Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why audio speaker produces the same sound wave?

  1. Apr 25, 2008 #1
    I played the audio song on my 8 Ohm audio speaker. I used my Computer Audio Port to play the song. I played that song on my speaker, it worked perfectly, but when I invert the audio signal 180' out of phase, the voice generated was the same.
    That is to say, when the speaker used to contract, it was expanding and when the speaker used to expand, it was contracting, but there was no difference at all in the sound wave generated, why was this happening?
     

    Attached Files:

  2. jcsd
  3. Apr 25, 2008 #2

    turbo

    User Avatar
    Gold Member

    The speaker creates pulsations in air pressure. In practical terms, it make no difference whether the speaker's voice coil initially retracts or extends when a note is hit, though I expect that some audiophile with $1000 speaker cables is going to challenge this. I have rebuilt and restored guitar amplifiers for many years, and when dealing with an amp with multiple speakers, I always observed speaker polarity to avoid cancellation effects. With single-speaker amps, the owner could never tell the difference if polarity was observed or not.
     
  4. Apr 25, 2008 #3

    rbj

    User Avatar

    it sounds the same, but it is not the same waveform. if they're driven by a sinusoid, one waveform will be 180o out of phase from the other.

    the reasons why it sounds the same are largely physiological and psychoacoustical: our hearing is essentially polarity insensitive. (not relatively between ears, if you reverse the leads on just one speaker of a stereo pair, you will notice a problem, but if you reverse the leads on both speakers, you will not hear the difference.)
     
  5. Apr 25, 2008 #4
    When a drum is hit, the surface that is hit moves in. But the surface on the other side moves out. So why don't they cancel out and no vibration should be produce.Condensation and rarefaction don't cancel out. Is that the reason they don't cancel out, because they are on opposite sides. As depicted in the below diagram.
     

    Attached Files:

  6. Apr 25, 2008 #5
    Written by: Kral
    There is disagreement among audiophiles as to whether it matters whether there is a phase inversionin the playback system or not. Certainly for steady state sine wave tones, it does not. However, consider this situation: You have a drummer whack his bass drum once. In the live scene, the initial wave reaching the ear is a pressure (higher than ambient pressure) wave. This is then followed by a rarefication (lower than ambient pressure) wave. If the audio reproduction system has a 180 degree phase shift anywhere along the signal path, then the initial wave reaching the ear from the speaker will be a rarefication wave, followed by a presure wave. Does this matter? This is the question that creates controversy among audiophiles. Some say they can hear the diffrerence, others say no. Certainly for most music, you will not be able to hear the difference (my opinion). It would be interesting to perform a double blind test with the bass drum or some other percussion instrument as the musical source material.
    Regards,
    Kral
     
    Last edited: Apr 25, 2008
  7. Apr 25, 2008 #6

    rbj

    User Avatar

    you need to set up the thought experiment better. of course they don't cancel out except conceivably latterally from the drum where the compression wave and rarefraction wave have the same distance to the listening ear.

    think of one monophonic amplifier driving two identical speakers but only one is wired with the polarity reversed. along a plane that is halfway between and perpendicular to the line connecting the speakers, theoretically the waves cancel out. but not at other locations where the distance to one speaker is shorter than the distance to the other.
     
  8. Apr 25, 2008 #7

    rbj

    User Avatar

    i'm in the Audio Engineering Society and there have been papers flying back and forth about this. conceptually, that is a tangible difference (the polarity of initial attack wave). but in your live setting, does the kick drum sound different from if your 4 meters in front of it vs. 4 meters behind it? how much of the difference is due to shadowing of the drummer sitting behind the kick drum?

    the only real answer to this is with well-designed (double) blind testing. and it has to be done so that the questions asked are objective and neutral (i.e. not which one sounds "better", but do the two sound the same or not). i'm a fan of what they might call AB testing rather than ABX testing. you can slip in an equal number of identical sounds and ask the same "are A and B the same or different?" question. subtract the number of false positives from the number of correct positives and same for the false negatives from correct negatives and then you remove any bias from the listeners.

    This kinda stuff has been done for Monster Cable (and those guys didn't like the result and still try to rationalize it away) and, i think, for certain sample rate experiments (where content above 22 kHz is removed to see if people can truly hear the difference - they can't).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?