Sound Transmission and Properties: Acoustics and Reflection
Sound Transmission and Properties
Media that Transmit Sound
The sounds we hear are typically transmitted through the air. However, any elastic substance, whether solid, liquid, gas, or plasma, can transmit sound.
Elasticity is the property of a material that allows it to change shape in response to an applied force and return to its original shape when the force is removed. Steel is an elastic substance, while putty is inelastic. In liquids and elastic solids, molecules are relatively close to each other, respond quickly to relative movements, and transmit power with little loss. Sound travels four times faster in water than in air and about 15 times faster in steel than in air. In solids and liquids, sound does not spread as well as in the air. While you do hit some submerged rocks, you will hear the click very well. Liquids and crystalline solids are excellent conductors of sound, much better than air. The speed of sound is higher in liquids than in gases and even higher in solids.
Sound does not propagate in a vacuum because it needs a medium. If there is nothing that will compress and expand, there can be no sound.
Speed of Sound in Air
We hear thunder after seeing the flash of lightning. This demonstrates that sound needs a considerable time to propagate from one place to another. The speed of sound depends on these conditions:
- Air (wind)
- Temperature
- Humidity
All sounds spread as quickly. The speed of sound in dry air at 0ºC is about 330 m/s, or about 1200 km/h. Sound travels faster in warm air than in cold air because the molecules of hot air move at a higher speed, collide with each other more frequently, and therefore can transmit an impulse in less time. For every degree rise in temperature above 0°C, the speed of sound in air increases by 0.6 m/s. At 20°C, it spreads at around 340 m/s.
Sound Reflection
Echo is the reflection of sound. The fraction of energy carried by the reflected sound wave is large if the surface is rigid and smooth and is lower if the surface is soft and irregular. The acoustic energy that the sound wave carries has reflected off the surface, i.e., it is absorbed by the surface. Sound is reflected off a smooth surface in the same way as light does. The incident sound angle equals the angle of the reflected sound. Sometimes, when sound is reflected off the walls, ceiling, and floor, the reflecting surfaces reflect it again, i.e., it is reflected several times. These multiple reflections are called reverberation. If the reflecting surfaces are highly absorbent, the sound intensity would be low, and the sounds would seem dull and lifeless. In an auditorium or concert hall, a balance must be found between reflection and absorption. The study of the properties of sound is called acoustics. Highly reflective surfaces should be placed behind the stage to direct the sound toward the audience. Sound and light follow the same law of reflection, so if a reflector is oriented to view a certain musical instrument, you can listen to it too. Plastic plates on the orchestra reflect both light and sound. It is very easy to adjust: what you hear is what you see.
Refraction of Sound
Sound waves are deflected when parts of their fronts travel at different speeds. This happens in “erratic winds” or when sound travels through air at different temperatures. This deviation of sound is called refraction. On a hot day, the air near the ground may be warmer than the rest, increasing the speed of sound near the ground. Sound waves move away from the ground, making the sound seem not to spread well. Different speeds produce the refraction of sound.