The Sone: Psychoacoustic Loudness Unit

Definition and Relationship to the Phon Scale

The sone is defined so that a loudness level of 1 sone corresponds to a pure tone at 1 kHz presented at a sound pressure level of 40 phon (equivalent to 40 dB SPL under free-field conditions). Loudness in sones doubles when the loudness level in phons increases by approximately 10 units for mid-frequency sounds. Mathematically, loudness N in sones relates to loudness level L_N in phons via N = 2^((L_N - 40)/10) for levels above 40 phon. The exponent reflects the psychophysical observation that equal increments in loudness correspond to equal ratios in physical intensity—a Weber-Fechner-type relationship but with a base-two scaling anchored at the threshold of comfortable listening.

Because the sone is perceptual, it has no direct SI dimension, yet ISO standards treat it as a defined unit for reporting loudness. When documenting measurements, specify the calculation method—ISO 532-1 (Zwicker method) or ISO 532-2 (Moore-Glasberg method)—because each method handles critical bands, temporal integration, and binaural effects differently. Mentioning the method ensures replicability and comparability when exchanging data with laboratories, regulatory bodies, or industry partners.

Historical Evolution of Loudness Measurement

Early studies of loudness by Fletcher and Munson in the 1930s charted equal-loudness contours using listening tests with telephone headsets. Their work introduced the phon scale, which equalises loudness to a 1 kHz reference. As consumer audio, broadcasting, and electroacoustic engineering advanced, practitioners needed a measure that conveyed how loud sounds felt to listeners, not just their physical intensity. This led to the sone, introduced by Stanley Smith Stevens in 1936 to provide a linear perceptual scale anchored to human judgement.

Subsequent decades refined experimental methods. Zwicker extended the concept by incorporating critical band theory, which recognises that the ear processes frequencies in bands with varying sensitivity. International standardisation arrived with ISO 226 (equal-loudness contours) and ISO 532, providing procedures to compute loudness from complex sounds using weighting filters, excitation patterns, and temporal integration. Modern approaches account for binaural listening, temporal masking, and spectral fine structure, enabling sones to represent not only steady tones but also broadband signals, impulsive sounds, and time-varying noise.

Psychoacoustic Concepts Underpinning the Sone

Critical Bands and Excitation Patterns

Loudness perception depends on how energy distributes across the basilar membrane. ISO 532 methods calculate specific loudness by analysing excitation patterns within critical bands measured on the Bark or ERB (Equivalent Rectangular Band) scales. The total loudness in sones emerges from integrating specific loudness across frequency. This integration explains why two tones with the same sound pressure level but different spectra can have different loudness values. Spectral peaks that fall into the same critical band produce masking, reducing the incremental loudness contribution of added components.

Temporal Integration and Time Weighting

Human hearing integrates energy over short windows, typically between 100 and 200 milliseconds. ISO 532 implementations therefore apply temporal averaging or running integration before converting to sones. Rapid fluctuations undergo partial integration, which is why impulsive sounds may register high peak sound pressure but a moderate loudness value. For product testing, specify the integration time constant, windowing function, and any gating applied so stakeholders can interpret the reported loudness correctly.

Binaural Summation and Spatial Factors

Listening with two ears typically increases perceived loudness by about 1 to 2 sones compared with monaural listening for moderate-level sounds. ISO 532-1 includes binaural correction factors when measurements represent real-world listening. Spatial distribution, such as diffuse sound fields or directional sources, also affects loudness. Documenting the measurement geometry, microphone array, and headphone calibration is crucial so that loudness data align with expected use conditions in architectural acoustics or consumer product testing.

Applications in Industry and Research

Product Design and Consumer Experience

Appliance manufacturers, automotive engineers, and consumer electronics companies rely on sones to evaluate how users perceive sound. Dishwasher and refrigerator labels often report loudness in sones because consumers understand that a 2-sone appliance sounds twice as loud as a 1-sone appliance. Development teams pair sones with objective metrics like A-weighted decibels to identify design changes—motor isolation, airflow optimization, or acoustic insulation—that deliver noticeable improvements in user comfort.

Environmental Noise Assessment

Urban planners and environmental consultants apply loudness models to predict community response to traffic, rail, and aviation noise. Loudness assessments complement A-weighted levels by highlighting spectral imbalances, tonal components, or modulation that increase annoyance. The sone-based perspective supports communication with stakeholders because it correlates more closely with perceived disturbance, enabling targeted mitigation measures such as barriers, low-noise pavements, or speed management.

Broadcasting and Audio Engineering

Loudness normalisation in broadcasting, streaming, and gaming aims to deliver consistent listener experience across programmes. Standards such as ITU-R BS.1770 use K-weighted loudness (in LKFS), but many engineers translate results into sones to communicate with multidisciplinary teams. Understanding how sones relate to phons, LKFS, and dB SPL helps manage dynamic range, avoid listener fatigue, and comply with regional regulations.

Hearing Health and Audiology

Audiologists use loudness scaling to diagnose hyperacusis, recruitment, and other auditory disorders. Sones quantify patient responses to calibrated stimuli, guiding the programming of hearing aids and cochlear implants. Documenting loudness growth functions in sones helps clinicians tailor compression strategies and evaluate rehabilitation outcomes alongside audiograms and speech intelligibility measures.

Working with Sones in Practice

When reporting loudness, include a table summarising the signal chain: microphone calibration level, weighting filters, sampling rate, and the ISO 532 implementation (software version, configuration). Provide plots of specific loudness versus critical band number to highlight spectral contributions. Where possible, report uncertainty by repeating measurements across subjects or by using Monte Carlo simulations of parameter variability. While sones are perceptual, rigorous documentation ensures reproducibility comparable to physical measurements in pascals or watts per square metre.

Many organisations pair loudness reports with complementary metrics such as sound power level and sound pressure level to provide a holistic acoustic profile. Include conversion graphs that map between sones and phons so stakeholders accustomed to decibel language can interpret results quickly. If you must combine multiple sources, first sum their spectra or sound pressure levels—using tools like the room reverberation time calculator to understand energy decay—before applying the loudness model. This avoids errors that would arise from adding sones directly.

Importance of the Sone in Contemporary Measurement

The sone captures human experience in a way that purely physical units cannot. It aligns acoustic engineering with user expectations, regulatory frameworks, and health considerations. By integrating sones with SI-based quantities like pascals, watts, and nepers, professionals ensure that perceptual insights inform design decisions without sacrificing measurement rigour. Whether you are an engineer optimising product sound quality, an audiologist tailoring therapy, or a researcher studying auditory perception, the sone provides a common language for communicating how loud sounds feel.

Continue exploring the measurement landscape with related guides on sound intensity, nepers, and practical tools like the noise exposure limit calculator. Together, these resources equip you to translate complex psychoacoustic phenomena into clear, actionable information for colleagues and stakeholders.