musical noise said:
Thats not precisely true since there is a continuum of wall impedances. I can define wave functions for a device that are absorptive at the walls and I can examine the propagation of said wave down the device. It will have characteristics of a waveguide (certainly not a horn since a horn has to have rigid walls or the assumptions are completely violated), but they will be different to be sure. The more rigid the walls the more like a normal waveguide the device will act. Its all an interesting study, one I may do at some point, but I don't feel that much would be gained and that the practicalities of making such a device would be prohibitive. So as interesting as it might be, it seems kind of academic to me.
The key phrase here (and admittedly this had not occurred to me when I was responding above) is the "continuum of wall impedances" What I was thinking of was of a soft material not unlike the foam used in your waveguides (perhaps even closed-cell foam) which is basically transparent to all frequencies presenting no acoustical impedance.
In that case, there would be no waveguide effect at all. Just a loss as if the driver were in free space. What one would want is an impedance about double or triple that of air. Then much of the wall reflections would be absorbed, but the main wave would also see a substantial effect of the waveguide principle.
Lipinski Sound This was the biggest commercial application I had seen, and the one that "was" using the term wave guide before, I thought. I now notice that they are no longer using that term, or even CD, so maybe they realized that was inaccurate.
Over the years though it does seem that others have done this. I'm not saying it's right, effective, or even a correct use of the terms controlled directivity or wave guide. My thought as to how it might be somewhat accurate though is that sound which radiates to the sides is absorbed by the foam and converted to heat, meaning that there aren't waves propagating around the baffle and diffracting, nor are they directed over much of a wide angle, thus the speakers directivity is controlled.
Over the years though it does seem that others have done this. I'm not saying it's right, effective, or even a correct use of the terms controlled directivity or wave guide. My thought as to how it might be somewhat accurate though is that sound which radiates to the sides is absorbed by the foam and converted to heat, meaning that there aren't waves propagating around the baffle and diffracting, nor are they directed over much of a wide angle, thus the speakers directivity is controlled.
Hello,
If I can try this parallel, a light guide glass fiber can be considered as a waveguide.
In the first generation of fibers used as light guide reflexions at the fiber boundary were used for the light to travel along the fiber but in moderns fibers, the structure of which induced a refraction index gradient, the light rays never touch the fiber boundary.
Why not an audio waveguide using a gradient refractive foam plug...
Though, it will surely be difficult to manufacture that foam, for sure...
Best regards from Paris, France
Jean-Michel Le Cléac'h
If I can try this parallel, a light guide glass fiber can be considered as a waveguide.
In the first generation of fibers used as light guide reflexions at the fiber boundary were used for the light to travel along the fiber but in moderns fibers, the structure of which induced a refraction index gradient, the light rays never touch the fiber boundary.
Why not an audio waveguide using a gradient refractive foam plug...
Though, it will surely be difficult to manufacture that foam, for sure...
Best regards from Paris, France
Jean-Michel Le Cléac'h
gedlee said:
Re: Refractive gradients
I came upon the whole foam idea from this view point. My idea was to use the foam as a form of acoustic lense by which I could further control the directivity. But alas this did not work because the index of refraction, as you say, is not great enough unless there is a lot of loss. Light reflection from a lense surface as well as internal losses in the glass are indeed the biggest problems in optical lense design.
But, when I actually listened to a foam filled waveguide I was amazed at what I heard. The harshness that I so long objected to was gone. It took almost two more years before I could explain this as a reduction in HOM. I had know that HOM would be possible and probably present, but I also knew that they would not be major waves in terms of amplitude. But then when we did our study of diffraction and how audible diffraction is even at low levels, then it all became clear. The HOM don't have to be large to be audible and the foam reduces them even further after the waveguide shape minimizes them as much as is possible with shape. Group delayed signals are not masked as nonlinear distortion effects are. They become more audible at higher levels, not less, and as such will always be a factor at some level.
auplater said:
I came upon the whole foam idea from this view point. My idea was to use the foam as a form of acoustic lense by which I could further control the directivity. But alas this did not work because the index of refraction, as you say, is not great enough unless there is a lot of loss. Light reflection from a lense surface as well as internal losses in the glass are indeed the biggest problems in optical lense design.
But, when I actually listened to a foam filled waveguide I was amazed at what I heard. The harshness that I so long objected to was gone. It took almost two more years before I could explain this as a reduction in HOM. I had know that HOM would be possible and probably present, but I also knew that they would not be major waves in terms of amplitude. But then when we did our study of diffraction and how audible diffraction is even at low levels, then it all became clear. The HOM don't have to be large to be audible and the foam reduces them even further after the waveguide shape minimizes them as much as is possible with shape. Group delayed signals are not masked as nonlinear distortion effects are. They become more audible at higher levels, not less, and as such will always be a factor at some level.
Re: Re: Refractive gradients
I'd expect these artifacts to be just as audible on baffle edges, driver frames, etc.
All the more reason for liberal use of rounding, flush mounting, beveling or rounding the inside lip of a driver mounting (cones are essentially acoustically transparent) and felt. Also it might make sense to explore more absorption on the inside of the enclosure close to the cones.
gedlee said:
I'd expect these artifacts to be just as audible on baffle edges, driver frames, etc.
All the more reason for liberal use of rounding, flush mounting, beveling or rounding the inside lip of a driver mounting (cones are essentially acoustically transparent) and felt. Also it might make sense to explore more absorption on the inside of the enclosure close to the cones.
In Badman's defense, isn't it true that at a certain point, if the frequency is low enough, and the density and rigidity of the diaphragm is low enough, it will pass through the cone, rather than reflect off of it? If a fabric dome such as is used in a tweeter was exposed to lower frequencies, won't those frequencies pass through it? Isn't this why fabric dome midranges require so much doping?