A sonar is an ultrasound device that is used to map the surface of the ocean. At a particular place the echo is heard \(2\) seconds after the sonar sends an ultrasound. How deep is the ocean there? Use \(1500\, \text{m/s}\) for the speed of sound in salt water.
16.
A person hears a sound level of 40 dB from an airplane engine. (a) If the eardrum has area of cross-section area of 0.4 cm2, how much energy does the eardrum receive is one minute? (b) If the sound intensity level goes up to 80 dB, by what factor has the pressure on the eardrum gone up?
Two strings on two cellos are tuned to the same frequency of \(247\, \text{Hz}\text{.}\) One of the strings is then tightened further increasing its tension by \(30\%\text{.}\) When the two cellos are played a beat is heard. Find the beat frequency.
18.
A piano key is to be tuned by using a \(440\, \text{Hz}\) tuning fork. The key loose and produces a sound that is lower that \(440\, \text{Hz}\text{.}\) The piano tuner hears 4 beats per second when the key is struck at the same time as the tuning fork. By how much should the cord be tightened?
19.
A tuning fork of frequency 256 Hz is set vibrating over an open tube. If the speed of air in the tube is \(344\, \text{m/s}\text{,}\) and a maximum height of \(1.5\) meters possible in the tube, at what height of the air column will resonance be heard?
20.
Two loudspeakers are connected to a \(2000\, \text{Hz}\) signal generator. A person while standing between the two speakers does not hear any beats, but when he runs from one speaker to another he hears a \(25\, \text{Hz}\) beat. How fast must he be moving away from one speaker and towards the other speaker? Use \(343\, \text{m/s}\) for the speed of sound.
Answer.
\(2.14\, \text{m/s}\text{.}\)
21.Doppler flow meter.
Doppler flow meters work with the principle of Doppler shift, whereby the transmitted frequency is altered by being reflected from particles and bubbles in the fluid. The speed of flow can be deduced from the shift in frequency. The wave is sent by a transducer at \(45\) degrees angle to the flow and are reflected back in the same line. In a particular flow an ultrasound of frequency \(10\, \text{MHz}\) is utilized and a shift of \(1000\, \text{Hz}\) is observed. What is the speed of flow? Use speed of sound in water to be 1500 m/s.
