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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