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Thread: the lowest frequency

  1. #1 the lowest frequency 
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    Do you know what is the lowest frequency of E.M. radiation a machine can detect? and the highest an oscillator can produce?


    Last edited by logic; June 4th, 2013 at 07:30 AM.
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    Quote Originally Posted by logic View Post
    Do you know what is the lowest frequency of E.M. radiation a machine can detect? ant highest it can produce?
    Zero, and if originated by a nuclear fission explosion, very high. jocular


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    For low frequencies it depends how big the machine's antenna is I think. The antenna needs to be a substantial fraction of the wavelength. If you're willing to build a machine with an antenna the size of a whole solar system, then I guess you'd be able to detect some really really low frequencies.

    Here's a wiki link on the 50hz range.

    Extremely low frequency - Wikipedia, the free encyclopedia

    If you get to very high frequencies in the Terahertz range, that's visible light. So if you count a flashlight as a "machine" then you can get pretty high.
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    X-ray machines go a lot higher than visible light.
    Its the way nature is!
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    Quote Originally Posted by kojax View Post
    For low frequencies it depends how big the machine's antenna is I think.
    If you get to very high frequencies in the Terahertz range, that's visible light. So if you count a flashlight as a "machine" then you can get pretty high.
    Thanks a lot Kojak, so it is practically impossible to pick up frequencies lower than 300 Hz.
    I thought modern technology could do without a traditional antenna.

    Is it possible to shorten the antenna measuring the frequency of a wave in a fraction of second?
    I mean can you pick up a wavelength of 100 km using a 100 m antenna and tuning the machine on 1/1000 sec.?
    Can it work that way=

    As to high frequencies, what is the practical obstacle/ limit for an oscillator?

    One more question: is it possible to generate an "inverse" oscillation that can cancel out an ordinary E.M oscillation?
    Last edited by logic; June 4th, 2013 at 07:59 AM.
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    Quote Originally Posted by AlexG View Post
    X-ray machines go a lot higher than visible light.
    ....or gamma rays higher still. But both start to invite the question of what one then counts as an "oscillator". I had taken the question to be a man-made one, rather than an electron in an atom or a sub-nuclear particle .
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    You were right, exchemist.
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    Quote Originally Posted by logic View Post
    Quote Originally Posted by kojax View Post
    For low frequencies it depends how big the machine's antenna is I think.
    If you get to very high frequencies in the Terahertz range, that's visible light. So if you count a flashlight as a "machine" then you can get pretty high.
    Thanks a lot Kojak, so it is practically impossible to pick up frequencies lower than 300 Hz.
    I thought modern technology could do without a traditional antenna.
    Most of the modern cell phones that don't have a visible antenna on them are also bragging about how they're in a "3G" or "4G" network. "4G" is the frequency they're broadcasting on. 4 Gigahertz. You can have a pretty short antenna at 4 Gigahertz.


    Is it possible to shorten the antenna measuring the frequency of a wave in a fraction of second?
    I mean can you pick up a wavelength of 100 km using a 100 m antenna and tuning the machine on 1/1000 sec.?
    Can it work that way=
    I'm not really a specialist at this, but I'm pretty sure that in order to tune an antenna to be sensitive to a radio signal the electrons in that antenna have to be oscillating in time with the signal. So the length of the antenna matters because they need to have enough space to change direction at the right times.

    I don't think you can fix that by changing how fast the machine oscillates exactly, but the antenna need not be entirely straight. In CB radios you can use a longer wire going to a slightly shorter antenna and count both the wire and antenna together to determine the whole length.

    Don't take my word for it on that, though. It might be possible, for all I know.



    As to high frequencies, what is the practical obstacle/ limit for an oscillator?
    It depends what kind of signal you want. For an analog radio, the main problem is buying transistors that can switch off and on fast enough to amplify the signal. I've looked at places like Fry's Electronics and online, and the fastest switching transistors I could find were about 6 gigahertz.

    Frequencies all the way up to visible light and beyond can be used for communication. However, I'm pretty sure that when they use visible light, the signal is encode digitally. For an ordinary radio signal like in your car, you can make the signal stronger and weaker to encode a sound/song/voice... etc. With light, I'm pretty sure there's no stronger and weaker, just on and off. It's a true digital signal. So the receiver has to be able to decipher digital signals.

    Not because it's impossible in principle to encode a light signal that way, but because of the limit on amplifying transistor tech. "On or Off?" is easier to measure than "how bright?".


    One more question: is it possible to generate an "inverse" oscillation that can cancel out an ordinary E.M oscillation?

    Yeah. You can cancel a radio signal. You can't cancel it like you can with sound, where you listen to the sound and then generate a canceling opposite sound in time to reach the listener's ear. Radio travels at the speed of light, so your canceling signal would never be able to catch up. With sound cancellation, the sound is only moving at the speed of sound, so your machine can listen, then calculate a good canceling sound and then send it to another speaker a short distance away in time to catch up.

    So the sender would need to be the one who is trying to cancel their own signal. Also if the canceling signal originates from a different location than the original signal, then it only cancels when it's being picked up from certain directions/locations.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by AlexG View Post
    X-ray machines go a lot higher than visible light.
    ....or gamma rays higher still. But both start to invite the question of what one then counts as an "oscillator". I had taken the question to be a man-made one, rather than an electron in an atom or a sub-nuclear particle .
    Yes, but we have man-made machines that produce x-rays.
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    Quote Originally Posted by logic View Post
    Do you know what is the lowest frequency of E.M. radiation a machine can detect? and the highest an oscillator can produce?
    jocular is wrong. If the frequency of the rate at which an EM field is chaning is zero then its a static EM field and it doesn't radiate. The requirement for radiaion to exist is that iit must have the field "detach" from the source, if you know what I mean. The field has to change in order for a change to propagate through space. There is only a lwer bound of zero but you can go as low as you wish. It's like asking How small can a real number be without being zero?
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    Quote Originally Posted by kojax View Post
    Quote Originally Posted by logic View Post
    Is it possible to shorten the antenna measuring the frequency of a wave in a fraction of second?
    I mean can you pick up a wavelength of 100 km using a 100 m antenna and tuning the machine on 1/1000 sec.?Can it work that way?
    .. I'm pretty sure that in order to tune an antenna to be sensitive to a radio signal the electrons in that antenna have to be oscillating in time with the signal. So the length of the antenna matters because they need to have enough space to change direction at the right times.

    I don't think you can fix that by changing how fast the machine oscillates exactly, but the antenna need not be entirely straight. In CB radios you can use a longer wire going to a slightly shorter antenna and count both the wire and antenna together to determine the whole length.
    One more question: is it possible to generate an "inverse" oscillation that can cancel out an ordinary E.M oscillation?
    ... if the canceling signal originates from a different location than the original signal, then it only cancels when it's being picked up from certain directions/locations.
    Thanks, kojak for the excellent and comprehensive explanations.
    1) . I did not mean to shorten the antenna by changing the speed of the oscillations, but by shortening the unit of time: after all the second is an arbitrary unit, if you put it at 1/1000 of the current second you are multiplying the frequency by 1000, right?
    2). what I meant is, suppose a signal is coming to your town: can you generate an inverse signal (of same frequency of course) that cancels it out / absorbs/ annihilates it? according to your reply that should be possible. Can you tell me something more on the characteristics od this inverse oscillations, how do you produce it and is its equation any different?
    ..and, if the inverse signal is of lower frequency, does it only affect the wavelength of the incoming signal and the signal can be picked up in another band?

    Thanks a lot
    Last edited by logic; June 4th, 2013 at 11:56 PM.
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    what I meant is, suppose a signal is coming to your town: can you generate an inverse signal (of same frequency of course) that cancels it out / absorbs/ annihilates it? according to your reply that should be possible.
    No, it's not. It can be done with sound, there are headphones which do just that, but it can't be done with radio waves. Electromagnetic radiation does not interfere with itself. You can jam a signal by producing a more powerful signal on the same wavelength, but that's sort of like not being able to hear someone because someone else is yelling in your ear.
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    Thanks, Alex, so there is nothing like an inverse oscillation, a sort of anti-radiation
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    Nope, afraid not.
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    Quote Originally Posted by PhyMan View Post
    jocular is wrong. If the frequency of the rate at which an EM field is chaning is zero then its a static EM field and it doesn't radiate. The requirement for radiaion to exist is that iit must have the field "detach" from the source, if you know what I mean. The field has to change in order for a change to propagate through space. There is only a lwer bound of zero but you can go as low as you wish. It's like asking How small can a real number be without being zero?
    Practically speaking, there is no DC. All sources of EM in existence have been "on" for only a finite time. So, jocular is not really wrong.
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    Quote Originally Posted by logic View Post
    Thanks, Alex, so there is nothing like an inverse oscillation, a sort of anti-radiation
    I'm not sure what you mean, so I'll answer the question you should be asking. An oscillation can be nulled out easily by its inverse. Indeed, this is why you can encounter dead zones in radio reception. Waves can get to you through multiple paths. If two, say, are 180 degrees out of phase, you'll get cancellation (or at least significant attenuation). In that sense, there does exist "inverse oscillation."
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    Quote Originally Posted by kojax View Post
    Most of the modern cell phones that don't have a visible antenna on them are also bragging about how they're in a "3G" or "4G" network. "4G" is the frequency they're broadcasting on. 4 Gigahertz. You can have a pretty short antenna at 4 Gigahertz.
    Minor nit: "4G" refers not to a 4GHz frequency, but to "4th generation" (of cellular) technology.
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    Quote Originally Posted by tk421 View Post
    Minor nit: "4G" refers not to a 4GHz frequency, but to "4th generation" (of cellular) technology.
    Didn't know that - always wondered about it but never got around to looking it up...
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    Quote Originally Posted by logic View Post
    1) . I did not mean to shorten the antenna by changing the speed of the oscillations, but by shortening the unit of time: after all the second is an arbitrary unit, if you put it at 1/1000 of the current second you are multiplying the frequency by 1000, right?
    You are not changing the frequency, just the units you measure it in. Tokyo is equally far away, whether I measure the distance in miles or kilometres.

    2). what I meant is, suppose a signal is coming to your town: can you generate an inverse signal (of same frequency of course) that cancels it out / absorbs/ annihilates it?
    This is possible in principle. We do see examples of such cancellation (the light and dark bands in a dual-slit interference pattern, for example). There are a couple of practical points:

    1. Noise cancelling microphones work by analysing the sound and then generating the same complex, changing pattern in anti-phase. This is possible because sound is relatively slow so there is time to receive the signal, analyse it and generate a response. EM radiation travels at the speed of light. By the time it arrives, it is too late to cancel it (unless you could do the analysis and generation in zero time or less).

    2. It can only work locally. Outside the zone of cancellation, there must be an area of reinforcement (conservation of energy).
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    Quote Originally Posted by tk421 View Post
    Minor nit: "4G" refers not to a 4GHz frequency, but to "4th generation" (of cellular) technology.
    [citation needed]
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    I remember being taught that light does not self interfere. Apparently I was taught incorrectly.

    Live and learn. Hopefully.
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    Photons do not interact with one another (*), which may be what you are thinking of.

    (*) Except at very high energies, where (apparently) gamma rays can be used to measure the density of photons in space.
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    Quote Originally Posted by Strange View Post
    Photons do not interact with one another (*), which may be what you are thinking of.
    You're right, that is what I was thinking of.
    Its the way nature is!
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by logic View Post
    Thanks, Alex, so there is nothing like an inverse oscillation, a sort of anti-radiation
    I'm not sure what you mean,
    I was hinting at anti-matter. By inverse oscillation I did not intend out-of phase, but a radiation that would absorb the signal no matter what phase, so it might just diminish the frquency
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    Quote Originally Posted by logic View Post
    I was hinting at anti-matter. By inverse oscillation I did not intend out-of phase, but a radiation that would absorb the signal no matter what phase, so it might just diminish the frquency
    In that case, there is no such thing (photons are their own anti-particles, effectively).
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    Quote Originally Posted by Strange View Post
    You are not changing the frequency, just the units you measure it in. Tokyo is equally far away, whether I measure the distance in miles or kilometres.
    What is the frequency of the note A (4)? they say 440 Hz/sec.
    But if (historically) they had chosen the second to be 1/43200 part of the day, now they would agree it is 220 Hz.
    So, what is the frequency of A4?
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    Quote Originally Posted by logic View Post
    What is the frequency of the note A (4)? they say 440 Hz/sec.
    But if (historically) they had chosen the second to be 1/43200 part of the day, now they would agree it is 220 Hz.
    So, what is the frequency of A4?
    The number depends what units you measure it in. That doesn't change the actual frequency.

    It is 6,740 miles from New York to Tokyo. Or is it 10,850 km? Do you think the Earth just got bigger as I typed that?
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    Quote Originally Posted by Strange View Post
    Photons do not interact with one another (*), which may be what you are thinking of.

    (*) Except at very high energies, where (apparently) gamma rays can be used to measure the density of photons in space.
    I agree. E.g. see - Remarkable idea: Scattering Light o Light at http://www.slac.stanford.edu/th/lectures/warsaw.pdf and http://en.wikipedia.org/wiki/Two-photon_physics
    Last edited by PhyMan; June 5th, 2013 at 04:21 AM.
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    Quote Originally Posted by AlexG View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by AlexG View Post
    X-ray machines go a lot higher than visible light.
    ....or gamma rays higher still. But both start to invite the question of what one then counts as an "oscillator". I had taken the question to be a man-made one, rather than an electron in an atom or a sub-nuclear particle .
    Yes, but we have man-made machines that produce x-rays.
    We do, but the "oscillator" responsible for creating the X-rays is an inner shell electron, not an artificial oscillator such as an antenna or cavity magnetron for example.
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    Quote Originally Posted by Strange View Post
    The number depends what units you measure it in. That doesn't change the actual frequency.
    ?
    What is the actual frequency of A4, please? what the actual wavelenght?
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    Quote Originally Posted by logic View Post
    What is the actual frequency of A4, please? what the actual wavelenght?
    In what units? Without that, the question has no meaning.

    What is the "actual" distance from Tokyo to New York?
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    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
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    Quote Originally Posted by logic View Post
    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
    According to Google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by logic View Post
    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
    According to Google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
    He shoulda picked megameters...
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    Or furlongs (3.38695267 kilofurlongs)
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    Those would be better... since Han Solo doesn't know what to do with a parsec.




    Ok... that wasn't one of my better jokes...
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by logic View Post
    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
    According to Google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
    the wavelength of A4 is related to the speed of light or the speed of sound?
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    Quote Originally Posted by logic View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by logic View Post
    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
    According to Google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
    the wavelength of A4 is related to the speed of light or the speed of sound?
    Logic, A4 (A above Middle C) is a musical PITCH, that is, a FREQUENCY, measured in units of reciprocal seconds i.e. "per second", also known as Hz. You can excite this frequency in a variety of media, and, depending on the speed at which waves propagate in the medium under consideration, you will get different wavelengths in order to achieve a frequency of 440/sec.
    The relation is v = f λ.

    It was you that introduced a musical pitch (i.e. sound waves) into the discussion. All Strange has done is work out what the wavelength of light with a frequency of 440/sec would be. The speed of sound in air at NTP is ~340m/sec. So in air, the wavelength of sound of 440Hz is v/f = 340/440 ~ 0.77m.
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    Quote Originally Posted by logic View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by logic View Post
    If you could pick up the oscillation of A4 with an antenna how long would it be in cm.?
    According to Google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
    the wavelength of A4 is related to the speed of light or the speed of sound?
    I assumed, given the topic of this thread and the fact you said "antenna", that you meant an electromagnetic wave of that frequency.

    If you meant a sound wave, then The Great Google says: speed of sound at sea level / (440 hertz) = 77.3386364 centimeters
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    Quote Originally Posted by strange View Post
    Quote Originally Posted by logic View Post
    Quote Originally Posted by strange View Post
    Quote Originally Posted by logic View Post
    if you could pick up the oscillation of a4 with an antenna how long would it be in cm.?
    according to google: the speed of light / (440 hertz) = 68 134 649.5 centimeters
    the wavelength of a4 is related to the speed of light or the speed of sound?
    I assumed, given the topic of this thread and the fact you said "antenna", that you meant an electromagnetic wave of that frequency.

    If you meant a sound wave, then the great google says: speed of sound at sea level / (440 hertz) = 77.3386364 centimeters
    snap!
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    Quote Originally Posted by exchemist View Post
    snap!
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    Quote Originally Posted by exchemist View Post
    The speed of sound in air at NTP is ~340m/sec. So in air, the wavelength of sound of 440Hz is v/f = 340/440 ~ 0.77m.
    Thanks, exchemist, 77 cm. Probably this is a bit off topic, but I introduced this oscillation to draw a comparison with antennas.
    The ear picks up an oscillation with wavelength 770 mm, by an 'antenna' a few mm long. That is a great achievement, or is it a completely different problem?
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by tk421 View Post
    Minor nit: "4G" refers not to a 4GHz frequency, but to "4th generation" (of cellular) technology.
    [citation needed]
    Sure thing. Here's one: 4G - Wikipedia, the free encyclopedia

    And here's more detail on 4G: What is 4G? A complete guide to 4G - PC Advisor

    And here's a list of frequency bands for the various cellular services: Cellular frequencies - Wikipedia, the free encyclopedia
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    Quote Originally Posted by logic View Post
    Quote Originally Posted by exchemist View Post
    The speed of sound in air at NTP is ~340m/sec. So in air, the wavelength of sound of 440Hz is v/f = 340/440 ~ 0.77m.
    Thanks, exchemist, 77 cm. Probably this is a bit off topic, but I introduced this oscillation to draw a comparison with antennas.
    The ear picks up an oscillation with wavelength 770 mm, by an 'antenna' a few mm long. That is a great achievement, or is it a completely different problem?
    It's a similar problem. An antenna needs to be a "significant" fraction of a wavelength in extent in order for the impedances to be "reasonable." The smaller the ratio of antenna length to wavelength, the smaller the impedance, and the more challenging the power transfer. An impedance matching element needs to be introduced to improve the power transfer efficiency. The cochlea, with its fluid-filled spiral shape, accomplishes this function. Its EM counterpart would be a short antenna that is coupled with an exponentially tapered waveguide (which similarly achieves impedance matching over a large fractional bandwidth).
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    Quote Originally Posted by logic View Post
    Thanks, exchemist, 77 cm. Probably this is a bit off topic, but I introduced this oscillation to draw a comparison with antennas.
    The ear picks up an oscillation with wavelength 770 mm, by an 'antenna' a few mm long. That is a great achievement, or is it a completely different problem?
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
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    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.

    ETA: The dependence of radiation resistance on dimensions also explains why woofers are large and tweeters are small.
    Last edited by tk421; June 5th, 2013 at 11:54 AM.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.
    But , thinking about the fluid-filled cochlea, the speed of sound in water is ~ 1500m/sec, making the wavelength of a 440Hz signal 1500/440 = 3.75m, instead of 0.77m, i.e. it goes the wrong way from the viewpoint of enabling the antenna to be smaller! There must be more to the cochlea than the antenna analogy suggests, I think.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.
    But , thinking about the fluid-filled cochlea, the speed of sound in water is ~ 1500m/sec, making the wavelength of a 440Hz signal 1500/440 = 3.75m, instead of 0.77m, i.e. it goes the wrong way from the viewpoint of enabling the antenna to be smaller! There must be more to the cochlea than the antenna analogy suggests, I think.
    Correct calculation, but not quite of the right thing. First, the cochlea isn't analagous to the antenna; it's a post-antenna impedance matching structure.

    That said, the question of dimensions still enters into the picture, and the answer is subtle. The calculation you carried out assumes a bulk-wave propagation mode. It turns out that the cochlea actually propagates surface acoustic waves, which have a much slower velocity. So, instead of optimizing for, say, 10kHz, the cochlea is able to provide reasonable impedance matching over a three-decade frequency range centered (geometrically) around 600Hz or so.

    Nature is awesome.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.
    But , thinking about the fluid-filled cochlea, the speed of sound in water is ~ 1500m/sec, making the wavelength of a 440Hz signal 1500/440 = 3.75m, instead of 0.77m, i.e. it goes the wrong way from the viewpoint of enabling the antenna to be smaller! There must be more to the cochlea than the antenna analogy suggests, I think.
    Correct calculation, but not quite of the right thing. First, the cochlea isn't analagous to the antenna; it's a post-antenna impedance matching structure.

    That said, the question of dimensions still enters into the picture, and the answer is subtle. The calculation you carried out assumes a bulk-wave propagation mode. It turns out that the cochlea actually propagates surface acoustic waves, which have a much slower velocity. So, instead of optimizing for, say, 10kHz, the cochlea is able to provide reasonable impedance matching over a three-decade frequency range centered (geometrically) around 600Hz or so.

    Nature is awesome.
    Aha. That's interesting. I was always crap at radio stuff, so thanks for the correction re impedance matching.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.
    But , thinking about the fluid-filled cochlea, the speed of sound in water is ~ 1500m/sec, making the wavelength of a 440Hz signal 1500/440 = 3.75m, instead of 0.77m, i.e. it goes the wrong way from the viewpoint of enabling the antenna to be smaller! There must be more to the cochlea than the antenna analogy suggests, I think.
    Correct calculation, but not quite of the right thing. First, the cochlea isn't analagous to the antenna; it's a post-antenna impedance matching structure.

    That said, the question of dimensions still enters into the picture, and the answer is subtle. The calculation you carried out assumes a bulk-wave propagation mode. It turns out that the cochlea actually propagates surface acoustic waves, which have a much slower velocity. So, instead of optimizing for, say, 10kHz, the cochlea is able to provide reasonable impedance matching over a three-decade frequency range centered (geometrically) around 600Hz or so.

    Nature is awesome.
    Aha. That's interesting. I was always crap at radio stuff, so thanks for the correction re impedance matching.
    I doubt that you were crap at this -- your intuition is excellent. The question of how a relatively short cochlea could do what it does puzzled scientists for a good long time. It was given to me as a homework problem in college, and in those pre-Google years, none of the students were able to get beyond the calculation that you did. Then the evil professor presented the answer, and used it to launch into a unit on other propagation modes (including various SAW modes). That's why I remember it so vividly.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    That is different. The ear detects changes in pressure. If anything, a smaller sensor is able react to/detect smaller changes than a large one.

    So I think the wavelength matching requirement only applies to transversal waves.
    Actually, it applies to longitudinal waves, too. The radiation resistance is still a function of normalized dimensions. That's why, for example, old-time phonographs used those giant horns. Before electronic amplifiers were developed, the only way to get reasonable sound volume was to use such impedance matching structures.
    But , thinking about the fluid-filled cochlea, the speed of sound in water is ~ 1500m/sec, making the wavelength of a 440Hz signal 1500/440 = 3.75m, instead of 0.77m, i.e. it goes the wrong way from the viewpoint of enabling the antenna to be smaller! There must be more to the cochlea than the antenna analogy suggests, I think.
    Correct calculation, but not quite of the right thing. First, the cochlea isn't analagous to the antenna; it's a post-antenna impedance matching structure.

    That said, the question of dimensions still enters into the picture, and the answer is subtle. The calculation you carried out assumes a bulk-wave propagation mode. It turns out that the cochlea actually propagates surface acoustic waves, which have a much slower velocity. So, instead of optimizing for, say, 10kHz, the cochlea is able to provide reasonable impedance matching over a three-decade frequency range centered (geometrically) around 600Hz or so.

    Nature is awesome.
    Aha. That's interesting. I was always crap at radio stuff, so thanks for the correction re impedance matching.
    I doubt that you were crap at this -- your intuition is excellent. The question of how a relatively short cochlea could do what it does puzzled scientists for a good long time. It was given to me as a homework problem in college, and in those pre-Google years, none of the students were able to get beyond the calculation that you did. Then the evil professor presented the answer. That's why I remember it so vividly.
    Yes, I can imagine: I've since been on the web and it seems there is a clever thing called the basilar membrane, the thickness of which tapers with distance from the oval window that transmits the vibrations to the cochlear fluid and thus causes excitation at different distances along it, depending on frequency. So it's quite intricate, evidently. Let's hope no creationists read this far, or we'll be inundated with claims of "intelligent design" !
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    Quote Originally Posted by tk421 View Post
    ETA: The dependence of radiation resistance on dimensions also explains why woofers are large and tweeters are small.
    Excuse me while I smack myself round the head a few times... (You would never know I did a course on speaker design once - a long, long time ago...)

    And excellent follow-up discussion.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by tk421 View Post
    ETA: The dependence of radiation resistance on dimensions also explains why woofers are large and tweeters are small.
    Excuse me while I smack myself round the head a few times... (You would never know I did a course on speaker design once - a long, long time ago...).
    Hell, I've slapped my forehead so many times that there's a permanent dent in it.

    --Cheers, tk
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    Quote Originally Posted by exchemist View Post
    Yes, I can imagine: I've since been on the web and it seems there is a clever thing called the basilar membrane, the thickness of which tapers with distance from the oval window that transmits the vibrations to the cochlear fluid and thus causes excitation at different distances along it, depending on frequency. So it's quite intricate, evidently. Let's hope no creationists read this far, or we'll be inundated with claims of "intelligent design" !
    They'd be even more excited were they to learn about the distributed mechano-electrical positive feedback along the basilar membrane that provides large amplification of the signal as it propagates. And how some pathologies can cause the gain to be too great and induce persistent oscillations (causing one form of tinnitus, audible with a small microphone inserted into the ear).
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Yes, I can imagine: I've since been on the web and it seems there is a clever thing called the basilar membrane, the thickness of which tapers with distance from the oval window that transmits the vibrations to the cochlear fluid and thus causes excitation at different distances along it, depending on frequency. So it's quite intricate, evidently. Let's hope no creationists read this far, or we'll be inundated with claims of "intelligent design" !
    They'd be even more excited were they to learn about the distributed mechano-electrical positive feedback along the basilar membrane that provides large amplification of the signal as it propagates. And how some pathologies can cause the gain to be too great and induce persistent oscillations (causing one form of tinnitus, audible with a small microphone inserted into the ear).

    No! That would be our counterargument, i.e. what a bloody awful "design" it can be, when it goes wrong!
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    Quote Originally Posted by logic View Post
    Quote Originally Posted by kojax View Post
    Quote Originally Posted by logic View Post
    Is it possible to shorten the antenna measuring the frequency of a wave in a fraction of second?
    I mean can you pick up a wavelength of 100 km using a 100 m antenna and tuning the machine on 1/1000 sec.?Can it work that way?
    .. I'm pretty sure that in order to tune an antenna to be sensitive to a radio signal the electrons in that antenna have to be oscillating in time with the signal. So the length of the antenna matters because they need to have enough space to change direction at the right times.

    I don't think you can fix that by changing how fast the machine oscillates exactly, but the antenna need not be entirely straight. In CB radios you can use a longer wire going to a slightly shorter antenna and count both the wire and antenna together to determine the whole length.
    One more question: is it possible to generate an "inverse" oscillation that can cancel out an ordinary E.M oscillation?
    ... if the canceling signal originates from a different location than the original signal, then it only cancels when it's being picked up from certain directions/locations.
    Thanks, kojak for the excellent and comprehensive explanations.
    1) . I did not mean to shorten the antenna by changing the speed of the oscillations, but by shortening the unit of time: after all the second is an arbitrary unit, if you put it at 1/1000 of the current second you are multiplying the frequency by 1000, right?

    Time is an arbitrary unit, and distance is an arbitrary unit. However the combination of the two is not arbitrary. The speed of light is approximately 300 million meters per second. If you change your unit of time, then you also need to change the speed of light. If I defined a second as 5 times it's current measure, then I'd have to also re-define the speed of light to be 1.5 billion meters per second. (Because remember that speed is defined as distance divided by time.)

    If the speed of light is different, then the wavelength of my signal is different by the same degree. If my signal was 100 mhz with a wavelength of 3 meters when a second was one second long, it will now be 500mhz with a wavelength of 3 meters when we redefine the second to be 5 seconds longer.

    You would get a higher frequency with the new definition, but the wavelength stays exactly as long as it was before.


    2). what I meant is, suppose a signal is coming to your town: can you generate an inverse signal (of same frequency of course) that cancels it out / absorbs/ annihilates it? according to your reply that should be possible. Can you tell me something more on the characteristics od this inverse oscillations, how do you produce it and is its equation any different?
    ..and, if the inverse signal is of lower frequency, does it only affect the wavelength of the incoming signal and the signal can be picked up in another band?

    Thanks a lot
    If the inverse signal is a lower frequency then it won't affect the higher frequency signal at all.

    As for the rest, the problem isn't just generating an anti-signal at the same frequency. You also need to make the anti signal contain all the same information. If the information is, for example, a guy on the radio talking, then the anti-signal won't cancel his voice unless you use his voice to generate it.

    So the trouble is that, in order to know what the guy is saying so you can cancel it, you need to first listen to what he's saying. By the time you've listened, it's already too late to generate a canceling signal, because your canceling signal would never catch up.
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by exchemist View Post
    Yes, I can imagine: I've since been on the web and it seems there is a clever thing called the basilar membrane, the thickness of which tapers with distance from the oval window that transmits the vibrations to the cochlear fluid and thus causes excitation at different distances along it, depending on frequency. So it's quite intricate, evidently. Let's hope no creationists read this far, or we'll be inundated with claims of "intelligent design" !
    They'd be even more excited were they to learn about the distributed mechano-electrical positive feedback along the basilar membrane that provides large amplification of the signal as it propagates. And how some pathologies can cause the gain to be too great and induce persistent oscillations (causing one form of tinnitus, audible with a small microphone inserted into the ear).
    No! That would be our counterargument, i.e. what a bloody awful "design" it can be, when it goes wrong!
    Oh, right! Then we could add that it works only over a narrow temperature range, and is prone to infection. Plus, kids find many ways to get objects stuck in them. A good engineer would have made the thing operate over a wide range of temperatures and be self-cleaning, obviously.
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    Modulating visible light wavelengths presents the problem of "tuning"; see, we can easily use resonant circuits to "tune" in to a specific frequency, then modulate that frequency and ship it out. R-C circuits do not work with the enormous multitude of frequencies present given a certain shade of visible light. Now, if use of a laser-light is made, in which only one specific wavelength (and thus, frequency) is present, we ought to be able to modulate that one way or another.

    I think. As you all know, I have been both wrong and right concurrently in this thread! jocular
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    Quote Originally Posted by logic View Post
    One more question: is it possible to generate an "inverse" oscillation that can cancel out an ordinary E.M oscillation?
    You might be interested in this: http://www.theregister.co.uk/2013/06...data_security/
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    Quote Originally Posted by kojax View Post
    Most of the modern cell phones that don't have a visible antenna on them are also bragging about how they're in a "3G" or "4G" network. "4G" is the frequency they're broadcasting on. 4 Gigahertz. You can have a pretty short antenna at 4 Gigahertz.
    I don't think the name 4G has anything to do with 4GHz frequency.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by tk421 View Post
    Minor nit: "4G" refers not to a 4GHz frequency, but to "4th generation" (of cellular) technology.
    [citation needed]
    Sure thing. Here's one: 4G - Wikipedia, the free encyclopedia

    And here's more detail on 4G: What is 4G? A complete guide to 4G - PC Advisor

    And here's a list of frequency bands for the various cellular services: Cellular frequencies - Wikipedia, the free encyclopedia
    From just the last of those:
    Frequency bands used in the United States
    Current / Planned Technologies Frequency (MHz)
    3G, 4G, MediaFLO (defunct), DVB-H 698–806
    GSM, IS-95 (CDMA), 3G, 4G 1,850–1,910 and 1,930–1,990
    3G, 4G 1,710–1,755 and 2,110–2,155
    4G 2,496–2,690


    And the PC Advisor one says:

    Technology
    4G 800MHz 1800MHz 2600MHz
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    So?
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