Date: 07.12.2007
Duration of activity: 6 hours
List of participants: Alessandro, Daniela, Samuele
Goal: Determine which frequencies will be produced by our sound sources, in order to maximize the sound following capability of our robots.
In this test session we wanted to do an extensive test of NXT sound sensor response on a wide range of sound frequencies. We hoped to discover some properties of some range of frequencies that can be useful for our project.
We needed at least to find some frequencies which yield sound sensor values proportional to the distance from the sound source, such that the robot can have some indication about the proximity with the sound source, using only sound sensor readings.
In this test we measured readings of sound sensor values, through pure frequencies sounds from 100 Hz to 10 kHz and were repeated putting the sensor at three different distances from the sound source, at 50, 100 and 200 cm.
As sound source we used speakers of a laptop, and we generated wanted frequencies using a small tone generator software - (NCH Tone-Waveform Generator).
The tests were performed by using a plastic cup on the sensor. As said before, plastic cups will help improving the directionality of the sound sensor, and since we will only use sound sensors with plastic cups, we performed tests under the same condition in which we will use sensors.
Values we used are not directly raw values, but they are the mean value of all samples taken during 1 second; we had to use mean values because of the very noisy lego sensor.
In some cases we also repeated tests, when we got "suspicious" readings, so oscillation of values must not be attributed to random sensor noise, it is always an almost reliable mean of values for the specified frequency.
Those are test results:
| frequency: Hz | value at 50cm | value at 100cm | value at 200cm |
|---|---|---|---|
| 100 | 2 | 1 | 1 |
| 200 | 4 | 1 | 2 |
| 400 | 9 | 4 | 9 |
| 450 | 16 | 14 | 8 |
| 500 | 30 | 18 | 11 |
| 550 | 41 | 23 | 14 |
| 600 | 53 | 30 | 38 |
| 650 | 52 | 31 | 38 |
| 700 | 50 | 15 | 13 |
| 750 | 85 | 14 | 37 |
| 800 | 81 | 43 | 11 |
| 850 | 91 | 74 | 22 |
| 900 | 90 | 67 | 45 |
| 950 | 93 | 77 | 93 |
| 1000 | 90 | 63 | 29 |
| 1500 | 93 | 81 | 37 |
| 2000 | 81 | 62 | 43 |
| 2500 | 90 | 76 | 57 |
| 3000 | 75 | 41 | 39 |
| 3500 | 93 | 47 | 6 |
| 4000 | 75 | 42 | 10 |
| 4500 | 93 | 93 | 60 |
| 5000 | 72 | 48 | 20 |
| 5500 | 93 | 65 | 39 |
| 6000 | 93 | 72 | 47 |
| 6500 | 80 | 50 | 29 |
| 7000 | 81 | 34 | 10 |
| 7062 | 67 | 32 | 3 |
| 7125 | 52 | 22 | 6 |
| 7188 | 36 | 13 | 4 |
| 7250 | 33 | 12 | 4 |
| 7375 | 13 | 5 | 1 |
| 7500 | 8 | 5 | 3 |
| 7750 | 7 | 3 | 3 |
| 8000 | 4 | 13 | 1 |
| 9000 | 4 | 4 | 0 |
| 10000 | 4 | 1 | 0 |
As it can be seen from this table, we sampled with high density where small variations in frequency gave big changes in readings, while skipping where responses were uniform.
These are two graphs obtained from the values above:
2D version
3D version
From the graphs above some sounds sensor features come out:
- The three shapes in the graph are similar in some way, and there is a decay of readings while the sound source gets far, which is an expected result, and a required condition for our robot to work. This means that near sources will give values higher than distant sources.
- It is better if we use frequencies between 1kHz and 7kHz, because sensor responses are more uniform and proportional to sound source distance.
- We can notice, by looking at the red line (which corresponds to the test with the closer sound source, 50cm) that the sensor response is not precise, when pushed to high levels, which means that we must consider every sampling above 70 as "loud sound" indistinctly, i.e. using threshold around 70-80 for loud sounds.
- It also comes out that there are some frequencies which are still perceived loud by the sound sensor even if they're distant (2.5kHz, 4.5kHz, 6kHz), while others fall down too fast (3.5kHz, 5kHz).
After some speculation during the tests, we figured out that the problem of that oscillation was given by the environment under which we performed tests: our small office. The sensor readings were probably affected by a phenomenon which in telecommunications field is called multipath fading.
The same sound waves that come out from the sound source arrive more than once to the sensor: first time the waves with a straight path, and immediately afterwards with a different path, after having bounced on the walls. These signals are identical, but with a phase shift, which in some cases can cause them to cancel out partly or completely.
Conclusion
We got some good results, like hints about frequencies to use for sound sources.
We will do further investigation on sensing different frequencies, and avoiding multipath fading, to clarify whether the frequency discerning capability is implementable or not.
References
http://www.mediacollege.com/audio/microphones/directional-characteristics.html
http://en.wikipedia.org/wiki/Multipath_fading
No comments:
Post a Comment