Absorption of ultrasonic waves in air at high frequencies (10–20 MHz)

Abstract

A high power and sensitive gated radio-frequency (rf) measurement system has been used to determine the absorption of compressional waves in air, in the frequency range from 10–20 MHz. The measured absorption, between 80 and 90 kPa and at room temperature (15 °C to 25 °C), has been found to be in good agreement with that predicted, by extrapolation, from low-frequency (below 1 MHz) data for the combination of the classical and rotational loss effects. The measured data fit a classical-plus-rotational absorption loss coefficient (αcr) for a 20 °C value of 1.83×10−11 Np/m. Above 500 kHz at normal atmospheric pressures, this coefficient becomes dominant, and the effects of variation in humidity are insignificant. The bounds for the parameter domain for this relationship to be valid are now pressures between 80 and 90 kPa, temperatures between 15 °C and 25 °C, and frequency/pressure combinations up to 200 Hz Pa−1. The velocity of ultrasound in air in the frequency range from 10–18 MHz has also been measured and is in good agreement with that predicted from theory. For example, at 83.3 kPa (625.5 mm Hg) and 24.4 °C, the measured velocity is 347.6±1 m/s at 10 MHz and that calculated, including the effect of humidity, is 346.4 m/s. Normalized velocity and attenuation data plotted against Knudsen numbers and a rarefaction parameter are in good agreement with those obtained at reduced pressure and lower frequencies.

The design and characterization of micromachined air-coupled capacitance transducers

“…Second, air is highly absorptive of ultrasound and has an absorption constant which depends on a number of parameters. Above 500 kHz, it increases with the square of the frequency, the square root of the temperature, and proportionally with pressure, while below 500 kHz humidity is an additional factor [23], [29]. The rapid increase of absorption with frequency restricts the higher frequencies to shorter propagation paths, and at 2 MHz (for standard temperature and pressure) only a few centimeters is practical.…”

“…5 and 6, and does not alter the general shape of the spectra in any significant way. As for attenuation, it increases with increasing frequency and at 2 MHz the loss can be estimated at 636 dB/m [23]. For the 4 mm path length employed here, this amounts to less than 2.5 dB.…”

Novel, wide bandwidth, micromachined ultrasonic transducers

“…Using the instrumentation shown, this represents the frequency response of the whole system, but includes a factor caused by atmospheric absorption. This can be removed to provide a better idea of the instrumentation response by correcting for this factor, using the known acoustic attenuation in air as a function of frequency [28]. After correction, the spectrum appears as in Fig.…”

Compensation network design for capacitive ultrasonic transducers

“…These properties in themselves have complex derivations due to the lack of measurable data the background of which can be found in [12]. The acoustic channel attenuation in air is appropriately captured by the work of Bond [13] using the molecular model derivation.…”