There are many numbers and figures appearing on a transducer specification sheet, this three part series serves as a guide to understanding the key parameters which define how a transducer may differ in implementation into an audio system. In Part 1 we will be covering DCR, Sd, and fs, with some elementary explanations which will allow the reader a basis for understanding specification sheets.
These parameters are:
- Power handling
- Mounting dimensions
DCR (sometimes labelled “Re”) refers to DC resistance of the voice coil. The importance of this parameter relates to how a loudspeaker is designed to interact with its amplifier. Typically, amplifiers are designed to optimally drive an electrical impedance of roughly 8 ohms. Loudspeakers are therefore designed to typically have this level of impedance. Now, a loudspeaker may be designed with a tweeter plus a woofer, wired in series; the typical impedance split between these two components would be for each to have a DC resistance of a little less than 4 ohms. If a tweeter is wired in series with two woofers which are wired in parallel, then the tweeter will have an impedance of 4 ohms, and the two woofers would have impedance of 8 ohms each.
The difference between DC resistance and impedance is that the transducer’s voice coil includes both DC resistance and coil inductance. These two factors are summed together to form the overall impedance level. This is one reason why DC resistance values often slightly undershoot the impedance value target of 4 or 8 ohms. The other reason that loudspeaker designs undershoot the impedance value target is that they can produce more SPL with a lower impedance, for a given input voltage signal…so long as the amplifier driving the loudspeaker can handle the lowered impedance.
Sd refers to the radiating area (the effective piston area) of the diaphragm of the transducer. A transducer functions by converting an electrical signal input to the voice coil, into a magnetic force driving the coil along the central axis of the transducer. The diaphragm, being attached to the voice coil, experiences the same motion as the voice coil, and through its motion pushes on the air in front of it, to create the outgoing pressure wave. The larger the radiating area, the more air can be displaced by the motion of the diaphragm, and the more sound pressure is generated.
Of course, diaphragms do add mass to the moving mass of this transducer, and this is a downwards “pressure” on the sensitivity. Additionally, the diaphragm also exerts pressure onto the air inside the loudspeaker cabinet, and the effective stiffness of the air inside of the cabinet varies with the square of Sd; this exerts a “pressure” upwards on f0. The result of this is that the Sd value for woofers tends to be dependent on the size of the internal air volume in the cabinet; large cabinets have large woofers, and small cabinets have small woofers.
fs (sometimes labelled f0) refers to the resonant frequency of the transducer. This is the frequency at which the motion of the soft parts of the transducer are going through their primary resonance, which is pistonic diaphragm motion. On an impedance curve, this frequency is identified as the impedance peak.
If you know the moving mass and the stiffness (or compliance) of your suspension system, you can calculate the resonant frequency from the standard mass-spring equation:
It should be noted that it is common that the value shown on the graph vs. in the table are often slightly different. This is because the measurement conditions are different, resulting in different diaphragm excursion levels. Transducer suspension system behavior is known to be complex, and the stiffness of the suspension system changes with excursion.
Check back next week for Part 2 where we will define Qts, Sensitivity and Bandwidth.