Driver TS Parameters: Mmd & Mms

If you’re comparing drivers in detail, it will help to understand some of the more intricate TS Parameters, as over time it will help you differentiate between different drivers and identify which applications they are more suited for.

For example, for a high power 18″ horn loaded bass bin, you will probably be looking for a driver with a high BL, and also a good strong cone. A strong cone will generally also be a heavier cone, so you’ll be looking for a driver with a relatively heavy cone.

For a 12″ mid driver, that wont be doing any bass, but mainly focused on vocal reproduction you will want a driver with a precise response, and a light cone.

Mmd is the mass of the moving parts of the driver;  the diaphragm, dust dome and voice coil. The diaphragm is the paper cone in a standard speaker. The voice coil includes the former and the copper wire.  Most definitions online for Mmd seem to just be copies of each other, citing that the surround and spider are included in the moving mass. A bit of further research has suggested that only part of the mass of the spider and surround should be included, as the outside edge of the surround, and the outside edge of the spider are both glued to the chassis, and therefore DO NOT MOVE.

Mms is commonly used in loudspeaker modelling software. It is Mmd plus the ‘air load’. The air load is the air just in front, and just behind the speaker cone that will tend to move back and forth with the cone. It’s just a few grams of air, but for mathematical modelling of speaker performance, needs to be added in. A larger cone will have a larger air load.

Mms is used to calculate other TS Parameters, such as Qes and Qms.

There is one final significant point with regard to the Mms, and that is the relationship to Fs  (Free Air resonance). A heavier cone generally has a lower resonant frequency, a lighter cone will generally have a higher resonant frequency. The other factor in the equation is  Cms which is a measure of the suspension compliance. The formula which connects Fs to Mms is as follows:

Fs_Formula

Click here to read more about Thiele Small Parameters: Fs (Free Air Resonance)

We’ve mentioned this before, and just to hammer the point home here, you really, really should be aware now that all driver parameters are dependent on each other. The fact that cone weight, voice coil geometry, magnet strength, cone stiffness all interact and affect each other in both positive and negative ways means that any speaker design always has some compromises. Some might call this optimisation – a driver specifically designed for the best sub-bass response will sacrifice mid and upper bass response.

To get lower frequencies for better bass, you will use a heavier stiffer cone. Whilst in theory, for lower resonant frequency you can keep increasing the cone mass, the drawback often be lower efficiency, and to counter  this you need a stronger magnet, and also a longer voice coil for better motor strength. Its unsurprising that in many instances, a compromise is settled on, which balances efficiency, resonant frequency, and cost.

To improve the mid range response of a driver, you would look at making the cone lighter, but that would potentially increase the resonant frequency, fine for mid-range drivers, not so good for subwoofers. As with most things in life, you can’t have your cake and eat it, and the only way to cover the whole frequency range more effectively is to use different drivers optimised for specific tasks.