how to lower the Fs of a driver

[Puggie]

I'm looking for subs to use in a aperiodic/infinite baffle setup in a car (see my other thread on here). I've found a couple of options all with Fs in the mid to high 30s and I'd really like to get it down to below 27Hz.

So could I take a PA driver with dual spiders and modify the top spider by cutting sections or the entire top spider away to reduce the suspension stiffness? would this have the desired effect of lowering Fs? I presume it would also reduce Qm and I have no idea how it would affect Vas etc.

any ideas guys and girls???

p.s. I'm looking at a sub with relatively affordable re-cones

[Rademakers]

Add weight to the cone.

Wkr Johan

[DSP_Geek]

Are you really sure you want to do this? I would highly recommend using a Linkwitz transform instead, since what you're really after is a lower F3.


Cheers,
Francois.

[Puggie]

Ahhh nice thought on the linkwitz but currently I'm trying to do everything passively or mechanically. Agreed, more mass on the cone will lower Fs but won't that also slow the sub down and make its transient response a bit 'loose'?

[GM]

Greets!

Only if it raises Qts too much:

mr = (Fs/Fs')^2

Qes' = Qes*mr^0.5

Qms' = Qms*mr^0.5

then: Qts' = Qes'*Qms'/(Qes'+Qms')

where:

mr = mass ratio
Fs' = new, lower Fs

GM

[DSP_Geek]

Originally posted by Puggie
Ahhh nice thought on the linkwitz but currently I'm trying to do everything passively or mechanically. Agreed, more mass on the cone will lower Fs but won't that also slow the sub down and make its transient response a bit 'loose'?

I bring it up because you may not like the effects of modifying your driver, particularly since modern woofers tend to have heavy cones so cutting slits in the spider will likely cause it to fail in fairly short order. Driver compliance has little effect on F3 in most boxes designed to fit in cars, anyway, since Vab tends to be smaller than Vas and thus dominates the equation Vaeffective = (Vas*Vab)/(Vas+Vab). Vaeff is what forms a resonant circuit with the equivalent mass (can't remember the name right now), ergo Fb = 1/sqrt(Vaeff*M). With Vab = Vas(original) and increasing Vas by a factor of 2 by cutting a _lot_ of slits in the spider, you get Vaeff going from Vas * 0.5 to Vas * 0.67, but that reduces your box frequency by only sqrt(0.5/0.67), or about 0.86.

In other words, the probability of destroying the driver is increased by quite a lot to gain only a sixth of an octave in low-end cutoff. In actuality, you don't even get that much since increasing Vaeff by that much also decreases the cabinet Q, so it starts rolling off a bit earlier.

Adding mass onto the cone comes with its own set of problems, increased stress on the suspension being one, and reduced efficiency being the other. You could derive the loss from the math, I suppose, but it's simpler to think of the voice coil/magnet assembly as a motor of known strength which now has to push a greater mass - it simply won't move it as far, hence the cone will produce less sound.

Long story short: you could spend a lot of time messing around with razor blades and plasticine, or you could short-circuit a bunch of grief and use the Linkwitz Transform before, instead of after, killing a couple of drivers. It's your choice.


Francois.

[Puggie]

Thanks for the input guys looks like I'm going to have to do some math!

Just to clear a few points up, the driver in question is a 15" PA driver with relatively low moving mass (so increasing mass even by only a small ammount could have a pretty profound effect)

you talk about 'most boxes designed to fit into car, this isn't your typical install, the drivers will be using the entire boot/trunk as the enclosure with the cabin and trunk entirely sealed off from each other. I will also be using aperiodic mats to restrict the airflow to limit cone excursion.

So looking at the Mass Ratio theory:

current Fs = 39Hz,
aimed Fs = 25Hz

(39/25)^2 =2.43

the current moving mass is 98g so I would need to increase it to 238g, now that sounds like a lot to me!

Doh, I'm going to have to go and do some maths, if anyone else feels like having a play, the two drivers I'm currently looking at are:
Volt R3825 (www.voltloudspeakers.co.uk)
and
Precision devices PD1550 (http://www.precision-devices.com/showdetails.asp?id=14)

Thanks guys.

[forr]

Have a look to Stahl's works to see how you can electronically modify the mechanic parameters of a loudspeaker. More dependable than a Linkwitz transform because it is less an "a priori" correction.

~~~~~~ Forr

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[phase_accurate]

Increasing the mass of a driver will definitely lower its efficiency (which is a function of fs^3) over its whole passband.
Special tunings combined with the appropriate electronics only affect the efficiency at the lower end.

LTF is recommended for closed boxes.

The ACE method by Stahl is recommended for vented alignments but you should only attempt it if you have enough patience and experience.

Just my $0.02

Regards

Charles

[forr]

Stahl's method changes the apparent parameters of the driver, so it can be applied to any kind of enclosures.
The trade for a lower frequency is of course a need for more power, however under 50 Hz, power distribution of the most demanding records falls at a 6 dB/o rate, so it is not as much as firstly thought.

~~~~~~~ Forr

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