Amplitude, loudness, and are crucial concepts in sound design. They determine how we perceive volume and intensity in audio. Understanding these elements helps create balanced, impactful mixes and shape the emotional impact of sound.
Measuring amplitude involves decibels and sound pressure levels. We'll explore peak and RMS levels, loudness perception, and . We'll also dive into , a key tool for controlling volume and shaping sound in modern audio production.
Measuring Amplitude
Decibel Scale and Sound Pressure Level
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Acoustic quantities, part 1: What are decibels? - Erlend M. Viggen View original
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(dB) logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity
Comparing (SPL) to a reference level, typically the at 1kHz (2×10−5 Pa)
Formula: SPL=20log10(p0p), where p is the RMS sound pressure and p0 is the reference pressure
Sound Pressure Level (SPL) measure of the effective pressure of a sound relative to a reference value, expressed in decibels (dB SPL)
0 dB SPL corresponds to the threshold of hearing at 1kHz, while 120 dB SPL is the
Every increase of 6 dB SPL approximately doubles the sound pressure and perceived loudness
Peak and RMS Levels
maximum absolute value of the waveform in a given time interval
Represents the highest point the waveform reaches, either positive or negative
Does not provide information about the average loudness or energy of the signal
Used to determine the and prevent (digital caused by exceeding the maximum level)
RMS (Root Mean Square) level average level of a signal over time, calculated by squaring the signal, finding the mean, and taking the square root
Provides a better indication of the perceived loudness and energy of the signal compared to peak level
is always lower than the peak level, with the difference depending on the (ratio of peak to RMS level) of the signal
Signals with a high crest factor (transient-rich sounds like drums) have a larger difference between peak and RMS levels compared to signals with a low crest factor (sustained sounds like synth pads)
Perception of Loudness
Fletcher-Munson Curves and Hearing Thresholds
Fletcher-Munson curves () represent the sound pressure level required for pure tones at different frequencies to be perceived as equally loud by human ears
The curves demonstrate that human hearing is most sensitive around 3-4 kHz and less sensitive at low and high frequencies
At lower sound pressure levels, the curves are more compressed, indicating that the perceived loudness difference between frequencies is more pronounced
Threshold of hearing lowest sound pressure level that can be perceived by human ears at a given frequency, typically around 0 dB SPL at 1 kHz for young, healthy individuals
The threshold of hearing varies with frequency, as described by the Fletcher-Munson curves
Age, exposure to loud sounds, and other factors can increase the threshold of hearing over time
Threshold of pain sound pressure level at which sound becomes painfully loud, typically around 120-140 dB SPL
Exposure to sounds above this level can cause immediate hearing damage and should be avoided
The threshold of pain is relatively consistent across frequencies, unlike the threshold of hearing
Dynamic Range Compression
Compression Parameters
relationship between the input and output levels above the threshold
Ratio of 2:1 means that for every 2 dB increase in input level above the threshold, the output level increases by 1 dB
Higher ratios (4:1, 8:1, 20:1) result in more aggressive compression, while lower ratios (1.5:1, 2:1) provide gentler compression
input level at which the compressor starts to reduce the gain of the signal
Signals below the threshold pass through the compressor unaffected
Lower thresholds result in more of the signal being compressed, while higher thresholds affect only the loudest parts of the signal
Peak level and RMS level detection methods used by compressors to determine the input level and apply gain reduction
Peak level detection responds to the instantaneous peak levels of the signal, resulting in faster attack times and more aggressive compression
RMS level detection averages the input signal over a short time window, providing a more accurate representation of the perceived loudness and resulting in smoother, more natural-sounding compression
Attack and Release Times
duration between when the input signal exceeds the threshold and when the compressor reaches its target gain reduction
Faster attack times (1-10 ms) catch transients and provide more aggressive control, while slower attack times (20-100 ms) allow transients to pass through and sound more natural
The appropriate attack time depends on the material and the desired effect (punchy vs. smooth compression)
duration between when the input signal falls below the threshold and when the compressor returns to its normal gain
Faster release times (20-100 ms) can cause pumping and breathing artifacts, as the compressor rapidly adjusts the gain
Slower release times (100-500 ms) provide a smoother, more transparent compression, but may not react quickly enough to sudden level changes
The release time should be set in relation to the tempo and rhythm of the material to avoid unnatural-sounding gain changes