Phasing type 1: Polarity
I noticed that a lot of people refer to a polarity invert as a phase button, phase flip or phase switch but not many referring to it as polarity. The simple concept of a polarity issue can easily be illustrated using a snare drum with one microphone at the top and another microphone at the bottom. As the drummer plays the snare drum the skin will move downward creating air movement away from the microphone, thus causing a vacuum and the microphone will create a negative impulse. The same snare stroke will create air movement toward the bottom microphone and thus creating a positive impulse on the bottom microphone, like the picture illustrates:
When these two impulses are combined they cancel one another, causing it to sound very thin. The solution would be to flip one of the microphone polarities (e.g. changing the negative impulse into a positive impulse) to get the two impulses to work together and prevent it from canceling out.
Phasing type 2: Timing difference
When two microphones are placed on the same instrument, but the one is placed closer to the sound source than the other, you will get a very short delay between the first microphone signal and the second microphone signal. Because sound takes some time to travel through the air, it will arrive at the microphones at different times, canceling frequencies (comb filtering). To illustrate this concept, I generated two identical 20Hz square waves that produce a lot of harmonics and routed both to a spectrum analyzer. The analyzer shows a fairly smooth response;
I delayed the one waveform by roughly 1.5ms and measured the response again, as shown here:
The orange line indicates the first analysis and the yellow line the analysis after the delay was introduced. In the picture below, you can clearly see the cancellation of frequencies. Because the delay was so small, most of the higher frequencies were affected. Longer delays will have a bigger effect on the lower frequencies.
This can also happen with one microphone. When you record an instrument like guitar, you wouldn't place the mic right against the instrument resulting in an unnatural sound. The standard solution is to place it between 30 and 60 centimeters away from the guitar to capture the natural ambiance of the instrument. As the sound-waves project from the guitar in all directions, the microphone captures the sound-waves. The problem is that some of the sound-waves bounce off the floor back to the microphone. The distance these reflected sound waves travel are farther than the direct sound from the guitar to the microphone, so shortly after the direct sound arrives at the microphone, the reflected sound also arrives at the microphone and once again causes frequency cancellation or comb filtering. This is the same reason why my drums never sounded right. I realized that it was my acoustically untreated room that caused comb filtering. I installed some acoustic treatment to absorb some of the reflections, and suddenly my drums sounded a lot better than before.
Some more advice to help you deal with phasing issues while recording:
- When using multiple microphones, make sure that you flip your mix to mono as this will quickly reveal phasing or comb filtering.
- Try to move away from walls when recording an instrument as it could cause comb filtering.
- If you have a small room, use absorption against the walls or gobos to reduce reflections.
- Delays longer than 30 milliseconds will be perceived as echoes and won't interfere with the original sound.
- If the difference between the original sound and the delayed sound is 9dB or more, the comb filtering or frequency cancellation is almost inaudible.
- 1ms is equal to roughly 1ft or 30cm so 1meter will be about 3ms. Remember this trick when you place your microphones in large rooms.
- When recording using two microphones, remember the 3 to 1 rule. If the mic-to-source distance were 1m (3ft), the mics should be at least 1x3 (3ft x 3) or 3m (9ft) apart to prevent audible comb filtering.