Last summer(2018) I deployed a simple audio recording device in our yard to sample the sounds of that ecosystem for thirty seconds every thirty minutes. Over the monitoring period from early in May through the end of October I accumulated around 8800 audio files. These were automatically analyzed and uploaded to a page on this website as they were recorded.
The images below are of a kind of pseudo-spectrogram, in the sense that it is irregularly sampled, built up from the power spectral density calculations for each thirty second recording for an entire month. I like these graphics because they show the general rhythms of the ecosystem. (click on any graphic for a larger view)
In May and June one can see the diurnal rise and fall of bird song. In early May, perhaps the evening calls of frogs and toads down at the pond. Then in mid-July the beginning of the most striking feature of these records, the appearance of the songs of the summer insects and the diurnal frequency changes with species and air temperature. In the fall months, one can see the tapering away of the insect sounds punctuated by abrupt changes caused by foul weather.
Over the next several weeks I hope to do a series of posts with more details in each month. I will also be making some changes to the recording hardware and software with the goal of getting two recorders out this year and earlier in the spring.
Unknown Spectral Feature - Chinook Helicopter
Almost a year and a half ago, I put out a request for help in understanding a shallow bowl-shaped feature in the spectrogram of the sound of aircraft flying overhead. My older son had already suggested that it was a multi-path interference pattern and his suggestion was supported a couple of months later by commenter Alberto Armani.
Finally last week, I took time to sit down and think clearly about how that would work. As it turns out, the model is very simple and seems to explain the shallow bowl-shaped feature in the the spectrogram. I need to make a couple of sketches of the geometry before I can put up a page on this subject but I’ll try to get it done in the next week or so.
I have added a page describing the ground reflection interference model here.
Unknown Spectral Feature - Air Tractor Fixed Wing Aircraft
My search for Doppler shifted sounds has resulted in a number of recordings of aircraft. As I studied those recordings for the Doppler shift, I noticed that there was also another feature in the spectrograms that I didn’t understand. There is a shallow bowl shaped feature centered on the time of closest approach, ie when the aircraft was roughly overhead. The “bowl” seems to be formed by harmonic content in the signal. The frequency increases not only as the aircraft is approaching, as one would expect for a Doppler shift, but also as it moves away. The two spectrograms accompanying this post, for the Air Tractor fixed wing and the Chinook helicopter, show the feature pretty clearly.
I have thought about refraction due to the temperature lapse rate as a function of altitude but the equations don’t show any frequency dependence. I considered source directivity but I see it in two very different aircraft. If you listen carefully to an airplane or helicopter flying over you can hear a frequency component that is increasing as the craft flies away from you…it seems to be a real phenomenon. I am stumped!
Please comment if you know what causes this feature or if you have a suggestion as to what it might be.
Unknown Spectral Feature - Chinook Helicopter
Last week my wife and I were bicycling on the Holmes County Trail north of Millersburg. A little over a mile from the Depot, the Trail crosses a large open field that stretches for about a mile. About half way across, I looked up to see a twin rotor Chinook helicopter coming toward me just a few hundred feet above the ground (I later discovered that my data and little snippets of my recollection do not support the low altitude. See the pages linked below). I have wanted to get a recording of an aircraft to see what information I can pull from the sound. Here was my chance. I have a Canon Powershot G10 camera that has a fine audio recorder built into it. I stopped the bike and fumbled around to get the camera out of my jacket. I turned it on and started punching buttons to make it record. All the time the aircraft was bearing down on me. I didn’t have a lot of time to observe it but I did get a recording as the helicopter went right over head. It was a perfect pass.
I got about 22 seconds of sound. When I listen to the recording it sounds like a helicopter going over. It’s pretty good but I was disappointed when I looked at the spectrogram in Raven Lite. I had hoped for a distinctive Doppler signature brimming with information but it just wasn’t there. I let it rest for a day or two.
Then I listened again. As the aircraft passed overhead and receded away from me, the thumping sound of rotors intensified. When I zoomed in on that part of the time domain waveform I could easily see pairs of impulses. Ahhh! I think there is some information here! I calculated the interval between a couple of pairs and calculated the frequency. This must be the blade passing frequency. The twin impulses are from some phasing phenomenon of the twin rotors. I had been so focused on getting the recording that I didn’t count the number of blades on the rotor. My brother-in-law is a helicopter pilot and instructor in Florida so I sent an email to him with some information and questions.
A little research on the internet in the meantime showed that a Chinook has three blades per rotor and the rotor speed is 225 rpm. I divided the frequency I calculated by three and multiplied by 60 to get the speed…206 rpm. A little low but I assume that the speed changes like it does on a car for instance. Then I got a reply from my brother-in-law. He had forwarded my questions to a friend of his who flies Chinooks. He said the speed is 225 rpm. Evidently the rotor speed is kept pretty constant. Then I realized that the machine was moving away from me…it was Doppler shifted to a lower frequency.
As soon as I have a little time, I’ll add another page ( done ) in the sound analysis section describing this recording and the operating information I am able to pull from it. It should be interesting to see how much I can learn about this helicopter from a 22 second audio recording.
Doppler shifted sound of the turbocharger on a passing truck
Most of my working life I have been involved with analyzing sound and vibration problems using very expensive, high precision microphones, signal conditioning amplifiers, and spectrum analyzers. I never got bored with it or tired of looking at those spectra in twenty nine years. There was always the prospect of seeing something in them that I had never seen before; they never let me down.
Last summer I was visiting a colleague at Michigan State University in association with my present job at Ohio State University. During one of the presentations on environmental sound one of the spectrograms showed a beautiful s-shaped feature that immediately caught my attention. No one seemed to know what it was. He had the sound embedded in the Powerpoint so I asked him to play it. It was a diesel truck driving past on the highway. Ahh! It was the signature of a doppler shift. I have found out since that it is probably the doppler shifted blade passage tone from the turbocharger as the frequency of the tone is quite high.
I found a free program called Raven Lite from the Cornell University Ornithological Laboratory that allows anyone to record sounds and then display their waveform and spectrogram. This is not on par with the analysis equipment that I used in my former life but the anticipation of what new things I might see in the next spectrogram is just as strong.
I have added a couple of new pages devoted to examples of some projects you can do with this software. The doppler shift analysis is still in the works; it needs a diagram or two and has a lot more equations to enter before I can publish it. I also am still looking for a signature that is as beautiful as the one I saw in Michigan but I will publish with the one I have if I don’t find one before everything else is ready.