diffraction power response

[thadman]

How significant are diffraction effects with regards to the total system?

Assuming the secondary energy sources coincide with the axial (ie direct) response, a significant degradation in sound quality may be observed.

However, if the secondary energy sources do not coincide with the axial (ie direct) response, a degradation in sound quality may not necessarily be observed as they may reinforce the reverberant field (ie >20ms).

Any thoughts?

Best Regards,
Thadman

[dlr]

Originally Posted by thadman

How significant are diffraction effects with regards to the total system?

Assuming the secondary energy sources coincide with the axial (ie direct) response, a significant degradation in sound quality may be observed.

However, if the secondary energy sources do not coincide with the axial (ie direct) response, a degradation in sound quality may not necessarily be observed as they may reinforce the reverberant field (ie >20ms).

Any thoughts?

Best Regards,
Thadman

See John K's paper here. Page three covers your question.

Dave

[thadman]

Originally Posted by dlr

See John K's paper here. Page three covers your question.

Dave

Thanks for the link

Diffraction, if considered separate from the direct response, may create a power imbalance if not dealt with.

However, beyond the baffle, let us consider minor obstacles, such as the faceplate of a tweeter. Diffraction may exist under such circumstances.

I do not believe a large amount of energy is required to significantly degrade sound quality if the energy coincides with the direct response (<3ms). The faceplate of a tweeter may be able to contribute such energy. However, if this energy were directed off-axis (away from the listener), it may not degrade sound quality and/or have a significant effect on the power response.

I am aware that Geddes' Summa displays an axial dip in the frequency response, however this hole assumes such a small area over the surface of the sphere of total radiated energy that he considers it insignificant.

[thadman]

How can we define power response with respect to diffraction effects, separate from the initial wavefront? I would expect, under most circumstances, that diffraction effects will be directive (ie not omnidirectional).

[dlr]

Originally Posted by thadman

How can we define power response with respect to diffraction effects, separate from the initial wavefront? I would expect, under most circumstances, that diffraction effects will be directive (ie not omnidirectional).

Power response is not a directional issue and you can't define it with respect to diffraction, since the latter is an axial consideration. Power response is the integrated response over a sphere with the driver at the center. The diffraction affects the power response together with the crossover as shown in John's page. It would take a system analysis to describe the diffraction pattern thoroughly for any given system, unique for each system. Diffraction in most systems is a rather complex situation and changes significantly with the axis.

Dave

[thadman]

Originally Posted by dlr

Power response is not a directional issue and you can't define it with respect to diffraction, since the latter is an axial consideration. Power response is the integrated response over a sphere with the driver at the center. The diffraction affects the power response together with the crossover as shown in John's page. It would take a system analysis to describe the diffraction pattern thoroughly for any given system, unique for each system. Diffraction in most systems is a rather complex situation and changes significantly with the axis.

Dave

Perhaps, power response was not the appropriate term. Of significant interest is the distribution of energy with regards to diffraction effects. For example, a tweeter faceplate. I understand the inherent complexity of such effects, however can any *rules of thumb* be made?

[dlr]

Originally Posted by thadman

Perhaps, power response was not the appropriate term. Of significant interest is the distribution of energy with regards to diffraction effects. For example, a tweeter faceplate. I understand the inherent complexity of such effects, however can any *rules of thumb* be made?

There's too much variability for there to be a rule of thumb, as shown in John's pages. Even for a circular baffle, the directionality of the driver(s) must be included. You have to analyze each case individually, with the possible exception of horns/waveguides in the region where their directivity essentially minimizes any diffraction artifacts.

Dave

[gedlee]

I guess that I don't see any relationship between the question and John's paper. There is nothing in John's paper that talks about diffraction. The "Baffle step" is an approximation to the lowest order diffraction from an enclosure, but it is a very course model.

In term of "power response" diffraction is going to be negligable, except maybe the lowest order, i.e. the baffle effect. But otherwise diffraction from edges or objects is not going to have any appreciable effect on the power response. These things become the most apparent in the impulse response, but even then, since the impulse response is usually shown linear and our hearing is logarithmic, diffraction that is audible could easily be too small to see in a linear amplitude impulse response. Waterfall plots alleviate the linear problem, but exacerbate the observation problem by not showing things in a way that makes diffraction easily recognized. Basically none of the current measures of speakers show this effect to any significant degree. Like THD, which is easily measured, but doesn't correlate at all to what we hear, diffraction is not easily measured but is likely to be highly audible. In a very real sense we look carefully at the unimportant things but ignore the important ones.

[dantheman]

Originally Posted by gedlee

I guess that I don't see any relationship between the question and John's paper. There is nothing in John's paper that talks about diffraction. The "Baffle step" is an approximation to the lowest order diffraction from an enclosure, but it is a very course model.

In term of "power response" diffraction is going to be negligable, except maybe the lowest order, i.e. the baffle effect. But otherwise diffraction from edges or objects is not going to have any appreciable effect on the power response. These things become the most apparent in the impulse response, but even then, since the impulse response is usually shown linear and our hearing is logarithmic, diffraction that is audible could easily be too small to see in a linear amplitude impulse response. Waterfall plots alleviate the linear problem, but exacerbate the observation problem by not showing things in a way that makes diffraction easily recognized. Basically none of the current measures of speakers show this effect to any significant degree. Like THD, which is easily measured, but doesn't correlate at all to what we hear, diffraction is not easily measured but is likely to be highly audible. In a very real sense we look carefully at the unimportant things but ignore the important ones.

So wouldn't we be better off with more rounded enclosures covered in foam?

Dan

[dlr]

Originally Posted by gedlee

I guess that I don't see any relationship between the question and John's paper. There is nothing in John's paper that talks about diffraction. The "Baffle step" is an approximation to the lowest order diffraction from an enclosure, but it is a very course model.

In term of "power response" diffraction is going to be negligable, except maybe the lowest order, i.e. the baffle effect.

Baffle step may be an approximation, but current software is rather good at predicting it. That was where I was placing the emphasis, since that is the most significant aspect of diffraction for most systems and does affect power response as well as the crossover design, as detailed to some degree in John's paper.

The on-axis influence above the step region is also important, certainly for typical non-horn/non-waveguide type systems. For those of us working with more classical systems, the part of diffraction we call step can be quite different between systems, so the difference in system power response can be appreciable between different designs. That was my focus. It may be that the OP was not as concerned with that and more so with the diffraction effects above the step area. If so, then yes, John's papers don't address that part of it. But for that case it's very hard to make any clearcut definitions of the influence since that also can vary dramatically from system to system and is an axial situation anyway.

Dave