Mixing Audio: The Haas 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 Haas Trick.With both, two identical mono signals are sent to a different extreme, and one of the signals is either delayed or phaseinverted. 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 Haas trick
The Haas trick was not invented by Helmut Haas, but it is, essentially, a demonstration of the Haas effect. Haas was interested in what happens when an initial sound is quickly succeeded by similar sounds from various directions. His findings teach us that the directivity of the sound is determined solely by the initial sound provided that (a) the successive sounds arrive within 1–35 ms of the initial sound and (b) the successive sounds are less than 10 dB louder than the initial sound. Although the successive sounds do not give any directivity cues, they still play a spatial role. The Haas trick simply involves an original signal panned to one extreme, and a ghost copy, which is delayed by 1–35 ms, sent to the other (see Figure 11.4).
The Haas trick is usually achieved in one of two ways. The first involves panning a mono track hard to one channel, duplicating it, and then panning the duplicate hard to the opposite channel and nudging the duplicate by a few milliseconds (Figure 11.5). The second involves loading a stereo delay on a mono track, and setting one channel to have no delay and the other to have a short delay between 1 and 35 ms.
The Haas trick results in a wide, open, spacious sound; although the sound can be localized to the non-delayed channel, there is some sense of unfocused stereo image. It can be applied during mixdown for three main purposes:
– To fatten sounds panned to the extremes – using the Haas trick on instruments already panned to the extremes can make them sound bigger, especially in a crossed arrangement. For example, it is common to double-track distorted guitars and pan each mono track to an opposite channel. Applying the Haas trick on each guitar track (sending the delayed duplicate to the opposite channel like in Figure 11.6) results in a fatter, more powerful effect.
– As a panning alternative – sometimes when panning a mono track all the panning options seem less than ideal. For example, in a sparse arrangement of three mono tracks – vocal, bass and guitar – chances are that both the bass and the vocal will be panned center. Panning the guitar to the center (apart from resulting in a monophonic mix) will place it in the busiest, high-masking area of the mix. Panning the mono guitar to one side or the other will cause stereo imbalance. By applying the Haas trick, we can open up the monophonic guitar sound, achieve some stereo width and place the guitar in a low-masking area.
– More realistic panning – the ear uses amplitude, time and frequency differences for the localization of sounds. A pan pot works by only altering amplitude; therefore the results are less natural. The Haas trick adds to the standard panning method time differences, and with a filter on the delayed channel we can also tuck on some frequency differences. However, this application is limited to instruments already panned to one extreme – we cannot pan the original signal anywhere else.
One of the settings we can control with the Haas trick is the amount of delay applied on the ghost copy. Different delay times give slightly different effects, so it is up to the ear to decide which is the best effect. One thing to consider is what happens when the mix is folded to mono. To be sure, the resultant combfiltering will have its impact (often loss of high frequencies), but some delay times, mostly longer ones, sum to mono more gracefully than others, so while checking in mono we look for the least-destructive delay time.
Another setting we can control is the level of the ghost copy. The Haas effect only applies if the ghost copy is less than 10 dB louder than the original signal. Depending on the delay time, it might not apply with figures even smaller than 10 dB. Assuming that both the original and the ghost copy are initially at the same level, boosting the level of the ghost copy will appear at first to balance the stereo image. With slightly more boost, a rather confusing, unnatural, yet interesting effect is achieved – it can be described as an audible delay where it is impossible to say which extreme is the delayed one.
Attenuating the ghost copy below the level of the original signal makes the effect less noticeable and the stereo image of the original signal somewhat less vague. Altogether, it is worth experimenting with different levels, and making the final decision based on taste and feel.
The Haas trick can be enhanced in two ways. First, applying a filter on the ghost channel – usually a low-pass filter – can bring about more natural results, and might even reduce combfiltering when the mix is folded to mono. This is especially useful when the Haas trick is used to achieve more realistic panning. Another enhancement can be used when the Haas effect is applied to fatten sounds. While using a delay on the ghost channel, we modulate the delay time with low rate and low depth (Figure 11.7). This gives a richer impact and bigger size to the effect. While the results are not always suitable, it is worth experimenting.
The Haas trick is often only expedient if used on one, maybe two instruments in the mix. When used on more, it can clutter the extremes and result in an overall unfocused stereo image – doing more harm than good to the mix. Like with many other mixing tricks, overdoing is inadvisable.
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.