We are well familiar with the ISO requirements and comply with the general practices. Onda is committed to delivering the highest quality products and services. We have stringent internal quality processes that offer the framework to support hundreds of calibrations and recalibrations of various acoustic instruments each year. However, we have not gained 3rd party accreditation because of the excessive overhead which we are unable to justify at this time. As we understand, some established national laboratories do not have ISO accreditation for their acoustic measurement services either. We understand the importance of quality and would welcome a visit from you or any of our customers.
Yes, Onda's calibrations are accomplished through the use of reference equipment calibrated at a National Laboratory such as National Physical Laboratory (NPL) in the UK or Physikalisch-Technische Bundesanstalt (PTB) in Germany. The particular reference equipment and national laboratory used for any calibration is referenced on the calibration certificate. National Institute of Standards and Technology (NIST) no longer provides traceability for ultrasonic field measurements.

There are two general considerations for determining the maximum pressure at which a hydrophone can be used: (1) the linearity of its preamplifier, and (2) hydrophone damage

1. Linearity of Preamplifier

Amplifiers have a voltage limit, beyond which they become non-linear and saturate. This is a reversible problem. That is, if it happens it does not damage the hydrophone or preamp. Although the data is invalid, the hydrophone can still be re-used within the amplifier’s specified voltage range. For Onda’s modular amplifiers (i.e., AH-20x0, AG-20x0, and AH-1100), a maximum voltage output is provided as a specification. The formulas to calculate loaded sensitivity can be used to determine the expected acoustic pressure level of the signal to verify they will be within the linear range of the amplifier. For high-sensitivity hydrophone models, Onda also offers a signal attenuator (i.e., ATH-1000) to prevent preamplifier saturation. For membrane hydrophone models which have an integral preamplifier, the maximum linear pressure range is also provided as a specification.

2. Hydrophone Damage

Unlike preamp linearity, damage thresholds are much more difficult to provide. Hydrophones are intrinsically fragile, particularly near the sensing element, because they are designed to have high sensitivity to detect transient pressures. Different models have varying degrees of protection depending on the construction. To add to the complexity, the robustness of each hydrophone depends on several factors in the test environment. For instance, it is particularly important to minimize the dissolved oxygen content in water to limit bubble formation which increases the likelihood of cavitation damage. Other factors that contribute to the likelihood of damage include drive frequency, duty cycle, and temperature.

The Pressure Range plot should be used as a guideline but not taken in a strict sense given these dependencies. The following plot estimates the pressure range that can be measured with each hydrophone model. However, it should be noted these are only guidelines. It is important that the user considers the details of other factors that contribute to the actual pressure thresholds (e.g., aperture size, preamplifier noise, water quality, drive conditions, etc.).

As can be seen from the plot, the only pressures that are clearly acceptable for all circumstances are below approximately 100 kPa. This is particularly true for continuous or quasi-continuous waves, which at pressures higher than 100 kPa make the hydrophone more susceptible to cavitation damage. So from a conservative point of view, the only clearly safe conditions are below 100 kPa peak negative pressure.

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A 1-20 MHz calibration is offered as standard on all of our hydrophones. Our calibration capabilities extend from 30 kHz to 60 MHz.

Onda’s hydrophones are calibrated with respect to a plane-wave. The hydrophone is positioned so that its active surface is parallel with the planar wavefront, i.e., the direction of propagation of the wave is normal to the active surface of the hydrophone. For example, in the case of needle or capsule hydrophones, the active surface is at the tip, so the normal is the major axis of the needle or capsule.

The hydrophone’s sensitivity decreases as it is rotated with respect to the normal position. For needle and capsule hydrophones the sensitivity approximately follows formula 1:

D(theta,f) = [((1+cos(theta))/2] (2J1(k a sin(theta)) / [k a sin(theta)] (1)

Where:
  • theta = the angle relative to the normal
  • D = directivity function (voltage out of the hydrophone relative to voltage when an incoming wave is normal to the tip)
  • f = frequency
  • J1 = Bessell function of the first kind
  • k = wave number (2 pi / wavelength) = 2 pi f /c where c is the wavespeed = 1500 m/s in water
  • a = radius of the hydrophone (0.75 mm approximately for an HNC1500, for example)

For high frequencies and small angles the (1+cos(theta)) term becomes negligible and the formula becomes:

D(theta,f) = (2J1(k a sin(theta)) / [k a sin(theta)] (1b)

Furthermore, for small angles as well as low frequency Eq. (1b) may be approximated as:

D(theta,f) = 2J1(2 pi f a theta/c)/[2 pi f a theta /c] (1c)

From (1c) it can be seen that for small angles and low frequency the directivity will scale with theta*f where f is the frequency. That means that under such conditions you can take the directivity pattern acquired at one frequency f1, and estimate the directivity pattern at another frequency f2 from the same curve, replacing theta with theta * f1/ f2. For example, if at 5 MHz the directivity is 0.9 at 10 degrees, at 2.5 MHz it will be 0.9 at 20 degrees. Onda provides typical directivity patterns measured at 5 MHz for this purpose.

The user should be aware, however, that Eq. (1c) loses accuracy at very low frequencies, as can be seen from Eq (1), which is the more exact formula.

Hydrophones should be removed from the water as soon as possible (after 1 hour of soaking). It is advised that if they are left immersed longer than 24 hours they are removed as soon as possible and dried out.
If possible rinse the hydrophone tip in distilled water at the end of a measurement session and leave to air dry. Avoid touching the hydrophone tip!
The main consequence of heating a hydrophone to above 50°C is that the polymers in its structure begin to soften and so the device's structure and water sealant is compromised. At even higher temperatures, the piezoelectric material will de-pole and the hydrophone will no longer be capable of acoustic measurements.
The DC block powers the preamp and removes the DC component from the returned signal. It does not affect the gain.
The impedance set on the oscilloscope should match the output impedance of the hydrophone (e.g., 50 ohms with preamplifier and 1 M-ohm without preamplifier).
Yes, but a scanning system is required in order to perform a planar scan and then integrate the intensity over the plane. Typical uncertainties measuring total acoustic power with the hydrophone are typically more than 25%, whereas with the RFB they are typically less than 10% and can be as low as 5% for unfocused transducers. Making total acoustic power measurements with an RFB offers a significant throughput advantage.
Yes, but it has not been established on how hard each hydrophone can be driven without damage. Because the intended design for Onda’s hydrophones are to “receive”, the damage threshold will be highly variable from one device to the next.
A primary advantage with sending devices to Onda for calibration is that they can be inspected against manufacturing QC criteria. For all re-calibrations, Onda includes a free evaluation of any Onda hydrophone before commencing with the calibration. If a hydrophone issue is discovered, Onda will inform our customers and present the option to forego the costly calibration. To help us understand your calibration needs, please complete the hydrophone calibration questionnaire and send it to us for review.

An End-of-Cable Open Circuit (EOC) sensitivity represents the sensitivity measured at the hydrophone connector without the preamplifier. An EOC hydrophone calibration can be mathematically converted to a preamplifier-loaded calibration. See the Resources section for the white paper. Onda also can supply a software utility to combine the standalone calibration data sets. An EOC calibration offers more flexibility because it can be paired with different preamplifiers. However, the user can avoid the conversion by having hydrophones calibrated in a preamplifier-loaded configuration.

Please note, however, that as the frequency goes up, the mathematical conversion becomes invalid because cable transmission line effects become more important. In particular, cable resonance effects become more prevalent at high frequencies. In some cases for fairly non-linear pulsed waveforms, there may be significant waveform distortion because the pressure signal contains very high frequency harmonics which excite cable resonances. In general, we recommend that the cable length be less than a tenth of the electrical wavelength in the cable for any relevant frequency. The cable electrical wavelength is given by the cable phase velocity divided by the frequency. The phase velocity can be assumed to be 2 x 108 m/s, so for example, at 20 MHz the cable electrical wavelength will be 1 meter. So, results above 20 MHz for a 1m cable. Just bear in mind that in many practical applications non-linear harmonics go to much higher frequencies than 20 MHz.

Note that frequencies above 20 MHz require the calibration to be preamplifier-loaded to ensure sufficient sensitivity.

The HMB is an HMA back-filled with absorbing material. The backing dampens vibrations of the membrane which may occur during scanning. Free membrane hydrophones are susceptible to this issue, and it is commonly suppressed either by waiting for the vibrations to dampen before taking a measurement, or by using cross-correlation to filter them out. Because both of these solutions require additional time, the HMB is a good option for setups that use motorized scanning. The only other significant performance difference between the HMA and HMB designs is that the HMB is more sensitive. As mentioned in our datasheet, the HMB-0200 is about 4 dB more sensitive than the HMA-0200.