Pedestrian tunnel below Hayden Rd, Scottsdale, Arizona
The tunnel is north of Indian School Rd & south of Camelback Rd. It is connected to the Indian Bend Wash which has a series of parks and a golf course along its length. The tunnel is about 100 feet long. Its fundamental frequency should be ~ 5.6 Hz based on an open-open pipe model. Enclosures of this type have a high acoustical model density, where the modal density increases with frequency.
I stopped underneath the tunnel and clapped my hands with my hands in a cupping shape. I recorded the acoustic response using my Android Smartphone ASR app. There was some background noise due to vehicle traffic, etc. Sound file.
I identified a few prominent modal frequencies from a single clap response as follows using Matlab scripts.
The overall reverberation time can be estimated using the sound pressure decay from the following envelope.
The overall acoustic response envelope is nonlinear due in part to background noise. The drop rate from 40 to 41 seconds is 8.3 dB/sec.
The 366.5 Hz mode has an individual damping value of 0.03% which is very low, as determined by a damped sine curve-fit.
The 511.8 Hz mode has an individual damping value of 0.06%.
Note that there are several modes near 60 Hz and that these modes dominate the overall sound response from 41 to 43 seconds. Here is a band-pass filtered time history focusing on these modes. Refiltering is used for phase correction.
– Tom Irvine
Thank you!
I recalled your boat cruise recording during the presentation at Wroclaw Conference. I said: Tom Irvin likes the random processes as much as I do. But he treats them in a different way…
Thanks for keeping us “grounded” to the real everyday world
Tom — where did the 5.6Hz mode go?
Good question. My hand clap may not have appreciably excited the 5.6 Hz mode. Also, the frequency response of the Smartphone’s microphone may have been inadequate to capture it anyway. Note that sound energy below 20 Hz is called infrasound and can have adverse affects on the human body.