Other options to smooth baffle diffraction

[Dave Bullet]

I'm trying to improve an old pair of commercial Wharfedale speakers. The tweeter in one is stuffed, so thought I'd replace both with a pair of Seas 27TBFC/G.

I want to re-use the cabinets and baffle. Good news is the Seas tweeter is a drop in fit / replacement for the existing Wharfedale tweeter (flush mounts). Bad news is it is centered which makes the baffle diffraction "lumpy" around the 2KHz area, as can be seen.

Do you think this response would be "acceptable" or is the dip in the 2KHz area too severe? (the dip is around 3dB).

If it needs tuning - what options do I have apart from trying felt around the tweeter (the felt which I have yet to source and measure).

Design goal for both woofer and tweeter is 4th order acoustic LR slopes centered just above 2KHz.

Below is the modeled on-axis response.

BTW - I've yet to sim this crossover on the off-axis responses which I've yet to measure.

PS: Ignore the "tail" between 200 - 500Hz - it is an artifact of my nearfield splice

Thanks,
David.

[Dave Bullet]

Here is the reverse null with tweeter polarity reversed (incase anyone is interested)

[Dave Bullet]

And here is the crossover...

The 470uf cap is a leftover from the old xo which I have reused to shape the hump in the woofer from about 100Hz to 200Hz). I will take this out and see if the midrange is notably clearer (don't like having big and undoubtedly bipolar caps inline with any driver).

The baffle dimensions are
440mm (h)
250mm (w)

Tweeter centre is 95mm from cabinet top and centered on width of baffle (ie. 125mm from either side).

[dlr]

Originally posted by Dave Bullet
I want to re-use the cabinets and baffle. Good news is the Seas tweeter is a drop in fit / replacement for the existing Wharfedale tweeter (flush mounts). Bad news is it is centered which makes the baffle diffraction "lumpy" around the 2KHz area, as can be seen.

Do you think this response would be "acceptable" or is the dip in the 2KHz area too severe? (the dip is around 3dB).

I prefer to minimize diffraction as well, but keep in mind that an on-axis response, especially without diffraction control, is a bit misleading. Much of what you hear is related to the power response that is the total output on all axes. A measurement slightly off-axis will probably be closer to what you'll hear, so a big dip on-axis is a bit less audible that you might think, but don't let this stop you from going for optimum.

If it needs tuning - what options do I have apart from trying felt around the tweeter (the felt which I have yet to source and measure).

I have done a significant amount of testing felt. SAE Type F-11 or F-13 are probably the two best types to use for this purpose. You'll find a couple of pages at my site on the topic as well as a link to a couple of sources. One is in Australia, though I don't know if it's still valid.

Dave's Speaker Pages

The June 2005 issue of audioXpress has an article of mine on felt as well with a bit more detailed measurements and description about placement issues.

Dave

[Dave Bullet]

Thanks for the site Dave. Looks comprehensive.

I've never tried felt to smooth tweeter response so will read your site thoroughly and try some options. Just need to find a place in NZ to source the appropriate felt from.

Thanks once again,
Dave.

[EC8010]

The BBC LS3/5a used strips of felt with a 1/2" square cross-section arranged as a large rectangle on the tweeter front plate to deal with diffraction etc.

[OzMikeH]

Self adhesive felt is often sold for sticking under furniture to stop it scratching the floor.
I see you are across the moat from here, you should be able to get something in K-mart. I got a roll of stuff from there that is black, 40mm wide by 5mm thick. It was about a metre long rolled up in a bag with a cardboard tag stapled on. It only cost about $3.

A star shape around the tweeter with an odd number of points works very well. For a horn you could cut zig-zags in one side of the self adhesive strip. I'd try to make them sharper than 90 degrees and less than 1cm peak-peak.

[dlr]

Originally posted by OzMikeH
Self adhesive felt is often sold for sticking under furniture to stop it scratching the floor.
I see you are across the moat from here, you should be able to get something in K-mart. I got a roll of stuff from there that is black, 40mm wide by 5mm thick. It was about a metre long rolled up in a bag with a cardboard tag stapled on. It only cost about $3.

A star shape around the tweeter with an odd number of points works very well. For a horn you could cut zig-zags in one side of the self adhesive strip. I'd try to make them sharper than 90 degrees and less than 1cm peak-peak.

If it's what I've used for feet on chairs, it's not very effective. It's just way too dense. It has to allow the sound to pass into it so that it can be damped as it passes through. Too dense and there's mostly reflections.

dlr