There are two main components: (1) the hydrophone to detect the acoustic pressure and (2) the meter to process the acoustic signal.

When selecting the hydrophone, factors such as the frequency range, chemistry compatibility, robustness, size, and cost must all be taken into account. The HCT hydrophone has an acoustic bandwidth that can support frequencies from 20 kHz to 1.2 MHz. The HCT construction consists of a 3 mm Teflon shaft material with a mechanically isolated senor for localized measurements in actual process conditions.  It is ideal for hydrophone measurements submerged inside a cavitation vessel.  In contrast, the CMS-0300 is a novel sensor that is externally-mounted to the sidewall of an ultrasonic tank.  From the outside of the tank, it is acoustically coupled to continuously monitor the cavitation pressure inside the tank without disrupting the acoustic field.

To select the right meter depends a few different factors.  First, the importance of absolute versus relative measurements should be determined.  R&D and process development use cases may require absolute measurements from the MCT-2000 that include a traceable calibration.  To support process monitoring, relative measurements with the MCT-1200 may be adequate for quick spot checks or studies to characterize process trends (e.g., time).  The two meters are complementary where a scale factor in the MCT-1200 can match with a reference like the MCT-2000.  Another factor is the need to separate and quantify the cavitation pressure. When developing a process, it may be useful to acquire lower-level transient and stable cavitation data to define lower and upper limits of a process window to control the cleaning performance.

Both hydrophones are compatible with either meter.

To evaluate the performance of an ultrasonic system (e.g., cleaning tank), one must take into account the relationships between voltage, pressure, intensity, power, and frequency.

Hydrophones are designed to measure the mechanical sound pressure in water. Most hydrophone sensors are piezoelectric materials which convert mechanical energy to electrical energy. The acoustic pressure (Pa) detected is converted to a voltage output (V) which can be acquired and analyzed with an instrument such as an MCT-2000, MCT-1200, or even an RMS meter or oscilloscope.

Because each hydrophone has an inherent frequency response, an acoustic calibration of the hydrophone determines the sensitivity as a function of frequency. This is measured in units of electrical output per unit of physical pressure (V/Pa) over a frequency range (Hz) and is traceable to a primary calibration laboratory. Only with a calibration is it possible to make absolute measurements of physical pressure.

The acoustic waveform is the acoustic pressure as a function of time. By post-processing the waveform, the total pressure can be separated into the direct field and cavitation pressure. Click HERE for further information about how the pressure components are determined.

The acoustic intensity or acoustic power density (W/cm2) is the product of pressure and velocity at any location. Since particle velocity can be measured only under very specific conditions (typically not an ultrasonic tank), the approximation that velocity is the pressure divided by the impedance of the medium is commonly used. However, this approximation is only valid when away from the source – and when the wave propagates “cleanly” without the presence of reflections.

Ultrasonic systems consist of a transducer driven by electronics, such as a generator. The drive electronics delivers electrical power (W) to the transducers to generate the acoustic field. The efficiency () of an ultrasonic system can be estimated as the acoustic power inside the vessel divided by the electrical power delivered to the transducer. The efficiency for each ultrasound system will vary since it depends on many design factors.

For additional information about measurement units, please refer to other references found in RESOURCES.

Relative measurements are common and have some merit. They describe the difference between ‘condition one’ and ‘condition two’, and are useful for comparative studies with a single variable (e.g., time). However, ultrasonic cleaning is based on a complex multi-variable system, and it is important to measure in standard units that accurately represent the ultrasound physics that do the cleaning.

Some instruments measure on a unitless scale of, say, one to ten. Some simply output the measured voltage. Others output slick units such as ‘Cavins’ or Cavitation Intensity. These measured values represent the total acoustic signal, not just the cavitation, which requires spectral analysis quantifying different pressure components in frequency-domain.

Another unit of measure that has gained some traction is ‘Watts per Gallon’. It perhaps offers some usefulness as a relative measure. However, it should NOT be recognized as a standard unit of measure to characterize acoustic fields.

To prove this as an invalid unit of measure, let us consider an example by comparing two cleaning tanks of equal volume. Assume both tanks are 20 gallons; only each tank has different physical dimensions, say 12 x 16 x 24 in and 16 x 24 x 12 in. Also, assume identical generators are delivering 100 Watts to each tank. In both cases, 5 Watts per Gallon are evidently delivered to each tank. However, it would be inaccurate to claim this “power per volume” value as a useful measure of the cleaning performance at any given location. This is because ultrasound is directional and in any cleaning tank, there will be substantial acoustic reflections from many directions. Ultimately, the uniformity of the acoustic field is dependent on several system factors such as the tank geometry and construction, transducer configuration, and loading conditions. Not to mention, the electrical power being delivered to each transducer will result in some loss and not completely convert to acoustic power.

For reference, there is an analogous debate about the usefulness of ‘Watts per Gallon’ applied to fish aquariums. Here, ‘Watts per Gallon’ refers to the amount of light being delivered to a given size fish tank to indicate how well plants will grow. Here is one discussion thread similarly debunking this as a valid unit of measure.