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Third Section

After this first experiment, it will be easier to think about 1D sound. Now check the sound waveforms below (recorded on a Sunday morning, in Brochstein Pavilion at Rice University, length 10 seconds).

Picture of 2 waveforms
Recorded sound wave

It is easy to see that these two sound waves are exactly the opposite of each other by carefully checking some specific spikes.

Now if you play the first OR the second piece of data, you would hear some gibberish of background music, people talking, and spoon and plates clinking on a typical Sunday morning. That being said, you should realize that your ears are not as sensitive as your eyes in some way; they cannot detect phase difference monotonically! The second piece of signal, though being the negative version of the first piece, sounds identical to humans. Now if you play the two signals at the same time, you would get absolutely nothing, like what happened to your eyes before: silence!

To experiment with this more convincingly, try it out with your own speaker set. First make sure you have two audio channels. Open MATLAB, and type in:

recObj = audiorecorder; recordblocking(recObj,5); % try to make some noisenoiseData = getaudiodata(recObj); antiNoiseData = - noiseData; % invert!double_channel = [noiseData,noiseData];sound(double_channel,8000);

Now you can hear the piece you’ve just recorded. To examine our cancelling theory, type in this:

double_channel = [noiseData,-noiseData];sound(double_channel,8000);

Note that if you are listening to this by your earphones, you would still hear the sound, but notice a subtle difference: your brain automatically calculates the position of the sound source by combining the two sound signals from your left and right ears. If you were playing them with your loudspeakers, however, you would very likely get nothing. Try to place the diaphragms of the speakers right towards each other very closely, and the sound waves will cancel out.

This is the basic mechanism of ANC. It is widely applied in most noise cancelling systems, such as headphones (to enable one to hear music better), on cars (to reduce the noise of the engine and tires), and in cell phones (to reduce background noise in conversations). While this sounds useful and easy, the real world is not always so straightforward. Detecting the phase of a sound wave is hard. Usually a Phase Locked Loop is used to determine the phase of input noise and to generate a 180-degree phase-shifted "anti-noise" as described above. This algorithm requires a steady noise source (resembling a sinusoidal wave) and is therefore unable to filter out more complex noise such as the background noise of a crowd. Even if the input were a pure sine tone, the reduction would be no more than 10dB. The greatest restriction is the geometry of environment settings. The location of the microphone used to pick up the sound signal and the location of output loudspeakers determine the sound quality of the noise cancellation. As you may have noticed, when you put your two speakers right in front of each other with no space between them, the cancellation works best; as the speakers are moved away from each other, you can hear louder and louder noise. The sound waves tend to reflect off of the wall, or any other surface, sometimes reinforcing each other and resulting in unpredictable behavior. Another limitation is that high frequency noise has really short wavelengths, which makes the distance between ears non-negligible, so that the cancellation signal for the left ear can actually reinforce noise for right ear.

Questions & Answers

what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Elec 301 projects fall 2014. OpenStax CNX. Jan 09, 2015 Download for free at http://legacy.cnx.org/content/col11734/1.2
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