An amplitude modifier impacts the sound source's amplitude. There are many kinds of amplitude modifiers. The most basic is a gain effect. Compressors, expanders, de-essers, and multi-pressors are the most common. Yet, there are a larger range of amplitude changing effects. We use amplitude modifiers to control unwieldy sound sources and to create balance between sound sources.

Gain

Gain is the simplest amplitude modifier. Gain simply changes the overall amplitude of the signal. A -5 dB gain will remove 5 dB from the sound signal at all times.

Compressors

Compressors ensure that a sound signal never gets too loud. Compressors work by reducing the proportion of the amplitude over a threshold amplitude. So a sound can be soft, but we will limit the sound in how loud it can get. There are five parameters to compressors. First, is the threshold. We will reduce any amplitude over the threshold. Second, is ratio. The ratio determines how much we will reduce the amplitudes over the threshold. Gain is the third parameter. Sometimes, the compressor can impact the overall volume too much, so gain can restore the overall amplitude. The "knee" parameter determines how quickly the reduction will come into effect. A soft knee create a gradual change. A hard knee is a more noticeable change. Finally compressors have attack and release parameters. These parameters impact how quickly the compressor will respond to changes in amplitude.

Compressors can be dangerous. While convenient, compressors can cause the music to lose some of its liveliness. I advise erring on the side of using too little compression instead of using too much compression.

Expanders

An expander works identical to a compressor, yet the impact of the ratio parameter changes. Instead of decreasing the amplitude, an expander increases the amplitude. We can use this effect when a compressor removed too much signal previously. Or, we can use the effect on drums and percussive instruments to add more pop.

De-esser

A de-esser is a special type of compressor for vocals. As the name suggests, a de-esser can target a specific band of frequencies, such as "esss" sounds, and reduce the amplitude on specific ranges. A de-esser is common to give the sound a more natural appearance.

Multi-pressor

Finally, a multi-pressor is a collection of individual compressors for specific frequency ranges. So there would be one compressor for bass frequencies, one for low-mids, one for high-mids, and one for high frequencies, for example. We use multi-pressors commonly on the entire output for the music instead of on an individual instrument to help balance the mix.