Ultrasound & Infrasound Flashcards

All 8 cards in this deck

What is ultrasound?

Sound with a frequency greater than 20 000 Hz (20 kHz).

What is the approximate range of frequencies the human ear can detect?

About 20 Hz to 20 000 Hz (20 kHz).

What is infrasound?

Sound with a frequency less than 20 Hz.

True or false? Ultrasound is sound that is too quiet for humans to hear.

False. Ultrasound is defined by its frequency (above 20 000 Hz), not its loudness or amplitude.

How is ultrasound used to make an image of a foetus?

Ultrasound pulses are partly reflected at each boundary between different tissues, and the reflected pulses are processed into an image. It is used because it is non-ionising and safe.

How is infrasound used to explore the Earth's core?

Very low frequency waves from earthquakes or explosions pass through the Earth and are detected at the surface; how they reflect and refract reveals the structure of the core.

What are the steps to find the depth of water using sonar?
e.g. an ultrasound pulse returns after 0.02 s, speed of sound in water 1500 m/s

1. Send an ultrasound pulse down and time the echo: $t = 0.02$ s.
2. Total distance $= v \times t = 1500 \times 0.02 = 30$ m.
3. The pulse goes down and back, so depth $= 30 \div 2 = 15$ m.

True or false? Ultrasound can be used to detect cracks inside a solid metal object.

True. A crack reflects the ultrasound pulse back sooner than the far side of the object, revealing the fault.