FAQ: When I test a Converter+Booster+Horn, I obtain a Qm value with a frequency sweep between 18,2 kHz - 21,8 kHz with 200 data points. When I select “customize” and sweep the same region with 500 data points, I obtain a much higher Qm value. If the one with 200 data points is 2000 then the one with 500 data points is roughly around 7000. Which one is true and why is there a difference? Mr. F.D.
The higher the resolution, the more accurate the Qm measurement. The resolution is proportional to the number of points and inversely proportional to the frequency range. The Qm's accuracy dependence on resolution increases with Qm: it is barely noticeable when testing converters alone, but high for complete stacks, especially when large and composite horns are involved.
There is an inevitable trade-off between resolution and measurement duration, that is, it is not possible to have a high resolution, wide frequency range and fast measurement at the same time. Since most of the time the TRZ Analyzer typical users are tuning horns and troubleshooting ultrasonic stacks, we adjusted the presets to minimize the measurement time as much as possible and this approach have as side effect adding systematic error to high Qm values. For specific cases, for instance, to optimize the nodal point of winged horns, it is advisable to increase the measurement number of points and/or reduce the frequency range to increase the Qm accuracy as you did. For general horn tuning and troubleshooting, the Qm systematic error should not be a problem.
Learn more about testing converters and acoustic stacks.