Lesson 9

Phase

The mysterious phase phenomenon. It may seem daunting to get a grasp of how it works, but once you understand it, it’s really not that complicated, and understanding the interaction of sound waves will certainly help you get a better sound.
It’s important to understand phase issues, what causes them, and how to fix them when it’s possible because this is an issue that happens a lot, and has a considerable negative impact on the sound quality.


When 2 sound waves of the same source are joined together at different times, the peaks and valleys won’t match up, and this is what will cause phase issues. If a peak joins with a valley, they cancel each other out. If a peak joins with another peak, it doubles in size. Since the wavelengths of lower frequencies are much longer than the wavelengths of high frequencies, what will happen is some frequencies get cancelled out, and some get doubled up. This is called comb filtering, and it happens all the time.
Another effect of comb filtering is the creation of new, higher frequency sounds.
Let’s use this 150 hz sine wave as an example. You hear how it sounds by itself?
Now, let’s double it and put 15 milliseconds I’d delay in it. You can still hear the fundamental tone, but now there are also upper harmonics.
Here’s some examples of causes of comb filtering.

1- recording into a microphone with a wall nearby. The sound enters the microphone directly from the source, but also bounces off the wall and enters the microphone again a few milliseconds later.

2- recording a sound source with 2 microphones that are not equal distances from the source. Common examples of this are recording with close up mics and with room mics, or blending close up drum mics on the kick, snare and toms, with the overheads.
This can be alleviated by putting the precise amount of delay on the close up mics so that the waves line up. Let’s look at some examples and hear the difference between in and out of phase on a snare drum, and on guitar with room mics.

3- flanging effect- this is a method of intentionally creating comb filtering for a desired effect. In this effect, a sound source is duplicated, and reintroduced back into the original signal with a varying amount of delay, generally reciprocating in the range of 10 to 30 milliseconds. As the delay time changes, it creates a continually changing com filter which results in a sweeping sound.

4- speakers. Often a subwoofer will be integrated into a system, but the sub will be farther away than the main speakers. The overlapping frequencies, which would be the upper frequencies of the sub, and the lower frequencies of the speakers, do not line up perfectly when they reach the listener. This would cause what’s called group distortion, because it’s a specific group of frequencies that’s being distorted by comb filtering. Another potential cause of phase imbalance from speakers can be a poorly designed crossover. Sometimes the electronic circuit which separated the bass for the woofer and the treble for the tweeter might not send each signal at the exact same time, which will cause comb filtering on the overlapping frequencies.
Also, a bass port will cause comb filtering. Because the sound coming out of the bass port comes from the back side of the speaker, which is the exact opposite phase, and then travels a little ways so it’s slightly delayed... so anyways, when it blends in with the sounds directly from the speaker, it’s out of phase. This is why the bass from a sealed enclosure is generally preferred over a ported one. The reason bass port designs are so popular is because it’s an inexpensive way of boosting the volume of the lower frequencies, and if the bass port is tuned low enough, like below 40hz, the interference it causes with the direct sound from the speakers is minimal, and the additional bass can be beneficial to the overall sound.