Not the same, wire length is the length of the wire in the gap, coil length is the total depth of the wiring on the coil former which defines the xmax along with the gap depth.
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You wind several meter of wire on a coil that is say 20mm long. Relations in the picture below...
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Sorry if I was ambiguous, L is wire length in the voice coil.
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There is no "coil length", actually it is "coil height".
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To try and simplify matters...
The "electromechanical force coupling factor" is normally written as (B.L)^2/Rc, where L is the length of the conductor in the magnetic field of mean flux density B and Rc is its electrical resistance.
The coil resistance Rc can also be expressed as Q.L/Sc, where Q is the coil resistivity and Sc is its cross-sectional area.
Substituting then allows the electromechanical force factor to be rewritten as B^2.Vc/Q, where Vc is the volume of the coil in the magnetic field defined by B.
The coil resistance Rc and length L therefore make no difference to the sensitivity - that is, the sensitivity properly measured with constant power input. It is instead the VOLUME of the conductor in the field B that is the important factor, which will also likely be a non-negligible part of the total mass in the "mechanoacoustic force coupling factor" too.
That being said, there are secondary effects that are effected by Rc even when the power input is constant. As an example, the 4Ohm driver will require a current that is sqrt(2) times larger than the current in the 8Ohm driver for identical power input (and a voltage sqrt(2) times smaller to compensate). As a result, the 4Ohm driver will produce a relatively larger output of current dependent non-linearities.
The "electromechanical force coupling factor" is normally written as (B.L)^2/Rc, where L is the length of the conductor in the magnetic field of mean flux density B and Rc is its electrical resistance.
The coil resistance Rc can also be expressed as Q.L/Sc, where Q is the coil resistivity and Sc is its cross-sectional area.
Substituting then allows the electromechanical force factor to be rewritten as B^2.Vc/Q, where Vc is the volume of the coil in the magnetic field defined by B.
The coil resistance Rc and length L therefore make no difference to the sensitivity - that is, the sensitivity properly measured with constant power input. It is instead the VOLUME of the conductor in the field B that is the important factor, which will also likely be a non-negligible part of the total mass in the "mechanoacoustic force coupling factor" too.
That being said, there are secondary effects that are effected by Rc even when the power input is constant. As an example, the 4Ohm driver will require a current that is sqrt(2) times larger than the current in the 8Ohm driver for identical power input (and a voltage sqrt(2) times smaller to compensate). As a result, the 4Ohm driver will produce a relatively larger output of current dependent non-linearities.
Some years ago in the "Beyond the Ariel" thread there was a discussion about the 8 and the 16 Ohm versions of some compression drivers, same happened in the JCB III thread some months ago, now I'm very confused... 😱
I assume you mean gage not mass?? Mass shouldn't change. If you want a lower Re you use a heavier gage which also gives you less windings for the same VC height. Take a look at 2220H and J, 8 vs 16 ohms nominal. Mass the same Re drops as does Le for the 8 ohm version.
Rob 🙂
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Yes, thank you! And as you see, the quotient of both BL values is about sqrt 2, which accounts for the current quotient for the same output power. Hence - sensitivity remains the same.
I have a similar table for many JBL transducers that I've found at LANSING HERITAGE. And I had compared the 4 and 8 ohms versions of the 2225. But as you must be registered to that forum, same as here btw, a direct link would lead to nothing for most of us.
Best regards!
You can trade turns and wire thickness in a coil so that impedance changes but power remains the same - basically impedance as such is independent of the behaviour of the transducer - the same is true for electric motors for exactly the same reason.
Changing to 4 ohms from 8 ohms involves using wire 41% shorter and with 41% more cross-sectional area, carrying 41% more current at 70% of the voltage, leading to the same power consumption, same amp-turns, same current density, same coil mass, same linear force from the coil, all else being equal. The impedance is all that changes (both inductance and resistance).
In reality the wire thickness affects thing like the gap size, coil layer count, coil length, thermal dissipation and mechanical strength of the coil. This is where the impedance will have most effect. Impedance is selected as a compromise between easy of making the speaker and convenience of design of the amp. For higher power using lower impedance speakers avoids inconveniently high supply rails in the amp.
Changing to 4 ohms from 8 ohms involves using wire 41% shorter and with 41% more cross-sectional area, carrying 41% more current at 70% of the voltage, leading to the same power consumption, same amp-turns, same current density, same coil mass, same linear force from the coil, all else being equal. The impedance is all that changes (both inductance and resistance).
In reality the wire thickness affects thing like the gap size, coil layer count, coil length, thermal dissipation and mechanical strength of the coil. This is where the impedance will have most effect. Impedance is selected as a compromise between easy of making the speaker and convenience of design of the amp. For higher power using lower impedance speakers avoids inconveniently high supply rails in the amp.
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Actually, it is wire length in the magnetic field (B) which extends somewhat beyond the gap. Not easy to define it (L) because outside the gap magnetic field is lower, but not zero.
For underhung motors, the whole wire length L (the whole coil height) is in the gap.
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Well quite frankly the crossover design is going to make the biggest difference to the overall sound quality from the loudspeakers. So in designing a crossover it is better to aim at an 8 ohm impedance as a target while knowing that it will swing lower than that figure in practice.
Whether 'Low impedance = better sound ? ' is doubtful, when one considers the demands the crossover component values required to make it a success.
C.M
Whether 'Low impedance = better sound ? ' is doubtful, when one considers the demands the crossover component values required to make it a success.
C.M
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I disagree. The problem is that 4 Ohms is a stretch for many amps. Current limiting almost always sounds worse than voltage clipping and cross-over distortion is worse into lower impedance, and the damping factor is half. More important is that 4 ohms may overheat your amp. The extra 3dB is hardly noticeable unless you make a habit of constantly clipping your amp. So if you music is heavy metal, ya, go with 4 Ohms but if you are listening to jazz or classical then you probably want 8 or 16 Ohm.
There are two different aspects with 4 vs 8 ohms woofers. Low cost surround receivers do have a hard time with 4-ohms loudspeakers, most are declared for 6 - 16 ohms load. But 4-ohm woofers tend to have lower Qts parameter than 8-ohm version of the same model, producing less boomy bass (low-cost woofers generally have high Qts). ScanSpeak woofers are expensive, but have the same trend for Qts parameter. On the other hand, high quality professional woofers such as JBL 2226 have identical TS parameters, irrespective of their impedance.
From the point of view of designing a crossover the impedance with a 4 ohm design tends to swing lower than the 4 ohms impedance, the DC resistance of the voice coil of the woofer will generally be around 3 ohms for a start, exasperating the problem you've mentioned above. The tweeter will most likely be padded down with a series resistor anyway, but then bypass capacitors have their shunting effect too.
High Qts can be an issue in box speakers admittedly but all drivers have compromises, it's something we live with.
C.M
High Qts can be an issue in box speakers admittedly but all drivers have compromises, it's something we live with.
C.M
Yes I agree.I use some massive high current power amps to drive 4 ohm speakers [which like many nominal 4 ohm speakers they actually dip below 3 ohms in the bass],and resent having to use that sort of amp.I would much prefer to have speakers that are an easy load and which suit a wide range of amplifiers including those that have no negative feedback ,lowish damping factor and even current drive rather than voltage drive.Because it is those types which tend to sound best.