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Danmounter 
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 Topic: Thiele\Small QuizPosted: 16 August 2008 at 1:37am |
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Can anybody tell me either: How to convert the parameter Bl (Motor Strength) of a loudspeaker from the rather mysterious Beyma unit of N\A to the usual tesla metre?
Also, what exactly does N\A stand for? Any help will be gratefully received!
Dan |
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Centauri
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Posted: 16 August 2008 at 2:33am |
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As far as I am aware, they are one in the same. I always took N/A to mean "not applicable" - maybe they feel that T.m are not valid units of measure.
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Danmounter
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Posted: 16 August 2008 at 2:38am |
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Ahhh that's the logical answer, but I did want to get a second opinion. It's either that or it means (According to the S.I. Standard) Newton Amperes, which in turn makes no sense whatsoever! I did sort of hope it was Not Applicable...
Anyone else have two cents on this?
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ceharden
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Posted: 16 August 2008 at 3:25am |
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It does indeed stand for Newtons per Ampere. Which actually makes more sense than Tesla.metres if you think about what it's describing!
The force with which the voice coil will try to move when a current is applied! |
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Centauri
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Posted: 16 August 2008 at 3:47am |
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Well that would make more sense. The BL will vary with voice coil resistance, which will of course determine the current. If Tesla.metres means "Driver magnetic flux density x voice coil conductor length" (taken from Hornresp help file definitions), then the voice coil resistance would not be taken into account at all, rendering the BL figure pointless.
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Matute
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Posted: 16 August 2008 at 5:37am |
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"Hmm... difficult to see... always in motion is the future..."
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Danmounter
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Posted: 16 August 2008 at 2:29pm |
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I thought Newton metres per Ampere would make sense, but not Newtons on their own per se... The article Matias posted a link to has cleared the confusion up thank you! I didn't think to check the Hornresp help file...silly me!
Thank you to everyone :)
Dan
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David McBean
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Posted: 17 August 2008 at 10:52am |
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Hi Everyone, We know from Faraday’s Law that:
f = B * l * i
where:
f = force in newtons
B = magnetic flux density in tesla l = conductor length in metres i = electrical current in amperes Rearranging gives:
f / i = B * l
The units newton / ampere and telsa * metre are therefore equivalent.
It makes little sense however to express magnetic flux density * conductor length in newtons / ampere. Similarly it is not very logical to express force / current in tesla * metres. This is why tesla.metre rather than newton/ampere is used as the unit for Bl in Hornresp.
Just thought you would like to know :-).
Kind regards,
David
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David McBean
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Posted: 17 August 2008 at 12:22pm |
Hi Centauri,
Just to clarify - Bl and voice coil resistance are entirely separate parameters. Bl is a function of the magnet strength and voice coil conductor length - it is quite independent of the voice coil resistance.
Kind regards,
David
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Centauri
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Posted: 17 August 2008 at 12:47pm |
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Hey, David.
Did you join up to post in this thread? Thanks for the explanations. I haven't really delved into driver theory much at all, preferring to work with the application of the data. I hadn't actually realised that Bl IS actually the B and l from Faraday's Law, and as such, of course Telsa.metres would be the appropriate measurement units. As Bl is an indicator of motor strength, in the context of "motor strength" Newtons per Ampere would make more sense, which is maybe why Beyma use those units. In this case, maybe the parameter should be specified as "motor strength" rather than "Bl", with either Tesla.metres or Newtons/Amp being equivalent units. I had assumed there was a correlation between Bl and voice coil resistance due to the change in Bl you get when specifying voice coil resistance in WinISD, however, as neither flux density or conductor length are specified, this would be due to back calculation from other parameters. Very interesting stuff. Cheers Graeme |
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Contour
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Posted: 17 August 2008 at 8:09pm |
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I always thought mister Lorentz came up with F = B x i x L, isn't this force called a 'lorentz' force?
Best regards,
Walt
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David McBean
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Posted: 18 August 2008 at 9:09am |
Hi Walt, As I understand it, the equation f = B * l * i can be derived from either Faraday’s Law of Electromagnetic Induction (when considered “in reverse”) or from the Lorentz Force Law. I chose to credit Faraday rather than Lorentz simply because Faraday pre-dated Lorentz.
You will note that I was careful not to say that f = B * l * i was Faraday’s Law, but just that “we know from Faraday’s Law that f = B * l * i”.
The Lorentz Force Law is normally expressed as:
f = q * (E + v x B)
where:
f = force in newtons
q = particle electrical charge in coulombs E = electric field in volts per metre v = particle instantaneous velocity in metres per second x = vector cross-product B = magnetic field in teslas The discovery of the so-called “Lorentz Force” was actually made before Lorentz’s time, hence my reluctance to give him the credit :-).
Kind regards,
David Edited by David McBean - 20 August 2008 at 9:21am |
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Centauri wrote: