Mass Loading Transducers

I was browsing the internet the other day looking for interesting topics to get involved with. Someone asked what 8" subwoofer could play the lowest. Suffice to say, the person received many different answers of different variations. Some answered with their favorite driver, while others said that the answer lies in the enclosure design. Just to spark new conversation I briefly said that Mass Loading might be the answer to fit their needs. A tidal wave of e-mail came my way the next day from people who were interested in the technique of mass loading transducers. If there are that many interested people on the net, then there must be more interested people who don’t have Internet access.

It’s kind of hard to give a definite sequence of how passive mass loading should be done. There are too many variables that can screw up your whole idea. When you mass load the cone, you make the driver's suspension work a lot harder to damp the excursion. With the heavier moving mass, we create a lot more inertia. A standard cupped spider will have a difficult time if a generous amount of mass has been added, simply because the vertical side walls of the spider will flex causing poor damping, over excursion, possible surround "popping" which will result in very muddy bass and overall inaccurate sound. A flat spider will help out tremendously! A flat spider that has a progressive rate restoring force will be as good as it gets. Though this type of spider still isn't very common, there are a few companies using these and more are starting to take advantage of the benefits. A standard flat spider will still work well, but just allow less mass to be added.

I'll always start with breaking-in the driver by feeding it a 20Hz sinewave (for subs) for a half hour or so and then measuring the Thiele/Small parameters 5 or so times and taking the average as my actual parameters. Most manufacturers spec'd T/S parameters are way off. It amazes me that some people are so lazy, that they won't be more careful for the sake of the consumer who pays their paycheck! Anyway, forget the bitterness... Once I know what the moving mass (Mms) is, I'll start by adding 10% mass for cheaper drivers and 15% for better drivers. I use different materials for the final added weight, but for testing, I usually use modeling clay of very specific weights measured in grams and in the shape of rings to distribute the weight evenly. I'll re-measure the parameters and see if I am where I'd like to be. You should have a good idea regarding where you want your bandwidth to extend to before you start. When I need to add more, I'll do so in 5 gram increments for subs and 1-2 gram increments for midbass/midrange drivers. You will need to have a test enclosure with non-porous blocks (like 2x4 pieces) to adjust volume. With the intended xover, feed the driver a pink noise signal with RMS power to be assured that the driver isn't overloaded. I try to get the driver to play undistorted at least a quarter to half octave below the intended xover frequency. Another issue of utmost importance is the pole piece! How many of you guys/gals have seen a driver that has the cone and coil assembly popped right out of the gap?! This is a result of a normal length pole piece and an abusive "bass nazi punk" consumer! You know who you are (and I love you because you pay my paycheck)! Anyway, by extending the pole piece, the magnetic field becomes more even, or rather asymmetrical. Additionally the chance of popping the moving assembly out of the gap is eliminated. The reason most people do not use extended pole pieces is because they do not actually manufacture anything, do not have an option and because they are more expensive. I'm trying to make sure you understand why these features are desirable for mass loading a driver. Basically, it's a matter of add mass, test, add mass, test, etc. Here's the funny thing though... WHY IN THE HELL WOULD YOU WANT TO MASS LOAD A DRIVER? Most drivers today have plenty of Mms! This is the 90s! Engineers may be very strange, but they know what works. If an 8" doesn't play low enough in the vehicle, get a 10" and build an A-period membrane... Whatever. Most of the drivers that I mass load are midbass drivers so that I can use a lower xover point. Whatever your reason for mass loading may be, you can be sure that it is fun to experiment and learn...

by: Grizz Archer