Pyracoustics

After seeing this video last week, it is definitely on my list of things to do this year! I’m currently unsure on my exact design, or whether I will create a Ruben’s tube instead, but nonetheless it is still a really cool idea (and probably the most interesting visualisation of acoustics)!

One of my current ideas is to create a few smaller Ruben’s tubes of varying lengths, each driven by a driver playing the same source. Because of their different lengths, they will resonate at different frequencies, each causing a different pattern of flames as the same source is played. If these were placed together (maybe in a stacked pyramid style) as music is played they would make a really cool music visualisation.

What do you think of the pyro board, have you seen anything like it before? If so, let me know or also if you have any other ideas for things I could try and build.

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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/

Stationary Falling

As it is currently half term, I have had several family members vising this week, meaning I have been out and about in Salford and Manchester, often visiting places I haven’t been before. One of these trips out was to Airkix, an indoor skydiving tunnel by the Trafford Centre, where I was lucky enough to be treated to a flight with my uncle and cousin.

It was such a surreal experience, falling in a vertical tunnel, yet not moving anywhere, whilst spectators sit around drinking tea and watching you. Despite my clear lack of skill, it is something I would definitely recommend if you ever get the chance.

Obviously, when doing activities like this there are many risks involved, hence why we had to fill in forms saying we were fit to partake. Apart from the obvious risks (dislocated shoulders, falling, or crashing into walls) there are other aspects that need to be taken into consideration; the main one of these, which you may have already guessed, is your hearing.

I don’t know if you’ve ever been on the phone on a stormy day, but it is often really hard work when all you can hear is the wind, making the conversation very hard. Now this is one thing, but imagine winds of up to 180mph (the speed that this tunnel can reach) and imagine not only how hard it would be to hear, but also the effect from pressure it would have on your ears. Thankfully, before anyone goes into the tunnel, they are given earplugs in order to protect hearing, but do these make any difference?

First of all I calculated the sound pressure of 100mph winds (about what it took to lift me) as around 1,200Pa, which when converted straight into decibels, is around 150dB. This initially does look extremely high (the human threshold of pain is around 120dB) even with the earplugs, which have around 30-40dB attenuation; however a few factors need to be taken into account first to understand how this is safe. This pressure is obviously needed to lift a person, but may not necessarily directly relate to sound pressure.

There may be a very high pressure at a stationary point in the tunnel, but the ears are covered by the helmet which are also facing outwards, not down, so most of the air goes rushing past. This leads to probably the main principle of acoustics, which is that you need something to vibrate in order to make sound. In the tunnel, if wind is rushing past at around 100mph, which may make a lot of noise due to the fans, the vibrating air particles are flying straight past you so fast that none of them are around enough to cause high Sound pressures. There is also quite a distance from fans to receiver and the way these fans work is shown on the Airkix website.

The duration of the flights are only 1 minute long (around 15 seconds longer than the free-fall when parachuting) which shouldn’t increase the Leq (the equivalent noise level if played constantly over a given time) excessively based on the whole duration of the visit to the centre. This is often why parachutists may wear earplugs for the noisy and longer plane journey up, but not for the flight down.

One final major factor could be the psychoacoustics of the noise, being the sound of wind, which is familiar meaning the flier feels less threatened and can be perceived as less. This could be linked to the frequency distribution of wind, which may be such that it does not affect the human ear so much. There are also likely psychoacoustic effects of giving out earplugs, whether they make much difference or not.

Image

What do you think? Have you ever been skydiving or experienced high winds and noticed any effects on your hearing?