During an overnight survey of a river channel, an autonomous mapping raft checks that there is enough water beneath it for a supply boat. Its ultrasonic ran...
Question 1 Report
During an overnight survey of a river channel, an autonomous mapping raft checks that there is enough water beneath it for a supply boat. Its ultrasonic rangefinder sends brief sound waves into the water. Fig. 1 shows one pulse leaving the sensor, reflecting from the riverbed and arriving back at the sensor 0.084 s later. The velocity of ultrasound in this water is 1500 m/s. The equipment operates at 48 000 Hz, so the sound cannot be heard by a child standing on the bank.
Fig. 1
(a) What is meant by ultrasound? [2]
(b) Use the information and Fig. 1 to calculate the depth of water below the sensor. Give your answer in m. [3]
(c) Describe why the time recorded by the rangefinder is divided by two when calculating this depth. [2]
(d) Explain two advantages of using short pulses rather than a continuous sound signal for this measurement. [3]
Answer Details
(a) Ultrasound is sound with a frequency above \(20\,000\ \text{Hz}\). [1] It is above the upper limit of human hearing, so humans cannot hear it. [1]
(b) The recorded time is for the outward and return journey.
\[\text{total distance}=vt=(1500\ \text{m s}^{-1})(0.084\ \text{s})=126\ \text{m}\] [1]
\[\text{depth}=\frac{126\ \text{m}}{2}=63\ \text{m}\] [1]The depth below the sensor is \(63\ \text{m}\). [1]
(c) The measured time is for the pulse to travel to the riverbed and return. [1] The depth required is only the one-way distance. [1]
(d) Short pulses allow a separate returning echo to be detected after each transmission. [1] The time between sending and receiving can be measured clearly. [1] Echoes from different objects or depths are less likely to overlap. [1] It is also acceptable that the sensor can stop transmitting while listening for the reflection.
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