The majority of reported measurements on high intensity ultrasound beams in air are below 40 kHz and performed on standing waves inside of a guide. Here, experimental characterization of high intensity progressive and divergent sound beams in air at 300 kHz are presented. Measurements in this frequency range are challenging. Accurate characterization of high intensity sound beams requires a measurement bandwidth at least ten times the beam's primary frequency, as high intensity soundwaves steepen and form shocks and, therefore, contain significant signal power at harmonic frequencies. Hence, a measurement bandwidth of at least 3 MHz is required. Calibrated measurement microphones are generally not available in this frequency range. This limitation has been overcome by using a hydrophone with a calibrated response from 250 kHz to 20 MHz. A narrowband piezoelectric transducer is used as the source in this study, and it is capable of generating tone burst waveforms centered at 300 kHz with 160 dB sound pressure level surface pressure. Cumulative wave steepening and shock formation are observed in on-axis measurements. The source's surface vibration profile is measured using a scanning laser Doppler vibrometer, and the vibration profile is imported into a numerical wide-angle Khokhlov-Zabolotskaya-Kuznetsov simulation for comparison against measured on-axis waveforms.

The speed of light in air is dependent on the air's instantaneous density. Since air density is modulated by sound, sound in the air can be observed and measured using optical methods. One such optical method is Laser Doppler Vibrometry (LDV). Most commonly, LDVs measure the mechanical velocity of a surface. However, by placing a rigid reflector beneath a sound beam in air, it is possible to measure the time rate-of-change of optical refractive index and thus to measure dynamic changes in air density, or sound. In prior demonstrations, this method has been used to visualize sound fields in the audible frequency range and ultrasonic range underwater. Here, we present the first measurements of high-intensity airborne ultrasound beams in the frequency range spanning 100 kHz–300 kHz. We observe accumulated distortion, wave steepening, and weak shock formation as high intensity sound beams propagate. LDV measurements are compared against numerical simulations of the sound field. Advantages of the LDV technique are discussed, and we also attempt to quantify limitations of the technique which include spatial averaging of the measurand along and normal to the optical beam path.

Most commercial microphones rely on capacitive or piezoelectric readout of a vibrating diaphragm. Optical methods have also been used to read the motion of a vibrating microphone diaphragm. Most optical microphone demonstrations to date use free-space optics. We present the design of an optical interferometric microphone that uses a photonic waveguide embedded within the diaphragm. The deflection of the moving diaphragm causes a strain field within the diaphragm, and this strain in turn changes the optical path length of the waveguide. Light traversing the sensing arm of the interferometer combines with light traversing an on-chip reference arm to yield an output light power that depends on the instantaneous displacement of the diaphragm. The output of the combined beams is coupled from the chip to a photodetector to complete the interferometer. An advantage of such transduction method is the absence of a microphone backplate and its associated thermal-mechanical noise. Electrical noise associated with high input-impedance amplification, as required for small capacitive and piezoelectric sensors, is also circumvented. This embodiment does, however, introduce several implementation challenges including the routing of light onto and off the microphone diaphragm.

No abstract

The majority of reported measurements on high intensity ultrasound beams in air are below 40 kHz and are performed on standing waves inside a guide. Here we present experimental characterization of high intensity progressive and divergent sound beams in air at 300 kHz. Measurements in this frequency range are challenging. Accurate characterization of high intensity sound beams requires a measurement bandwidth at least 10x the beam's primary frequency, as high intensity soundwaves steepen and form shocks, and therefore contain significant signal power at harmonic frequencies. A measurement bandwidth of at least 3 MHz is therefore required. Calibrated measurement microphones are generally not available in this frequency range. We have overcome this limitation by using a hydrophone with a calibrated response from 250 kHz—20 MHz. A narrowband piezoelectric transducer is used as the source in this study. The source is capable of generating tone burst waveforms centered at 300 kHz and with 160 dB SPL surface pressure. Cumulative wave steepening and shock formation are observed in on-axis measurements. The source's surface vibration profile is measured using a scanning LDV, and the vibration profile is imported into a numerical wide-angle KZK simulation for comparison against measured on-axis waveforms.