how to lower the Fs of a driver

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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 ;)
 
Puggie said:
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.
 
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.
 
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
 
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

§§§
 
PHASE_ACCURATE

"It is of course applicable to closed and vented boxes (in fact to ANY type) but - apart from increased complexity - it wouldn't have any advantage" over an LTF within a closed box."


I would agree butl I made some simulations of LTF introducing slight errors between the compensation components and the unit to compensate. I was surprised to see that there were abrupt phase changes around the resonance. I would be glad to know if people have made the same simulation as mine and then have experienced the LTF : do these phase shifts really happen ?

What do you think ?

Stahl also claims advantages of reducing distorsion with his approach which is not very far from motion feedback, whatever he says.

~~~~~~~ Forr

§§§
 
I know about the distortion claims. There is even some mentioning that the more the artificial parameters dominate the lower the distortion would be. I am not so sure about that since the BL nonlinearity of the motor would also have influence on the way the ACE method works (he even cared about asymmetric excursion dependant Le variations in his paper !).

If you attempt extreme corrections with an LTF and you have large component tolerances at the same time I could imagine that you'll end up with a ragged amplitude response with a subsequently bad phase-response.

Regards

Charles
 
forr said:
I would agree butl I made some simulations of LTF introducing slight errors between the compensation components and the unit to compensate. I was surprised to see that there were abrupt phase changes around the resonance. I would be glad to know if people have made the same simulation as mine and then have experienced the LTF : do these phase shifts really happen ?

I haven't done the sims, but the LT is a low-Q polynomial divided by another low-Q polynomial, both of which show smooth changes in gain and phase relative to frequency, so a gut check tells me a reasonable change in one shouldn't produce a drastic change in the final result.

A Butterworth (Q = 0.707) sealed alignment has phase = 90 degrees at resonance, about 35 degrees an octave up, and about 130 degrees an octave down, so I'd guess the phase changes were on the order of 10 degrees for about a 15% mismatch, with a transition from leading to lagging [1] between the cabinet frequency and the erroneous LT frequency. Even though the transition occurs over a small region, it looks worse than it is because the phase difference is also fairly small, hence the effect on group delay will not be overly large.

[1] or vice versa, depending on whether the error is higher or lower than the cabinet resonance.
 
I've seen a number of different methods for raising mms, from VAF (who use portions of 'felt' that is also used as tweeter surrounds) to DIY attempts using coins glued to the cone. the only problems with weight added is the surround - will it handle the extra weight if mounted vertically or will it affect xmax if mounted horizontally?

If you're worried about efficiency after doing this, then do what I do - get 2:)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.