5.7 Sound
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Sound waves are longitudinal waves (meaning that they vibrate in the same direction that they are travelling in). They are produced by objects vibrating, such as a speaker. Sound waves travel through solids, liquids and gases and travel at different speeds through each of these. They travel the fastest through solids and the slowest through gases. Sound waves are unable to travel through a vacuum and this is why space is quiet.
Sometimes the sound will travel into someone’s inner ear and reach their ear drum. Humans can only hear sounds with a frequency between 20 Hz (low pitched noise) and 20,000 Hz (high pitched noise). As we get older, our hearing deteriorates and the range of noises that we can hear diminishes.
Sound waves can be reflected. Hard surfaces will reflect sound waves. Soft surfaces absorb sound waves. This is why an empty room tends to echo and a room that has some furnishings in, such as, chairs, sofas, carpets, rugs etc. doesn’t have an echo because the furniture absorbs the sound. The reason why there is a delay with an echo is because of the time that is takes for sound to travel. Sound waves travel through air at about 340 m/s. This is significantly lower than light, which travels at around 300,000,000 m/s. The difference in speed between light and sound is why we see lightening before we hear lightening.
Sometimes the sound will travel into someone’s inner ear and reach their ear drum. Humans can only hear sounds with a frequency between 20 Hz (low pitched noise) and 20,000 Hz (high pitched noise). As we get older, our hearing deteriorates and the range of noises that we can hear diminishes.
Sound waves can be reflected. Hard surfaces will reflect sound waves. Soft surfaces absorb sound waves. This is why an empty room tends to echo and a room that has some furnishings in, such as, chairs, sofas, carpets, rugs etc. doesn’t have an echo because the furniture absorbs the sound. The reason why there is a delay with an echo is because of the time that is takes for sound to travel. Sound waves travel through air at about 340 m/s. This is significantly lower than light, which travels at around 300,000,000 m/s. The difference in speed between light and sound is why we see lightening before we hear lightening.
Sound waves will have different characteristics. For example, the higher frequency, the higher the pitch of the sound will be. Also, the bigger the amplitude, the louder the sound will be. The amplitude is the height of the wave around the rest point.
Doppler Effect
When an object that is creating noise is travelling towards the listener, the noise sounds slightly higher pitched than if the object was stationary. Also, when an object that is creating noise is moving further away, the sound sounds lower than if the object was stationary. Next time an ambulance, fire engine or police car passes you with their sirens on, listen out for the change in sound. This phenome is known as the Doppler effect. The reason why the sound sounds higher when the object emitting noise is moving closer to you is because the sound waves are hitting you more frequently and a higher frequency sound waves have a higher pitch. On the contrary, when the object creating sound moves further away, sound waves are hitting you less frequently, resulting in the sound having a lower pitch.
The Doppler effect happens to both longitudinal waves (sound) and transverse waves (light: see the red shift section).
When an object that is creating noise is travelling towards the listener, the noise sounds slightly higher pitched than if the object was stationary. Also, when an object that is creating noise is moving further away, the sound sounds lower than if the object was stationary. Next time an ambulance, fire engine or police car passes you with their sirens on, listen out for the change in sound. This phenome is known as the Doppler effect. The reason why the sound sounds higher when the object emitting noise is moving closer to you is because the sound waves are hitting you more frequently and a higher frequency sound waves have a higher pitch. On the contrary, when the object creating sound moves further away, sound waves are hitting you less frequently, resulting in the sound having a lower pitch.
The Doppler effect happens to both longitudinal waves (sound) and transverse waves (light: see the red shift section).