Speed of Sound

You can imagine the propagation of sound similar to the propagation of waves when a stone is dropped into the water. These waves spread circularly in all directions with the same speed around the stone. Sound spreads in the same way as invisible waves in the air around a stationary sound-source (e.g. speakers). That’s why they are called sound waves. In the case of water waves, the distance between two waves (or rather between the two highest points of the waves, called crests; the low-lying points in between are called the troughs of a wave) is of no great importance, in contrast to sound waves. In the case of sound waves, the distance between the crests of the wave determines the wavelength. From the wavelength, the frequency can be calculated in turn. The frequency of a sound wave tells us how high the tone is of the sound we hear.

Why does the siren of the ambulance sometimes sound higher and sometimes lower?

If you stand next to a stationary car (with its engine running), you always hear the same pitch, regardless whether you stand next to the car, behind the car or in front of the car, since these sound waves emit waves with the same speed in every direction. If the car is moving, it follows the sound waves in one direction (forwards), whereas it moves away from the waves when you look backwards. This leads to a shift in the distances between the crests of the wave. They are pushed together in the front of the car, and pulled apart behind the car. This means the sound waves have a different frequency (thus a different pitch) in front of the car than they do behind it. That is the reason why you perceive the pitch of the siren of a passing ambulance differently. When the ambulance approaches you, you hear a high tone, when it moves away, a lower one. This effect is called the Doppler effect, named after its discoverer Christian Andreas Doppler born in 1803 in Salzburg, Austria.

The Big Bang

As is the case for light, the velocity of propagation of sound behaves differently in different materials. For us, the propagation in the air is the most important thing. It is approximately 331 m/s or 1192km/h. But some planes can go faster. When they reach the speed of sound, a loud bang can be heard. We already know the reason: Like for the car, the crests of the waves of the sound waves are pushed together in the direction of movement. When the airplane flies at the speed of sound - or even faster - the following picture arises:

When the plane flies at the speed of sound, the crests of the waves are pushed together to a front. This front is called the sonic barrier. When flying through this sonic barrier, a loud bang is generated, as you can hear many sound waves at once.
A well-known example of a supersonic aircraft is the Concorde. It reaches speeds of more than 2200km/h, which is almost twice the velocity of sound!