What's with the sharp edges on speaker boxes these days?

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#5
Dr Geddes seems concerned about box edge diffraction, uses generous radii

I believe he claims due to his listening test results he weights early time delay distortions like baffle edge, mouth diffraction, HOM in horns as more audible defects than many nonlinear/THD distoritons in typical loudspeakers


how about some pointers to threads, reference materal rather than off handed dismissal
 
#6
Melo theory said:
Well, Charlie, you could just tell me.
🙂
Click to expand...

Knowledge is power, but in order to have a conversation about the info that leads to the knowledge you must first have some basic understanding of the fundamentals. This is the path I am trying to lead you down...

First Google "Olson diffraction" and read the first link from True Audio. What do you think the differences in the graphs are telling you for different shaped boxes?

Let's start there.

-Charlie
 
#8
I'll say it simply- most chamfers are too small to show a large effect on diffraction. They're more for decoration. You need large chamfers or radii to have a significant effect at midrange and lower treble frequencies. A little felt will have considerably more effect.
 
#9
SY said:
I'll say it simply- most chamfers are too small to show a large effect on diffraction. They're more for decoration. You need large chamfers or radii to have a significant effect at midrange and lower treble frequencies. A little felt will have considerably more effect.
Click to expand...

OK, now we are getting somewhere. The post above is hitting on the high point of the topic - most people use their router to put a 1/2" 45 degree chamfer or roundover along the edge of the baffle. In theory, this DOES result in smoother frequency response (oooh, aaah!) but in practice it doesn't. Why? The size of the chamfer is inversely related to the frequency above which is has an effect. So the 1/2" roundover can only help things above something like 4000 Hz (guesstimating here). But what is happening at 4000Hz? Only the tweeter is operating and its dispersion is causing that part of the wavefront that is emitted to the side, towards the cabinet edge, to be down in level by maybe 12dB, so any effect is muted at best.

You could make the roundover HUGE so that it is effective at lower frequencies where it will do some good. But making a 4"-6" roundover turns out to be somewhat difficult, although there are some special curved MDF products that can do it. Mostly it just looks stupid.

Why go to all this trouble in the first place? The goal is to smooth out the ripples that tend to occur with rectangular cabinets with sharp edges. Most people want their loudspeaker to have a smooth frequency response without major peaks or dips. But it turns out there are other clever ways to minimize these ripples even if you have perfectly square edges, like playing with the position of the driver on the baffle until the ripples are as low as possible using a diffraction simulator (software). This is what I do (now).

Sure, in the past I had some speakers with rounded or chamfered edges. I didn't build the cabinets. The cabinet maker put these edge reliefs there for cosmetic reasons and because a perfectly square edge is prone to damage, so the edge has to be relieved anyway. But I don't believe that the edge profile has anything to do with the sound or has much impact on the frequency response, it's just there.

I was hoping you (Melo) would get a grain of curiosity about these things. There is lots of physics behind all of this. Just cutting to the answer often leaves out large and sometimes important points about WHY something is the way it is. That is why the journey to the destination is often times more important than the destination itself.

-Charlie
 
#10
SY said:
I'll say it simply- most chamfers are too small to show a large effect on diffraction. They're more for decoration. You need large chamfers or radii to have a significant effect at midrange and lower treble frequencies. A little felt will have considerably more effect.
Click to expand...

This is a common misconception. Assume a hemispherical wavefront, then consider that the angle of incidence varies the effective size of any edge termination.

This is borne out by one of our local's measures, but I don't remember whose site off the top of my head. He was surprised by the effect of something like 1/2" roundover down near 1200Hz.
 
#11
CharlieLaub said:
OK, now we are getting somewhere. The post above is hitting on the high point of the topic - most people use their router to put a 1/2" 45 degree chamfer or roundover along the edge of the baffle. In theory, this DOES result in smoother frequency response (oooh, aaah!) but in practice it doesn't. Why? The size of the chamfer is inversely related to the frequency above which is has an effect. So the 1/2" roundover can only help things above something like 4000 Hz (guesstimating here). But what is happening at 4000Hz? Only the tweeter is operating and its dispersion is causing that part of the wavefront that is emitted to the side, towards the cabinet edge, to be down in level by maybe 12dB, so any effect is muted at best.

You could make the roundover HUGE so that it is effective at lower frequencies where it will do some good. But making a 4"-6" roundover turns out to be somewhat difficult, although there are some special curved MDF products that can do it. Mostly it just looks stupid.

Why go to all this trouble in the first place? The goal is to smooth out the ripples that tend to occur with rectangular cabinets with sharp edges. Most people want their loudspeaker to have a smooth frequency response without major peaks or dips. But it turns out there are other clever ways to minimize these ripples even if you have perfectly square edges, like playing with the position of the driver on the baffle until the ripples are as low as possible using a diffraction simulator (software). This is what I do (now).

Sure, in the past I had some speakers with rounded or chamfered edges. I didn't build the cabinets. The cabinet maker put these edge reliefs there for cosmetic reasons and because a perfectly square edge is prone to damage, so the edge has to be relieved anyway. But I don't believe that the edge profile has anything to do with the sound or has much impact on the frequency response, it's just there.

I was hoping you (Melo) would get a grain of curiosity about these things. There is lots of physics behind all of this. Just cutting to the answer often leaves out large and sometimes important points about WHY something is the way it is. That is why the journey to the destination is often times more important than the destination itself.

-Charlie
Click to expand...

You are assuming that I haven't researched edge diffraction?
I've been designing and building Speakers for a long time.
I wanted to have a discussion about this, and the first reply I get is snarky and enigmatic.
I for one do believe in the smearing effect of edge diffraction.
And you are right, the chamfer or roundover is in proportion to wave length.
If you have a wide dispersion tweeter on a narrow baffle, you will get a glitch in an impulse response measurement, and that will effect FR. And what tweeters don't have full 180 degree polar response at 4 kHz? I hardly doubt that amplitude is 12 db down.
Now, you can use the crossover transition from tweeter to mid in relation to the baffle width to even out this effect (due to different positions on the baffle), but I hardly think commercial speakers have this in mind.
I think it's more of a production cost issue.
 
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#12
OK, it looks like you already know all of this, so end of "discussion" I guess? It is for me at least.

For what it's worth, when recently talking to a moderator of this board he suggested that replies on issues such as these are framed so as to "lead" the OP along towards the answer instead of just giving it directly. So FYI that was the tact that I was on - I wasn't trying to create a snarky (your words) reply. The mod is trying to stimulate extended discussion. Lots of people read these threads later to learn things.

My apologies that I didn't pick up on your learned state of mind vis a vis diffraction by your short opening post. If this is the kind of response I get, I might as well just save my breath.
 
#17
badman said:
This is a common misconception. Assume a hemispherical wavefront, then consider that the angle of incidence varies the effective size of any edge termination.

This is borne out by one of our local's measures, but I don't remember whose site off the top of my head. He was surprised by the effect of something like 1/2" roundover down near 1200Hz.
Click to expand...

I was led to believe anything under two inches radius would be pointless, but perhaps the source of that gem was thinking along the lines of the full spectrum.
Which gets me to thinking…. Take a typical / popular speaker design where the baffle starts to rolls off at the tweeter and gets shallower as it gets to the mid etc. (only using this picture because I have seen it recently, not picking on this build)

This would be seemingly backwards based on the information above…
 

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#18
I found a small explanation by Joseph D'Appolito. It's difficult to find information about how chamfering will affect this.

----------------------
Strassacker: Speaker Building, Components
"Edge Diffraction:
A conceptual picture of the edge diffraction process is shown in Figure 2. The source is driven with a pure tone producing a hemispherical wave front progressing outward along the disk surface. When the wave reaches the edge of the disk it is suddenly forced to expand into a much larger volume. The original wave continues to expand outward wrapping around the disk and diffracting to the rear with no change in phase. As the wave expands from a half space into a full space various conservation laws tell us the pressure must drop. The pressure drop at disk edge, however, causes a second wave to be launched at the disk edge traveling in the forward direction. The phase of this wave is reversed relative to the original wave. One way to view this is to consider the drop in pressure to be caused by the generation of a second wave at the disk's edge with opposite polarity to the original or incident wave.

The forward propagating diffracted wave will interfere with the original wave causing response ripples as the diffracted wave alternately reinforces or diminishes the on-axis frequency response."

jd_beugung2.gif
 
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