15 Nov 2012

Part 3: A Guide To Reading Transducer Specification Sheets

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In Part 3 of our Guide to Reading Transducer Specification Sheets, we will be concluding with explanations on understanding Xmax, Power handling and Mounting dimensions.

Xmax

Xmax is an indication of how much excursion the diaphragm can make before the performance of the transducer starts to become non-linear.  The larger the value, the louder the transducer can play before distortion effects become discernible in the sound quality.  As woofers naturally have more excursion than tweeters, a large value for Xmax is intrinsically more valuable for woofers than for tweeters.

Power Handling

Power handling is a term which relates to reliability tests which the transducer design is successfully subjected to.  These tests ensure that the product is reliable during expected applied input signals, for an extended period of time.  A higher value for the power rating implies that the transducer can be played at a louder signal level than otherwise.

There are a number of different testing standards for power handling tests.  One must use caution when interpreting values, therefore, as the signal bandwidth, type of test tone, and duration of the test may all be different between different testing standards.

Operating transducers at power levels higher than the rated power may expose the transducer to early failure, due to audible defects, “bottoming” (moving parts coming into hard contact with non-moving parts), and general fatigue and failure of the moving components.

Mounting Dimensions

The mechanical drawing dimensions shown on the specification sheets relate to:

  • The outside envelope of the transducer (red circles)
  • The mounting depth of the transducer (blue circle)
  • The size of the enclosure cut-out hole needed for the transducer to fit into the enclosure (green circle)
  • Positioning and size of the mounting holes (purple circles)

Parameters Not Discussed

The fundamental parameters not discussed are as followed:

  • BL – this is the motor force constant, relating the conversion constant for current traveling through the voice coil, into magnetic force acting along the voice coil’s axis, by way of the Lorentz force law.
  • Cms – this is the compliance of the transducer’s suspension system.
  • Mms – this is the moving mass of the transducer (diaphragm, voice coil, dust cap, glues, etc.)
  • Rms – this is the mechanical damping in the transducer’s suspension system.

These parameters are typically derived by transducer measurement systems, from calculations and formulae relating these parameters to the parameters discussed above.  Some of these equations are:

These equations, coupled with knowledge of the typical transducer equivalent circuit model, allows for the calculation of BL, Cms, Rms, and Mms from fs, Sd, Re, and Qts.

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