On the 01/25/2016, we began the second week of Biosensor project. The subject for this week was “Forces”, and it includes mechanic vibrations, sound vibrations, electrostatic forces, magnetic forces etc...
What force did we choose ?
We decided to work on sound wave. Before explaining what we did, it is important to figure out what a sound wave is to better understand our project.
An acoustic wave is a mechanic disturbance (compression then expansion) which is propagate into a media without material transport. The period is the time that the waves take to complete one cycle (it means to go back in the same position). The frequency is the number of times, that a phenomenon (here, the sound wave) occurs per time unit. The Humans can hear sound with frequency between: 200 - 20,000 Hz.
Illustration of acoustic wave on a microphone
An interesting video to understand what a sound is :
What was the project ?
We choose to work on rusty red cockroaches as biological sensor and an iPhone with Dicibel Ultra app as electric sensor. So our question is “How do both cockroaches & an iPhone sensor react to sound?” The aim is to determine which is the best sound sensor to detect high-frequency sounds.
At the beginning of the project, we wanted to identify both the sound frequencies which scares cockroaches and the one that attracts them. But why did we choose cockroaches (it’s so dirty!) ? This is because they are sensitive to vibrations! They have many receptors on their legs and their antennas which are very responsive to vibrations. Indeed, they can sense small vibration in the air. That’s why it is so difficult to catch them. Moreover, it is original to study this hated organism.
To emit our high-frequency sounds, we used “Frequency Sounds generator”, an app that can produce sounds from 20 Hz to 20,000 Hz. To receive these sounds, we use Decibel Ultra, another smartphone application. Why this paticular application? Because it is accessible to a large number of person (it is free on appStrore & Android) and it measures sound level and its frequency with high precision. The first idea was to test sound with a frequency higher than 20 000Hz named ultra sound but our phone application cannot produce there frequency.
How did we the experiment ?
Figure 1 : Visual protocol
To measure if cockroaches are sentitive to high-frequency sound, we place only one cockroach into a transparent plastic tube because they cannot climb on wall, it allow us to see the movement of the cockroach as well. Then we placed the sound transmitter in front of the tube and the camera facing the recipe to film all the tube because we want to analise their film to calculate cockroach’s speed. Concerning the electronic sensor, we just replace the tube by the iPhone on the cockroach position marks and we note the value given by the application.
What are the results ?
Figure 2. Sensor responses to varying frequency
The values given by the iPhone receptor ( “Decibel Ultra App”) were quite precise, except for the value given at a 20 000Hz frequency. Indeed, at 20 000Hz, the receptor doesn’t detect the sound wave accurately and gives a value clearly underrated to reality (accuraty represents if a measure correponds to the reality).
For our cockroaches, our results were difficult to interpret. At all given frequencies, our cockroaches didn’t have clear responses. Sometimes they didn’t move at all, others they were repulsed, then attracted, then repulsed again before stopping, etc… As such, we did an average of their speeds at each frequency to create this graph.
What is the conclusion ?
Thanks to these results, we can deduce that cockroaches are less efficient than our IPhone to detect high-frequency sounds. But, it was really hard to detect and understand their reaction in front of sound waves. Their behavior was really different from one individual to another. That said, we can see on the graph that our application was not really good to detect sound with frequency greater than 20 000 Hz.
Why our experience didn’t work as expected ?
Our results, difficult to interpret may they be, are certainly due to some of these causes. First, our cockroaches were placed near crickets in the shop we bought them from which may explain their lack of particular observable behavior when in contact with sound waves. During the experiment, we could hear noise coming from the street, so our biological sensor wasn’t solely in contact with our sound frequencies. Also, with time, they were getting used to receiving high-frequency sounds, so they reacted less and less, though we did try to minimize this bias by exposing them in a random order to our souds. Additionally, we stressed them by moving them from one environment to another. We also discovered that their behavior was dependant on light conditions by doing our protocol during night time and during day time which caused clearly different behavior in our cockroaches.
Want to know more about the subject ?
An scientific article by S. Shaw on the "Detection of airborne sound by a cockroach 'vibration detector': A possible missing link in insect auditory evolution" Journal of Experimental Biology. (en)
Video about "A saptial robot built on the basis of cockroach sensors !" by the NASA
By Fairouz Gzara, Juliette Delahaye, Nathan Touboul and Nicolas Silva - @The_SpiderSpeed
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