Exploring Sound Within Structure Acoustics Around the Dartmouth Campus

1. Exploring Sound Within StructureAcoustics Around the Dartmouth Campus JB Cholnoky & Cyrus Tingley Math 5 • Professor Barnett Spring 2007

2. Project Purpose & Goals • 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.

3. Project Sites, Methods, & Materials • We chose three different venues as our subject structures: Leverone Fieldhouse, the steam tunnels beneath the Green, and an empty fuel storage room attached to the Dartmouth power plant. • The basic procedure in each location included the production of a loud sound signal, which in conjunction with any resulting reverb, resonance, or echoes was recorded on a laptop using Audacity.

4. Project Sites, Methods, & Materials • 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.

5. Steam Tunnels

6. Steam Tunnels

7. Steam Tunnels • 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.

8. Steam Tunnels • 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.

9. Power Plant Fuel Room

10. Power Plant Fuel Room • We stumbled on this site basically by accident while touring the plant with FO&M - a large, relatively empty concrete room once used to store fuel. • Dimensions are as seen on the diagram. • This room had incredible acoustic qualities - though not exactly what we expected.

11. Power Plant Fuel Room • Surprisingly, this room did not produce the echoes we expected - considering the sound our voices made when we walked in, the room having a booming, amplifying effect on them. • Most notable about this room was its tremendously long reverb time. • Listen to several of our Audacity recordings from this room - for both higher- and lower-volume noises, the sounds hangs around for quite some time - without echoing though!

12. Power Plant Fuel Room • Peak frequency of signal = 464 Hz • Calculating reverb time: T60 = 0.161 (V/S) • V = (30.48m)(6.096m2) = 1133m3 • S = (AC of concrete)(SA) = (0.04)(780.4m2) = 31.2 • [Absorption Coefficient for Rough-Finish Concrete at 500Hz found online] • T60 = (0.161)(1133/31.2) = 5.85 sec • By zooming in our recording in Audacity, we found that the sound appeared to have a timespan of 12.613 sec -> 18.49 sec, or 5.877 sec. • At this site, we were thus overwhelmingly more successful in accurately measuring and calculating the room’s reverb time.

13. Leverone Field House

14. Leverone Field House • Leverone was the first site we recorded at and while we initially believed there to be nothing special at work, our results from Leverone were in fact the most remarkable. • Dimensions/layout of the building can be seen on the following diagram. • What we discovered at one location in the building was a perfectly clear flutter echo.

15. Leverone Field House • 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.

16. Leverone Field House • Check out our Audacity recording! It was only by slowing the sound down 70-90% that we were able to both see and hear the echo. • A “flutter echo”, as we learned in class, is the sound of repeated clicks gradually fading away - continuous ‘bouncing’ of sound within a structure. • The time before you hear the echo: T = 2x/c • According to Audacity, the initial sound arrived at 3.99 seconds, and zooming in showed echoes arriving at 4.096 sec, 4.198 sec, and so on - thus the time delay was approximately .104 seconds or so. • We suspected that the echo was produced by the sound we made at Site #3, bouncing off the ceiling. • At that point, the height of Leverone is roughly 24.5 m. • Plugging .104 sec into the equation referenced above: • .104 = 2x/340 m/s • X = 17.68 m.

17. Leverone Field House • 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.