Doppler Effect

Today I received a Tweet with a question regarding a principle of acoustics, which I aim to answer in this post.

The effect that causes this pitch change is known as the Doppler Effect and regards the change in frequency of a sound source for an observer moving relative to its source. To understand how it works, a few basic principles must be underlined.

Sound is created by pressure changes in the air, which is caused via vibrations of an object (e.g. a speaker). As the speaker vibrates outward, pressure is increased in front of it and as it moves back inwards pressure is decreased again because it is compressing and decompressing the air particles in front of it. These pressure changes travel through air, away from the speaker, at a fixed speed of 344 metres per second. To play a lower pitch (or frequency) sound, the speaker vibrates slower (less vibrations per second). As the pressure variations (sound waves) move away from the speaker at the same speed no matter what frequency is made, there is a greater distance between the sound waves at lower frequencies (as the sound wave has had more time to travel further between the speaker vibrations).

Now back to that siren… As the fire engine travels, the sound it is creating from the siren, travels away from it in all directions. This next step best illustrated with a diagram:

Doppler Effect

Watch a video that animates this

The lines around the fire engine represent the sound waves travelling away from it. As the engine moves forward, the speed at which the sound waves travel away from it (in the same direction the engine is traveling) decreases because it is like the engine is ‘chasing’ the sound waves. This causes the distance between sound waves to decrease. This also means that the engine is ‘driving away’ from the sound waves travelling away behind it and, like said previously, a greater distance between sound waves relates to a lower frequency.

This means that if you are standing on the side of the road and an ambulance drives past with its sirens on, as it comes toward you, it sounds higher as the distance between sound waves is short, but as it has gone past the distance between sound waves increases and it has a lower pitch. This effect creates the classic sound we are used to when we want to recreate the sound of a car going fast, because the greater the speed of the vehicle, the closer its speed is to the speed of sound, the shorter the distance between the wavelengths in front of the vehicle and the more noticeable this effect becomes.

If you are in the vehicle however, this effect is unnoticeable because you are at a constant distance from the sound source (e.g. the siren/engine) meaning that the sound does not change relative to you. Also, if you are standing on a path and a vehicle goes past very slowly, it is not noticed as the difference between the speed of sound and the speed of the vehicle is so great that there is no noticeable effect.

As an aside, you may wonder what would be to happen if a vehicle did go faster than the speed of sound. This can happen, but only in planes as 344 metres per second is the same as 770mph. A sonic boom is created when this happens and is the sound of the shock waves as the object passes this speed.

Of course as some of you may be aware, the Doppler affect has been mentioned on TV before in the Big Bang Theory and I couldn’t really write a blog post on it without including the clip…

Screaming in Space

So you all know the phrase that none can hear you scream in space, after all its true, there are no air particles for the sound to vibrate which is how sound travels. But is there any sound, I mean can you see sound in space? The idea of an audio tour of space seems really bizarre for this reason alone, however the BBC have managed to make a program from it, so it must be possible right?

The Sun

The idea is that by observing movements of patches of varying gas densities on the sun, any differences can be linked to vibrations and can be recreated for us to listen to. The movements, however are very low in frequency, below what we could hear anyway (we can hear as low as 20 Hertz, which is 20 vibrations per second). In order to hear this, it has been sped up, which kind of seems like cheating a bit, but I guess it doesn’t mean it can’t exist just cause we can’t hear it!

This reminded me of one of my fellow students blog posts (at University of Salford) a few weeks ago about a very low frequency in space. This sound had a frequency of 0.0000001 Hertz, now that is what you call bass!

If you get a chance, have a watch of the BBC’s short clip (its only 3:40) and let me know what you think and what your favourite sounds are!

Notes

[1] Image of the sun – http://en.wikipedia.org/wiki/Sun

In the Mode

Yesterday we had a lab experiment in the university lab facilities, specifically the small reverberant chamber; this lab was the creation and measurement of room modes. In acoustics, everything has a frequency that it naturally vibrates at, like a guitar string or the air in a bottle when you blow across it; well the same is the same for a room.

If a loudspeaker is placed in a room and the frequency is found where the wavelength of the sound wave (produced from the loudspeaker) is the same as (or an integer multiple of)  that of the dimensions for the room, then what is known as a standing wave is set up. This is where when the wave is reflected between the two walls and it does not move or emit any energy but instead creates ‘nodes’ and ‘antinodes’ which are pressure minimum and maximum points respectively. As this wave is set up, the wave from the speaker interacts with the wave reflected from the wall which causes the equal and opposite signals to cancel each other out at nodes and or vice versa and double at anti nodes. A graphic representation is given in the diagram below. As the frequency is doubled, double the number of half wavelengths are set up between the walls, as is shown in the diagram also.

Standing wave

You may not notice room modes often in everyday scenarios as spaces are often acoustically designed to avoid them. Use of absorption (as mentioned in one of my previous posts) reduces reverberation and thus the reflected signal and room mode.  We carried out this experiment in the reverberant chamber because of this, so that they would be easily set up. Rooms with matching axis dimensions or integer multiple dimensions (e.g. a cube or a room 2m x 4m x 8m) are a bad idea as this means that at one frequency all room modes are going to resonate and will become more prominent as a result; in this case 4x louder than in just 1 dimension.

The video shows how the frequency starts loud at 1 wall then reduces in the middle and gets louder at the other wall; this is the effect of the room mode. You may have expected, if the theory is true, that there would be no sound in the middle, however this is not the case because of sound waves propagating in the other axes as the room was not cubic.

You may find it hard to hear on your speakers (but with footsteps and speech being more audible); this is because I shot this video on my iPhone 5. The graph below shows the frequency response for an iPhone microphone where 0dB on the y axis means that the mic picks up the input signal at the same level it is heard, with below this being picked up less and it being amplified greater than it is above 0dB. You will see that it is mainly amplified at mid/high frequencies because this is roughly the frequency of speech, the main requirement for a phone microphone! Low frequencies are reduced because they are not needed so much (e.g. to reduce that annoying wind howl in the background as you are trying to make a phone call).

iPhone Mic Response

This is why you cannot hear it very clearly in the recording, even though in reality the speaker was producing up to 100dB, which is why we needed to wear ear defenders.

Loud Noise

It was an interesting concept to experience, but one I recommend; if you have a spare few minutes and have a reasonably good set of speakers, try out this Tone Generator and try and find the resonant frequency of the dimensions of the room. Note that there will be a different mode set up for each different dimension in the room.

To find the right frequency, you need to use the simple equation: Frequency = 340÷Dimension (in Metres). You can also use integer multiples of the dimension (e.g. 2 or 3 times the length of the room). To get you started, here are some example values so that you can get an idea of the frequency region you will need:

Room Modes

Once you find a frequency you seem to think may be resonant, then have a walk round and listen for how the sound varies at different points in the room, then comment below and let me know what you heard!

Notes

[1] https://www.soundonsound.com/sos/dec07/articles/acoustics.htm

[2] iPhone mic and speaker responses http://blog.faberacoustical.com/2010/ios/iphone/iphone-4-audio-and-frequency-response-limitations/