Isobaric (Series vs Parallel)

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Ok, so with an isobaric sub you are essentially 'creating' a new driver with slightly different T/S Parameters.

The Mechanical Resistance, Stiffness and Mass all double. Which is logical.

Cms is 1 / Stiffness so that halves.

The magnetic field strength effectively stays the same but the voice coil wire length doubles so the Bl doubles.

Vas is a function of Cms and so the Vas halves which is why we can use enclosures of half the size. This all makes sense.

What I'm struggling with is the fact that if you wire them in series then the Re doubles but if you wire them in Parallel then the Re halves. So there is a 4 times difference between series and parallel.

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Looking at the above equation. If we double Mms and Re and top of the division and double Bl^2 then the driver Q stays the same and so the response will stay the same.

However if re is not doubled but halved then the Qes will be reduced by a factor of 4 which will in turn reduce Qts by a significant amount. Therefore the entire response of the system will be altered quite dramatically. This doesn't seem logical to me but i can't see what I'm missing??

So if we wire isobaric drivers in Series then the response will be unchanged but if we wire them in parallel it completely changes. Is this correct or have I missed something fundamental here?
 
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So if we wire isobaric drivers in Series then the response will be unchanged but if we wire them in parallel it completely changes. Is this correct or have I missed something fundamental here?
The sensitivity will change, but frequency response of the isobaric pair will be unchanged regardless of whether the pair are wired parallel or series, or in the case of double voice coils, series parallel, parallel or series.

If you have a pair of speakers,an amp, a dB meter and test tones you can easily verify this fact for yourself.
 
One related point on isobaric configuration: The drivers are better wired in parallel, because they aren't truly "isobaric". The compliance of the air between them means that they are loaded differently, will not move exactly in unison, so will reflect different impedances. The difference can be quite large if used in a resonant enclosure such as a ported enclosure or tapped horn.
 
The magnetic field strength effectively stays the same but the voice coil wire length doubles so the Bl doubles.

Hi schmeet,

It is only for two drivers connected in series that Bl is doubled - Bl stays the same for two drivers connected in parallel.

If it is assumed that Fs does not change (not strictly correct), then the value of Qes will remain constant.

Single driver:

Qes = Re / (2 * Pi * Fs * Cms * Bl ^ 2)

Two series isobaric drivers:

Qes = [2 * Re] / (2 * Pi * Fs * [Cms / 2] * [2 * Bl] ^ 2) = Re / (2 * Pi * Fs * Cms * Bl ^ 2)

Two parallel isobaric drivers:

Qes = [Re / 2] / (2 * Pi * Fs * [Cms / 2] * [Bl] ^ 2) = Re / (2 * Pi * Fs * Cms * Bl ^ 2)

Kind regards,

David
 
Thanks David,

I know its not a perfect situation, but for the sake of modelling:

Cms is Halved
Mms is doubled
Rms is doubled

Bl is doubled in series
Bl is the same in parallel

Re is doubled in series
Re is halved in parallel

Lvc is doubled in series
Lvc is halved in parallel

I can then work out the other parameters from this and just model as a single driver..
 
Hey richie00boy.

But Bl is B (Magnet strength in Teslas) multiplied by L (length of the voice coil wire). Current is not involved.

Bl * Current gives you the force applied by the magnet.

The magnet field strength of the magnet doesn't change and so I can only assume that the length of the wire is considered to be doubled in series and the same in parallel. Not completely sure why that would be though.
 
Maybe i am allowed to use this picture for illustration:

B stays the same regardless whether wiring the two
voice coils in series or parallel.

Series wiring means doubling the length of the wire in the field
but maintaining the same cross sectional area of the wire as in
the single driver case.

(Bl) doubles, because wire length (l) doubles.
Rg doubles, because wire length (l) doubles.


Parallel wiring instead means (effectively) doubling the
cross sectional area of the wire in the field instead while
maintaining the same length of the wire as in the single driver
case.

(Bl) stays the same, because B and l stay the same.
Rg halves, because the cross sectional area of the wire doubles.

__________________

However this illustration does not account for the wire to be wound
as 2 separate coils and only holds for low frequencies.

It does e.g. not explain why resulting voice coil inductance Lvc halves
in the parallel case, which is because in fact we have 2 coils in parallel.
 
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I propose the following reasoning.

Let's start with the well known formula

F = (Bl) * I <==> (Bl) = F / I

Then (Bl) can be considered as the ratio of the driving force, F, to the current in the voice coil, I.

If between different configurations, the current through the circuit is maintained at a constant value I :


Single driver

Fsingle = (Bl)single * I

(Bl)single = Fsingle / I


Series Isobarik


Current I goes through each voice coil, each motor gives the same force as in the single driver case, and both forces add :

Fser = Fsingle * 2

(Bl)ser = Fser / I = (Fsingle * 2) / I = (Fsingle / I) * 2 = (Bl)single * 2


Parallel Isobarik

Current I is equally split to I/2 into each coil, the force of each motor is halved compared to the single driver case, and both forces add :

Fpar = (Fsingle / 2) * 2 = Fsingle

(Bl)par = Fsingle / I

In this configuration, for the same current, the force is the same as for the single driver case.
Hence the wire can be considered as having the same length l with its resistance halved compared to the single driver case.


Edit : similar thinking to Line Array but I did not read his post before sending this one.
 
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So for LF enclosure design the only thing relevant
happening - besides voltage sensitivity changing -
is Vas halves for the compound driver, regardless
if wiring series or parallel.

Qes and Qms remain unchanged.

One could even short circuit one of the two driver's
voice coils and connect only one of them to the amp.

Having one coil disconnected and open would rise Qes.
 
This paper on BL seems to imply that low-BL drivers have worse impulse response than high BL drivers.

This is rather interesting, as I've long noted that high BL prosound drivers tend to sound 'tight', but I'd assumed this was purely a function of their response shape. Someone with a better grasp of the relation between impulse response and frequency response could likely provide a better commentary than I can.

http://www.extra.research.philips.com/hera/people/aarts/RMA_papers/aar05pu3.pdf

"Because Bl influences the transient response of normal
drivers and low-Bl drivers in particular, a transient response analysis is presented next. In [24] the response to a sinusoid which is switched on at t 0 was calculated. Those results showed that the medium- and high-Bl driver systems rapidly converge to their steady-state response, while this is not generally the case for low-Bl drivers. This can also be seen by calculating the impulse response.
"

Either way, it's a good lunch time read.
 
Could it be the case that a series isobaric configuration, with each driver having the full current flow, they would experience higher I^2*R losses and thus go into thermal compression sooner?
It would seem to me that the parallel setup may have advantages in power handling.
This of course is only an issue in some circumstances, but I have always believed that THD is markedly reduced and SQ improved by running LF speaker systems at less than half their power capability and also much less than Xmax.

Dave

Footnote: In working with dual linear alternator systems, the series vs. parallel question seems to always end up with no clear winner, so this is one reason I pose this question.
 
A Watt is a Watt...
Be it in series or parallel, result is the same.
The only difference is, for series the Voltage needs to be higher, parallel lower.
Series the current will be lower. parallel→higher current.
Result: the same power. So it will generate the same heat. (do the maths) ;)
 
Parallel connection is preferred over series connection for isobaric drivers, especially in cases where there is significant pressure on the "inner" driver such as smaller than optimum enclosures or ported enclosures - basically, any situation where one driver experiences different forces than the other.

Shadydave, it alo applies to linear actuators driving uncoupled loads which may differ in weight / resistance, such as amp racks that are heavier on one side then the other... :)
 
A Pascal is a Pascal

A Watt is a Watt...
Be it in series or parallel, result is the same.
The only difference is, for series the Voltage needs to be higher, parallel lower.
Series the current will be lower. parallel→higher current.
Result: the same power. So it will generate the same heat. (do the maths) ;)

I knew there was a good explanation why it didn't really matter.
Thank You

Dave

PS
it alo(sic) applies to linear actuators driving uncoupled loads which may differ in weight / resistance, such as amp racks that are heavier on one side then the other... :)
OK, just wondering why one may have linear actuators on an amp rack:confused:
 
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