Sound waves are longitudinal waveA wave that moves in the same direction as the direction in which the particles are vibrating.. They cause particles to vibrate parallel to the direction of wave travel. The vibrationsRepeated movements back and forth (about a fixed point). can travel through solids, liquids or gases. The speed of sound depends on the mediumA material through which a wave can be transmitted (propagate). through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuumA volume that contains no matter. because there are no particles to carry the vibrations.
Tim Peake uses an alarm clock to help explain how sound travels
When a sound wave meets a boundary it may be:
reflectedThere is a reflection when waves bounce off a surface.
refractedDeflected from a straight path. Bent.
absorbedEnergy is 'taken-in' by the material and the internal energy of the material will increase, eg infrared radiation from the Sun is absorbed by the surface of the Earth.
Whether a sound wave is reflected, refracted, or absorbed depends on the densities of the materials either side of the boundary. If the densities are very different then reflection is more likely.
Sound waves across boundaries
When sound waves move from one medium to another, there will be changes to the velocity (or speed), frequencyThe number of waves produced each second. The unit of frequency is hertz (Hz). and wavelengthThe length of a single wave, measured from one wave peak to the next. of the sound wave. This change in velocity can also result in a change of direction of the sound wave - also known as refraction.
For example, refraction occurs when sound travels from warm air into cold air. When this happens:
the wavelength of the sound wave decreases
the frequency of the sound wave stays the same
the velocity of the sound wave decreases (since wave speed = frequency × wavelength)
This is why sound travels further at night, when it is cooler.
