Sound and Ultrasound

Sound and Ultrasound

1. Sound waves are LONGITUDINAL

   • they are caused by mechanical vibrations in a substance
   • vibrations are PARALLEL to the direction of travel

2. Human ears can hear frequencies from 20Hz to 20,000Hz

   • the ability to hear higher frequencies declines with age

3. Sound waves can be reflected to produce ECHOES

   • When calculating the distance of a reflecting surface from a person, remember that an echo goes there and back (divide by two) E.G. Sound travels at 340m/s. A person hears an echo 0.4 seconds after shouting - how far away is the reflecting surface? (340 x 0.4) = 136 136/2 = 68m
   • only hard, flat surfaces can reflect sound
     • e.g. flat walls & floors
   • Soft objects/surfaces absorb sound
     • e.g. carpets, curtains & furniture

4. Sound waves can also be DIFFRACTED

   • the WAVELENGTH should be of the same order as the size of the obstacle or gap that diffracts the waves

5. FREQUENCY

   • determines the PITCH of a sound
     • the higher the frequency the higher the pitch
   • = the number of vibrations each second
   • measured in Hertz (Hz)

6. AMPLITUDE

   • determines the LOUDNESS (VOLUME) of a sound
     • the greater the amplitude, the more energy the wave carries and the louder the sound
   • the 'height' of the wave

7. OSCILLOSCOPE

   • a device used to display the shape of an electrical wave
   • a short wavelength on the screen corresponds to a high frequency

8. ULTRASOUND

   • waves with frequencies higher than 20,000Hz (20MHz)
   • non-ionising (safer than X-rays)
   • used for pre-natal scanning or to break down kidney stones so that they could be removed
   • Ultrasound scanners use a transducer to produce and detect ultrasound waves
     • Again, the echoes are going there AND back, so the calculations will probably be halved again
     • pulses of ultrasound are directed into the body
       • they partially reflect from the tissue boundaries
         • the reflected pulses are detected by the transducer

9. s = v x t

   • distance (between interfaces in various media) = wave speed x time taken