Transducer Quality "Q" - Airmar
2009 Sep 04
"What is “Q”?
“Q” stands for quality and is a measure of the sharpness of the response of the piezoceramic element to the frequency that is supplied to it. In other words, “Q” describes how precisely the frequency must be output to achieve the best performance from the transducer. It answers the questions: “What is the piezoceramic element’s best or resonant frequency?”, “How well does the piezoceramic element work on either side of its resonant frequency?”, and “How long will the transducer continue to ring after a transmit pulse?” Engineers have a standard method for determining “Q”. It is the operating frequency divided by the bandwidth. For non-engineers, it is helpful to think of frequency and bandwidth in terms of volume and tuning-in your favorite radio station. The operating frequency is that spot on the radio dial where your favorite station comes in the loudest and clearest. The bandwidth includes the frequencies slightly above below the best spot, but where the station can still be heard. Engineers use the standard of three decibels below the peak Transmitting Voltage Response, shown as -3 dB. If we could hear the transducer sound waves, the -3 dB point is where the sound would be half as loud. As we turn the dial above and below our station, the signal begins to fade, so we can’t hear it as well. The points above and below the radio station on the dial at which our radio is half the volume of the correct radio station frequency determines our bandwidth.
The “Q” of a transducer model can be determined by analyzing its Transmitting Voltage Response graph. The resonant frequency and peak TVR is at 50 kHz. At 3 dB below TVR, the frequencies are 47.6 kHz and 52.8 kHz, giving us a bandwidth of 5.2 kHz (52.8 kHz - 47.6 kHz = 5.2 kHz). To determine “Q”, the resonant frequency is divided by the bandwidth giving us a “Q” factor of 9.6. (50 kHz ÷ 5.2 kHz = 9.6). “Q” factors range between 1 and 40. At 9.6 this model’s “Q” factor is relatively low. The sounder manufacturer need not be as precise in setting the echosounder’s drive frequency when a transducer has a lower “Q”.
The lower the 'Q' the better the transducer, for example:-