acoustics musical instruments

Acoustics and Ultrasonics

Laser Doppler vibrometers are indispensable tools for challenging work in the field of acoustics and ultrasonics. They visualize vibration directly at the source of sound generation: on a membrane or any other moving surface. Their non-contact measuring principle is precise, highly linear and free from influence.

Optomet laser vibrometers have proven their value in the development of loudspeaker systems, in the construction of musical instruments, the development of ultrasonic sensors, microphones, mobile phones and ultrasonic sonotrodes but also in the acoustic design of consumer goods, white goods, automotive and aircraft components or in the validation of FE models.

Sound optimization of musical instruments

Laser vibrometers from Optomet support manufacturers of musical instruments in the optimization of musical instruments with regard to sound quality, durability and user wishes.

The examination of instrument strings is practically impossible with conventional contact sensors, as these are difficult to attach to the strings. On the other hand, a mass loading by sensors such as accelerometers would falsify the dynamic response of the measurement object. A similar problem arises with the sound boxes of guitars and string instruments as well as the sound boards of pianos and grand pianos.

Optomet Laser Doppler vibrometers enable instrument manufacturers to systematically and contactlessly investigate vibration amplitudes, resonance frequencies, damping and the effects of the choice of materials in the further development of the musical instrument.

The vibration analysis of resonance bodies allows the detection and visualization of surface vibrations on the body of an instrument. Vibration modes or hidden sounds can be clearly identified and classified in the frequency spectrum. This enables a clear analysis of the unique sound of guitars, violins, pianos, drums and many other instruments.

A further quality assessment and sound characterization is the time representation of the vibration propagation. This method clearly determines the time course of the wave propagation over the resonance body.

 

 

Interferometric sound field measurement

The propagation of sound waves in a medium (e.g. air) causes a spatial and temporal fluctuation in density. Since the refractive index and thus the speed of light change with the density of the medium, the changes in density caused by the sound waves can be made visible with the Laser Doppler Vibrometer.


To do this, the laser beam that passes through the sound field to be measured is scanned on a static white surface and the reflected signal is detected. In contrast to the typical vibrometry applications, the interferometrically measured phase differences do not result from the movement of the reflecting surface, but from the variation of the transit time from the vibrometer to the reflector and back to the measuring device, which are caused by the density fluctuations.
Due to the fluctuations in the refractive index, the transit time of the laser beam varies from the Laser Doppler Vibrometer (LDV) to a reflector that is located behind the sound field to be measured and back to the vibrometer. A white wall can serve as a reflector, for example. This change in transit time leads to a phase change detected by the vibrometer.


Density and thus also pressure fluctuations caused by the sound waves can be visualized in this way with the OptoSCAN software. Application examples are the measurement of the sound field for the development of ultrasonic transducers and loudspeakers. The three-dimensional geometry of the sound field can also be reconstructed using tomographic methods.

Ultrasonic transducer

Ultrasonic transducers are often used in non-destructive testing methods or ultrasonic signal transmitters. The picture shows the sound field of a divergent ultrasonic transducer which was measured with an Optomet laser scanning vibrometer.

Ultrasonic Welding

Ultrasonic welding is a process for joining of thermoplastic and metallic materials by means of mechanical vibrations at frequencies of 20 kHz and more. The displacement amplitude in this process is typically in the two-digit micrometer range. Due to the relatively high frequencies, this results in vibration velocities of several meters per second. These high velocities in turn ensure the necessary energy input into the welding point. 

A laser Doppler vibrometer is an excellent solution for measuring and optimizing the complex welding process in all details. For example, a scanning laser vibrometer can measure and visualize the operating deflection shape of the sonotrode.  

For this purpose, Optomet LDVs combine highest displacement resolution (sub picometer) with high velocity reserves (up to 25 m/s) and an extremely high sampling rate (160 MSamples/s).

A laser vibrometer helps you develop efficient tools for ultrasonic processes, validate FE models, determine simulation parameters, align sonotrode and converter and carry out end-of line tests on your products or incoming tests on piezoceramics  and much more.

 

 

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