Felt or foam walled waveguide?

[dlr]

Originally Posted by speaker dave

No, I think these two cases are hard to separate. A sound wave going down a rigid surface can't expand sideways. It is only when it hits the corner that its energy is allowed to expand (diffract) around the bend. This act of expanding signals a change of acoustic impedance which causes the reflection as well. This is just like a signal going down a transmission line: when it hits a change of impedance (open, short, or just different) a reflection is caused.

I was trying to envision it, but that probably just makes it more confusing.

To have a hard edge, where energy diffracts around the corner and then to replace it with a hard edge with flush absorption, appears to me to be the same case. Rather than continuing along a hard surface the wave is suddenly allowed to bend into an absorbing medium. To the extent that the absorber is a perfect absorber, the sound wave goes around the corner never to be seen (or heard from) again. No difference.

I see your point. Not being familiar with the wave equations, one question I have is what is the form of the impedance? That is, is it primarily resistive or reactive and if the latter, what is that reactive impedance? Certainly it's a function of frequency.

There is one additional factor, however. There will be two changes in the acoustic impedance with a damping medium. The first will be at the baffle/felt interface, the second will be at the edge of the felt. The key is in the difference in the two impedances. My suspicion is that due to the issue of angle of incidence, the difference is very small. Were it large, we should measure a more significant change in the on-axis response (when comparing with vs. without).

Dave

[speaker dave]

And how do you avoid the other textbooks?

I never see the term "diffracting off" of an object but rather diffracting around.

"Bending of light" is a common ussage, as shown by the dictionary definition that I quoted. Bending of light also occurs when transiting a medium of varying refractive index (as does bending of sound when going through a temperature gradient).

It may be "symantic"s but I think it is clearer to refer to diffraction as just the bending around an object and reflection as the re-radiation from the transition. That one typically is found with the other is understood.

David

[speaker dave]

Originally Posted by dlr

I was trying to envision it, but that probably just makes it more confusing.

One of my college physics teachers had the coolest apparatus that was this long snake of wooden bar-bells tied together with springs. You could tweek one end of it and see the wave work slowly down to the other end. If the end was unattached a negative wave came back. If it was rigidly fastened the wave came back in phase. There was even an oil damper that absorbed the wave with no return and a second line of smaller size (different impedance). It was great for visualizing wave phenomona.

I see your point. Not being familiar with the wave equations, one question I have is what is the form of the impedance? That is, is it primarily resistive or reactive and if the latter, what is that reactive impedance? Certainly it's a function of frequency.

I'm not very good at the wave equation either. My understanding is that the acoustic impedance for a plane wave is a resistive 42 ohms per square cm. of the wave front. As a wavefront expands it is presumed to quickly become a plane wave (a sphere of large radius). I'm guessing that the "bending" of the wave around the edge means that it is, at least temporarily, not a plain wave and the impedance change stems from that. (I'm sure someone will correct me if my physics is in error.)

There is one additional factor, however. There will be two changes in the acoustic impedance with a damping medium. The first will be at the baffle/felt interface, the second will be at the edge of the felt. The key is in the difference in the two impedances. My suspicion is that due to the issue of angle of incidence, the difference is very small. Were it large, we should measure a more significant change in the on-axis response (when comparing with vs. without).

Dave

My suspicion is similar. Also, if an absorber performs well its impedance should approach that of free space, shouldn't it?

David S.

[forr]

Hi Speaker Dave,

My suspicion is similar. Also, if an absorber performs well its impedance should approach that of free space, shouldn't it?

So -6 dB SPL in the axial response compared to an infinite baffle
and no more of the so-called "baffle effect" ?

[dlr]

Originally Posted by speaker dave

My suspicion is similar. Also, if an absorber performs well its impedance should approach that of free space, shouldn't it?

I can't give an answer. Part of the problem in considering this is that thinking in terms of impedance in this way, how does it relate to the fact that we're considering an issue of 3-dimensional space?

We also have to consider where the material is positioned in terms of performance. This relates to your question. What characteristic of felt, for example, do we consider good performance? Absorption only? Change in speed of propagation of the wave? Both? Would this be the same goal for both locations?

This goes back to the problem of understanding the mechanism of how felt is an aid in minimizing diffraction when placed on the front baffle. It's a different issue altogether if used, say, to form a waveguide. I doubt that the properties that make it useful on a baffle are the same properties that one would want in a waveguide.

Dave

[DBMandrake]

Originally Posted by gedlee

In all the textbooks that I have read diffraction is that wavefront that is sent off from the edge, in this case. This would make the forward zone the direct plus diffraction field and the "shadow zone" the diffraction only. I think your distinction here between an edge reflection and an edge diffraction is symantic.

In all my writing and use of the term, the diffraction wave propagates in all directions not just into the shadow zone. And I believe that this is consistant with the physics literature on the sibject as well.

That's how I understood it too. The wave bending around the sharp edge causes a drop in pressure to occur at the bend, launching a second out of phase wave from the diffraction point - but this wave propagates in all directions from the sharp edge, as shown by this diagram:

When a wave hits an object in space we refer to how the sound "diffracts" off of it. There is no distinction made between the sound that "reflects" off of it from the sound that diffracts off of it. It's all the same thing.

In the case of a tweeter on a baffle where is the reflection though ? The wave that reaches the sharp edge of the baffle has been running along parallel to the plane of the baffle to get there.

Unless the tweeter dome physically protrudes above the plane (which some do I guess) there is no chance for a reflection to occur, only diffraction.

I'm also not sure that I understand how diffraction and reflection can be one and the same - if a sound wave from afar impacts an infinite baffle and reflects off it, how is this equivalent to diffraction when an infinite baffle has no finite dimension for the sound to diffract around ? And yet an infinite plane will quite happily reflect a sound wave...

Also a reflection will reflect at the incident angle from the surface much like a beam of light in a mirror, yet diffraction will cause radiation in all directions from the sharp edge where diffraction is occurring as in the above diagram - diffraction becomes a secondary sound source while a reflection is simply a mirror image of the original source location. A sound wave reflection from a boundary is also in phase with the original at the point of reflection, while diffraction re-radiation is out of phase with the original at the point it occurs. Doesn't seem like the same phenomenon to me.

[megasat16]

Originally Posted by gedlee

In all the textbooks that I have read diffraction is that wavefront that is sent off from the edge, in this case. This would make the forward zone the direct plus diffraction field and the "shadow zone" the diffraction only. I think your distinction here between an edge reflection and an edge diffraction is symantic.

In all my writing and use of the term, the diffraction wave propagates in all directions not just into the shadow zone. And I believe that this is consistant with the physics literature on the sibject as well.

When a wave hits an object in space we refer to how the sound "diffracts" off of it. There is no distinction made between the sound that "reflects" off of it from the sound that diffracts off of it. It's all the same thing. In the equations we use two terms to dscribe the field everywhere - the direct field and the diffraction field. In the shadow zone there is only the diffraction field, everywhere else the field is the sum of the direct field and the diffraction field.

That's understood and agreed.

Originally Posted by DBMandrake

That's how I understood it too. The wave bending around the sharp edge causes a drop in pressure to occur at the bend, launching a second out of phase wave from the diffraction point - but this wave propagates in all directions from the sharp edge, as shown by this diagram:



In the case of a tweeter on a baffle where is the reflection though ? The wave that reaches the sharp edge of the baffle has been running along parallel to the plane of the baffle to get there.

The image above and explanation I don't get it. I think Diffraction requires an object in space. How do the baffle and it's edge fits in the Diffraction?

[speaker dave]

Originally Posted by DBMandrake

In the case of a tweeter on a baffle where is the reflection though ? The wave that reaches the sharp edge of the baffle has been running along parallel to the plane of the baffle to get there.

Unless the tweeter dome physically protrudes above the plane (which some do I guess) there is no chance for a reflection to occur, only diffraction.

I'm also not sure that I understand how diffraction and reflection can be one and the same - if a sound wave from afar impacts an infinite baffle and reflects off it, how is this equivalent to diffraction when an infinite baffle has no finite dimension for the sound to diffract around ? And yet an infinite plane will quite happily reflect a sound wave...

The reflection being discussed isn't due to sound hitting a surface from some angle above the surface, rather just the reflection of the wave that has been traveling across the surface.

As to diffraction and reflection being one and the same, we aren't really talking about conventional reflections, only those that come along with the act of sound diffracting around a discontinuity, the secondary re-radiation from an edge. Your infinite baffle won't have any edge reflections.

David S.

[speaker dave]

Originally Posted by megasat16

The image above and explanation I don't get it. I think Diffraction requires an object in space. How do the baffle and it's edge fits in the Diffraction?

The term diffraction applies here also. Diffraction means the bending (expanding) of the wave into the void after the boundary conditions change in any way. Sound can hit an obstacle in space (wall with an opening in it) or it can be traveling down a wall that ends.

David S.

[LineArray]

Hi,

some kind of "lossy" or "defracting" WG i made two
years ago as an explorative prototype, which may
read more polite than "quick and dirty" ...

"A" is the driver in the stuffed tube without WG

"B" is with WG attached

As far as i remember angles were

blue: on Axis
red : 20 degrees
green: 45 degrees (quite in line with the WG's "walls")

distance about 0.7m


A very different approach from felt of course ...

but absorption could also be added in several ways
outside the WG's walls.

Kind Regards