Mixing Audio: The Out-of-Speakers Trick
By Roey Izhaki
When two identical waveforms are not in phase, but each is played through a different speaker, the result is quite different from combfiltering. Two mixing tricks are based on such a stereo setup. The following outlines one of them – the out-of-speakers trick. With both, two identical mono signals are sent to a different extreme, and one of the signals is either delayed or phase inverted. To distinguish the two we will call the unaltered signal the original signal and the copy, which is either delayed or phase-inverted, the ghost copy.
The out-of-speakers trick
With the out-of-speakers trick we keep the ghost copy time-aligned to the original signal, but we invert its phase (Figure 11.8). This means that the sound arriving at one ear is phase-inverted with the sound arriving at the other. In nature, this happens (for specific frequencies only) when the sound source is located to one side of the head.
The two phase-inverted signals emitted from each speaker first travel to the near ear, but shortly after both arrive at the far ear (Figure 11.9). The outcome of this is sound that seems to arrive at both sides of the head at the same time. Since different frequencies are localized slightly more forward or backward, the final effect is of sound coming from around you, rather than simply from your left and right.
The out-of-speakers trick can make some people overwhelmingly excited when heard for the first time – not only does sound appear to arrive from outside the speakers, it also seems to surround you in a hallucinatory fashion. To add to the excitement, an instrument on which the out-of-speakers trick is applied will disappear completely when the mix is folded to mono (provided that the original and the ghost copy are at exactly the same level, as they usually are with this trick).
The trick, however, is only in full effect when the listener’s head is on the central plane between the two speakers. Also, our ears only localize sounds coming from the sides based on low-frequency content. The wavelength of high frequencies is too short compared to the distance between our ears. High frequencies can easily change their phase a few times while traveling the distance between our ears. Scientifically speaking, 1 kHz is roughly the frequency below which side localization is experienced. Consequently, it is mostly frequencies below 1 kHz that give the impression of surrounding the head. The out-of-speakers trick is therefore mostly effective with low-frequency sounds.
A sound that appears to surround your head in a hallucinatory fashion is clearly the definition of anti-focus. We can apply the out-of-speakers trick on an instrument, but we should not expect the instrument to relate well to the sound stage we are building. What’s more, the instrument will disappear in mono, so it would not be wise to apply this trick on important instruments, say the beat of a hip-hop track. The out-of-speakers trick is used as a special effect, a sonic gimmick, and is usually applied on the least important instruments, or ones that appear for a very short time in the mix.
To conclude, it should be noted that the out-of-speakers trick is not strictly vinyl proof. In fact, the worst-case scenario for vinyl cutting is phase-inverted low frequency content on the left and right channels. For this reason, mastering engineers sum the low end of the mix to mono, which cancels out, to some degree, any instruments to which this trick has been applied. If the mix is to be cut on vinyl, mixing engineers are advised to submit a vinyl edit in which this trick is omitted.
The above is an excerpt from Roey Izhaki’s book Mixing Audio, 2e. Roey Izhaki has been involved with mixing since 1992. He is an academic lecturer in the field of audio engineering and gives mixing seminars across Europe at various schools and exhibitions. He is currently lecturing in the Audio Engineering department at SAE Institute, London.