Stretching sound: Communicating with velocity

A proposed solution to the dangers caused by drivers who ignore the speed limit on Britain’s motorways. This system communicates directly with drivers on the motorway who are exceeding the 70mph speed limit. A driver who is travelling over the speed limit will hear a high pitch ring as they drive along the motorway. Once the driver reduces their speed below the speed limit they will no longer hear the ring. This ring will act as a warning to unaware drivers who carelessly allow their speed to drift above the limit. It will also act as a preventative measure for drivers who intentionally choose to break the speed limit on the motorway. Drivers will find that prolonged exposure to the high pitch ring will act as an annoyance persuading most to reduce their speed. Meanwhile anyone moving at or below the speed limit (even if they are close to a speeding vehicle or if they are a nearby pedestrian) will not hear the ring. How does it work? Have you ever listened to an ambulance siren as it drives past you in the street? As the ambulance drives towards you the sound of its siren is relatively high pitch, then as it passes you and drives away the pitch appears to lower. The cause of this shift in pitch can be attributed to the Doppler effect.
	Doppler's Principle: The principle that a wave will appear to shift in frequency because of the relative motion of the observer of the wave and the wave's source.
As the ambulance moves towards you, as a result of its movement, the frequency of the sound waves gets squashed together, raising the frequency and the pitch of the sound that you hear. As the ambulance moves away the frequency of the sound waves gets pulled apart, lowering the pitch of the sound. It is this basic principle, for creating an apparant shift in frequency due to relative motion, that is used in the proposal for warning drivers when they exceed the speed limit. The following images demonstrate how waves are stretched in relation to movement.

This photograph shows the projected image of water waves being produced by a stationary source. (The wave source is indicated by the black circle.) The waves are regularly spaced as they move away from the source.		This photograph shows waves being created by a source which is moving from right to left. Notice how the waves to the left of the source are closer together than the waves to its right. The waves moving in the opposite direction to the movement of the source (ie. waves moving left to right) have been stretched, they are now further apart and have a lower frequency. The waves moving in the same direction as the source (ie. right to left) have been squashed and now have a higher frequency.
The previous examples demontrates how waves are affected if the source of the wave is moving and the observer is stationary. This principle also works in the same way if the source of a wave (eg. a speaker) were stationary and the observer (eg. a driver) was moving. The highest pitch that the human ear can hear is roughly 20 000Hz depending on the individual. Any sound waves with a frequency higher than this are inaudible to the human ear. However if you move away from a sound of this higher frequency the rate at which the sound waves reach you will get stretched and the pitch will lower. If you move away from this sound fast enough the pitch will lower enough for you to hear a noise. This noise will sound like a high pitch ring. In fact sound can be produced at an exact frequency so that anyone standing still or moving slowly will hear nothing, but anyone moving away from the sound's source at around 80mph or more will hear a high pitch ring. The proposal is for speakers to be stationed at regular intervals along the motorway, leaving drivers in continuous audible range of the nearest speaker. The speakers will produce a continuous high frequency sound wave which will only be audible to drivers who are exceeding the speed limit. The frequency of sound that will be observed when: Driving at 100 miles/h = 19 406 Hz (audible) Driving at 80 miles/h = 20 000 Hz (audible) Driving at 70 miles/h = 20 297 Hz (inaudible) Standing Still = 22 373 Hz (inaudible) In order to calculate this apparant shift in frequency we can say: F = (1 – v ÷ u) f u = The speed of sound v = The speed of observer f = The frequency of the sound produced F = The frequency of the sound observed The frequency of the sound produced (f) = 22 373 Hz The speed of sound (u) = 331.4 + 0.6 T T = Temperature The speed of sound used in the calculations above was 337.4 m/s calculated for 10°c To convert the speed of the observer (v) into m/s: 0.621 miles = 1 km 100 miles/h = 161.03 km/h = 44.73 m/s 80 miles/h = 128.82 km/h = 35.78 m/s 70 miles/h = 112.72 km/h = 31.31 m/s Additional Note: As there are instances when it is necessary for emergency vehicles to exceed the speed limit a small audio device will be added to vehicles of this nature to block out the noise. An in built microphone will ‘listen’ to the high pitch noise entering the vehicle, then a microchip in the audio enhancer will create a mirror image sine wave (the same frequency and amplitude as the original, but 180 degrees out of phase) and emit it from an in-built speaker. As a result of destructive interference this device will cancel the original noise creating silence in relation to the high pitch ring.