This post is also available in: 日本語 (Japanese)
When building a home theater or a home studio, what kind of speakers should we choose? We should choose the combination of speakers which does not cause destructive interference when speakers play the same signal.
Destructive interference is the following phenomenon. When the speaker A and speaker B play the same sound, following waveform comes out from each speaker, each sound wave is canceled out. It causes the same condition that nothing is playing, and we can not hear the sound.
Think of a geometric graph, Sin (sine) and Cos (cosine). It will make it easy for you to understand destructive interference. Which frequency is played with reverse phase or positive phase? It depends on the speaker’s characteristics. In order to prevent the interference phase, the ideal pattern is to select all the same speakers.
The next ideal selection is choosing the same type of speaker (same manufacturer, same series). You may want additional surround speakers when you upgrade your acoustic environment from 5.1ch to 7.1ch or 9.1ch. It’s important to select the same type of speaker as what you already have as much as possible. If you choose without considering destructive interference, you may degrade your acoustic environment by “upgrading” the system with inappropriate speakers.
Even if you can’t get the same type of speakers (same manufacturer, same series), you can use different types of speakers which have the same network characteristic. In this case, you should know detailed information about the speaker’s characteristic before you buy them.
There are following types of speakers.
Full range speaker: it plays all frequencies from one place
2-way speaker: it plays above the crossover frequencies from a tweeter and plays below the crossover frequencies from a woofer (you can see 2 circles in a speaker)
3-way speaker: it plays high frequencies from a tweeter, mid frequencies from a squawker, low frequencies from a woofer
4-way speaker: it plays high frequencies from a tweeter, mid-high frequencies from mid-high, mid-low frequencies from a mid-bass, low frequencies from a woofer
5-way, 6-way + speaker: each speaker plays more fragmented frequencies.
For example, there are reverse phase tweeters and positive phase tweeters. A reverse phase tweeter is connected with the reverse phase. (high frequency -, low frequency + ) A positive phases tweeter is connected with the positive phase. (high frequency +, low frequency +)
So the following combinations are suitable.
1) Both are full-range speakers.
2) Both are 2-way speakers and both tweeters have reversed phase.
3) Both are 2-way speakers and both tweeters have positive phase.
4) A full-range speaker and a 2way speaker whose tweeter has a positive phase.
However, in the case of 2), the crossover frequency of both speakers should be the same or almost the same. Otherwise, destructive interference will occur.
For example, if the crossover frequency of speaker A is 800Hz and crossover frequency of speaker B is 2kHz, destructive interference will occur. Speaker A plays low-frequency (below 800Hz) with positive-phase and high-frequency (above 800Hz) with reverse phase. Speaker B plays low-frequency (below 2kHz) with positive-phase and high-frequency (above 2kHz) with reverse phase. So frequencies from 800Hz to 2kHz cancel each other and destructive interference occurs.
If you feel somewhat boxy, there is a possibility of destructive interference, and a particular frequency may be off. Even if you choose the same type of speakers, destructive interference can occur because of the position of speakers. (I’ll mention how to avoid destructive interference in another article)
Choosing speakers, which do not cause destructive interference when they play the same signal, is essential.