An article from Acoustic Image their latest newsletter on the subject of phase invertion. Click here for the online version. If you want to know more about it, check also the incredible interview with Steve Rodby on amplifying double bass.
What’s “Phase?” And why would you want to invert it?
We recently introduced a “Phase Invert” switch on our two-channel preamp. This is a very useful feature (I’ll explain why in a bit), but it has prompted some players to ask a basic question: “What is phase and why does it need to be inverted?”
Audio signals (in fact, all periodic signals, such as radio waves) have two important characteristics: amplitude and phase. Amplitude, measured in volts, refers to the magnitude of the signal while phase, measured in degrees, is the position of the peaks and valleys of the signal relative to a fixed reference. To try to understand these concepts better, let’s take a simple sine wave signal. The figure below shows a sine wave in which the amplitude (vertical axis) is varying with time (horizontal axis):
Several cycles of the wave are shown. One cycle is the portion of the wave that includes a positive peak and a negative peak, representing 360 degrees of phase relative to the starting point of the wave (the first positive peak of the wave is 90 degrees from the start and the first negative peak is 270 degrees from the start). The frequency of the wave is the inverse of the time between positive peaks (Hz = cycles per second).
Phase can also be a relative measurement between two waves (in other words, one wave is the reference for the other). The figure below shows two waves of the same frequency that have the same amplitude. As you can see, there is a “zero” phase difference between them (their peaks line up). These waves are (you guessed it) “in phase.”
The next figure shows two waves where the peaks of one wave are lined up the valleys of the other. There is a 180 degree phase difference between them and they are, not surprisingly, said to be“out of phase.”
If the two signals of the last two examples are added together (i.e., their amplitudes are added at every instance of time), an interesting thing happens: for the in-phase signals, we get a signal with the twice the amplitude. For the two signals out of phase, the result is nothing; the two signals cancel each other out.
So you can see how phase is important. If two signals representing the same instrument are out of phase, the result can be pretty bad if both are added together. Let me illustrate by example.
I recently heard from a customer who bought a new Coda R two-channel combo to replace his older Contra one-channel combo because he liked being able to “blend” two pickups on his bass together. He found that the combination of the two gave the sound he wanted. In his email, he said that something was wrong with the new combo. “Something is missing, it just doesn’t sound like the old setup,” he said. You know where this is going. It turns out that his two pickups were out of phase with each other, and as a result, the signals were cancelling resulting in a thin sound. When I explained what was happening, he reversed the phase of one of the pickups and the result was an immediate improvement in the sound.
So, that’s why we put a phase reverse switch in each channel of our two channel combos. That switch reverses the phase of the signal by 180 degrees giving you some control of phase interactions between two pickups or a mic and a pickup.
It turns out that the phase reverse switch can also be useful in controlling feedback in a system that uses a single pickup or microphone. When standing in a certain position relative to the amplifier, it’s possible that, at a given frequency, the amplitude of signal could be at the maximum value (the peak in the examples above) in the position in which you are standing. That high amplitude could interact with the pickup or mic to cause feedback. Reversing the phase (putting the best “phase” on things?) reduces the signal at that frequency to a minimum value (the valley in the examples above), reducing the amplitude of that frequency at your position and eliminating feedback in the process.
Filed under: Acoustic Image, Amplification, Gear, Steve Rodby by admin
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