Ambient ASF-G Epic Podręcznik - Strona 9

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ASF-G

6. Mains Hum

When using the ASF-G indoor, some general details have to be considered:

The ASF-G 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

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electromagnetic radiation should be kept away from the water to be recorded in.

(witching power supplies, fluorescent lamps, CRT displays or TV sets, etc.)

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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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

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down 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

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into 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 80 cm wooden block,

even though the wavelength underwater wouldn't suggest this.

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