Pulse Echo Measurements

In pulse-echo techniques a short pulse of waves is sent out, reflects from a boundary, and the reflected pulse (the echo) is detected, often by the same transducer that sent it. The pulse travels to the reflector and back, so if the echo returns a time t after the pulse was sent, the distance d to the reflector is given by2d = vt    so    d = vt ÷ 2Uses include medical ultrasound scanning, sonar for measuring the depth of the sea, radar for tracking aircraft, and detecting cracks inside metal parts. The pulses are often displayed on an oscilloscope, where the time base converts the horizontal gap between the transmitted pulse and the echo into a time.Worked exampleUltrasound travels at 1540 m s^-1 in soft tissue. An echo returns 65 μs after the pulse is sent.d = vt ÷ 2 = 1540 × 65 × 10^-6 ÷ 2 = 0.050 m = 5.0 cmLimitationsThe detail that can be seen is limited by the wavelength: features smaller than about one wavelength cannot be resolved, so a higher frequency (shorter wavelength) gives more detail. However, higher frequencies are absorbed more, so they cannot reach as deep.The pulse duration also matters. If a pulse is too long, echoes from two boundaries close together overlap and cannot be told apart. A new pulse must not be sent until the echo from the previous one has returned.