Narrow baffles and radiused edges

[bremen nacht]

I'm told that both the above improve stereo imaging.

I've built a pair of boxes and it was no hardship to make them narrow (good SAF) and I'm still prepared to radius the edges if it'll help.

But can anyone explain (in words of less than one syllable, I'm feeling delicate) WHY narrow baffles and radiused edges have an effect?

Thank you!

[filgor]

Originally posted by bremen nacht
I'm told that both the above improve stereo imaging.

I've built a pair of boxes and it was no hardship to make them narrow (good SAF) and I'm still prepared to radius the edges if it'll help.

But can anyone explain (in words of less than one syllable, I'm feeling delicate) WHY narrow baffles and radiused edges have an effect?

Thank you!

I bet this will kick up some debates!

It really is a big word topic so sorry for all the sylables!

Before you get lost in the physics of things it might be simpler to start with the fact that if the width of your front baffle is narrow, meaning not much wider than the speaker diaphram then rounding the edges is not as critical. I would round them anyway because it improves the look of your cabinet.

I would argue that it affects the smoothness of your speakers response curve. Whether or not it improves stereo imaging.. well thats up for debate.

Any way it is to do with ripples in the frequency response caused by varying phase shifts that occur between the driver and the baffle edge. As sound waves propagating from the driver reach the edge of the enclosure a phase shift occurs which is frequency dependant.
Depending on the phase shift it can cause either an increase or decrease in output. Since the phase shift is frequency dependant you end up with ripples.

There are heaps of sites that can explain the effects. Not really in simple terms and few mention the fact that if the driver diaphram is large relative to the baffle width then the ripples will be far less significant.

Oh and I haven't even mentioned the 6dB drop in sensetivity that happens below these ripples even if they are small.

Before I frustrate you.. Heres a link to start you off. It talks about experiments done by Olson. Note first that these experiments were carried out using a very small driver on much larger baffles to maximise the horrible ripples that can occur.

http://www.trueaudio.com/st_diff1.htm

I hope this helps you out before you get bombarded with big words and passionate opinions

[filgor]

Opps forgot to clarify something!

the smaller the baffle width the less the phase shifts and this limits the ripples to higher frequencies. If the driver diaphram is wider it is more directional at high frequencies so they travel forward and dont "see" the edges of the baffle. Hence the idea of making the baffle thin compared to the driver.

Rounding the edges reduces the amplitude of the ripples that do occur because the baffle edge is less well defined.

[Curmudgeon]

There was an article in Audio Express two or three years ago that addressed the rounded edges/diffraction issue. Conclusion was that a radius of 4 inches or more produced the best results; not that easy to make, although some of the layered construction techniques make it less awkward.

On the speakers currently under (inteminable) construction, we use layers of 2" high density foam (60psi blue foam) built up in a shape somewhat like a teardrop, with the lower half sliced off to match the top of the woofer cabinet. With really good drivers, every change we made to reduce diffraction had an audible effect. We also found that the use of felt on the front and around to the sides and top helps also. The felt should be high wool content, and 3/8" thick is not a bad starting point. We get ours from McMaster Carr. Routing an inset so that the speaker front frame is flush with the cabinet also helps. If the driver is not inset, the felt can provide a buildup to give you a flush surface.

I'm not sure if diffraction issues would affect imaging though. I think the first order imaging issue is precise matching of the two speakers. But then I don't pay a lot of attention to imaging, regarding hyper-precise placement as a recording artifact; closing my eyes listening to a live performance never results in pinpoint imaging.

[filgor]

Looks like Curmudgeon agrees that it really isn't relavent to imaging.

It is only relavent to smoothness of the frequency response.


To almost completely eliminate diffraction ripples I follow a simple rule:

Radius the edges most of the width of the distance between the driver frame and the box edje. ie. if there is a 1.1 inch gap between the driver frame and baffle edge give it a 1 inch radius curve.
Alternatively 45 degree chamfer to within 10mm (3/8") of the divers frame also works well.

[Curmudgeon]

Afterthought. If the mid and tweeter are mounted so that they are not equidistant form the edges, it helps spread the spectrum of the diffraction effects, which also reduces the severity. As an example, the tweeter center might be 2, 3 and 5 inches from left side, right side, and top. (Non-integer relations, just like box interior. ) As a result, you'll want to make the two front baffles mirror images to maintain imaging.

With the blimp the diff effects are tamed so well that we did not bother with asymmetry.

[filgor]

Originally posted by Curmudgeon
Afterthought. If the mid and tweeter are mounted so that they are not equidistant form the edges, it helps spread the spectrum of the diffraction effects, which also reduces the severity. As an example, the tweeter center might be 2, 3 and 5 inches from left side, right side, and top. (Non-integer relations, just like box interior. ) As a result, you'll want to make the two front baffles mirror images to maintain imaging.

With the blimp the diff effects are tamed so well that we did not bother with asymmetry.

Hey Curmudgeon,

Bremen Nacht hasn't replied..

Do you think we spooked 'im!

[bremen nacht]

Originally posted by filgor



Hey Curmudgeon,

Bremen Nacht hasn't replied..

Do you think we spooked 'im!

ho ho, no, I don't spook that easily!

I did reply but for some reason it never appeared. Anyway, I said something along the lines of:

thanks for your very clear and delightfully myth-busting answers. I agree with the inappropriately-named Curmudgeon that pin-point stereo doesn't really exist. What I like is the effect of sounds appearing to emanate from places other than a straight line between the speakers, and to hear instruments apparently closer or farther away from my chair than the speakers are. As you say, that generally has more to do with the recording (and the listening room) than the speakers.

My speakers are 12" Tannoy HPDs and the cabinets are 14" wide, so radiusing probably isn't necessary or practicable.

And as the tweeter is a horn that develops to the full diameter of the bass driver's 'cone', that should take care of diffraction from the tweeter.

Thanks!

[Curmudgeon]

Chuckle. You might want to try experimenting with a relatively high wool content felt (75% at least) about 3/8" thick on the front panel around the driver to see if you notice a difference. Perhaps extend it back a few inches along the sides and top....

With a little effort you can make a really ugly speaker.

Felt grades F11 and F13 are useful; F11 has a bit more wool content, but is denser, F13 allows for better absorption. I'd expect that the felt grades are not international though. McMaster Carr has good specifications on the felt that they carry.

[dnsey]

With a little effort you can make a really ugly speaker.

It's hard to improve on some of the vintage Tannoys in that respect

(I'm a fan of Tannoys - have a pair myself - but some of the '60s and '70s cabs...)