Measurement and Analysis of Chirp Acoustic Signals
Remote, contactless measurement of acoustic vibrations across a wide frequency range, using Ommatidia Q2 Laser Doppler Vibrometer.
This test demonstrates the capability of Ommatidia’s Q2 Laser Doppler Vibrometer to generate, record, and optically measure wideband acoustic chirp signals. The results confirm accurate signal reproduction and reliable vibration measurement without physical contact.
Objective of the Test
The objective of this test was to demonstrate Ommatidia’s Q2’s ability to accurately measure an acoustic signal in a broad frequency range, i.e. to be used as a remote sound sensing instrument. The results obtained show that Q2 faithfully measures the programmed chirp, showing that the signal measured matches the reference signal generated during the test.
Equipment Used
The equipment used in the test was the Ommatidia LiDAR Q2 Laser Doppler Vibrometer, part of the Q Series of instruments based on FMCW Laser RADAR (Frequency Modulated Continuous Wave) technology. The Q2 is a high resolution instrument designed to measure the velocity or displacement of a surface without physical contact.
The system incorporates a multichannel coherent detection architecture composed of 65 simultaneous laser beams. Thanks to its photonic integrated circuits (PICs), the Q2 can process multiple optical channels in parallel, enabling simultaneous measurement of vibrations.
In this experiment, data from a single beam were used, taking advantage of the Q2’s capacity to extract individual beam measurements. This approach enables precise analysis of the local behavior of the system without processing all 65 channels.
Figure 1: Ommatidia's Q2 laser Doppler Vibrometer
Experimental Setup
The experimental setup enabled the Q2 to perform signal generation, acoustic excitation, and optical vibration measurement simultaneously.
Figure 2: Experimental setup
Chirp Generation
The Q2 was configured to emit a 100–8000 Hz triangular chirp (1600Hz/s ramp, 700mV amplitude) through its analog output. The signal was simultaneously reinjected into the Q2, allowing the generated chirp to be recorded back by the Q2. This recording corresponds to the spectrogram shown in Figure 3. Besides the main signal (1) we can observe some other slight features:
- second (2) and third (3) harmonics of the chirp
- broadband noise at the end of the first frequency ramp up
- constant signal at 1KHz and some harmonics
Figure 3: Spectrogram of the chirp generated by the Q2
Figure 4: Internal RGB camera during measurement
Acoustic Reproduction via Loudspeaker
The generated signal was injected to a loudspeaker. The loudspeaker membrane vibrated following the frequency sweep, presenting resonances, nonlinearities, and artefacts characteristic of acoustic transducers. The Q2 laser beam was directed onto the membrane to measure its real vibration during excitation.
Optical Measurement Using the Q2 Laser
The Q2 Laser Doppler vibrometer measured the vibration of the membrane of the loudspeaker. In Figure 4 we see the image captured by the Q2. The dotted line is a virtual representation of the measuring points, although only one laser beam was used in this experiment.
Results
The Q2 recorded the membrane’s velocity v(t) using Doppler interferometry. This signal was stored in the HDF5 file for later processing. Using v(t) (Figure 5), the Short Time Fourier Transform was computed to obtain the measured spectrogram (Figure 6), showing distortions introduced during physical reproduction.
The velocity signal measured on the loudspeaker membrane (Figure 5) shows amplitude variations and noise typical of mechanical transducer response. From this signal, the reconstructed spectrogram (Figure 6) was obtained, where the chirp retains its general shape but appears more distorted and less sharp due to white noise (vertical lines in Figure 6), resonances and nonlinearities inherent to the loudspeaker.
Figure 5: Velocity signal v(t) measured optically by the Q2
Figure 6: Spectrogram reconstructed from v(t)
Conclusions
The test confirms that the Q2 is capable of
The comparison between the ideal and measured signals shows that the observed deviations arise primarily from the mechanical behavior of the loudspeaker rather than from the Q2 generator, whose performance is robust and accurate.
