Loudness is measured by listeners assigning numbers relative to a reference sound.

Evidence

In a common implementation of the loudness balancing procedure, we measure the level of a sound of interest (the test stimulus here) that matches the level of a 1000-Hz stimulus (the reference stimulus). For these studies, a 1000-Hz tone is always the reference sound, and it is presented at some fixed stimulus level (e.g., 40 dB SPL). The two sounds (the 1000-Hz reference and the test stimulus) are always presented in succession (never simultaneously) and sometimes to opposite ears if the listener has normal hearing. In a typical experiment, the method of adjustment is used where the listener adjusts the level of the test sound so that its loudness is equal to that of the reference sound. The level of the 1000-Hz tone in dB SPL is the loudness level of the test sound, measured in phons. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745 (#expands, via linked claim)

Stevens’ law says that a sensation belonging to the category of intensity sensations grows with physical intensity according to a power law. In consequence of that law, we have to assume that a relative change in loudness is proportional to a relative change in intensity. Specific loudness is developed using this power law. Instead of using the total loudness as the value in question, specific loudness is used. Additionally, instead of intensity, excitation has to be used. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1262 (#expands, via linked claim)

Categorization

• XREF: connects to: Loudness is measured subjectively against 1 kHz reference tones — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1169; Loudness depends on frequency, angle, duration, and envelope, not just level — Toole, Floyd E, Sound reproduction- the acoustics and…, 903; Loudness grows as a power law of physical intensity so relative changes match — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1262 (+6 more)

Broader Connections

• The phon scale defines loudness relative to equal-loudness matches at 1 kHz #expands (0.90) — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 745
• Loudness grows as a power law of physical intensity so relative changes match #expands (0.85) — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1262
• Categorical loudness measurements use fixed repeated stimuli spanning the dynamic range #expands (0.85)
• Loudness depends on frequency, angle, duration, and envelope, not just level #expands (0.78) — Toole, Floyd E, Sound reproduction- the acoustics and…, 903
• Loudness perception is essentially logarithmic #expands (0.78) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 611
• Hearing forms loudness by integrating sound energy over about 100-200 ms #expands (0.75) — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1171
• Loudness depends on more than physical sound intensity alone #expands (0.75) — Dijk, Pim van, Physiology, Psychoacoustics and…, 2610
• Loudness perception is probabilistic, with a response distribution at each stimulus level. #expands (0.75)