Why not just increase mass?

[Pashley]

If you look at the physics of a simple spring-and-weight resonant system, the resonant frequency varies inversely with the square root of the mass. Double the mass, reduce the frequency to .707 times original, et cetera.
Simple Harmonic Motion

One could fairly easily do that with a speaker. Clearly it would need more power to control the added mass. Would it run out of excursion sooner? I am almost certain this is a bad idea, but I'm not sure why, so I figured I'd ask.

On a related note, with an isobaric pair of speakers, is the vibrating weight two speakers plus a column of air? If so, is that enough to affect resonance significantly?

[tiefbassuebertr]

Originally Posted by Pashley

If you look at the physics of a simple spring-and-weight resonant system, the resonant frequency varies inversely with the square root of the mass. Double the mass, reduce the frequency to .707 times original, et cetera.
Simple Harmonic Motion

One could fairly easily do that with a speaker. Clearly it would need more power to control the added mass. Would it run out of excursion sooner? I am almost certain this is a bad idea, but I'm not sure why, so I figured I'd ask.

On a related note, with an isobaric pair of speakers, is the vibrating weight two speakers plus a column of air? If so, is that enough to affect resonance significantly?

Follow brand do this - so I think:
:: Ground Zero :: Subwoofer / GZPW 18SPL
http://www.ground-zero-audio.com/13_...al_gzpwspl.pdf
http://www.ground-zero-audio.com/13_download/gzpw.pdf

however, there are the highest requirements in terms of mechanical processing and bonding of diaphragm/surround/viocecoil as well as the max. possible input power.
By car hifi woofers such constructions often to find (MTX, JL-Audio, Earthquake, Bumperspeakers and many more).

[chris661]

Efficiency would drop.

It's also difficult to add weight to the cone, when you have to...
- keep it looking reasonable
- keep the mass central

I tried this method. You need more motor power to properly control the cone - so you'll have to modify the motor structure. Generally speaking, all the other parameters will end up out.

Chris

[Conrad Hoffman]

A ring of big plumbing solder around the dust cap, potted in black RTV works. As said above, the efficiency gets bad in a hurry and the driver manufacturer usually had things pretty well optimized to begin with. Never really solved a problem this way.

CH

[tiefbassuebertr]

Originally Posted by Conrad Hoffman

A ring of big plumbing solder around the dust cap, potted in black RTV works. As said above, the efficiency gets bad in a hurry and the driver manufacturer usually had things pretty well optimized to begin with. Never really solved a problem this way.

CH

Yes, indeed no great efficiency - have a look to follow datasheet:
:: Ground Zero :: Subwoofer / GZPW* 12SPL
http://www.ground-zero-audio.com/13_...al_gzpwspl.pdf
Impedance: 2x 1 ohm (for 2 ohm or 0,5 ohm impedance)
SPL 2.83 V/m at 2 ohms (4 W): 92 dB; that means SPL1W/m: 86db (122 db/m/4KW) or
SPL 2.83 V/m at 0,5 ohms (16 W): 92 dB; that means SPL1W/m: 80db (116db/m/4KW)
power handling: 6500 W

Advantage: low VAS value and thus only very small enclosure (chamber) necessary
Disadvantage: even by very high output power limiting acoustical output power. Transducer very expensive. Vented box with undistorted sound only with passive radiator. In other cases only closed box possible.
Recommended only in special cases with very very little available space.

If you want to have the other extreme, i. e. very low mass by low resonance frequency "fo" at the same time, efficiency is very good. There are now very high values of VAS and thus the needed chamber is very large. This is my favorite design (see photos from my website), because I can design a vented loudspeaker design without any unwanted effects.

The best example therefore was the Focal Audiom 15AX unfortunately obsolete:
# X-max +/- 2,5 mm
# power handling: 175 Watt
# Impedance: 8 Ohm
# fo: 23 Hz
# Qts=0,31
# Vas=635 liters
# RDC=6,1 Ohm
# 96 dB (1W; 1m)
http://www.falcon-acoustics.co.uk/Units/Audiom15AX.pdf

An nearliest replacement are the 40RCA15 from Davis
Davis Acoustics - Haut-Parleurs

[event horizon]

Originally Posted by chris661

Efficiency would drop.

I tried this method. You need more motor power to properly control the cone - so you'll have to modify the motor structure. Generally speaking, all the other parameters will end up out.

Chris

Indeed, Vas as well as Qts will both increase & as you pointed out, the efficiency will drop

With the isobaric speakers Fs (free air resonance) will stay the same & so will driver Qts, however the Vas will be halved so a box of half the volume can be used. You don't get something for nothing though as driving both coils will need double the power output..

If you want to go lower in a sealed box i'd suggest keeping the driver the same & employ a Linkwitz Riley transform to force the driver to go deeper

[MaVo]

Max spl is proportional to sd*xmax, adding mass doesnt change the output of the system. And for a different frequency response, just use a different enclosure or eq.

[tinitus]

Actually you should get more output.......below 30-40hz
And thats what a sub is about, isnt it

And with lowered Fs it also goes lower, thus greater Xmax is needed

But you can use coating to make an ordinary woofer to go lower
The loss in sensitivity will be at higher frequency, where sensitivity isnt needed anyway
Cleaner sound, and slightly more low bass output....lower Fs and higher Qts

Whether you want to mess with expencive drivers in that way is another concern
In these modern times some kind of Eq appears to be the obvious choise, yes

Coating a midrange driver is a totally different game tho

Anyway, you have to look at compliance stiffness in relation to weight
I dont know why there isnt a commonly specced compliance to weight ratio found anywhere

Ofcourse a woofer with relatively low Fs yet still having a lighter cone will be more "responsive" to mass(mms) changes

[LineArray]

Adding mass to a driver with very low Q and high fs can
result in a more attractive BR alignment.
The question is whether the driver is able to perform
the larger excursion needed for lower frequencies ...

A typical "high fs / low Qts" driver is often not built for
large excursion.

What happens to the driver:

Qts rises
fs lowers
Vas stays the same (there was a wrong post before ...)

For a BR enclosure the tuning ratio for comparable alignment
h=fb/fs is lower for the mass added driver.

also alpha=Vas/Vab is lower for comparable alignment
which means box volume to increase relative to equivalent volume
of the speakers suspension and cone area when mass is added
to the driver.

Concerning equalizing:

An equalizer cannot be used to influence the lower frequency limit
far below fb (tuning frequency of the BR box).

The amounts of eq needed in the rolloff range of a BR Box will
lead to excessive cone excursion and little effect.

So not everything, which can be done by adding mass and lower
BR tuning can also be done using an equalizer.

Anyway if the Qts gets higher than 0.4 ... 0.5 by adding mass,
i doubt this making sense for BR, since cabinet size increases
very fast then and the alignments possible with those high
Qts drivers are of questionable use.

Best Regards

[Rademakers]

The Fs is related to the efficiency in the following way:

10 * log (Fs^3 * Vas / Qes * 9.6e-8) = efficiency in % (Vas in liters).
10 * log (Fs^3 * Vas / Qes * 9.6e-10) + 112 = sensitivty in dB/W/m.

Best regards Johan