How does a compressor control dynamic range, and what does each parameter do?
Dynamics processing: the compressor and its parameters (threshold, ratio, attack, release, knee, makeup gain), gain reduction, limiting, the noise gate and expander, and creative uses such as controlling peaks, adding punch and parallel compression.
A focused answer to the Edexcel 9MT0 dynamics content, covering the compressor and its parameters (threshold, ratio, attack, release, knee, makeup gain), gain reduction, limiting, gating, and creative compression.
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What this dot point is asking
Edexcel wants you to control dynamic range with a compressor and to explain every parameter (threshold, ratio, attack, release, knee, makeup gain). You must also know limiting, gating and expansion, and the creative uses of compression such as adding punch and parallel compression. Component 4 routinely asks you to describe compressor settings for a named source and to justify them, so the parameters must be precise.
The answer
What a compressor does
Compression is used everywhere: to keep a vocal consistent, to tighten a bass, to add punch to drums, and to glue a whole mix together. Understanding it is essential for Component 4.
The parameters
These interact. A fast attack with a high ratio clamps peaks hard; a slow attack lets the initial transient through, preserving punch, before the body is compressed. A short release can cause audible pumping; a longer one is smoother.
Limiting, gating and expansion
Creative compression
Examples in context
When a vocal sits steadily in a busy mix, a compressor with a moderate ratio and careful attack and release is evening out the level. When a tom track is silent except on the hits, a noise gate is removing the spill in between. When a drum bus sounds powerful yet still natural, parallel compression is adding density under the preserved transients. Dynamics processing is how you control and energise the levels in a mix.
Try this
Q1. What does the threshold control on a compressor set? [1 mark]
- Cue. The level above which compression begins.
Q2. A compressor is set to a ratio of . For every dB above the threshold, how many decibels come out? [1 mark]
- Cue. dB.
Q3. State what a noise gate does and one typical use. [2 marks]
- Cue. It mutes or reduces signals below the threshold, for example to remove mic bleed between notes.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 9MT0/04 20205 marksA vocal track has an inconsistent level that jumps between quiet and loud phrases. Explain how you would use a compressor to even it out, describing the role of the threshold, ratio, attack, release and makeup gain.Show worked answer →
A compressor reduces the dynamic range by turning down signals that rise above a set level. To even out the vocal, set the threshold so the louder phrases cross it while the quietest passages stay below it, so only the loud parts are reduced. Set a moderate ratio (for example or ) to control how much the parts above the threshold are turned down without over-squashing. Set the attack fast enough to catch the peaks but not so fast that it dulls the consonants and transients; set the release so the gain recovers smoothly between phrases without pumping. Finally, apply makeup gain to raise the now-compressed signal back up, so the quiet phrases sit louder relative to the (controlled) loud ones, giving a consistent, present vocal.
Markers reward threshold = level at which compression starts, ratio = amount of reduction, attack = how fast it acts, release = how fast it recovers, makeup gain = restoring level, and the overall effect of a more even, consistent vocal.
Edexcel 9MT0/04 20234 marksExplain the difference between a compressor and a noise gate, and give one typical use of each in a mix.Show worked answer →
A compressor acts on signals above the threshold, reducing their level to control loud peaks and even out dynamics. A noise gate acts on signals below the threshold, reducing or muting them, so it removes quiet unwanted sound. They work at opposite ends of the level range.
A typical use of a compressor is to even out and add sustain or punch to a vocal, bass or drum, keeping a consistent level. A typical use of a noise gate is to remove background noise, mic bleed or hiss between notes, for example gating a tom track so it is silent except when the tom is hit, tightening the drum sound.
Markers reward compressor acts above threshold (reduces loud parts), gate acts below threshold (mutes quiet parts), plus a valid use of each.
Related dot points
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A focused answer to the Edexcel 9MT0 EQ content, covering the frequency bands, high-pass and low-pass filters, shelving and parametric EQ, cut and boost, the Q control, and subtractive, corrective and creative equalisation in a mix.
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A focused answer to the Edexcel 9MT0 mixing content, covering setting levels and the static balance, frequency balance and masking, the three dimensions of a mix, creating depth, bus routing and the mixdown.
- The decibel as a logarithmic ratio: the power formula and the amplitude (voltage) formula, dBFS and headroom, the relationship between decibel change and perceived loudness, and dynamic range.
A focused answer to the Edexcel 9MT0 decibel content, covering the decibel as a logarithmic ratio, the power and amplitude formulae, dBFS and headroom, how decibel changes map to perceived loudness, and dynamic range.
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Sources & how we know this
- Pearson Edexcel A-Level Music Technology (9MT0) specification — Pearson Edexcel (2017)