Precise Acoustic & Ultrasonic Measurements with LDVs
Acoustic and Ultrasonic Measurements with Innovative, Precise Measurement Technology
Laser Doppler vibrometers (LDVs) are indispensable tools for demanding acoustic and ultrasonic applications. They visualize vibrations directly at the source of sound generation: a diaphragm or another moving surface. Their non-contact measurement principle is precise, highly linear, and unaffected by environmental influences.
Optomet laser vibrometers have proven their value in the development of loudspeaker systems, musical instruments, ultrasonic sensors, microphones, mobile phones, and ultrasonic sonotrodes, as well as in the acoustic design of consumer goods, household appliances, automotive and aircraft components, and the validation of FE models.
Sound Optimization for Musical Instruments
Laser vibrometers from Optomet help musical instrument manufacturers optimize their instruments in terms of sound quality, durability, and design characteristics.
Investigating instrument strings with conventional contact sensors is virtually impossible because the sensors are difficult to attach to the strings. In addition, mass loading from sensors such as accelerometers would distort the dynamic behavior of the test object. A similar issue arises with the resonant bodies of guitars and bowed string instruments, as well as the soundboards of upright and grand pianos.
Optomet laser Doppler vibrometers enable systematic analysis of vibration amplitudes, resonant frequencies, damping, and the effects of material selection during musical instrument development—all without physical contact or any resulting impact on the instrument's characteristics.
Vibration analysis of resonant bodies enables surface vibrations on an instrument body to be detected and visualized. Vibration modes and hidden tones can be clearly identified and classified within the frequency spectrum. This provides a clear analysis of the unique sound of guitars, violins, pianos, drums, and many other instruments.
Time-domain vibration analysis provides an additional means of assessing quality and characterizing sound by clearly determining how waves propagate across the resonant body over time.
Interferometric Sound Field Measurement
The propagation of sound waves through a medium (e.g., air) causes its density to fluctuate in space and time. Because the refractive index—and therefore the speed of light—changes with the density of the medium, a laser Doppler vibrometer can visualize the density variations caused by the sound waves. This method is a key component of interferometric measurement and delivers precise, reproducible results.
For this purpose, the laser beam passing through the sound field under measurement is scanned across a stationary white surface, and the reflected signal is detected. Unlike typical vibrometry applications, the interferometrically measured phase differences do not result from movement of the reflecting surface. Instead, they arise from variations in the propagation time from the vibrometer to the reflector and back to the instrument, caused by fluctuations in density. These results play a crucial role in sound field analysis and in reconstructing the three-dimensional geometry of the sound field.
Applications include sound field measurements for the development of ultrasonic transducers and loudspeakers. The three-dimensional geometry of a sound field can also be reconstructed using tomographic methods, a technique known as sound field tomography.
FEM in Acoustics: Precision Through Simulation
Finite element methods (FEM) play a key role in acoustics, enabling the precise simulation of complex sound fields and vibration patterns. FEM allows resonant frequencies and mode shapes to be analyzed and optimized in detail.
Laser Doppler vibrometers from Optomet are the perfect complement to this method, providing experimental data for validating simulations. This combination enables the efficient development of acoustic components such as loudspeakers and sonotrodes, as well as the optimization of sound insulation across a wide range of applications.
Practical Example: The Ultrasonic Transducer
Ultrasonic transducers are frequently used in nondestructive testing or as ultrasonic signal sources. The image shows the sound field of a divergent ultrasonic transducer measured using an Optomet Laser Scanning Vibrometer.
Ultrasonic welding with vibration analysis
Acoustic ultrasonic vibrations at frequencies of 20 kHz and above are used to join thermoplastics and thin metal parts. Vibration velocities of several m/s provide the energy input required for the melting process.
The measurement range of up to 60 m/s offered by Optomet's single-point and scanning laser vibrometers provides detailed insights into the complex welding process. They enable users to estimate simulation parameters, validate finite element models, and fine-tune horn and anvil designs.
A 48-bit digital signal output reveals even the smallest vibrations when they are superimposed by vibrations with much larger amplitudes.
The long service life of the SWIR laser source, even in continuous operation, makes Optomet vibrometers ideal for end-of-line testing and quality control, as well as for inspecting piezoceramics sourced from suppliers.
The high signal levels of Optomet's SWIR vibrometers eliminate the need for reflectivity-enhancing surface treatment.
Improving ultrasonic horns
Optomet's Scanning Laser Doppler Vibrometers make it possible to measure the entire surface of an ultrasonic horn and visualize its deflection shapes. Unwanted high-amplitude vibration modes can occur particularly at the edge of the horn, significantly affecting the welding result.
The causes and origins of such problems can be efficiently identified using vibrometry. Validating finite element models provides a sound basis for systematically improving the ultrasonic process and equipment.
Vibration measurement in research, development and industry – Optomet offers the right solution for your requirements.
Contact