Accurate weights for speaker Vas measurement - diyAudio
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Old 28th December 2002, 12:46 AM   #1
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Lightbulb Accurate weights for speaker Vas measurement

One way of loudspeaker Vas measurement is by measuring the resonant frequency, then measuring it again with a small amount of extra mass added to the cone. The change in resonant frequency equates to the Vas somehow. Not everyone has a set of suitable accurate weights lying around (or so I thought). Then I realised that coins are readily available, non magnetic, and accurate composition and size, therefore mass. Seeing Australians are the best loudspeaker diyer's in the world here is a suitable link to the Mint with the relevant info. http://www.ramint.gov.au/making_coin...in_designs.cfm
Actually, the 5, 10 and 20 cent coins are 2.83, 5.65 and 11.3 grams respectively, a ratio of 1:2:4 like binary so it would be easy to make any mass in 2.83 gram steps. 2.83 grams is 1/10 ounce if that makes more sense. Decimal money in imperial measurements. Unbelievable.
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Old 28th December 2002, 02:09 AM   #2
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I've always made do with Silly Putty and a gram scale, but you've given me an idea.....

I think the twice weekly Air NZ/Air Fiji flight to Nadi leaves Los Angeles on Saturdays (tomorrow for me).
A couple of quick hops (Nadi-Nausori-Savusavu-Matei), and my wife and I could be at the Taveuni Country Club maybe 24 hours after leaving the house, pouring down Fiji Beer and cautiously sipping Bounty Rum. Perhaps you could meet us there.
A 1 km hike down the hill to Waiyevo, another km or so south on the dust rut that serves as a road, and we'd be standing on the International Date Line.
Bring along at least $5AU, I'll bring a reasonable amount of Silly Putty, and we can do the first Audio Related event in the world in 2003.
My wife is a saint when it comes to my audio expenditures, but this is one that she'd be up for 110%.
What do you think?

Have fun and HAPPY NEW YEAR,
Steve
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Old 28th December 2002, 04:05 AM   #3
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Using coins is not a bad idea but how would You fix them without damaging the speaker and without adding weight again?
Better IMO is some elastic material that does not harden for instance some kind of silicion or the like (I don`t know what`s "Silly Putty" but maybe it`s similar to what I mean). It has to stuck (though easy removable) without leaving nasty marks - try out first.
You can make a string of this and place it around the dustcap.
This way the diaphragm is loaded symetrically too which would be hard with a coin. To place the coin in center of a cone is not always possible (Phase-Plug or fragile dustcap).
You could make a nice set also. Take something of it to a place where they have a precision balance (or is it called scale?) for small weights, a pharmacy (or a dental labaratory) for instance and You can have it weigh out there. Keep the stuff in empty and marked (with weight) film containers (You know that cylindric black plastic boxes with cover).
2,83 gramms is useful only for very lightweight cones which are rare. As You need mass in the order of the cone weight a set of 5, 10, 15, 20, 30, 50 gramms pieces or similar is more suitable and probably would cover almost all Your needs for Vas measurements.
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Old 28th December 2002, 04:29 AM   #4
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The trip to the skies of the Southern Cross would be a dream, but Silly Putty is the real deal. It seems to do no harm to even paper cones, yes you can shape it into nice rings - it makes into ropes easily, like dough, and it sticks well but peels off easily. If memory serves, I've even used rings of it to adhere coins to cones! A quick search turned up this site, there's no doubt tons more....

http://www.hungry.com/~tspencer/putty/

Have fun,
Steve
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Old 29th December 2002, 09:04 AM   #5
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Quote:
Originally posted by selder
A 1 km hike down the hill to Waiyevo, another km or so south on the dust rut that serves as a road, and we'd be standing on the International Date Line. What do you think?
Steve
I think if people from all over gathered there and waited till midnight for the year to tick over, all the while eating and drinking, you could call it the International Late Dine.
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Old 29th December 2002, 10:06 AM   #6
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Hi Ciclotron,

For the weight I used M8 to M16 steel plate rings and large key-holding rings. I attached them to the cone with 3 strips of 5 mm wide double sided adhesive tape (with weak adhesive). The tape stripes also form neat handles to remove the rings afterwards.

I calibrated the weight of the rings with an accurate post-weighting scale at my work. But now I stick with the delta volume method. Terracotta flowerpots are excellent for this.
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Old 29th December 2002, 11:11 AM   #7
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Quote:
Originally posted by Pjotr
Hi Ciclotron, But now I stick with the delta volume method. Terracotta flowerpots are excellent for this.
Hmmm.. that's not a bad idea at all. Their shape would make them very non flexing, you could measure their volume by filling them with water, and you could attach the board to the pot rim with a few handfuls of silicone rubber stuff. Time to go shopping methinks. Thank you!
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Old 29th December 2002, 11:54 AM   #8
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Quote:
Their shape would make them very non flexing, you could measure their volume by filling them with water, and you could attach the board to the pot rim with a few handfuls of silicone rubber stuff
Be careful to use acid-free silicone stuff. I do not use it: I clamp the pot between two 18 mm MDF boards hold together with 4 threaded rods and seal the speaker holding board to the pot with foam rubber.
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Old 29th December 2002, 12:58 PM   #9
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I use the thing to stick poster on the wall (tac'N Stik). It is blue and there is two 28 gram square that can easily stick on the cone.
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Old 30th December 2002, 11:41 PM   #10
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I use nickles (5g each) attached with very thin double faced tape. It works extremely well. The delta compliance method has never given me reliable data, there are just too many variables.

Rob
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