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Mains Hum
When using the ASF-1 MKII indoor, some general details have to be considered:
The ASF-1 MKII is working with highly sensitive piezo sensors and therefore also sensitive to
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electromagnetic and electrostatic fields.
Water picks up electrostatic radiation. Therefore, electronic devices known for electromagnetic
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radiation should be kept away from the water to be recorded in. ( switching power supplies,
fluorescent lamps, CRT displays or TV sets, ...)
To avoid the hydrophone from picking up hums, the water you are recording in should be
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grounded.
Often, the easiest way is to connect the audio ground of your mixer / recorder with the water.
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ATTENTION: Never do so when recording from a boat and your mixer / recorder is connected to
the electric ground of the vessel. If you are experiencing inducted mains hum ALWAYS
disconnect the ground of your mixer / recorder from the power supply ground (e.g. use battery
power instead) before you connect the audio ground with the water.
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Underwater Acoustics
The acoustic underwater differs in many ways from the airborne sound we are used to. The sound
velocity underwater is varying between 1450 and 1550 m/sec depending on temperature, salinity and
depth. This results in 4 to 4.5 times bigger wave lengths as well as in a special sound phenomenon
called the SOFAR, also known as Underwater Sound Channel. Horizontally sound can travel over huge
distances whereas vertical propagation is strongly attenuated. Here are some useful advices:
Sound Channels (USC, SSC,...) can be avoided by knowing their local depth
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To avoid the noise from braking waves on the boat carcass, take long enough cables to go down
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deep enough or prepare to record in greater distance to the vessel.
Take time for detailed micro-phasing when recording in pools or artificial environments as
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bigger wave lengths underwater lead to greater distances between sound maxima and minima.
Never forget how far low frequencies can spread underwater. Therefore, avoid touristic areas
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for documentary shots.
Water owns a reflection factor close to 1. Thus airborne sound virtually can't couple directly into
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water. This phenomenon can be used. If barriers reach close to the water surface (reefs, wave-
breakers, e.g.) you can use them as acoustic barriers, independent of the current depth. This
way you may find acoustic cover in a pool with 1m depth behind an 80cm wooden block, even
though the wavelength underwater wouldn't suggest this.
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©2018, Ambient Recording GmbH
ASF-1 MKII