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!

Tube Work

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Last week I was doing a lab for an assignment on my course that was quite interesting, so I thought I’d share it with you. In architectural acoustics, we often require different a Reverberation Time (RT) depending on the scenario (e.g. a concert hall needs a longer RT compared to a classroom) and one way of changing this is though changing the amount of absorption in the room.

You will probably be aware that different materials absorb in different amounts; this is a property called absorption coefficient, with a value between 0 and 1, where 0 is not absorbing at all (e.g. a hard concrete reflecting surface) and 1 is fully absorbing (e.g. an open window). Once absorption coefficient is known then the amount of the material needed to reduce the RT by a specified amount can be calculated; this experiment is to calculate the absorption coefficient (and other properties) of absorbing materials.

One alternative way of measuring absorption coefficient of a material is by measuring the RT in a reverberant chamber, then adding a known amount of the absorbing material and measuring this again and calculating absorption coefficient by the difference this makes. The problem with this method is that it requires expensive equipment and takes a large amount of an absorbing material to notice any significant difference.

An impedance tube is an easier and cheaper alternative method, hence why this method has been used. The idea of this experiment is to have a speaker at one end of a closed tube and some of the sample at the other end, then using a movable microphone in the tube, the sound pressure at several different points along the tube is measured. Using the pressure values measured and distances between them, we can then calculate absorption coefficients (at different frequencies) using different equations.

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