Exploring Sound Within Structure Acoustics Around the Dartmouth Campus. JB Cholnoky & Cyrus Tingley Math 5 • Professor Barnett Spring 2007. Project Purpose & Goals.
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Given the often-overlooked presence of sound and acoustics at work all around us, the purpose of this project was to apply the theories and methods learned in Math 5 to our everyday surroundings here on the Dartmouth campus.
We hoped to discover interesting and relevant acoustic anomalies in campus buildings, explored with sound recording and analysis.
To produce sound, we struck together two pong paddles (made of laminated plywood roughly .75 cm thick) - this was sufficiently loud and quite easily repeatable. Each time, the edge of one paddle was hit sharply against the flat center of the other.
A highly sensitive Logitech digital microphone was used to record sound directly to Audacity.
Courtesy of FO&M, we explored a 300-foot straightaway in the tunnels located between the SE and NE corners of the Green.
Tunnel dimensions were as seen on the diagram - however, the presence of two large stream pipes running along one wall is not appropriately detailed - we are quite sure this effected the results we got.
Nevertheless, we did discover some interesting acoustics; surprising given our expectation of one, there was no echo apparent in the tunnels - instead we found an example of near-perfect reverb.
Because the tunnel turned at 90-degree angles at either end of the 300-foot subject section, it behaved much like a closed room and sound resonated accordingly.
Calculating reverb time (without accounting for pipes in the way):
T60 = 0.161 (V/S); V = 544.4m3
Peak frequency = 557 Hz [from Audacity]
S = (AC of concrete)(SA) = (.02)(904.5m2) = 18.09
Absorption Coefficients for Concrete: Click Here
T60 = (0.161)(544.4/18.09) = 4.85 seconds
By zooming in on our Audacity recording, we found that the sound began at 4.85 sec, ending at 6.26 sec (see Audacity), implying a reverb time of only 1.41 seconds.
However, while this does not match the calculated value, the difference can be attributed to perhaps human error and/or lack of microphone sensitivity; but most obviously, the calculated T60 value is for an empty tunnel hallway and thus would be longer than the reverb time in reality, which is effectively much shorter because of the pipe’s interference in space, insulating covers, etc.
We recorded at 3 locations - #2 and #3 provided the best results. The flutter echo was heard best when sound was produced at #3 and recorded at #2 - we believe the echo was in fact sound bouncing off the roof of the building - our experimentation with recording/production locations supported this.
We believe the large amount of nets/flags/screens/material hanging from the ceiling and at the far end of the building stifled any echoes we may have heard traveling end-to-end in Leverone.
Thus a calculated reflective distance of 17.68 m does not correspond to any known/relevant distance within Leverone.
We suspect that the echo was in some manner bouncing off the roof above our heads at Location #3 - 24.5 m was a measurement estimated by JB as FO&M was unable to furnish us with complete architectural plans.
In conclusion, it would be nice to definitively know the path and distances associated with the flutter echo we heard - though any number of factors could’ve interered with the connection we sought to make.