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Post by indiehouse on Mar 2, 2021 13:11:31 GMT -6
Ok, basic question, but I want to revisit/relearn like a noob. What is a resonance and how do you determine it's frequency? How do you differ between a resonance and a fundamental?
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Post by ragan on Mar 2, 2021 14:13:37 GMT -6
Ok, basic question, but I want to revisit/relearn like a noob. What is a resonance and how do you determine it's frequency? How do you differ between a resonance and a fundamental? Loosely put, a resonant frequency is a frequency at which you get more bang for your buck with excitation. Meaning it takes less input energy to get a big response. Doesn’t matter the system, could be a mass on a spring, could be a room and sound waves, could be a circuit - certain frequencies will generate bigger responses. Have you ever played on a stage where some note just goes crazy? Like you’re playing through your sound check and every time you go to the chorus, it seems like all you can hear is the bass for that one chord/note? It’s probably because the stage or drum riser or some structure is resonating strongly at that frequency. Tap a piece of metal, or thin piece of plywood or something. You’re giving it an impulse and you’ll hear some note/tone louder than all the others. That’s a resonant frequency making itself known. I bet matt@IAA could wax about mechanical resonance for days. Same with svart and signals. Oh and the fundamental is just the lowest of the resonant frequencies.
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Post by matt@IAA on Mar 2, 2021 14:39:20 GMT -6
Ragan did a good job. Maybe expand a bit. A resonant frequency is one that a system will respond to sympathetically. Every system has multiple resonant frequencies. Each resonant frequency corresponds to what is called a mode shape. We deal with these things all the time intuitively. Simple one is a guitar string. If you pluck it, you get the resonant frequency of the string. As you increase or decrease the tension, the string gets more or less stiff and the frequency moves up and down - tuning. If you put a mic'd or electric guitar up to the speaker/amp, it will begin to resonate by feedback. BUT! Not if you put it head on, so the string is pointing straight out from the speaker. That's because the mode shape, the shape the string takes from that exciting frequency, has a certain directionality to it. You're dealing with mode shapes when you dampen drums. When you strike an object, you excite all of its mode shapes and resonant frequencies at the same time. You can actually measure them if you have an accelerometer. By putting pads or goo or a ring in certain places on a drum youre reducing or eliminating certain mode shapes that correspond to certain frequencies. So, when you hit the drum those frequencies can't ring. Every resonant frequency corresponds with a specific mode shape. Back to the guitar string - there is a mode shape at the string's open note. If you pluck a 12th fret harmonic, you excite the second mode, which is double the frequency. If you pluck a 7th fret you get the third mode. 5th fret you get the fourth mode. 4th or 9th fret you excite the fifth mode. Each one of these corresponds to increasing number of nodes on the string which don't move and a different mode shape-frequency. Like this (not my image): A guitar string is simple...a drum is not. Here are some drum mode shapes. Then imagine something that isn't more or less two dimensional. You get all kinds of crazy possible mode shapes. And each mode corresponds to a specific frequency. OK. A fundamental is a couple of things: - the lowest excitable resonant frequency of a system (with its corresponding mode shape) - the lowest frequency component of a complex vibration mode or signal This can come out in a lot of ways. In an amplifier, the note you put will be a fundamental, and all distortion will be harmonic overtones on top of that in integer multiples of the fundamental frequency. On that guitar string, the fundamental is the open string so you can talk about the E string's harmonic overtones as multiples of 82.41 Hz, but G will be multiples of 196 Hz. If you play the same note on multiple instruments (say, A at 440Hz), the difference between how those things sound is their overtones, and the 440 Hz they make is the fundamental.
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Post by svart on Mar 2, 2021 14:48:02 GMT -6
Both ragan and matt@IAA are correct. A resonance is also a frequency that will oscillate more easily than others from a given stimulus. Sometimes this results in higher amplitude than the incident signal, but sometimes they do not. It depends on the method of resonance and the amount of power applied.
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Post by jmoose on Mar 2, 2021 15:06:29 GMT -6
Ok, basic question, but I want to revisit/relearn like a noob. What is a resonance and how do you determine it's frequency? How do you differ between a resonance and a fundamental? To some degree you can hone in on resonant & fundamental frequencies with an RTA. Resonant frequencies will show more level. Resonance to me is similar to harmonic activity. Its generally, not always, but often above the fundamental and can be related to sustain. This can manifest in good ways and bad ways... A good example would be saying an acoustic guitar or drum is resonant. It has a great sustain that we find pleasing. Bad example would be an overly resonant stage, where every time someone plays an F natural the room sounds like it wants to explode.
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Post by matt@IAA on Mar 2, 2021 15:20:18 GMT -6
If you get out of the mechanical side, audio circuits have resonance too. In a mechanical circuit we talk about stiffness, mass, and damping. For audio, we talk about capacitance, inductance, and resistance.
In audio resonance you have a frequency where a circuit has the same capacitance and inductance. At that frequency the impedance goes to some minimum or near-zero. The LC filter has a center (resonant) frequency of 1 / sqrt (L*C) where L in inductance and C is capacitance. The R resistor in an RLC sets the Q, or how sensitive the "system" to that frequency. The Q is the center frequency * L / R.
If you put that in series in a circuit with the RLC portion going to audio 0V, at the resonant frequency you have cut the signal, and how wide you cut is based on the Q.
If you take that same circuit and put it in a feedback loop, that portion of the signal will not be added in as negative feedback, and so at the resonant frequency you have boosted the signal.
Ta da, now you know how *all* EQs work.
The only difference is how you make the filter (you can make filters using inductors, resistors, capacitors, or just resistors and caps, or use transistors to make a gyrator, etc). (Ok, passive EQs dont have feedback but you get the point).
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Post by indiehouse on Mar 2, 2021 15:32:29 GMT -6
This is all fantastic info, thank you so much. How then, would you separate a good resonance from a bad resonance? For example, trying to eq out resonance from a vocal or acoustic? What is it that one should listen for? I often can find multiple areas where an eq sweep will reveal a boost in level at a particular frequency. But if I cut every one of those, then I am left with something less than desirable.
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Post by ragan on Mar 2, 2021 15:42:45 GMT -6
This is all fantastic info, thank you so much. How then, would you separate a good resonance from a bad resonance? For example, trying to eq out resonance from a vocal or acoustic? What is it that one should listen for? I often can find multiple areas where an eq sweep will reveal a boost in level at a particular frequency. But if I cut every one of those, then I am left with something less than desirable. In audio, it's mostly a totally subjective thing. A voice can be honky and so you scoop out some 2-4kHz or whatever and then it's not as honky but it's also...boring, dull, flat, whatever. Or a bass guitar is booming around 100Hz and you want to tame it. Same kinda deal, tame too much and you've got a wimpy signal left. There's not really an objective method for handling resonance. Best practice is to keep an eye out for awkward resonances when you're tracking and address them there, rather than try to fix them later.
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Post by plinker on Mar 2, 2021 16:16:37 GMT -6
Excellent Question, and excellent discussion.
I'm learning a lot!
Thanks!!!
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ericn
Temp
Balance Engineer
Posts: 15,014
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Post by ericn on Mar 2, 2021 16:43:11 GMT -6
Everything has a resonance frequency, the original Bagend ELF system that went down to 8HZ was great for finding what the resonance freq of a structure was. Oh and the damage one can do while try to notch it out.
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Post by schmalzy on Mar 2, 2021 17:10:08 GMT -6
Ragan did a good job. Maybe expand a bit. A resonant frequency is one that a system will respond to sympathetically. Every system has multiple resonant frequencies. Each resonant frequency corresponds to what is called a mode shape. We deal with these things all the time intuitively. Simple one is a guitar string. If you pluck it, you get the resonant frequency of the string. As you increase or decrease the tension, the string gets more or less stiff and the frequency moves up and down - tuning. If you put a mic'd or electric guitar up to the speaker/amp, it will begin to resonate by feedback. BUT! Not if you put it head on, so the string is pointing straight out from the speaker. That's because the mode shape, the shape the string takes from that exciting frequency, has a certain directionality to it. You're dealing with mode shapes when you dampen drums. When you strike an object, you excite all of its mode shapes and resonant frequencies at the same time. You can actually measure them if you have an accelerometer. By putting pads or goo or a ring in certain places on a drum youre reducing or eliminating certain mode shapes that correspond to certain frequencies. So, when you hit the drum those frequencies can't ring. Every resonant frequency corresponds with a specific mode shape. Back to the guitar string - there is a mode shape at the string's open note. If you pluck a 12th fret harmonic, you excite the second mode, which is double the frequency. If you pluck a 7th fret you get the third mode. 5th fret you get the fourth mode. 4th or 9th fret you excite the fifth mode. Each one of these corresponds to increasing number of nodes on the string which don't move and a different mode shape-frequency. Like this (not my image): A guitar string is simple...a drum is not. Here are some drum mode shapes. Then imagine something that isn't more or less two dimensional. You get all kinds of crazy possible mode shapes. And each mode corresponds to a specific frequency. OK. A fundamental is a couple of things: - the lowest excitable resonant frequency of a system (with its corresponding mode shape) - the lowest frequency component of a complex vibration mode or signal This can come out in a lot of ways. In an amplifier, the note you put will be a fundamental, and all distortion will be harmonic overtones on top of that in integer multiples of the fundamental frequency. On that guitar string, the fundamental is the open string so you can talk about the E string's harmonic overtones as multiples of 82.41 Hz, but G will be multiples of 196 Hz. If you play the same note on multiple instruments (say, A at 440Hz), the difference between how those things sound is their overtones, and the 440 Hz they make is the fundamental. And this graph of the drum modes is why you can't REALLY tune a drum to a note. Those all happen fairly simultaneously. You tune a drum to a whole bunch of notes and depending on a million other things you get something that sounds like a note (and other notes). Brains are dumb. Drums are fun.
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