SINAD Signal-to-Noise And Distortion ratio

What SINAD is and what it measures

SINAD stands for Signal-to-Noise And Distortion. It collapses two different impairments — broadband noise and harmonic distortion — into one ratio against the desired signal.

The figure is reported in decibels. Under the standard audio definition, the numerator is signal-plus-noise-plus-distortion and the denominator is noise-plus-distortion alone, so the ratio is always at least 1 and the dB number is always positive — a higher dB value means a cleaner signal. A second convention used in some communications contexts ratios the pure test-signal power against the residual and can therefore go below 1 (negative dB), so the always-positive rule applies specifically to the audio convention used by Audio Precision, Stereophile, and ASR.

The canonical stimulus is a single sine wave, almost always at 1 kHz because it sits in the middle of the human hearing band; 400 Hz is the common alternative in radio and communications work. Audio Science Review (audiosciencereview.com), founded by Amir Majidimehr, established SINAD as the single headline figure of merit for DACs and amplifiers in the consumer audio enthusiast community, publishing a continuously updated SINAD-rank chart with results grouped into colored tiers and a full FFT plot alongside every measurement.

How SINAD is calculated

Under the audio-industry definition, SINAD in decibels is computed as 10·log₁₀[(P_signal + P_noise + P_distortion) / (P_noise + P_distortion)] — the ratio of the total measured signal power (the test tone plus everything else in the band) to the residual noise-plus-distortion power that remains after the test tone is notched out.

SINAD is the reciprocal of THD+N expressed in dB. A THD+N of 0.1% (which corresponds to −60 dB) is the same measurement reported the other way around as a SINAD of +60 dB; the two numbers describe the same residual energy, with THD+N expressing it as a fraction of the test tone and SINAD expressing it as a ratio of the test tone to the residual.

That reciprocity only holds when both numbers are computed over the same bandwidth. The residual is RMS-summed after the test tone is notched out, and that residual changes with the analyzer's input bandwidth. Audio Precision notes that an analyzer with, for example, a 22 kHz input bandwidth will only capture the third harmonic of test tones up to about 7 kHz — so reported SINAD numbers are only directly comparable when the analyzer bandwidth (commonly 20 Hz–20 kHz, sometimes 22 kHz, sometimes A-weighted) is the same.

What good SINAD numbers look like

SINAD converts directly into an equivalent ideal-converter resolution via ENOB = (SINAD − 1.76) / 6.02, where SINAD is in dB. The 6.02 dB/bit factor converts decibels (a log₁₀ representation) to bits (a log₂ representation), and the 1.76 dB term comes from quantization error in an ideal ADC. This is why a real 24-bit DAC with a measured SINAD of around 120 dB has an ENOB of roughly 19.6 bits, not 24 — the missing bits are eaten by analog noise and distortion.

Published just-noticeable-distortion data suggests SINAD figures well below modern DAC measurements are already audibly transparent in pure-tone listening. A summary of psychoacoustic studies (citing ISO 226-2003 hearing-threshold curves and Zwicker & Fastl's Psychoacoustics: Facts and Models) reports that hard-clipping at 1 kHz becomes just noticeable around a SINAD of about 65 dB, with lower-order 3rd-harmonic distortion thresholds in the 50–60 dB range; above ~80–90 dB SINAD at the listening level, single-tone harmonic distortion is reliably below masking thresholds for typical program material.

Modern well-engineered DACs and amplifiers routinely measure with SINAD figures well beyond those audible thresholds. As Headphones.com puts it, noise and distortion are no longer concerns from affordable, well-engineered amplifiers and DACs, which consistently measure beyond the limits of human hearing and are therefore audibly transparent for music playback. Note: ASR's exact analyzer bandwidth, headline-SINAD reference level by product class, and the dB cutoffs for its color tiers are not published in a single canonical methodology post; the rank table itself remains the authoritative reference.

What SINAD does and doesn't capture

A single SINAD figure collapses two perceptually different problems — broadband noise and tonal harmonic distortion — into one number, so two devices with the same SINAD can sound completely different: one may have benign low-order distortion masked by the fundamental while the other has audible high-order spurs or hiss. As Headphones.com puts it, SINAD simply combines all of the unwanted parts of the signal into one number and has trouble capturing what the listener actually hears, which is why ASR publishes a full FFT plot alongside every SINAD figure.

SINAD is also distinct from three closely related metrics that each isolate one piece of what SINAD lumps together. SNR measures only the noise floor relative to signal, with no distortion. THD measures only harmonically-related distortion components — per Audio Precision, the RSS summation of the harmonic components divided by the RMS level of the fundamental signal — with noise excluded. IMD measures the sum and difference frequencies created when two or more tones are played simultaneously, which a single-tone SINAD cannot reveal at all.

This is why a single 1 kHz SINAD measurement is insufficient on its own. Multitone tests (for example, a 32-tone signal spaced across the audio band) excite intermodulation products and reveal jitter and sample-rate-conversion errors that single-tone SINAD cannot. Audio Precision states the case directly: multitone distortion is generally better at detecting real-world problems involving clock jitter and sample rate conversion than traditional THD+N measurements, and for DACs in particular a single-tone SINAD will not show jitter sidebands that fall on frequencies absent from the test stimulus.