Sheldon said:
Yes I thought about foam too, *once* I feel the need for.
What I've read from the French guys, its no good idea to have a rough contour - I think this even applies to acoustic foam.
But in case of AMT there is a possibility to make small horizontal blades the height of the bar's - maybe 1/3 or half the horn depth ? - its the way silecers are made for airconditioning systems.
You know AMT's are not completely open :
http://www.mundorf.com/deutsch 1.1/lautsprecherchassis/index.htm
Michael
mige0 said:
Hi Michael
Complicated isn't it!
There are several ways that a wave can propagate through a horn or waveguide device. The first, most predominate and most well known is the main wave that moves along the axis of the device. This wave can, and will, have reflections - from the mouth or a diffraction slot, etc. These reflections create a comb filter effect of a periodic peak and dip in the response. These peaks and dips are all minimum phase, will have a decay rate as any resonance does, they have the same effect at all polar angles and they can be corrected - exactly - with EQ.
Then there are the HOM. They DO NOT travel along the axis, but at angle to it, refelcting off of the walls as they go. They can also circle arround the axis. Since they have to travel at the same speed in air as the main wave, they will not travel along the axis at the same speed as the main wave, because they travel a greater distance. For this reason they are called dispersive waves. They can be reflected from the mouth and diffraction slots as well and will also create peaks and dips in the response. But they are very different than the peaks and dips in the above example. They are not minimum phase, they cannot be corrected with EQ except at a single point and thats because their effect is different at every field point (in general).
Now further to this is the subjective response to all this. The ear will mask the minimum phase resonances much more than it masks the nonminimum phase ones from the HOM, or any diffraction (cabinet edges etc.) for that matter. It has also been shown that the audibility of nonminimum phase effects increase with SPL level (less masking!), making them sound like "nonlinear distortion", except that the effect itself is linear.
As to how HOM are created, ANY discontinuity in the walls shape or slope will generate diffraction and hence HOM. For example, the main waves reflection off of the mouth creates both main waves and HOM wave reflections. Both types of wave move back down the horn and reflect off of the diaphragm and back out to the mouth again, and on we go to another round, until these waves have dissipated. There are lots and lots of ways to generate HOM and in fact all contours for real sources create HOM - some more than others. Even an OS waveguide will generate HOM, but it can be proven that this exact contour generates the lowest levels of HOM possible. All other devices will have more HOM than an OS. The HOM created by the mouth reflection will be independent of the type of horn that is being used. Thats why proper mouth treatment is so critical for any device.
Complicated enough for you?
gedlee said:
Thanks for laying out the subject as simple as possible - I guess I got a glimpse – you say that its the ongoing diffraction along the contour (plus its further reflections) not especially the big horn mouth reflection that's summarised in HOM ?
Can I conclude from the above (min / non-min phase) behaviour that I could - most easily - detect HOM doing on axis measurements at different distances and compare for different decay in CSD for example?
Michael
mige0 said:
I don;t understand this question.
mige0 said:
I don't see what distance has to do with anything. I would determine that portion of the response that is nonminimum phase - this can be done from the magnitude and phase - in the axial response This is likely to be the HOM portion of the response. I would be tempted to do this at several polar angles because it may well be that some HOM have no response on-axis. If there is a coherent non-minimum phase response at several positions then this would clearly be the HOM. If its all random then its probably noise. The HOM likely won't show up in the CSD at all.
Hi Earl,
Correct me if my knowledge of horns is not so complete, but I was under the impression that horns, even if HOMs are not present (or considered in the analysis) are dispersive and therefore, not MP.
The other issue I would raise is, assuming that the only non MP component in the amplitude was due to HOM how could you separate them out? Given a measured amplitude and phase we can always construct the MP response corresponding to the amplitude. We could then determine the deviation of the measured response from MP, but give that non MP phase component there is no relationship between it and any amplitude that would allow us to back out the HOM contributions to the amplitude. All that can be said is that given the measured amplitude and phase, the response can be decomposed into a MP component and an all pass, non MP component.
Correct me if my knowledge of horns is not so complete, but I was under the impression that horns, even if HOMs are not present (or considered in the analysis) are dispersive and therefore, not MP.
The other issue I would raise is, assuming that the only non MP component in the amplitude was due to HOM how could you separate them out? Given a measured amplitude and phase we can always construct the MP response corresponding to the amplitude. We could then determine the deviation of the measured response from MP, but give that non MP phase component there is no relationship between it and any amplitude that would allow us to back out the HOM contributions to the amplitude. All that can be said is that given the measured amplitude and phase, the response can be decomposed into a MP component and an all pass, non MP component.
mige0 said:
Depending of cut-off horn, the height of sawtooth is equal to one quarter of the wavelength lowest.
Per example for 1000Hz, we have 8,5cm (OK, it's much and ugly).
But I recognize that's theorical, I think it takes at least an octave higher (or not, i never try the sawtooth, but the american army, yes).
mige0 said:
In my opinion, the waveform shape is the worst case; we have the same distance to the straight end and to transducer. We will have the same effet of a tube, but it's relatively easily to equalize.
The sawtooth, on the contrary, is varying the distance and using the rough contour when it's a problem usually in terms of diffractions.
Hello,
I would be careful, because the height of the slot is approximately 125mm, while the case of the driver measures 160mm instead, so the distance between the driver's surfaces will be 35mm minimum. This may end up with heavy interferences, measured by Dr. Makarski on 6 magnetostats in line (probably the best available ones).
It would be interesting, if one knows his measurements of the surface SPL distribution (only some mm distance from the surface), because I believe to remember, this strongly affects the WG functionality.
Moreover the horizontal opening of the driver is 25mm, a 31mm "slot" will narrow the beamwith too early.
Cheers, Timo
brucemck2 said:
I would be careful, because the height of the slot is approximately 125mm, while the case of the driver measures 160mm instead, so the distance between the driver's surfaces will be 35mm minimum. This may end up with heavy interferences, measured by Dr. Makarski on 6 magnetostats in line (probably the best available ones).
It would be interesting, if one knows his measurements of the surface SPL distribution (only some mm distance from the surface), because I believe to remember, this strongly affects the WG functionality.
Moreover the horizontal opening of the driver is 25mm, a 31mm "slot" will narrow the beamwith too early.
Cheers, Timo
john k... said:
Hi John
In Horn theory, horns are clearly dispersive at cutoff in the equations, but as my articles show this is exactly where horn theory is the least accurate. In reality the cutoff is not sharp like predicted and I don't believe that it is strongly dispersive. However, as I said before ALL contours for real sources generate HOM so that means that ALL contours would have to have some non-MP part, which I think answer your question - all horns and waveguides are dispersive - some more than others. Perhaps this is a key to sound quality.
I do believe that at any useful frequency where a waveguide has "gain" that the main wave is MP (it may be nonMP at very low frequencies), but clearly the HOM are not as they are absolutely dispersive. (As usual, I don't think about acoustics in MP terms.)
Your analysis of the MP and Non-MP components being decomposed is, in fact, exactly the way that I would approach the problem too. But, I have not actually done that to know if it works or not. As I said somewhere, its been a long time since I have had the time to do something "analytic". It's good to be busy I suppose, but I wish that I had some time to do some technical stuff.
I'd love to see someone do the analysis as you describe it to see what the MP and non-MP parts of the response looks like. To see if a diffraction type horn does have a greater non-MP part as I expect - or not. That would certainly be a good set of data.
tiki said:This may end up with heavy interferences, measured by Dr. Makarski on 6 magnetostats in line (probably the best available ones).
It would be interesting, if one knows his measurements of the surface SPL distribution (only some mm distance from the surface), because I believe to remember, this strongly affects the WG functionality.
Moreover the horizontal opening of the driver is 25mm, a 31mm "slot" will narrow the beamwith too early.
Cheers, Timo
Timo
Nice post, because its good to see people using polar maps!! They really tell a more complete story than any other type of measurement. That example is a classic case of a bad response, and it is quite obvious in that link. Thanks
But the second link does not make sense. The greatest output is at 90 degrees? And its resolution is sadly lacking.
The surface SPL is related to the surface velocity, which of course, has a strong impact on any waveguides performance. Almost a dominate impact.
Thanks Earl,
once learned, these maps are very easy to read and useful for comparison purposes.
One should keep in mind, that the first link shows a vertical response of a "line" source of course.
The second link does not seem to be that bad to me: it shows only 0 - 90 degrees (quarter plane), the viewer may be willing to let the 90 degrees be on axis and 0 degree orthogonal to the axis (a matter of file naming while combining the measurements). It would be very easy to mirror the picture to get the half plane pattern.
What do you mean with lacking resolution? The smoothing is 1/24 octave and the 3dB-steps between the levels are for clarity, as you wrote anywhere the -6dB line is a measure for the radiation angle of the waveguide.
The radiation pattern itself is not good, I do agree fully.
Cheers, Timo
once learned, these maps are very easy to read and useful for comparison purposes.
One should keep in mind, that the first link shows a vertical response of a "line" source of course.
The second link does not seem to be that bad to me: it shows only 0 - 90 degrees (quarter plane), the viewer may be willing to let the 90 degrees be on axis and 0 degree orthogonal to the axis (a matter of file naming while combining the measurements). It would be very easy to mirror the picture to get the half plane pattern.
What do you mean with lacking resolution? The smoothing is 1/24 octave and the 3dB-steps between the levels are for clarity, as you wrote anywhere the -6dB line is a measure for the radiation angle of the waveguide.
The radiation pattern itself is not good, I do agree fully.
Well, "Hier liegt der Hase im Pfeffer", as the Germans say. Because the AMT driving (Lorentz) force is directed orthogonal to the SPL, the SPL reaction force operates against the mechanical stiffnes of the folded foil only which cannot be evenly distributed across the whole AMT surface.
Cheers, Timo
Way, way OT, but since I am an airline pilot I could not resist replying to this comment by Lynn Olson:
"OK, I made a little mental decision to slow down our social visits a bit, and no air travel for sure, since the damned airlines are too cheap to provide all-fresh air from the vents, and mostly use poorly filtered recycled air to save a little of bit of fuel. Although not mentioned by the CDC to spare the airline industry, airplanes were the obvious vector, since there were cases on record of one influenza-sick person boarding the airplane at the beginning of the trip, the plane sitting idle on the runway for three hours, and the majority of the passengers sick by the time they got to their destination."
The use of "recycled air" has nothing to do with the "cheap" airlines, and has everything to do with the aircraft manufacturers and the design of airliners in general.
Conditioned air on airliners comes from one of two sources: pre-conditioned air from an external source or from the onboard air cycle machines.
For pre-conditioned air the plane is literally hooked up to an external air conditioner - this typically happens when the plane is parked at the gate. The conditioned air is pumped directly into the plane's air distribution system and does not include any air recirculation. (Next time you fly, look for a big yellow hose hooked up to the plane while its parked at the gate - that's the conditioned air supply)
The other option is the use of the on board air cycle machines a.k.a the air conditioning packs. These are the ONLY way to provide conditioned air while the plane is on the move. These packs take a source of high pressure compressed air and use a combination of air-air heat exchangers, compression and expansion turbines to condition the air (this conditioned and compressed air also is what pressurizes the cabin - no packs, no pressurization!). The source of compressed air used by the packs is fresh, outside air tapped directly off the compressor section of the engines (or from the auxiliary power unit, APU, which is a small gas turbine located in the tail of the plane).
The packs are good at supplying conditioned and compressed air but generally at a relatively low volume. To increase the flow of air though the distribution system (including those little round vents above the seats) a "recirculation fan" is generally used. This fan takes cabin air and mixes it with the conditioned air coming from the packs. So in the end, the air that is moving about the plane is a mix of fresh outside air (as supplied by the packs) and recirculated cabin air. The recirculation fan typically has an on/off switch in located on the flight deck. If the fan is turned off, the airflow through the cabin will noticeably decrease, but the amount of fuel being used by the engines (to supply air to the packs) will not change one bit!
So, the use of recirculated air is inherent in the design of an airliner's air conditioning system and is not used because the airlines are cheap. This is a very effective system and is typical of most airliners regardless of their manufacturer.
It is true that much of what the airlines do are driven by costs (they struggle to keep costs at a minimum yet still manage to lose billions) however, the use of recirculated air is completely unrelated. And, yes, while the airlines do make a pretty effective disease vector, the same can be said of any high-speed mass transit.
This not meant to be a bash on Lynn in any way. I always enjoy and am often inspired by his writings but only as they relate to audio!
"OK, I made a little mental decision to slow down our social visits a bit, and no air travel for sure, since the damned airlines are too cheap to provide all-fresh air from the vents, and mostly use poorly filtered recycled air to save a little of bit of fuel. Although not mentioned by the CDC to spare the airline industry, airplanes were the obvious vector, since there were cases on record of one influenza-sick person boarding the airplane at the beginning of the trip, the plane sitting idle on the runway for three hours, and the majority of the passengers sick by the time they got to their destination."
The use of "recycled air" has nothing to do with the "cheap" airlines, and has everything to do with the aircraft manufacturers and the design of airliners in general.
Conditioned air on airliners comes from one of two sources: pre-conditioned air from an external source or from the onboard air cycle machines.
For pre-conditioned air the plane is literally hooked up to an external air conditioner - this typically happens when the plane is parked at the gate. The conditioned air is pumped directly into the plane's air distribution system and does not include any air recirculation. (Next time you fly, look for a big yellow hose hooked up to the plane while its parked at the gate - that's the conditioned air supply)
The other option is the use of the on board air cycle machines a.k.a the air conditioning packs. These are the ONLY way to provide conditioned air while the plane is on the move. These packs take a source of high pressure compressed air and use a combination of air-air heat exchangers, compression and expansion turbines to condition the air (this conditioned and compressed air also is what pressurizes the cabin - no packs, no pressurization!). The source of compressed air used by the packs is fresh, outside air tapped directly off the compressor section of the engines (or from the auxiliary power unit, APU, which is a small gas turbine located in the tail of the plane).
The packs are good at supplying conditioned and compressed air but generally at a relatively low volume. To increase the flow of air though the distribution system (including those little round vents above the seats) a "recirculation fan" is generally used. This fan takes cabin air and mixes it with the conditioned air coming from the packs. So in the end, the air that is moving about the plane is a mix of fresh outside air (as supplied by the packs) and recirculated cabin air. The recirculation fan typically has an on/off switch in located on the flight deck. If the fan is turned off, the airflow through the cabin will noticeably decrease, but the amount of fuel being used by the engines (to supply air to the packs) will not change one bit!
So, the use of recirculated air is inherent in the design of an airliner's air conditioning system and is not used because the airlines are cheap. This is a very effective system and is typical of most airliners regardless of their manufacturer.
It is true that much of what the airlines do are driven by costs (they struggle to keep costs at a minimum yet still manage to lose billions) however, the use of recirculated air is completely unrelated. And, yes, while the airlines do make a pretty effective disease vector, the same can be said of any high-speed mass transit.
This not meant to be a bash on Lynn in any way. I always enjoy and am often inspired by his writings but only as they relate to audio!
Hi Earl,gedlee said:
But that is where the problem lies. If the radiated SPL from the horn, wave guide, or any other source like a 2-way speaker with typical crossover what you have is an amplitude with non MP phase. It is possible to determine the MP associated with that amplitude. And then it is possible to determine an allpass response (flat amplitude) which contains the correction to non MP. The total response is the product of the two. But given that non MP response there is no way to determine what part of the amplitude is from an MP response. It's like saying I have $0.25. Now tell me what coins do I have in my hand? You need more information than is contained in just the resulting sum.
John
I understand what you are saying completely. As I said, I have not tried to isolate the non-minimum phase part at all.
Perhaps all that one would need is the all-pass phase as this would yield the excess group delay from the dispersive modes. This might contain the desired information even if it did not yield the amplitude domain effects. (and maybe its not possible to even talk about any amplitude domain effects!?) Just guessing here.
I understand what you are saying completely. As I said, I have not tried to isolate the non-minimum phase part at all.
Perhaps all that one would need is the all-pass phase as this would yield the excess group delay from the dispersive modes. This might contain the desired information even if it did not yield the amplitude domain effects. (and maybe its not possible to even talk about any amplitude domain effects!?) Just guessing here.