# Medical Physics: Sound

A. What is Sound?

 1. Sound is a wave that propagates from a source and gradually fades away as it travels. 2. Sound propagates thanks to the movement of particles. These particles can be in the form of a gas (= air), a fluid (=water) or a solid (=a wall). 3. Sound propagates because particles collide against each other; at each collision, the next particle is moved further. 4. In contrast to light, which does not require a medium, sound does need a medium. If you take away the medium (thereby creating a vacuum), then there is no sound. 5. Light propagates forever as long as it is not blocked. Sound however always fades away, as the energy gradually dissipates after increasing number of collisions. 6. Sound propagates much slower (330 m/sec) then light (300,00 km/sec). 7. The difference in speed is well illustrated in a thunderstorm where there is always first a flash and then, after a few seconds, a thunder. 8. In fact, you can calculate how far the thunderstorm is located by counting the seconds after a lightening (21, 22, 23 …). If there are 3 seconds between flash and thunder, then the thunderstorm is approximately 1 km away (3 seconds x 330 m/sec = 990 meters).

B. What do you need to know about sound?

 1. Sound is a WAVE, it can be described by its wavelength and by its amplitude, just as with light. 2. The wavelength determines the frequency of the sound. The unit is Hertz (= Hz) and a sound with a frequency of 1 Hz has a wavelength of 1 second. 3. Human beings can hear sound in the range of 20 Hz – 20.000 Hz. (Can you calculate the period of this range from these two values?) 4. Another word for frequency is “pitch” (especially the British use this term) 5. So, if someone talks at a HIGH frequency, she talks with a high pitch. 6. If you talk at a low frequency, then you talk with a low pitch. 7. The amplitude determines the loudness of the sound. If the amplitude is large, then the sound is loud. If the amplitude is small then the sound is soft. 8. The loudness of a sound is described in decibels (=dB). We can hear from 1 to approximately 120 dB. Sounds louder than 120 dB are painful and dangerous. Normal speech is at the level of 20-30 dB. larger?

C. What can we do with sound?

 1. In principle, because sound is also a wave, sound can be reflected and refracted, just as with light. 2. Especially reflection is very common with sound waves. For example, an echo is a good example of a reflection. 3. In a large curved room, it is also possible that sound waves are reflected to a focal point; voices, softly spoken, will be amplified on that spot. It works like a parabola (see Light). 4. Our own ear, the fleshy part that sticks out of our skull, is actually a wonderful collector of sound waves 5. If people cannot hear properly, they will often put a hand behind their ear and slightly cup it, thereby increasing the collector’s area. 6. Some animals can even rotate (= turn) their ear towards the source of interesting sounds (dogs, camels, horses etc). 7. In medicine, a stethoscope is an important collector of sounds. These sounds, such as the heartbeat, originate from the body and are transmitted, through tubes, and the air inside the tube, to the ears of the physician. 8. Sound waves are also used in imaging machines such as ultrasound that makes it possible to visualize, for example, the unborn child. This is much healthier than using x-rays (because this might damage the fetus). In fact, an ultrasound machine is a good example of reflection and refraction of sound waves. 