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Topic: Am I seeing the term "resonance" used two ways?

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  1. #1

    Am I seeing the term "resonance" used two ways?

    About a filter\'s resonance:

    I seem to be seeing the term \"resonance\" used two ways:

    1. To mean a volume boost at the cutoff frequency. (Defined this way in the Plugsound manual for their VSTI\'s.)

    2. To mean Q, the breath of the filter across the area around the cutoff frequency. (Not sure where I\'ve seen this, but it was original understandingh of what resonance is.)

    Am I confused, or are different people using this term to mean very different things?

  2. #2
    Senior Member Bruce A. Richardson's Avatar
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    Re: Am I seeing the term "resonance" used two ways?

    In short, I\'d say yes. People call different things resonance. I don\'t think there\'s any official resonance algorithm, but rather many types of filter behaviors that can potentially excite a resonance in the signal. The end result of a swept flanger is resonance, even though it\'s approached as a combination of modulated delay and feedback. Digeridu players affect resonance by changing the shape of the vocal cavity, or vocalizing against (or with) the frequency of their aperture.

  3. #3

    Re: Am I seeing the term "resonance" used two ways?

    Actually 1 and 2 are the same for low-pass and high-pass filters.

    When you lower the Q you make the cutoff very smooth and shallow. As you increase the Q the filter cutoff gets sharper, until it reaches an optimal point where the cutoff is as sharp as possible, with no additional boost. (One example is a Butterworth filter, which, if memory serves, has a Q of 0.7071.)

    As you continue to increase the Q, the slope is sharper, but then you get a bump in the response, just before cutoff. When the Q is extreme, the boost is huge and narrow, and frequencies in that range will resonate.

    Does that help?

  4. #4

    Re: Am I seeing the term "resonance" used two ways?

    Bruce makes a good point about the broader definition of resonance.

    First, I\'ll broaden the filter explaination... There are also bandpass filters. As you up the Q they get really focussed. They will only pass one frequency - and they pass it really well. That\'s also a resonance - and a very tuned one.

    On the other hand, if you invert that filter, it\'s band stop. These are really useful at 60 Hz. Or 50 Hz in Europe. They can have a high Q, but it\'s more of an anti-resonance. They damp out the chosen frequency. So here \"resonance\" doesn\'t really describe the aural sensation.

    But on the broader scale, resonance describes the tuning of a guitar string, a horn\'s tube, or a flute\'s length. It can describe the sound of your bathroom, or the feedback you get when you hold your hand near the ball of a microphone. At low rates it can apply to the swinging of a pendulum, or the under-damped swinging of the Tacoma-Narrows bridge - which was destroyed by a simple wind storm. It\'s anythng that\'s tuned, in which the vibrations can build upon one another, as in re-sound.

    That\'s probably way more than you were looking for :-)

  5. #5

    Re: Am I seeing the term "resonance" used two ways?

    I\'m still confused. It may help me understand if I limit myself to talking about resonance in a low pass filter:

    If a LP filter is set to 1k for, say, a piano multisample, and the resonance is set to 1, how does this give resonance, in the sense of a greater sense of vibration, to the area around 1K?

    My understanding is that the lowpass filter will cut off frequences above 1K on the filtered samples. Wouldn\'t there be less resonance, in this case, since fewer tones are interacting? And there isn\'t actually a boost, is there, but instead an apparent lowering of volume in the area most affected by the filter area, which makes notes that aren\'t too affected by the filter only seem louder?

    Sorry to seem as confused as I clearly am, here.

    Can someone point me to a good site with information about how Resonance settings affect the different filters? My main focus is pianos, and although I seem to get the best results with either a LP filter on the upper octaves set at 1kh or a little above, with Resonance set at 0 or not much above, or with a 2-pole LP filter set around 2.1kh, and the Resonance set at 0 or not much above, I\'m still experimenting, and need, clearly, guidance.

    Thanks for any suggestions or information.

  6. #6

    Re: Am I seeing the term "resonance" used two ways?

    I\'ve always understood it to be #1 - feedback. When you reach for the Resonance control on an analog synthesizer, that\'s what you expect to happen.

  7. #7

    Re: Am I seeing the term "resonance" used two ways?

    Originally posted by Jake Johnson:
    I\'m still confused. It may help me understand if I limit myself to talking about resonance in a low pass filter:

    If a LP filter is set to 1k for, say, a piano multisample, and the resonance is set to 1, how does this give resonance, in the sense of a greater sense of vibration, to the area around 1K?
    <font size=\"2\" face=\"Verdana, Arial\">Great question.

    First, the cutoff frequency, in this case 1k, is *always* reduced in gain by 3 db. It\'s the transition point between the passed, lower frequencies, and the stopped, higher frequencies.

    So, imagine a graph that is at 1 (unity) at 0 Hz, is at 0.7071 (-3 dB) at 1 kHz, and is at 0 at infinitely high frequencies. Now draw some transitions through the 1 kHz point. The slow, gradual transitions are the low Q ones. There\'s no resonance effect. Now draw some really steep transitions through the 1 kHx point. Those are high Q. The problem is that the steep slope can\'t turn the corner fast enough as it approaches 1 (unity) on the low-frequency side. It overshoots, so there\'s a bump in the curve above unity. Maybe it\'s sharp enough that it has a gain of 2.0 or higher. That means that the frequencies around 950 Hz get a big boost. And it sounds like a resonance at that point.

    It\'s really just a side-effect of the way that electronic filters work. And it happens to sound way cool.

  8. #8

    Re: Am I seeing the term "resonance" used two ways?

    So the smaller the Q, the longer and more gradual the ramp, and thus there is less of a bump in volume, and the inverse is true with a large Q? (Bigger bump.)

    And resonance is, in other words, just a sudden rise in volume of the frequencies just below the cutoff, followed by the dip caused by the LP filter.

    Thanks much. I think I\'m getting it.

  9. #9

    Re: Am I seeing the term "resonance" used two ways?

    (And by the way--my point of departure for the original question--does anyone have any suggestions for other filter settings I should try on pianos?)

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