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…

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To Silence All The Gun Nuts…

This article was written by my course mate and good friend Dom Attwell, who writes about the perception, theory and practical effectiveness of gun “silencers”. A very good article worth reading, one of the best I’ve read for a few weeks!

DomJohnAttwell - Acoustics

The confession:
To begin, I must iterate that I am not a gun specialist. I have never even held a firearm that didn’t shoot BB-pellets or lasers. That said, my kill-death ratio on computer games is out of this world… literally (I mostly play Halo).

The Question:
I get a lot of questions concerning the efficiency and plausibility of gun silencers. Hollywood tends to take great artistic liberties with these ‘magical’ devices, with the explosive discharge of these lethal weapons sounding more like a kitten sneezing.

Handgun with SilencerMotorbike Silencer

Ever heard a gunshot at close range? You would definitely know if you had…

The Life Story: (if inpatient just jump to ‘The Gist’…)
Late last year, our course was given an ‘acoustic tour’ of the Halle Orchestra’s Bridgewater Hall in central Manchester. While there, we conducted an experiment to measure the hall’s reverberation time, which involved creating an impulse response. Due to the hall’s…

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Seeing is Hearing

As kids we get taught about the 5 senses: sight, hearing, smell, taste and touch. They are each perceived by a different part of the body, i.e. sight with eyes and hearing with ears. But what if what we see clashes with what we hear, can we really hear with our eyes?

This short video is a really interesting view over an aspect of psychoacoustics looking into this. What makes this effect even better is that even when you know what’s happening, you can’t stop your brain from interpreting it as different.

Maybe we should stop relying on our eyes so much and start hearing with our ears, besides that was what we have them for!

Notes

[1] IoA SoundBites Spring 2014

[2] BBC 2 ‘Is Seeing Believing’ Documentary

20/20 Audition

So you know what 20/20 vision is, it’s when you have perfect vision. But what is the equivalent for your ears? Well I think it should be called 20/20 audition; let me explain why…

20/20 vision is defined as how well you can see at 20 feet away, with 20/20 being what you should be able to see with clarity. However if you have 20/100 vision, you have to be 20 feet away to see something that someone with perfect sight can see at 100 feet away. “So what does that have to do with sound” I hear you cry, “last time I checked you can’t see sound!”. If you said that, you would be right, however the 20/20 part is to do with how we measure hearing.

Hearing is measured by how quiet you can hear sound (your hearing threshold) at different frequencies. Because sound is created by vibrations, we measure frequency by the number of vibrations per second (which we call hertz or Hz for short). 20Hz is the lowest we can hear, which literally means 20 vibrations per second and 20,000Hz is the highest we can hear, which is 20,000 vibrations per second. For hearing tests, a sound of a known frequency is played and the listener adjusts it to the quietest point at which they can only just hear it. This level is then compared to what the level should be for perfect hearing and the difference is the hearing loss at that frequency.

Hearing Loss

As we become older, we lose sensitivity in our ears, particularly at higher and lower frequencies, meaning the range that we can hear is reduced. As perfect hearing goes from 20Hz to 20kHz (20,000Hz), this is why I think it should be called 20/20 audition (audition is just another word for hearing). From what I could seem to find online, there wasn’t a term for perfect hearing like there is for sight, only “normal hearing”, which seems a bit boring to me. Do you agree with me, or if not what do you think perfect hearing should be called?

Notes

[1] What is 20/20 vision? – http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/visual-acuity

[2] What is normal hearing? – http://www.bio.net/mm/audiolog/1998-June/004067.html

[3] How is hearing measured? – http://www.dosits.org/science/soundmeasurement/hearingmeasured/

Plane Annoyance

There are some noises in life that are really annoying and others that we just learn to live with; aircraft and airport noise are often subject to a lot of debate as different people would put them into different categories. But with it being such a hot topic, how do the airports keep track of their noise emissions?

First of all, we must understand what causes aircraft noise. [1] There are 2 sources of noise from aircraft: air friction (as it passes over the wings or fuselage) and engine noise. These both occur in the direction of the planes travel (i.e. most sound travels in the direction of approach/descent) rather than travelling sideways from the runway.

There are 2 main methods that are used to keep track of airport noise, both with their advantages/disadvantages. The first method is to measure the Sound Level created by one aeroplane taking off, then to add levels accordingly depending on the number of flights that will occur. An average Sound level for the whole period of time can then be calculated, depending on the number of flights. This is often used in prediction of airport noise, as it is quick and easy. However it is not always accurate because each plane makes a slightly different level.

The second method is to measure what is known as an LAeq. This is where a fixed microphone is set up so the sound level can be measured over a prolonged period of time for many departures/arrivals. Because the sound level varies with time (i.e. increasing when planes take off) it is then averaged. From this, the constant equivalent level (hence the eq part of LAeq) that would have to be made to create the same overall noise level can be calculated. At Norwich International Airport (NWI), my local airport, 3 fixed microphones are placed on the airport perimeter, so that the noise levels are constantly monitored and do not exceed regulations.

If an LAeq measured over a time period of 1hour is taken and reads 75dB,* this does not mean that it is constantly below 75dB. This could however mean that it is 90dB for 2 minutes and 45dB for the rest of the hour, which would give an average equivalent noise level of 75dB for the hour. The graph below shows this example:

LAeq

One problem with airport noise is that some planes take off at night, which is a time where the noise is generally considered more of a nuisance. For this reason another measurement can be made using the LAeq, which takes this into account. The night day and evening measurements are done separately and then the evening one has 5dB added to it and the night one has 10dB added to it, because they are perceived as louder at night. These new values are then averaged again over the whole time period of a 24-hour day.

With these measurements, noise maps can be made, where ‘contour’ lines mark different dB levels. In 2005 NWI commissioned an external body to produce a noise map for its surrounding areas. [2] The values used for these maps depend on a day-to-day basis, but are an average over a period of time. Factors such as wind direction/speed and flight path can affect these, but measures are taken by airports to take this into account (e.g. flight paths over lesser populated areas).

The map below shows the LAeq noise contours of the airport over a 16-hour period [3]:

NWI LAeq Noise Contours

This next map is similar to the one above, but this time is based on measurements over a 24-hour period with the evening and night penalties added, so it is more representative of the general perception of the level [3]:

NWI Lden Noise Contours

You will see, as stated before, that the noise mainly travels in the direction of flight and the levels are greater towards the ends of the runways; this is because of the noise created by the engines and the friction on the wings in takeoff/landing. In order to get an idea of comparison, 65-70dB is roughly the level of a busy restaurant or a busy A-road, which is 20m away. The background level for a housing area would generally be expected to be up to around 50dB without noise from the airport.

A general rule of thumb for sound is that it will decrease at -6dB for every distance doubling (e.g. if you are at 5m and it is 85dB then at 10m it will be 79dB). This rule is true if there are no reflections, so in reality it is often not quite this much. This rule can be seen on the maps where the distance between the contours increases the further away from the runway.

For many, the airport can be seen as a nuisance and should be made to reduce noise; in ways this has happened as planes have become quieter, but despite this there has been an increase in the number of planes. For local authorities, noise complaints need to be assessed as whether the source (i.e. the airport) has many pros (e.g. local jobs). Also, the type of nuisance needs to be addressed as to whether it is interfering with the individual’s enjoyment of their land or whether it has affected many.

What are your views on your local airport; do you feel like the disadvantages of the noise outweigh the benefits of the airport?

Notes

*dB stands for decibels which is the units we use to measure sound.

Acknowledgements

Thank you to Norwich International Airport for kindly supplying me with the noise survey data in order to write this blog.

References

[1] What causes aircraft noise? – http://www.gatwickairport.com/business-community/aircraft-noise/air-traffic-noise-explained/causes-of-aircraft-noise/

[2] Norwich Airport Noise Policy – http://www.norwichairport.co.uk/content.asp?pid=52

[3] Department for Environment, Food and Rural Affairs Airport Noise Action Planning for NWI (Noise Contour Mapping) – http://www.norwichairport.co.uk/downloads/airport_datapack_2013_norwich_airport_egsh.pdf

Wingardium Leviosa

Here’s a short video you hopefully may find interesting, it’s a more extreme visualisation of standing waves that I wrote about doing in some lab sessions a few weeks ago. As shown in the video, the way it works is by using 4 lines of loudspeakers all facing each other playing the same simple tone. As they interact with each other, they create a standing wave, meaning parts of the waves interact constructively and others destructively. This basically means that some parts of the wave move a lot and some don’t move at all.

As the small objects are dropped inside (they must be small enough to fit into the wave and light enough for the wave to be able to oppose gravity) the part of the wave that is constantly moving most traps them. As the air particles move up and down (about 1,500 times a second) they manage to oppose gravity and hold the object, so that it is levitating.

You may notice that that the polystyrene pieces are spread out and not all constantly spaced; this is because at the nodes of the standing wave (the bits with destructive interference where the sound wave does not move up and down) have no strength against gravity. When the pattern moves around in air, that is because the pattern of the loudspeakers change, causing the standing wave to move.

You may now be wondering if we had big enough speakers, could we levitate a person… Well theoretically I would say yes, I can’t see why not, however unfortunately in practice I think by the time you’d reached a loud enough Sound Pressure Level, your ears would have been exploded and you would probably be half way to mashed potato! As nice a thought as it is, if you want to fly it’s probably best to just go to a wind tunnel instead!

Selective What?

So we’ve all heard of selective hearing, it’s the process whereby we listen in to things depending on whether we want to or not. I often remember being told by my mum that I only listened to things I wanted to hear and never when I was asked to do jobs around the house… (surprise surprise!) But is this actually a real thing?

Well in short yes, it is and is also known by some as the ‘Cocktail Party Effect‘ which is where we selectively pick out and listen in on the conversations we want to hear amongst a general background noise of many conversations. When I read about this, it reminded me of a story we were told at school in assembly once, which went something like this…

There were once two guys walking down the road together, trying to hear themselves and have a conversation over the noise of the cars going past. Then one of them stopped, turned to the other and said “Can you hear that grasshopper?”. “Of course I can’t! That’s crazy, you can’t hear that” replied the second man, “It’s far too noisy to hear something so quiet!” However, the first man knew he was right and replied “No, I did hear it and I bet you could hear it as well if you wanted, you just don’t want to”.

At this he pulled a small coin out of his pocket and dropped it onto the pavement below; as he did so several people around them stopped and looked down as if to check if it was them who had dropped their money. “See” said the first man, “They can all hear the coin, just like I can hear the grasshopper, the only difference is what we’re listening out for”.

This, although a very basic idea of what I’m talking about, does represent the theory quite well. The effect is something that is of particular interest to neuroscientists and also in the realm of psychoacoustics. It is linked to the brain’s capability to focus on particular information that it is interested in or can relate to (i.e. when you hear someone mention the name of the town you’re from). Studies have now been done in audio signal processing to create a program that can separate out different signals from a general noise (e.g. separating a piece of music from a signal that has talking over the top of it). This helps us to work  out how the brain works and also may have the potential to be used for security and data analysis in the further.

Coffee Shop

Because of this effect, people often find it hard to work when conversations are going on around them, which could be part of the reason why coffee shop noise is considered by many to be good for creativity. Background noise for many, is useful as it is so common now in our everyday life that a lack of it can often seem unnatural. With careful acoustic design through sound diffusion and by using large volume spaces (i.e. high ceilings) sound in these places can easily be dispersed giving a general background noise but where individual conversations are hard to pick out. Nowadays, as open plan offices are becoming more and more common, this knowledge is becoming more and more vital in order to create a creative and calm working atmosphere.

Where do you find it best to work, do you like the idea of open plan offices/coffee shops or do you work best with complete silence? Have a listen to Coffitivty and see if you find it easier to work to!