It’s a bit like thinking a DI box’s job is simply to flip the phase and match impedance and it does those perfectly so nothing else happens, when in fact it has a frequency response which changes the sound on the way to the mixer.
Modern PA amplifies have DSP processors inside them. The DSP is different for each speaker driver. That's how the drivers are electronically controlled to produce the desired overall speaker system directivity.
In reality there is no perfect equipment. There are idealized models of equipment, is all. The models are often very useful to understand. If we need to add more complexity to better understand real physical systems then we can do that as needed.
Some things ARE utter nonsense. That is one of them.
Best of luck on your future endeavors.
You are seriously barking up the wrong tree. The definition of 'feedback' is that the source is receiving and outputting the signal of its AUDIO output. Electronics may reduce the output at the offending frequency or delay the signal of the offending frequency, rendering the output 'untrue' - the textbook definition of distortion.
Compared to the regular dudes - I'm an idiot, but even I know you're trying to prove 2+2 doesn't = 4 if you use the right calculator.
The frequency shifter idea appears to rely on a couple of factors: (1) frequency shifting involves phase shifting, they are closely related, and (2) feedback systems build up into a state of oscillation over a period of time, and the rate of feedback signal growth is exponential with time; it also depends on phase. Therefore, if we shift frequency (phase) faster than the feedback signal growth can develop into sustained oscillation, we may make oscillation less likely to occur. Again, that the word 'feedback' is used in sound reinforcement to mean 'oscillation of the feedback system' can add some confusion to exactly what is going on.
I see why this idea is difficult to believe. You’re saying that what we term ‘feedback’ only occurs when there is an input transducer like a microphone and an output transducer like a speaker in the same acoustic field (near to each other) receiving (necessarily) the same/similar signal, and so it becomes exponentially amplified.
But what I’m trying to point out is that this is not the only possible mechanism.
When you tap a cone of a woofer, it moves several times - the first much more, but then there is a sequence of smaller movements before the cone finally comes to a complete stop. The similarity is that, in the case of what I said in the first paragraph above, there is the ‘pure signal’ (for example a singer) and then a sequence of repeats of that sound- in this case so great that there is a continuing increase in volume. But I point out, the extra SPL that ANY resonances or reflections in a certain situation (albeit from the woofer, the amplifier or the room etc) within the vicinity of the problem (mic-speaker) will contribute to that feedback.
What I’m suggesting is that these two sources of feedback interact with each other, which ought to be obvious. And that, the presence of this smaller amount of feedback will add to the total, and lead quicker to the point at which the signal is ‘exponentially amplified’-as I put it at that particular frequency.
But what I’m trying to point out is that this is not the only possible mechanism.
When you tap a cone of a woofer, it moves several times - the first much more, but then there is a sequence of smaller movements before the cone finally comes to a complete stop. The similarity is that, in the case of what I said in the first paragraph above, there is the ‘pure signal’ (for example a singer) and then a sequence of repeats of that sound- in this case so great that there is a continuing increase in volume. But I point out, the extra SPL that ANY resonances or reflections in a certain situation (albeit from the woofer, the amplifier or the room etc) within the vicinity of the problem (mic-speaker) will contribute to that feedback.
What I’m suggesting is that these two sources of feedback interact with each other, which ought to be obvious. And that, the presence of this smaller amount of feedback will add to the total, and lead quicker to the point at which the signal is ‘exponentially amplified’-as I put it at that particular frequency.
Peter,
The example of tapping on a woofer doesn't hold up in reality, when considering the frequency range where Thiele and Small did their work. Most woofers have Qts <0.5, which means they gradually return to their rest position when mechanically excited. NB: there must be an amplifier connected to the speaker, or we end up with Qms, which might be >10.
The sort of resonance you're talking about will be evident in the frequency response, as a high-Q peak. A peak in the response is obviously likely to cause feedback problems, especially if the mic happens to be positioned in such a way as to amplify that peak further (whether that's down to the acoustical environment, or imperfections in the mic's polar pattern).
The reason that better speakers tend to be more feedback-proof can be attributed to a few factors:
- Higher-end monitors tend to be point-source. That means that, while microphone polar patterns aren't perfect (as I alluded to earlier), they can be accurately aimed for where they're effective over most of their frequency range.
- Better speakers will have flatter frequency response curves, which carries obvious benefit.
- Finally, better speakers will tend to control their directivity better. That means the sound goes where you want it, and when you start to think about nearby reflective surfaces, that can be really beneficial in terms of ensuring the mic can reject the sound coming out of the monitor. If the monitor is spraying a load of HF content at the back wall of the stage (not useful), that'll reflect and be picked up the mic.
Hope that's useful.
Chris
The example of tapping on a woofer doesn't hold up in reality, when considering the frequency range where Thiele and Small did their work. Most woofers have Qts <0.5, which means they gradually return to their rest position when mechanically excited. NB: there must be an amplifier connected to the speaker, or we end up with Qms, which might be >10.
The sort of resonance you're talking about will be evident in the frequency response, as a high-Q peak. A peak in the response is obviously likely to cause feedback problems, especially if the mic happens to be positioned in such a way as to amplify that peak further (whether that's down to the acoustical environment, or imperfections in the mic's polar pattern).
The reason that better speakers tend to be more feedback-proof can be attributed to a few factors:
- Higher-end monitors tend to be point-source. That means that, while microphone polar patterns aren't perfect (as I alluded to earlier), they can be accurately aimed for where they're effective over most of their frequency range.
- Better speakers will have flatter frequency response curves, which carries obvious benefit.
- Finally, better speakers will tend to control their directivity better. That means the sound goes where you want it, and when you start to think about nearby reflective surfaces, that can be really beneficial in terms of ensuring the mic can reject the sound coming out of the monitor. If the monitor is spraying a load of HF content at the back wall of the stage (not useful), that'll reflect and be picked up the mic.
Hope that's useful.
Chris
Speaker cone resonance can be viewed as a 'memory' effect. It can be reduced by connecting the speaker to close-by amplifier with a high damping factor. In any case, cone resonance is by nature exponentially decreasing. An amp with a high damping factor speeds up that exponential decay. Maybe in some case exponential growth of acoustic feedback in the presence of exponentially decaying cone resonance could interact in some way, but in most cases its probably not going to happen at the same frequency for the speaker cone natural resonance and the acoustical feedback path phase shift and gain. It would be a special case if were. In that case the most practical solution might be to notch out the cone resonance a little, and or otherwise increase mechanical damping of the speaker a little.
Of course it does. No matter how faithful the movement of the cone via the motor, the diaphragm still resonates. Anything else is to assume an infinitely stiff cone and somehow infinitely stiff and infinitely compliant suspension.
Whilst this may seem trivial, in practice it makes a difference.
Whilst this may seem trivial, in practice it makes a difference.
Speaker cone resonance has a natural frequency. For the cone to ring or resonate, the cone must receive excitation at that frequency. It won't resonate at just any frequency or with just any input signal. It means for it to affect feedback, the natural acoustic feedback frequency would have to be pretty close to the natural cone resonant frequency.
Look, and without meaning to be offensive, because some of the points you raise, it appears that you might benefit from some more related science and engineering studies. We are happy to help out as we can, but what we can do in large part depends on what you want us to do. If you have questions, we can try to answer. We can try to point to relevant educational materials. That sort of thing. OTOH, if you are in a state like (and I'm not saying you are) like you have already decided the earth is flat or something and you don't want to hear otherwise, then of course we probably can't help.
So, if you want to think about the problem you originally posed, and you want the benefit of conversation with other, possibly in some specialized way more experienced members, to get a better handle it it, then we are happy to do what we can to assist.
Look, and without meaning to be offensive, because some of the points you raise, it appears that you might benefit from some more related science and engineering studies. We are happy to help out as we can, but what we can do in large part depends on what you want us to do. If you have questions, we can try to answer. We can try to point to relevant educational materials. That sort of thing. OTOH, if you are in a state like (and I'm not saying you are) like you have already decided the earth is flat or something and you don't want to hear otherwise, then of course we probably can't help.
So, if you want to think about the problem you originally posed, and you want the benefit of conversation with other, possibly in some specialized way more experienced members, to get a better handle it it, then we are happy to do what we can to assist.
Ok this thread has turned into an argument.
As long as there are no devious edits, it’s clear to me this started with the contributor who, without any explanation denounced my ideas as ‘utter nonsense’.
There is no reason for this. A few more taps on the keyboard would have explained why.
Other contributors have laid out there arguments, with which I would have been happy to interact with, but the above was unnecessarily distracting.
It comes down to peoples expectation of ‘respect’ which is commonly skewed.
Don’t expect me to respect you, whatever your achievements if you come at me like that. I respect what is respect worthy. And unfortunately this leaves me with a scepticism about your achievements in general. Only you can correct this.
As long as there are no devious edits, it’s clear to me this started with the contributor who, without any explanation denounced my ideas as ‘utter nonsense’.
There is no reason for this. A few more taps on the keyboard would have explained why.
Other contributors have laid out there arguments, with which I would have been happy to interact with, but the above was unnecessarily distracting.
It comes down to peoples expectation of ‘respect’ which is commonly skewed.
Don’t expect me to respect you, whatever your achievements if you come at me like that. I respect what is respect worthy. And unfortunately this leaves me with a scepticism about your achievements in general. Only you can correct this.
But there is sound everywhere in the situation were imagining, and the cone does not only resonate at its fs.
The criticism you carefully set out here would make sense if I had been the one with the lack of an open mind, how is this condition demonstrated by any of what I say, and where could it be clearer than in denouncing my carefully delineated ideas someone decides to utterly block the idea?
I’m sorry, I have done nothing wrong.
I think the problem is that my ideas tend to be quite iconoclastic, and I will express them clearly as such.
This should not be a problem if the ‘icon’ can swiftly explain the flaw, but that is not what has been demonstrated here.
This should not be a problem if the ‘icon’ can swiftly explain the flaw, but that is not what has been demonstrated here.