Explanation of acoustic feedback, a sound reflection
As mentioned above, it is known that acoustic amplification often means acoustic feedback. It is seldom talked about that this sound feedback is actually two different feedbacks. It is a sound wave reflection and the sound waves travel in the air with a phase correlation.
If there is ringing feedback in an audio system and the feedback is controlled with a notch filter etc, it is likely that there is also antiresonance.
Acoustic resonance
When people talk about audio feedback, they usually mean ringing and oscillating in the sound system, a positive acoustic feedback is an acoustic resonance. The acoustics have resonant frequencies. Sometimes a sound feedback is not oscillating but close to, which is heard.
And antiresonance - empty sound ?
This is what is seldom mentioned. Just as feedback may cause ringing etc, it can also lead to the opposite, that the level of certain frequencies are lowered. The result is that the sound may sound empty in a way that is difficult to otherwise find out. It may actually be difficult to equalize unless the phase pattern is changed by doing some rearrangements.
Resonant or antiresonant frequencies in the frequency responese
The key is to maintain as flat frequency responce as possible in an acoustic amplification system. Next chapter is about that.
How to handle acoustic feedback
The audio can be more or less in resonance due to so called reflected standing waves. It affects the frequency responce.
Vatrious ways that may help are :
1. Relocating the microphones, instrument/playing location, speakers, reduce sound reflection by using sound absortion techniques and/or anything else.
2. Using a feedback/notch filter. (Only to handle resonant feedback.)
3. Phase switch.
4. Parametric equalizer. (To handle both resonant and antiresonant feedback.)
5. Frequency response analyzer. To look also for smaller resonances in the frequency response.
6. Besides, there is a technique that does not seem to be used. Using an all-pass filter with a variable phase shift for a variable frequency. All-pass filters don't change the level, just the phase.
Feedback filter (Notch filter)
A feedback filter is done with a notch filter, like a parametric equalizer/filter with a variable frequency
but always negative (variable) gain and rarely variable q-value/bandwidth. In notch filters, the bandwith is quite narrow. When set at a resonant frequency it may help out.
Because it could adversely affect the instrument's tone, it is important first trying to follow the first point as above.
Phase reversal switch
Like a most studio preamps, some acoustic preamps, as mentioned above, have a simple inverting phase switch.
Drawbacks using a phase switch
Even though feedback at a certain frequency may disappear when using a phase switch, which just inverts the signal's polarity, it can mean 2 things.
1. When the signal is phase reversed and resonance of a frequency is reduced, there may instead be too much of antiresonance on that frequency.
It lowers the level of that frequency which could give an empty sound. The frequency then has to be somewhat re-gained ( => equalized) with a parametric equalizer.
That's why notch filters are easier to use where the gain can be varied, if used in the first place, or a parametric equalizer can be used as a notch filter.
2. Resonance may occur on another frequency.
As an example, resonance at 1kHz. If the phase is reversed, shifted a 180 degrees, there will no longer be a resonance. Instead there could be resonance on the half frequency, 500Hz, or another frequency.
Parameteric equalizer
Can be set to work as a notch filter by setting the gain as low as needed.
If there is a control for the Q-value/bandwitdh, it can be set to narrow so that the filter has less effect on the instrument's or guitar's tone.
A parametric eq can compensate for antiresonant feedback (increased gain on the frequency).
Tone control
At least 2 or rather 3 tone controls are required. Tone controls
set the overall tone. Bass and treble set the lowest and highest frequencies. If there are 3 tone controls, the mid control should cover a very wide mid range, preferably ca 500 - 5kHz. (However for an even 3 band ratio, the bands would be 20 - 200, 200 - 2k and 2k - 20kHz.)
For acoustic guitars it also depends on the
pickup and the strings. Equalizers can provide control to the player to improve nuances in the sound. There are two major types of equalizers, graphic and parameteric equalizer.
More equalizer bands may seem better although a combination of a graphic and parametric equalizer may be the best. An extra equalizer may add a more detailed control over the sound.
The best preamps should have 2 or 3 tone controls, at least a 7 band graphic equalizer and parametric equalizer/filter controls. Few preamps have this, if any. Therefore, an additional graphic and/or parametric equalizer could be an idea. There are small and handy simple equalizer pedals, both graphic and parametric which could be a useful addition after a preamp.
Graphic equalizer
Some acoustic preamps have a graphic equalizer.
Many acoustic guitars already have built in tone controls (bass/treble) or instead a 3 or 4 band graphic equalizer. Some preamps have two or three tone controls and also a graphic equalizer with 7, 8 or even more bands.
Parametric equalizer
They are used to :
1. Setting the instrument's or guitar's tone. When a parametric equalizer is used for this, it should rather be called a parametric filter.
2. Used in order to equalize or reduce room acoustics.
Both these circumstances may be combined.
Like already mentioned above, a parametric equalizer/filter may of course also be used as a feedback/notch filter to reduce or eliminate resonant feedback (low gain on the frequency) or to compensate for antiresonant feedback (increased gain on the frequency).
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