SPL in the impulse response, but that wavefront then does not necessarily "contain all frequencies" It would only do so if there is no group delay or dispersion (in the physics sense).
2) What comes "rolling after" is not necessarily due to "irregularities". For example, there is a "tail" to the impulse response in any 2 dimensional wave propagation, such as from a line array. This is not due to any "irregularities", its due to the physics. So, for example, if the wavefront expands dominately in one direction then there will be a tail in the wavefront in that region no matter if the wave eventually expands in 3D or not. Theoretically there is no "tail" to the impulse response in an OS waveguide, but there would be one in a cylindrical or elliptical waveguide.
Michael, its just not as simple as you want to make it.
gedlee said:
Its by far less complicated as you like to put it – just let us generously focus “on the whole” and not pick after the hair in the soup...
gedlee said:
Thats Ok – but not so important for what I'm after.
And besides that I think there always is a tail – even for your axisymmetric wave guide *if* the source (or virtual source) is other than one dimensional (point source)
But anyway...
Lets get one step further with my example.
Say we take Grand Canyon and shape it into a horn – put a glass roof, and invite a few thousand fans of DIYAUDIO as audience sitting right in and in front of our mega-horn.
Then we ask Zeus to assist and throw one of his lightnings into the throat.
We have instructed our lovely audience to raise arms once they hear something.
Watching our experiment from a bird's eye view we see the arms going up like when wind goes through a corn field.
Now - so far so clear – having defined what is meant with the term “wave front”.
Telling us that there is only one thing important to the term “wave front” - time of flight.
Meaning – the contours of subsequent “wave fronts” – like shown in the pix of W.M. Hall simply ain't possible
What we should focus on is that time of flight depends *only* on the distance nothing else (well - in the first place).
Quite in contrary to the term “sound field” which basically mirrors “what comes after”
So – from the above we could say the wave front is independent of the horn contour *if* we look at horns of the same throat dimension - *and* at a distance where the horn contours are of the same included angle - *and* as long as we can see the throat from any point where we draw the “wave front”.
Quite in contrary to the sound field / pressure distributuin which strongly is affected by the contour of the horn.
Michael
panomaniac said:
I guess that I must be the one who doesn't get it.
Think of a sunami as an impulsive wavefront, it's well defined, passes once and has a tail. Now think about the same beech on a normal day with waves coming in. There are still wavefronts, they come in continuously, one after the other, typically only one wavelength, but different heights all along the beach - the "wavefronts" are still quite obvious. Its all the same ocean, its just a different waveform.
The plots in Bjorn's paper are "waves on a beech" not a sunami. There are wavefronts.
panomaniac said:
no need to get lost by that little bit of voodoo dancing, panomaniac - its just Earls lovely habit to avoid exercising in concession speeches
The term “wave *front*” describes points in space where time of arrival are the same (rising hands in audience).
“Impulse response” in contrary describes the *quality* of what can be heard at this points in space – two things to keep apart.
Michael
Hello,
I used the grid on the measurements made by Hall like a grid of oscilloscope.
And i used the phase value of the horn axis (red line) to transfer it on the grid.
The amplitude value is arbitrary:
It's obvious that it is not measured an 800Hz sinusoid.
So, I think there are measure the effect of diffraction and not the front wave.
By the way, i have made a english translation of my site, i hope it's good:
http://thend.chez-alice.fr/audio-en/Bonjour.html
I used the grid on the measurements made by Hall like a grid of oscilloscope.
And i used the phase value of the horn axis (red line) to transfer it on the grid.
The amplitude value is arbitrary:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
It's obvious that it is not measured an 800Hz sinusoid.
So, I think there are measure the effect of diffraction and not the front wave.
By the way, i have made a english translation of my site, i hope it's good:
http://thend.chez-alice.fr/audio-en/Bonjour.html
Thanks tend for your work, ans also that you brought your page back!
Below the according sound field / pressure distribution calculated by CARA (for the W.M. Hall measuements):
most striking for such a simplified simu – no?
and the corresponding wave front propagation:
QED
Michael
An externally hosted image should be here but it was not working when we last tested it.
Below the according sound field / pressure distribution calculated by CARA (for the W.M. Hall measuements):
most striking for such a simplified simu – no?
and the corresponding wave front propagation:
QED
Michael
I dont understand
Are you saying that sound wave hits the room air like if its was like a brick wall
Obviously it will "hit" the air and loose support from waveguide, spread out, and loose intensity
Maybe a very small part will bounce back, but I suppose not like if it hit a wall
Or it will be hit by the next coming wave
Are you saying that sound wave hits the room air like if its was like a brick wall
Obviously it will "hit" the air and loose support from waveguide, spread out, and loose intensity
Maybe a very small part will bounce back, but I suppose not like if it hit a wall
Or it will be hit by the next coming wave