Subtle design change - will it hurt?

[Billyo]

Hi all,

I'm part of the way through building Tony Gee's Low Budget Speakers(LBS) - but due to having toddlers and kids around, I can't have the drivers naked, otherwise they'll get poked with a curious finger as soon as I turn my back.

I've decided to screen them with speaker cloth and been turning over ideas about how to best incorporate the slight design change.

Tony's original design for the LBS floorstanders is with a 45 degree chamfer, 22mm wide running down both vertical edges of the front baffle, with the drivers rabbeted flush with the baffle.

Tony doesnt have a picture on his site anymore, but there is a pic here from someone else - although they seem to have not done the chamfer at all (or at least its a lot smaller than 22 mm).



I was thinking of doing away with the chamfer and substituting with a wooden frame to hold the speaker cloth - the wood of the frame would have a triangular cross section with a 90 degree angle pointing outwards and the two sides that are *not* against the baffle measuring 9 mm,

The frame itself would be held onto the baffle by means of those super strong little neodymium magnets that I'd embed into the frame and baffle - they would easily reach through the speaker cloth and a thin layer of wood putty over the magnet embedded in the baffle.

Rather than rabbet the baffle to flush mount the drivers, I'd leave them a bit proud of the baffle (since they're covered by cloth anyway).

My question is, will this proposed design have any nasty side effects, such as causing audible diffraction? Is it important to keep the frame profile as low as possible, or minimise the outwards facing angle?

I'm spending a lot of time (and a lot more money than I originally thought as well!) on these speakers - being my first build, I want to get them as close to perfect as I can and don't want to inadvertently stuff it up by making a silly design change...

[AllenB]

Your changes would increase diffraction that may be audible but you could alleviate this with felt, i.e. cut it to the shape of the baffle around the drivers and have it as thick as the grille frames.

[Charles Darwin]

I'd make the cloth frame from half-round material rather than triangular.

[sreten]

Hi,

Make sure you leave enough clearance for full forward driver excursion.
(A mistake easy to make, even seen it in commercial speakers.)

rgds, sreten.

[Billyo]

Thanks for the fast responses!

sreten - thanks for that warning. According to the Monacor site, the woofer has a linear excursion of (XMAX) ±5.5mm (presumably at the "center" of the cone). Not sure how that translates to the outer edge that will be closest to the speaker cloth - but I'd think that leaving at least, say, 8 mm between the driver frame (of which the outer edge of the cone is at the same level) and the speaker cloth should be safe enough - what do you think?

Allen and Charles - how about combining semi -circular material and using the thick felt?

Would I be better off recessing the drivers into the baffle a little like the original design, or is it better having them proud?

[mondogenerator]

From Weems' book. he recommends using spacers/standoffs to mount the grille suspended say 1" from the baffle, to help reduce the diffraction effects. You could cover the rear of the grille frame in felt instead OR in addition to the baffle as you see fit. And it looks cool

Half round material would be good...id be tempted to make out of MDF and route the profile to suit once ive sorted the fixings.

[sreten]

Hi,

According to Zaph Audio tests tweeters should be recessed. especially
round ones, but bass mids don't have to be. Using felt or foam on the
baffle is another way of dealing with the edge of the tweeter.

8mm sounds sensible. Pity Tony Gee has now ruined his site.

rgds, sreten.

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[joe58]

Could you not just build the grille frame out of wire like coat-hangers are made from? Then at the corners just have pieces of triangular wood to attach the magnets to? Would the metal be thin enough not to 'interrupt' the sound waves. Just a thought, although my lack of knowledge, so may not work. You could even then make the baffle curve round the chamfers at the side. You would have to be very careful with the glue though when glueing it to the wire.

[wintermute]

I went through the same process a year or so ago though I do have chamfered baffles which made it quite a bit harder! I'm actually going to redo the grills as I wasn't 100% happy with my first attempt (which was somewhat ambitious).

1st observation, I used 3mm magnets. bigger (unless you have absolutely flat surfaces) would have been better! even 1 layer of speaker grill cloth between the magnets is enough to weaken the attraction considerably. The other issue is the smaller the magnets the more accurate you have to be with ensuring they line up exactly! not as easy as it may sound!

Felt is also something I'm planning on incorporating with the new grills. I found a good supplier in Aus. http://www.feltshop.com.au/categories.asp?cID=1

joe58 I also considered wire like you have suggested. the thing I was concerned about is that it might ring at certain frequencies. One way to find out I guess

Tony.

[joe58]

wintermute: Ah didn't think of that.