06 Nov 2012

Part 2: A Guide To Reading Transducer Specification Sheets

No Comments Sound Quality, Transducer Design, Transducers, Wireless

In Part 2 of our guide to reading transducer specification sheets we will be covering Qts, Sensitivity and Bandwidth with some elementary explanations, which will allow the reader a basis for understanding specification sheets.

Qts

Qts is the total quality factor for the transducer.  Quality factors are a measure of the “strength” of a resonance (for the lack of a better phrase), so for the transducer the Qts relates to the primary, pistonic resonance.  A few points to keep in mind:

  • The lower the Qts, the more damping there is in the system (mechanical or otherwise).
  • A Qts value of 0.7 for a transducer means that the transducer which has a “maximal flat” frequency response.  Such a transducer is well suited to an infinite baffle type system (like in-wall applications, or “very large cabinets”).
  • A Qts value between 0.4 and 0.6 for a transducer means that the transducer can be well suited for ported enclosures.
  • A Qts value of 0.4 or less for a transducer means that the transducer can be well suited for use in sealed enclosures.

Qts can be calculated from other parameters, using the following equations:

Sensitivity

Sensitivity relates to the frequency response to the transducer.  Specifically, as shown in the picture below, it gives an indication as to the frequency response level (dB SPL) at a frequency which corresponds to the minimum in the impedance curve, at a frequency above fs.

The controlling factors for sensitivity are moving mass Mms (the less Mms is, the higher the sensitivity), the motor force factor BL (more BL means higher sensitivity), coil resistance (more DCR means less sensitivity), and radiating area (more radiating area means more sensitivity).

Sensitivity is often quoted in two different ways:

  • Sensitivity given a certain fixed input voltage to the transducer (usually 2.83 Vrms), measured at a 1 meter distance in a semi-anechoic chamber.
  • Sensitivity given a certain fixed input power to the transducer (usually 1 W), measured at a 1 meter distance in a semi-anechoic chamber.

The second measurement nulls out the impedance factor from the sensitivity calculation (impedance changes are often easy to accommodate in transducer designs), allowing for an “apples to apples” comparison to be made between competing transducers.

The value in the sensitivity number is that it relates how easy it is for the transducer to reproduce normal sound pressure levels.  Normally, speech lies in the 70-80 dB range, and we play music in the 80-90 dB range.  The higher the sensitivity value, the easier it is for the audio system to play both at a normal sound level, but to accommodate SPL “spikes” associated with things like FX in movies (as an example).  “Easier” in this context means “less power required”.

Bandwidth

Bandwidth in this context relates to the frequency range used in the power handling tests.  This bandwidth is meant to relate to the frequency range expected to be used in operation of the transducer in typical audio system applications.  The low frequency or “start” of the bandwidth usually corresponds to a frequency somewhat lower than fs.  The high frequency or “end” of the bandwidth corresponds to the start of the roll-off in the frequency response, or a large spike in the frequency response to the break-up behavior of the diaphragm at those high frequencies, or an expected cross-over point for a woofer (a frequency at which the system stops sending signals to the woofer, instead sending them to a midrange driver or to a tweeter).  Often the bandwidth is also the range used for frequency response testing, at the end of the transducer production line; during such a test, the deviation in the sound pressure level from that produced by a “golden sample” is evaluated against acceptable limits.

“Sweep test bandwidth” is a slightly different term, referring to the frequency range used for the swept sine tone listening test usually conducted at the end of a transducer production line.

Check back next week where we will conclude our series by covering Xmax, Power handling and Mounting dimensions.

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