In order for the transducers to transmit sound waves into the water they are applied with energy to make them vibrate. After a certain signal has been emitted, the power supply is turned off to make the transducers stop vibrating and prepare them to listen for echoes. However, the acoustic activity around the transducers won't stop immediately, but are damped with time instead. This is called ringing and will affect the measurements from the area closest to the instrument head.
To minimize the interference from ringing, the instruments are programmed to wait a short time before they start to listen for returned signals. The purpose of this is to give the transducers time to settle down first. As echoes nearby reach the instrument first, this means that there is a region right above the transducers where echoes are ignored. This period when no measurements are made is called blanking, and blanking distance is the vertical distance this constitutes to (see Figure 1). When configuring the instrument, the blanking distance is the vertical distance from the instrument head, so the blanking area along each beam is adjusted based on the transducer angle.
The minimum blanking distance is based on the distance of which the sound travels during attenuation of ringing. The transducers of the different frequency instruments require different time to stop vibrating and the blanking distance will thus vary with acoustic frequency. In general, lower frequencies require longer blanking distances. The minimum blanking distance also depends on the strength of the returned signals. With strong echoes, as in water laden with particles, the blanking distance can be smaller than when the echoes are weak, such as in very clear water.
When planning a deployment, the instrument software sets a default blanking distance. This can nevertheless be changed within a given range, depending on the instrument frequency. The position of measurement cells is shifted when changing the blanking distance, and blanking distance (along with number of cells, cell size and scattering condition) affects the total profiling range.
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