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The frequency response of a device under test (DUT) is a useful tool for sound and vibration analysis. The NI Sound and Vibration Measurement Suite provides two distinct techniques to measure the frequency response. The swept-sine technique performs single-tone measurements at each test frequency. The FFT-based technique measures the response over the entire acquisition bandwidth. The following table lists the basic differences between swept-sine and FFT-based techniques for measuring frequency response.

Swept-Sine Frequency Response	FFT-based Frequency Response
Single-tone excitation Can measure harmonics Arbitrary test frequencies Longer test time for many test frequencies Better dynamic range possibility	Broadband excitation Cannot measure harmonics Linearly spaced frequency resolution

Swept-sine measurements provide superior dynamic range over FFT-based measurements because you can optimize the signal level and input ranges at each test frequency. FFT-based techniques must specify a signal level and input ranges appropriate for the maximum broadband response.

The following front panel shows the simulated frequency response function for a four degree of freedom system.

The peak at 17.6 Hz has a magnitude roughly 1,000 times larger than the peak at 5.8 Hz. To use an FFT-based technique, use broadband excitation to excite the entire frequency range of interest to measure the frequency response. This situation forces you to set the input range so that the overall response does not overload the DUT or the acquisition device. Therefore, when you measure the response at 5.8 Hz, you lose 60 dB of measurement dynamic range. The swept-sine technique allows you to tailor the excitation amplitude to the specific test frequency, preserving the full measurement dynamic range.

FFT-based measurements are limited to a linear-spaced frequency resolution determined by the sample rate and the block size. When the response changes rapidly, this frequency resolution may not yield enough information about the dynamic response. Also, a linear resolution may yield an excessive amount of information in frequency regions where the dynamic response is relatively constant. Swept-sine analysis has the ability to test arbitrary frequency resolutions that are linear, logarithmic, or adapted to the dynamic response of the DUT. When the frequency resolution is adapted to the DUT dynamic response, you can test more frequencies in regions where the dynamic response is of interest to the application and less where it is not.

The main benefit of swept-sine analysis is the ability to measure harmonic distortion simultaneously with linear response. FFT-based analysis offers a speed advantage for broadband measurements with many test frequencies.