Reference
Glossary of Terms
Definitions of audio, acoustics, and signal processing terms used throughout Sounn and this documentation.
A
- Axial Mode
- A room mode occurring between two parallel surfaces. The strongest type of room mode, causing significant bass peaks and nulls.
Related: Room Mode, Tangential Mode, Oblique Mode
C
- Coherence
- A measure of how consistent a measurement is across multiple averages. Values range from 0 to 1, with 1 indicating perfect consistency. Low coherence suggests noise or nonlinear distortion.
- Comb Filtering
- A series of regularly-spaced peaks and dips in frequency response caused by interference between direct sound and a delayed copy (such as a reflection).
- Convolution
- A mathematical operation that applies a filter (impulse response) to an audio signal. Used to implement FIR room correction.
D
- dB (Decibel)
- A logarithmic unit used to express the ratio between two values. In audio, typically used for sound pressure level (SPL) or signal levels. A 6 dB increase represents a doubling of amplitude; 10 dB sounds roughly twice as loud.
- dB SPL
- Decibels relative to the threshold of human hearing (20 micropascals). Used to measure absolute sound pressure level. Normal conversation is about 60 dB SPL; measurement signals typically target 70-80 dB SPL.
E
- EQ (Equalization)
- The process of adjusting the frequency balance of audio. Sounn uses EQ to correct room-induced frequency response problems.
F
- FIR (Finite Impulse Response)
- A type of digital filter with a finite-length impulse response. FIR filters can provide linear-phase correction (no phase distortion) and can correct both magnitude and phase. The trade-off is added latency.
- Frequency Response
- A measurement of how loud different frequencies are at a given position. An ideal frequency response for listening is smooth with gentle roll-offs at the extremes.
G
- Group Delay
- The time delay experienced by different frequencies passing through a system. Ideally constant across all frequencies (indicating time-aligned arrival). Variable group delay causes phase distortion.
H
- Harman Target Curve
- A target frequency response developed by Harman International based on extensive listening preference research. Features mild bass boost and gentle treble roll-off. Widely considered to match average listener preferences.
- House Curve
- A target frequency response with more bass boost and treble roll-off than flat or Harman curves. Popular for home theater and listeners preferring warmer sound.
- Hz (Hertz)
- The unit of frequency, measuring cycles per second. Human hearing ranges from about 20 Hz (low bass) to 20,000 Hz (high treble). Room modes typically occur below 300 Hz.
I
- IIR (Infinite Impulse Response)
- A type of digital filter (including traditional parametric EQ) that theoretically has an infinite impulse response. Efficient to compute but introduces phase shifts correlated with magnitude changes.
- Impulse Response
- The output of a system when given an instantaneous input (impulse). Captures all information about a system's behavior including frequency response and time-domain characteristics. The basis for FIR correction.
L
- Latency
- The delay between audio input and output. FIR filters add latency proportional to their length. Important for video sync and real-time applications.
- Linear Phase
- A characteristic of filters (like FIR) that delay all frequencies equally. Preserves waveform shape and transient accuracy. Contrast with minimum phase (IIR) which delays different frequencies differently.
M
- MLS (Maximum Length Sequence)
- A type of test signal that sounds like white noise but has precise mathematical properties allowing accurate measurement of impulse response. Sounn uses MLS for room measurement.
- Modal Frequency
- A frequency at which a room mode occurs. Determined by room dimensions. At modal frequencies, sound energy builds up creating peaks (at pressure maxima) and nulls (at pressure minima).
N
- Null
- A point in space or frequency where sound cancellation occurs, resulting in significantly reduced level. Room modes create nulls at specific positions. Difficult to correct with EQ because boosting a null wastes amplifier headroom.
O
- Oblique Mode
- A room mode involving all six room surfaces. The weakest type of room mode, typically 6 dB lower than axial modes.
- Octave
- A doubling of frequency. From 100 Hz to 200 Hz is one octave. Smoothing is often specified in fractions of an octave (1/6 octave, 1/3 octave).
P
- Parametric EQ
- An equalizer that allows adjustment of frequency, gain, and Q (bandwidth) for each filter band. The building block of IIR room correction.
- Peak
- A frequency or frequency range with elevated level compared to surroundings. Room modes create peaks at pressure maximum positions. Can be corrected effectively with EQ cut.
- Phase
- The timing relationship of a wave cycle, measured in degrees (0-360) or time. Phase relationships between frequencies affect how transients are reproduced.
Q
- Q (Quality Factor)
- A measure of filter bandwidth. Higher Q means narrower bandwidth. In parametric EQ, Q determines how much of the surrounding frequencies are affected by a filter.
R
- Room Mode
- A resonance caused by sound reflecting between room surfaces. Creates position-dependent peaks and nulls in bass response. The primary cause of bass problems in small rooms.
Related: Axial Mode, Tangential Mode, Oblique Mode, Standing Wave
- RT60
- Reverberation time - the time for sound to decay by 60 dB after the source stops. Indicates how 'live' or 'dead' a room sounds. Ideal RT60 for home listening is typically 0.3-0.5 seconds.
S
- Sample Rate
- The number of audio samples per second, measured in Hz. CD audio is 44,100 Hz (44.1 kHz). Higher sample rates (48, 96, 192 kHz) allow capturing higher frequencies but require more processing.
- SBIR (Speaker-Boundary Interference Response)
- A frequency response dip caused by destructive interference between direct sound and a reflection from a nearby wall. Frequency depends on speaker-to-wall distance.
- Smoothing
- Averaging frequency response data over a range of frequencies, reducing the appearance of fine detail. Used to prevent over-correction of minor variations. Specified in fractions of an octave.
- SNR (Signal-to-Noise Ratio)
- The ratio between desired signal level and background noise, measured in dB. Higher is better. For room measurement, 40+ dB SNR is excellent.
- SPL (Sound Pressure Level)
- A measure of how loud sound is at a given position. Measured in dB SPL. Safe listening levels are typically 70-85 dB SPL.
- Standing Wave
- A wave pattern that appears stationary due to interference between waves traveling in opposite directions. Room modes are standing waves. Also called eigenmodes or resonances.
Related: Room Mode
T
- Tangential Mode
- A room mode involving four surfaces (two pairs of parallel walls). Typically 3 dB weaker than axial modes.
Related: Room Mode, Axial Mode, Oblique Mode
- Taps
- The number of coefficients in a FIR filter. More taps allow correction of lower frequencies and longer time-domain problems, but add more latency. Typical FIR filters use 16,384 to 65,536 taps.
- Target Curve
- The desired frequency response after correction. Options include Flat, Harman, House, and custom curves. Personal preference determines the ideal target.
Related: Harman Target Curve, House Curve
- Time Domain
- Analysis or correction based on how audio changes over time (as opposed to frequency domain). FIR filters can correct time-domain problems like modal ringing.
W
- Waterfall Plot
- A 3D graph showing frequency response over time. Useful for visualizing how long different frequencies ring in a room. Reveals room mode decay behavior.