Calibrated vs. Non-calibrated Hydrophone Use

Question

I am interested in collecting underwater acoustic data from a small boat and am finding the cost difference to be quite significant between calibrated and non-calibrated hydrophones. What are the limitations in the use of the data I collect if I decide to go with the non-calibrated hydrophone? And if I did go with a calibrated hydrophone, how frequently would I need to have it re-calibrated?

Answer 1

The main difference between a calibrated and non-calibrated hydrophone system, besides the obvious cost difference from places like Aquarian Audio, Cetacean Research Technology, etc. is that one can be used for measuring sound pressure levels (i.e. how loud something really is compared to all other sounds) and the other cannot.

Hobbyists or people who own yachts and sailboats probably don't need a calibrated system unless they want to be citizen scientists and share their data with acousticians. But anyone doing science should probably have a calibrated system if they can afford it.

What calibrated means is that measurements have been taken of the actual hydrophone after it has been built to know exactly how many Volts are produced in the piezoelectric for 1 microPascal of sound pressure at as many frequencies as possible. (This is why you will see sensitivity ratings on hydrophones in the unit "dB re 1 V/µPa". It means how many dB are registered on the hydrophone relative to 1 Volt and 1 microPascal of pressure.)

Sometimes the hydrophones are "spot checked" - meaning only one frequency or a few frequencies is/are calibrated. The remaining frequencies are interpolated to create a "frequency response curve". Other times, the full frequency response curve is measured so the hydrophone is calibrated for all frequencies. Full frequency response curves are nice so you can see where the "roll-off" happens - meaning which frequency and above you should not trust in your data because the hydrophone isn't sensitive enough to accurately record those frequencies of acoustic pressure.

These calibrations are done with pistonphones (spot-calibrating) or in tanks or small open water ponds/lakes. Depending on the set-up it can be very expensive. There are seven calibration tanks around the world that have matching standards. One of these is at NIOT in Chennai, India. These seven are the most accurate but also probably the most expensive. In the United States, Naval bases are a common place to find a calibration tank and at some universities like the University of New Hampshire or Scripps Institution of Oceanography. If you had the budget you would send your hydrophones to them before and after each experiment to make sure that the calibration didn't shift during deployment. But not everyone has that kind of budget. In that case, perhaps get your hydrophones calibrated before each deployment, especially if they have taken a beating during previous experiments or have sat on the shelf for a long time. For a smaller budget, a pistonphone can be purchased for about $5000 USD so that at least some spot-checking can be performed by the researcher in their own lab whenever needed.

But why use calibrated hydrophone systems? Because then you can compare "loudness" in different environments or between different animals in different pods, etc. Using a calibrated system means you can measure the actual quantity of acoustic pressure you recorded from a boat or a dolphin or a snapping shrimp. This means you will have actual numbers to compare against other actual numbers. A nice analogy is say you want to measure the speed of cars driving past your house. If you have a calibrated hand-held speedometer, you can calculated the actual speed of each car and calculate things like average car speed on the street. If you don't have a calibrated speedometer, you will have to rely on your perception of speed. You'll likely be able to tell which cars are relatively faster than others and say things like "many cars go quite fast on this street", but you won't have actual numbers to back up your information.

If you don't have a calibrated system, you can still do some research with that data, but only qualitatively so. For example, measuring the bandwidths of sperm whale clicks in a spectrogram, or determining the thematic structure of humpback whale song, or listening to the difference between a ringed seal and a bearded seal sounding like aliens are all aspects of recorded data that can be evaluated without calibrated data. But you won't ever be able to calculate the source levels of any of those sounds without a calibrated hydrophone.

So in summary, if the goal is to enjoy listening to the ocean or analyze qualitative aspects of marine animal communication, uncalibrated data are ok. But if the goal is to measure any sort of "loudness" quantity of the recorded data, and calibrated system is a must. This usually means a calibrated hydrophone plus a DAQ and a higher price.

Answer 2

It depends you what you wish to collect data on: If you want to know what species of fish and marine mammals there are in different habitats based on sounds in the audio range below 20 kHz most hydrophones will do a good job. However as soon as you wish to speak to noise levels, received levels of calls, probability of detection ect, you will need to know the sensitivity, frequency response and directionality of your hydrophone. You don't need to buy a calibrated hydrophone to know that, but it requires a bit of calibration using a dedicated setup to get that info.

If you take care of a hydrophone it will hold the calibration for decades, but you still need to calibrate often. If after 3 years you find that the hydrophone is partly broken (crystal is cracked typically) how can you know when it happened in the previous three years and hence what data to trust or not..

We have a methods paper where some of the issues around hydrophones are covered:

Madsen, P.T., Wahlberg, M. (2007), “Recording and quantification of ultrasonic echolocation clicks from free-ranging toothed whales”, Deep-Sea Research, Part I, 54, 1421-1444.

Answer 3

If your study needs to know the true received level of a sound (e.g. studies of source level, noise measurements and sometimes density estimation) then you need a calibrated hydrophone. Otherwise an uncalibrated hydrophone is usually fine. However there are some other considerations that are very important for data quality when selecting a hydrophone;

1. Frequency response: Usually a hydrophone consists of some sort of piezo electric ceramic that converts sound pressure into a voltage. How much voltage is produced per received pressure level (sensitivity) will vary depending on frequency. At some point, as frequency increases a ceramic will hit its resonance frequency and sensitivity will decrease significantly. As ceramics get smaller they stay more sensitive at higher frequencies. So larger ceramics may not be suitable for recording very high frequency species, such a as harbour porpoises (100 - 150kHz). Also, how well do you know your recording chain? - the hydrophone might be calibrated but the cables, amplifier, DAQ card may not. This is why it's often important to calibrate entire recording chains instead of just the hydrophone.

2. Directionality: The elements inside hydrophones can be different shapes, for example cylinders or spheres. The type and how the ceramic is potted and mounted will have some effect on directionality, i.e. how sensitive the hydrophone is for sound received at different angles. For example, Hydromoths, which use a MEMS based microphone underwater are highly directional which essentially means they monitor over a significantly smaller area than their "on-axis" performance would otherwise suggest.

3. Mounting. How a hydrophone is mounted can have a marked effect on the quality of recording. If a hydrophone is near a large acoustic reflector (e.g. air filled housing) then reflections can interfere with received signals causing distortions and making data analysis harder. This is one of the reasons we avoid putting microphones in air filled underwater housings.

Answer 4

As said by @Marinebioacoustics it depends on your application.

If you only wanted to detect and count animals, then you can use SNR and sensitivity is removed. Only if you wanted to describe the sound characteristics then one would expect sound pressure values given in Pa.

In general, do not overvalue the purpose of calibration, because there is calibration and calibration.

A proper calibration of a hydrophones would give you sensitivity and frequency response for all frequencies of interest.

By definition the receiving sensitivity of a pressure sensitive hydrophone is given by the ratio of open-circuit voltage and free-field pressure. Both quantities are theoretical values: if you measure hydrophone voltage, it is not anymore open-circuit, and free-field pressure means that you need uniform sound speed without boundaries close by.

In short, there exist hardly reliable calibration data below 1 kHz as you need very large calibration tanks to remove surface and boundary reflections.

Also, I have seen sensitivity data of multiple identical hydrophones (ceramics only) and there are visible differences. The question now arises to which extent are the hydrophone different or how much contribute measurement errors. If you add preamplifiers, where capacitors are only specified to 5 or 10%, the amplitude and frequency response is easily modified.

Most hydrophone manufacturers give you a sensitivity that is valid within a frequency band limited by -3 or -6 dB. This means that your received level estimation could vary up to 3 or 6 dB. Sound propagation is such that depending on the distance you easily can have variations of a couple of dB, So it is hard to know if some variations are due to hydrophone or propagation.

A final observation: It is not only the hydrophone that counts, but also the whole system. Depending on the frequency, there may be resonances in the whole structure (recorder housing, glider, floats etc) that amplify or absorb incoming sound and must be considered or removed.

Answer 5

For low frequencies (<1500 Hz or so), the hydrophone sensitivity is the same in air and in water, so it is a good approach to calibrate hydrophones in air relative a microphone if you want to know the details of LF frequency responses beyond what a pistonphone can tell you. It does require and an-echoic room, but most engineering schools will have one available for use in off-hours.