First off, a little bit of terminology, a speaker what you can buy readily as a whole component. The driver is the component that goes inside of a cabinet/box. See the pictures below:
Speakers
Drivers
The Thiele/Small parameters is a set of data that tells you everything about the physical and electrical properties of a driver. If we were to think of this as food, the Thiele/Small parameters would be the ingredients to a cake (driver). Using the ingredients, we can calculate how they come together and would taste, or in a driver’s case, sound. Dropping the analogy, the Thiele/Small parameters give information that is useful for determining frequency response and box size as well as box type (box type will be another post).
Thiele/Small parameters were created from the hard work of (wait for it!) A.N. Thiele and Richard Small, who, through experimentation, came up with a method of measuring certain features (parameters) of a driver and how they mathematically influence the design in which the driver is to be used. Many times in the history of speakers, one or two individuals can be seen as the grandfather of a certain specific field.
Qualitatively, the parameters can be described as follows:
Fs [Hz]: The resonant frequency of the driver (think like a vibrating string). Typically a lower fs is better; frequencies below the resonant frequency are less efficient and in some cases can cause large excursions (the cone, the part you see moving too far in and out).
Qms [unitless]: The mechanical damping of the driver which is basically frictional and elastic loss. A higher Qms indicates lower mechanical loss.
Qes [unitless]: The electrical damping of the driver through eddy currents and what not; the electrical equivalent to Qms.
Qts [unitless]: The total damping of the driver; however, it is not simply Qms plus Qes.
Re [ohms]: DC resistance of the driver’s voice coil; see picture below.
Le [henries]: inductance of the voice coil.
Cms [m/N]: The stiffness of the driver
Vas [L]: The volume of air, when compressed to one liter that matches the Cms.
Vd [L]: The volume of air that the cone will move
BL [T-M]: Strength/power of the magnet, the higher the better!
Mms [g]: Mass of all the moving parts (cone, voice coil, etc)
EBP: Efficiency Bandwidth Product, it determines what kind of box necessary, I’ll post next week
Xmax [mm]: how far in/out the cone can move
Sd [m^2]: surface area of cone
Phew! That was a lot of information without a whole lot of meaning… at least for now! These parameters are to be used in formulas or calculators (either online or through computer programs like winISD) to aid in the design of the box the driver has to go in to produce optimal sound quality and performance. So what should you take away from this post? Aim for drivers with low Fs, as performance typically dwindles below that frequency. Smaller Vas will require a small speaker enclosure, but efficiency (see previous post) will suffer as this requires a heavier cone and a stiffer assembly in general.
Don’t worry or be intimidated by all of these parameters; most of the time you will just be copying and pasting these numbers into formulas without regards of their actual meaning.
As an fyi, here’s a datasheet for a driver I recently purchased, these parameters can be seen on the left side.
Tags: bedell, design, parameters, passion, rcl1213, small, speaker, thiele
Kevin-
T-M means Tesla Meter, it’s the product of the strength of the driver’s magnet and the voice coil’s length. I just included the dash because TM would look a little funny, and generally on most datasheets it says the units as [T-M]. Because the Bl is the product of voice coil length and magnet strength, a higher Bl will result in a louder, more efficient speaker.
One quick question: what unit is T-M. I would expect T to mean Teslas but I have no idea what a capital M means or what a dash means when we talk units. Great post.
Wow, this doesn’t even seem like English! It’s really cool to learn about something I don’t know a lot about! Neat post, and thanks for the summary at the end!