Brüel & Kjær Sound & Vibration Measurement A/S

Narrowband FFT Analysis in a Hand-held Analyzer – A Product Engineer’s Tale


Courtesy of Courtesy of Brüel & Kjær Sound & Vibration Measurement A/S

“Wheeeeeeek… When pronounced in a falsetto voice, that’s the noise the brand new product makes. Hurts the ears, I’ve heard it so many times. Always the same pitch, at least to my ears. The unusual thing is that not all the units make the sound. The helical gear drive is supposed to be as quiet as possible and now I’ve got to fi gure out what the problem is.”

If the above strikes a chord, then the new Type 2250 FFT analyzer is the tool to use. The max.  hold function lets me store the frequencies of transient signals, and for the fi rst time, compare the results to the linear averaged signal in the same measurement. 6400 lines of resolution is probably good enough to let me cover the 0 – 20000 Hz audible frequency range with one measurement. Once I get the maximum level, I’ll expand the display to look at the details. In this case there is a hint of sideband behaviour around the peak at 980 Hz. It’s the details that will tell me where to look for the cause of the squeal.

Type 2250 FFT provides a unique dynamic zoom selection capability that lets me simply drag the cursor over the screen to select the next zoomed frequency range. Easy, intuitive, clever! Now with a tighter frequency span – down to as small as 100 Hz – the frequency details just pop up. The symmetrical delta cursor places the frequency spread between sidebands at just under 3 Hz. Turning on the Correct Frequency display option fi ne tunes the measurement a little better by interpolating the fi lter overlap caused by the Hanning weighting that Type 2250 FFT engages automatically when continuous signals are measured.

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