Goal:
Our final program takes an acoustic wave signal, strips out all audible frequencies using a Fourier transform, then shifts a range of the ultrasound frequencies into the audible range in order to qualitatively investigate the properties of the audio signal. Our initial goal was to record our own ultrasound frequency signals for analysis, but we were not able to procure a microphone sensitive enough to record ultrasound. Instead we choose an online video claiming to repel rodents by emitting an audio wave with ultrasound frequencies that effect rats (found here: https://www.youtube.com/watch?v=l5x1Bb9UTT0). On first inspection, the signal seemed to only play what resembled white noise, but we needed an accurate representation of the frequencies present in the audio file before making any more assessments. We hypothesized that the video did not contain ultrasound frequencies capable of repelling rodents.
Investigation:
Using the frequency analysis methods we implemented earlier in our project we found that the video actually contained frequencies above 20 kHz. But we were still skeptical because these higher frequencies were about 80% less powerful than the frequencies present in the audible range. This may have been a tactic used to trick the user in believing that high frequency sounds were playing. We wanted to examine what the high frequencies sounded like so we took a 5 second clip from the audio file to scrutinize.
(Please find the 5 second clip here: https://drive.google.com/file/d/0B4quZV7NVNf7YU16blYwcUxXYXc/view?usp=sharing)
We started by removing any audible frequencies from the clip then shifted the ultrasound range into the audible. The resulting sound was again white noise, but at a lower volume.
(No audible frequencies: https://drive.google.com/file/d/0B4quZV7NVNf7clBFTlJIT1I5bzQ/view?usp=sharing , Shifted signal: https://drive.google.com/file/d/0B4quZV7NVNf7amhqVkZ6NjBtUXc/view?usp=sharing)
A qualitative analysis of this new wave does not lead us to believe this video would repel anything. White noise is not an abrasive sound unless played at extremely high volumes, but anything played at a high enough volume can be found abrasive. White noise is generally considered to be the exact opposite and there are white noise generators that are sold on the premise that they are calming and relaxing, and are used to help people focus or sleep. The video we analyzed is not exactly what we would expect in a rat repellent, it seems like this video aims to be more of an aid that would assist in rat mediation.
(code here: https://drive.google.com/file/d/0B4quZV7NVNf7cldUbG5WQUFWTXM/view?usp=sharing)
Here is the clip from the original 5 minute video. Notice the amplitude of this signal.
Here are the various operations we performed on the Fourier transform of the clip.
No audible frequencies are present in this new signal. Please note the difference in amplitude in this signal from the original signal.
The new, shifted audio signal.
We then decided to make a code to demonstrate what a real rodent repellent signal might sounds like. We start by assuming that sounds that are irritating to humans are also irritating to rodents. A sound that is often considered to be annoying to us is a pair of high frequency sinusoidal waves that are similar in frequency but not identical. This is a very dissonant sound, and even at low volumes can grab the negative attention of someone around you (which can happen if you accidentally sound them in a public space while coding a rat repellent). So we also built a code that generates what we consider to be an abrasive sound, but is well above the audible range so we cannot hear it. We decided on the frequencies of a distressed rat baby (http://www.ratbehavior.org/rathearing.htm). Unfortunately, Matlab’s audiowrite function was not able to save this as a .mp3 file, so please download the code play and use the sound command found to play it on your machine.
(code here: https://drive.google.com/file/d/0B4quZV7NVNf7OW1kQld5QW1KVkk/view?usp=sharing)
Future Work:
If we were to continue to research methods and applications frequency shifting we would write a code that transposes frequencies. Transposition maintains the harmonic ratios of the frequencies present in the wavefunction by multiplying them by a constant instead of linearly shifting them. Changing the sampling frequency at playback also maintain these ratios but in doing so, distorts the audio signal by stretching or compressing the wavefunction. Out next code would have been to transpose frequencies without changing the length of audio sample. We would also like to to detect our own ultrasound frequencies with an ultrasound microphone for qualitative analysis.
