Onda Corporation - References

Choosing a Hydrophone

Sensitivity versus effective area: as with most sensors, voltage amplitude is proportional to the area of the active element. More sensitive devices have a larger sensing area, hence lower spatial resolution and narrower acceptance angle (directivity) with a specific frequency (or wavelength).

Interference with the acoustic field: in general, it is preferable to use a sensor that does not affect the parameter being measured. However, the choice of hydrophone depends on the nature of the acoustic field. Continuous wave fields are most affected by large reflecting structures in the neighborhood, so the needle type is preferred -- keeping in mind that the hydrophone may have a reflecting support structure. Pulsed fields are more forgiving because the reflections can be separated by time, so membrane types may be acceptable.

Fragility: Hydrophones are intrinsically fragile, especially near the sensing element, because their purpose is have high sensitivity to transient pressures. Different models have varying degrees of protection depending on construction.

Immersion: Although all Onda hydrophones are sealed, water is a relatively aggressive solvent, and it even migrates through plastics. For this reason we recommend that the hydrophone be taken out of the tank when it is not actively used.

Size: Needle hydrophones afford the smallest size, followed by the capsule design, and membrane types are extremely large compared to the sensing element.

Cost: In general, membrane devices are more costly, followed by capsule and then needle hydrophones. Within each type, the smallest devices are the most difficult to make and hence the most expensive.

Onda Hydrophones at a glance:

Membrane Hydrophones (HMA and HMB)
For most pulsed wave applications that require a flat response over a broad bandwidth, membrane hydrophones are the type of choice. With their uniform sensitivity these devices provide accurate reproduction of a pulse including harmonic distortion up to 45 MHz. The single membrane HMA model has a traditional laminar construction that does not significantly affect the wave as it interacts with the device. The backed membrane HMB model has additional ruggedness because of its lossy backing, and because the backing is carefully designed to match the properties of water it also does not affect the acoustic wave -- as long as it does not contain large amounts of energy. If exposed to a high intensity field such as from a lithotriptor, the energy absorbed in the backing is likely to generate blisters and destroy the device.

Capsule "Golden Lipstick" Hydrophones (HGL)
This unique design, created by Alan Selfridge, bridges the gap between membrane and needle hydrophones. It has an extremely flat sensitivity similar to membrane devices, yet it does not have a frame that affects the acoustic field and is very convenient for moving around a tank. Onda's AH amplifiers mate directly to these hydrophones, allowing for a water-tight, optimal configuration for measuring acoustic fields with minimal disturbance.

Needle Hydrophones (HNZ and HNV)
The ceramic-based HNZ models have the most sensitivity for their size, but their frequency response is less flat. The polymer-based HNV model needles have a smoother frequency response, with about 10 dB less sensitivity for a given size. Normally, frequency response varies somewhat among individual devices. Throughout the industry there are no standards on the range of variability that can be found among devices.

Note that because of their needle shape, you can poke them into objects like foams or soft materials, which makes them very convenient for many experiments. However, it is also very easy to scratch the device and remove isolation around the edges. The active element has very little separation between its two electrodes, so even a slight tap against a solid object can completely destroy the hydrophone. Any fixture that requires inserting the hydrophone through a hole is NOT recommended. Instead, we recommend the use of "V" groove holders with clamping bars, into which the hydrophone can be laid without threatening the fragile tip.

Needle Reflecting Hydrophones (HNR)
These needle hydrophones have a flat-faced tip, about 2.4 mm in diameter, although the active element diameter is much smaller (see data sheet). The angular responses are about what we expect for the specified aperture, and the sensitivity tends to roll off above 10 MHz. The main advantage these devices have is that they are TOUGH, and able to withstand pressures and cavitation that would ruin a needle type device. A typical use might be to measure fields in ultrasonic cleaners ("Megasonic") and other high intensity fields. One peculiar characteristic of this design is a 2 to 3dB sharp resonance near 2 MHz.