Equal-loudness contours curl up at the low and high ends, making bass and treble harder to notice at low volume

What it is

Plotting the sound pressure level needed at each frequency to sound "equally loud" produces an equal-loudness contour (originating in the early Fletcher–Munson measurements, later standardized as ISO 226). This curve is not flat — it dips in the middle and curls upward at both ends: making a low-frequency (bass) or high-frequency (treble) sound seem as loud as a mid-frequency sound (near where most speech energy sits) requires noticeably higher physical sound pressure. The shape of this curve is also not constant — the lower the overall loudness level, the steeper the upward curl at both ends; the higher the loudness, the flatter the curve becomes. This means that turning the same piece of audio down noticeably thins out its bass and treble more than it would at higher volume — not because the frequency content has actually changed, but because the ear's sensitivity to those frequencies is simply worse at low loudness to begin with.

Why it happens

The upward curl at both ends comes from the frequency-selective transfer of sound through the outer and middle ear: the ear canal itself has a physical resonance range that transmits mid frequencies (roughly where most speech energy concentrates) most efficiently, with efficiency dropping off toward both the low and high ends, requiring greater physical sound pressure to produce an equivalent response in the inner ear.

The fact that the curve's shape changes with overall loudness comes from a frequency-dependent compression in loudness growth itself: at higher sound pressure levels, loudness in the low-frequency range grows comparatively faster as pressure increases, which partly offsets the disadvantage of poor transfer efficiency, flattening the curve at high loudness. At lower sound pressure levels this compensation has not kicked in yet, so the poor sensitivity at both ends is fully exposed, and the curve curls up more sharply.

Studying it

The standard method is loudness matching: a reference frequency (commonly 1000 Hz) is fixed at a given sound pressure level, and listeners adjust a test tone at a different frequency until it subjectively sounds equally loud, recording the sound pressure level needed to achieve the match. Repeating this across many frequencies and reference loudness levels and connecting the results produces the equal-loudness contour for a given loudness level. This measurement has historically been repeated and revised multiple times (the early Fletcher–Munson data was later updated by the ISO 226 series of standards), and individual variability and differences between measurement methods (free field, diffuse field, headphone presentation) remain an active area of research.

Where it stops holding

• Laboratory equal-loudness contours are typically measured under calibrated free-field or diffuse-field conditions; applying them directly to an arbitrary consumer headphone or speaker is inaccurate, because the playback device's own frequency response adds a further layer of coloring on top of the ear's own contour. The two layers are combined in practice, so the contour's raw values should not be treated as a device-independent absolute standard.
• Hearing loss significantly reshapes an individual's equal-loudness contour, and certain types of hearing loss amplify the sensitivity gap at both ends that is otherwise normal at low loudness for the general population; such listeners cannot simply have the general-population standard contour applied to them.
• This entry explains only the loudness difference caused by frequency. It is an independent mechanism from loudness's own nonlinear growth with sound pressure (the loudness-versus-pressure relationship); the two combine to jointly determine the final result of "how loud a sound at a specific frequency and sound pressure level actually sounds."

Applying it

• This explains the physical and perceptual basis behind the loudness compensation ("loudness") feature common in consumer audio equipment: boosting bass and treble gain relatively more at low playback volume and easing off that boost at high volume matches the ear's actual sensitivity changes better than applying one fixed equalization curve across the whole volume range.
• When bass or treble content (a low, deep warning tone, a sharp cue sound) needs to remain identifiable across different playback volumes, do not tune the spectral design at one reference volume and assume the effect holds everywhere — identifiability of such content clearly worsens at low volume in particular.
• How to check: run listening tests on the same audio material at several different playback volumes, confirming whether low- and high-frequency content is noticeably less identifiable than mid-frequency content at low volume, rather than only auditioning at the single reference volume habitually used during design.

Related

• Same group: A3.02.1 Loudness does not scale linearly with sound pressure, so volume controls should step by loudness, not pressure · A3.02.2 Ambient noise sets the usable loudness floor · A3.02.4 A brief sound's perceived loudness is lower than a sustained sound at the same sound pressure · A3.02.5 A uniform volume number across devices does not guarantee uniform perceived loudness
• Nearby: A3.01 Audible frequency range
• Search terms: equal-loudness contour · Fletcher-Munson · ISO 226 · loudness compensation