Ever wondered why your woofers have that Extra-Loud note?

Discussion in 'Archived Threads 2001-2004' started by GeorgeTW, Dec 9, 2002.

  1. GeorgeTW

    GeorgeTW Stunt Coordinator

    Nov 16, 2002
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    Ever wondered why your woofers have that one extra-loud note?
    You may have known this is the resonant frequency, but do you really know what makes the frequency resonant, and why it's louder than the others are?

    Normally, Impedance is the product of 3 combined opposition forces. These are:
    1. Resistance - Opposition to Current
    2. Inductance Reactance - Inductor's Opposition to AC Current
    3. Capacitive Reactance - Capacitor's Opposition to AC current.

    The last 2 forces are opposite reactances, and create phase differences that are also opposite, anywhere from 0-180 degrees, depending of the applied frequency. When they both become exactly 180 out of phase with each other, they cancel each other out. Whatever frequency this cancellation occurs at, is called the Resonant Frequency.

    Resonance is a special condition of Impedance.
    By definition, Resonance is the condition where Inductance Reactance equals Capacitive Reactance. Being opposite forces, when they equal each other, they become ZERO.

    All that is left of the 3 opposition forces is Resistance, and another rule of a Resonant Frequency is that
    Impedance = Resistance (Z = R)

    Okay, now that I've beaten that horse to death, here is where the over-simplification starts.

    Where you normally had 3 forces, you now only have 1.
    Your opposition is reducing by 2/3, so now power is increasing by 2/3, BUT ONLY AT THAT ONE FREQUENCY.
    The sudden drop in Impedance allows your receiver to suddenly increase Wattage by an equal amount. This is why your resonant frequency sings so loudly.
  2. Brian Fellmeth

    Brian Fellmeth Supporting Actor

    Jul 30, 2000
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    Well, I for one disagree with just about every word in the above post.
  3. Scott Falkler

    Scott Falkler Second Unit

    Oct 23, 2001
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    I'll second that emotion.
  4. ChrisWiggles

    ChrisWiggles Producer

    Aug 19, 2002
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  5. Allan Jayne

    Allan Jayne Cinematographer

    Nov 1, 1998
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    What is being described is the resonant frequency of the part of the circuit that includes the speaker voice coil. It will be different given the same amp (or A/V receiver) depending on the make and model of speaker system since the voice coil inductance so varies. It may or may not be within the range of human hearing.
    This is different from the acoustic resonant frquency of the speaker cone, which also differs between makes and models. If you were to remove the speaker grill and gently tap the woofer cone with your ear up against it, you will get an idea of what this resonant frequency is. It is not unusual for a woofer to have a resonant frequency of as much as 100 hertz.
    I do not know how the resonant frequency of the speaker circuit interacts with the acoustic resonant frequency of the speaker to produce a given frequency response (with or without boominess or shrillness at one frequency) of the audio system as may be measured by a metering or graphing device sitting in the middle of the room with a microphone, or perceived by you the listener.
    I suppose that what the original poster is trying to claim is that if the amplifier is outputting more power because of the resonant frequency of the circuit at the resonant frequency of the speaker cone, there will be a substantial increase in loudness at that frequency.
    There is also a concept called damping (electronic; within the circuitry) that I know little or nothing about, except I thought I read somewhere that damping is supposed to counteract the tendency of the speaker cone to vibrate more vigorously at its resonant frequency, and more specifically counteract the tendency of the cone to overshoot what would be accurate reproduction of large waveforms. But I suppose that damping for this reason would have to have something adjustable or calibratable about it. This is not adjusting of the graphic equalizer.
    Video hints:

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