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An expander is said to make quiet signals quieter. The ratio, just like with compressors, is expressed with the input-output notation. We can see from Figure 20.2 that, with a 1:2 ratio, 20 dB fall below the threshold for input signals results in 40 dB fall for output signals. Visually speaking, the portion of the signal below the threshold is stretched downward to be made quieter. It is interesting to note that with the 1:100 ratio, the transfer function of the expander looks like a gate. Indeed, a gate is often an expander with a large ratio, and most processors are an expander/gate rather than one or the other. Commonly, we have a control that lets us dial ratios between 1:1 and 1:100. On some hardware units, a switch will toggle a gate into an expander with fixed ratio, often 1:2.

Figure 20.2 The transfer function of an expander with different ratios.

Expanders rarely provide a soft-knee option, which means that the transition between treatment and no-treatment at the threshold point can be quite harsh. As with compressors, one way to reduce this artifact is to use the attack and release. In order to understand how the attack and release work, it is worth remembering how a compressor works: the amount of gain reduction is determined by the threshold and ratio settings, then the attack and release slow down changes in gain reduction. An expander works on exactly the same principle, only the amount of gain reduction is determined by the signal level below the threshold, not above it (the undershoot, not the overshoot amount).

This is worth stressing: both the attack and release on an expander simply slow down changes in gain reduction. More specifically, a gain change from –40 to 0 dB will be slowed down by the attack, and a change from 0 to –40 dB will be slowed down by the release. Neither the attack nor the release has any relation to the threshold setting. If the signal level fluctuates below the threshold, the attack and release will affect respective changes in gain reduction. Once the signal overshoots the threshold, the applied gain reduction will diminish as fast as the attack allows, and once down to 0 dB the expansion stops. As the signal then drops back below the threshold, expansion starts again, and the release starts to take effect.

The very common hold function on an expander can be implemented in two different ways: the compressor style or the gate style. A compressor-style hold simply alters the release rate, so gain reduction starts very slowly. A gate-style hold freezes the gain reduction for the hold period once the signal has dropped below the threshold.

Most expanders also offer a range control. This parameter defines the maximum amount of gain that will be applied on the signal. For example, a range of –20 dB means that the signal will never be attenuated by more than 20 dB. Figure 20.3 illustrates this.

Figure 20.3 Range function on an expander. The gray line denotes the curve of a plain 20 dB attenuation, which sets the bottom limit for the expansion effect. With the resultant ratio curve (black line), the maximum amount of attenuation will never exceed 20 dB.

In practice

While the technical descriptions above might seem somewhat complex, the principal usage of expanders in mixing is very simple—a softer alternative for gates. It was stressed in the previous chapter that gates tend to be obstructive, mostly due to the sharp transition between treatment and no treatment. An expander, with its ratio, provides a more gradual transition between the two. Whether it is large or small attenuation we are after, an expander can give smoother results. The idea is that while a gated signal jumps from one level to another (as the threshold is crossed), an expanded signal slides. Figure 20.4 demonstrates these differences. There is an important outcome to this sliding nature of expanders. While on a gate the attack and release are often employed to soften the gate’s operation, on an expander this responsibility is consigned to the ratio, allowing the attack and release to concern themselves with the musical aspects of the signal dynamics.

Figure 20.4 Gate vs. expander. On the top row, both the gate and expander are employed to drastically attenuate the input signal. While the gate toggles instantly between large attenuation and no attenuation, an expander slides between the two. Even when small range is used, as in the bottom row, the same sliding nature of expanders results in less obstructive effect.

However, there might be some issues with expanders when large ranges are involved. If we assume that large ranges are used when we want to mute signals below the threshold, only gates can ensure such behavior. Expanders can make the very quiet signals inaudible, but signals right below the threshold might still be heard. In order to make these signals less audible, we have to increase the ratio. The issue is that the higher the ratio, the more the expander behaves like a gate—the ratio curve becomes steeper and level changes slide faster. In some situations, signals right below the threshold can only be made inaudible with very high ratios, which effectively turn the expander into a gate. In a way, expanders are more suitable for small range settings.

Another scenario where expanders have an advantage over gates is when we want to de-emphasize the quiet sounds in a gradual fashion. If a gate could talk, it would say: “If it’s above my threshold I keep it, if it’s below I attenuate it.” If an expander could talk, it would say: “Well, if it’s below my threshold I attenuate it, and the quieter it is the more I attenuate it.” While on a gate, all signals below the threshold are attenuated by the same amount, expanders let us keep more of the louder details (those right below the threshold), and less of the very low details (which might not be heard anyway). This is suitable in situations where it is actually what is above the threshold that we are trying to emphasize, but we do so by attenuating what is below the threshold (which is still important). An example would be adding some dynamic life to a flat drum loop. Figure 20.5 shows a typical transfer function for these types of applications.

Figure 20.5 Gradual attenuation using an expander. This type of expansion lets us keep more of the loud details below the threshold, and less of the very low level details.

Upward expanders

Why upward expanders are so hard to find is baffling. Both with compressors and gates, we can make the loud signals louder, but we have to take a back-door approach—make what is below the threshold quieter then bring the whole signal up. Upward expanders are designed to make loud signals louder, which makes them an ideal tool for accenting the natural attack, accenting or reviving transients, reconstructing lost dynamics, or adding liveliness, snap, or punch.

Figure 20.6 shows the transfer function of an upward expander with a 0.5:1 ratio. The rational number denotes upward behavior (some manufacturers will write the same ratio as 1:2). It can be seen from Figure 20.6 that for a 20 dB input rise above the threshold, the output rises by 40 dB. With these specific settings, an input signal entering the expander at 0 dB would come out at +40 dB. Naturally, this would result in hard clipping or overload on most systems. In practice, the ratios used in upward compression are very gentle and do not often go below 0.8:1. Still, if the input signal ever hits 0 dB, the output will exceed the 0 dB limit. Thus, often the output level control is used to bring the overall output level down.

When it comes to upward expansion, we no longer talk about gain reduction, we talk about gain increase. In that sense, the attack controls how quickly the gain increase can rise, while the release determines how quickly it can fall. If a snare is being treated and the threshold is set to capture the attack, shorter attack means more of the natural attack, and shorter release means less of what comes after it. Figure 20.7 demonstrates this.

Both the attack and release functions on upward expanders tend to be far more transparent than on a compressor, since in most cases the expander operates with the signal direction, not against it. Despite the dominance of compressors and gates, upward expanders provide an advantageous choice when our goal is making something louder. Both compressors

Figure 20.6 An upward expander. Signals below the threshold remain at the same level, while signals above the threshold become louder based on the ratio settings. With the settings shown in this illustration, any input signal above –20 dB will exceed the 0 dB at the output and might overload the system.

Figure 20.7 The effect of attack and release on upward expansion. The shorter the attack, the quicker the signal above the threshold is boosted, resulting in a stronger effect. Long release can be used to boost the natural decay.

and gates are designed to reduce the gain; upward expanders are designed to increase it. If we want to emphasize the impact of a kick, upward expansion is likely to bring natural results, as it simply reinforces the already rising attack.