Sound And Ultrasound
Visão Geral
Hold a finger against your throat and hum. What you feel is the whole of this topic in one sensation: something is shaking, and because it is shaking you can hear it. Every sound that has ever reached you began as a vibration and arrived as a vibration, carried across the room by air that squashes and stretches millions of times a second without ever travelling anywhere itself. Take the air away and the vibration has nothing to ride on, which is why the loudest explosion in space would reach you in complete silence.
This lesson takes that idea and turns it into physics you can calculate with. You will learn why a sound is high or low, why it is quiet or deafening, and where the two limits of your own hearing sit. Then you will cross those limits deliberately: above 20 000 Hz lies ultrasound, inaudible to you but reflected back from every boundary it meets inside a steel bolt, a ship's hull or a human body. Time those reflections, multiply by a speed, and a pulse you cannot hear turns into a picture of something you cannot see.
Objetivos
- Sound waves are longitudinal waves and cause vibrations in a medium, which are detected as sound. The range of human hearing is about 20 Hz to 20 000 Hz. No details of the structure of the ear are required.
- The pitch of a sound is determined by the frequency of vibrations of the source. Its loudness is related to the size of the amplitude of the disturbance.
- Sound waves can be reflected (echoes) and diffracted.
- Ultrasound is acoustic (sound) energy, in the form of waves with a frequency above the human hearing range.
- Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans.
- Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.
- The distance, s, between interfaces in various media can be calculated using: s = v×t where v is wave speed and t is time taken. Students may be required to use and interpret data from diagrams of oscilloscope traces.
- Ultrasound waves can be used in medicine. Examples include prenatal scanning and the removal of kidney stones.
Mapa mental
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Nota de Aula
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Use esta secção de avaliação como uma oportunidade para reforçar a tua compreensão do tema e identificar quaisquer áreas onde possas precisar de estudo adicional. Não te deixes desencorajar pelos desafios que encontrares; em vez disso, vê-os como oportunidades de crescimento e melhoria.
- What type of wave is a sound wave? A. A transverse wave B. A longitudinal wave C. An electromagnetic wave D. A wave that needs no medium Answer: B
- What is the approximate range of frequencies that a human can hear? A. 2 Hz to 2000 Hz B. 20 Hz to 20 000 Hz C. 200 Hz to 200 000 Hz D. 20 000 Hz to 2 000 000 Hz Answer: B
- A guitar string is plucked harder but is not shortened or retuned. What happens to the sound? A. It becomes louder and higher in pitch B. It becomes louder with the pitch unchanged C. It becomes higher in pitch with the loudness unchanged D. It becomes quieter and lower in pitch Answer: B
- A pulse of ultrasound is sent downwards from a ship and its reflection from the seabed is detected 0.20 s later. The speed of sound in seawater is 1500 m/s. What is the depth of the seabed? A. 75 m B. 150 m C. 300 m D. 600 m Answer: B
- Why can a single pulse of ultrasound be used to locate several different boundaries inside the body? A. The pulse is completely reflected at the first boundary it meets B. The pulse is only partially reflected at each boundary, so part of it continues deeper C. The pulse speeds up each time it crosses a boundary D. The pulse is absorbed at each boundary and re-emitted at a lower frequency Answer: B
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