8. Psychoacoustics (Perception Layer) · Concept 2 of 18
Equal-Loudness Contours
The modern, updated version of those same curves showing which pitches need more push to feel equally loud.
Bass and 'air' sit in valleys: they need far more dB to feel as loud as the easy 2-5 kHz mids.
What it is
Standardised curves (ISO 226) showing how much level each pitch needs to SOUND equally loud to human ears.
Key facts
Standard: ISO 226:2003, revised ISO 226:2023 - replaced the old 1933 Fletcher-Munson curves.
Loudness unit = phon. A contour's phon number = its dB SPL value at 1 kHz (the reference pitch).
0 phon = threshold of hearing; 1 kHz tone at 0 dB SPL sits on the 0-phon line by definition.
Ear is MOST sensitive 2 kHz-5 kHz (ear-canal resonance ~3.4 kHz adds up to +10-12 dB gain).
Bass is the big sag: to match a 1 kHz tone at 40 phon, a 20 Hz tone needs roughly +60-70 dB MORE level.
Curves FLATTEN as level rises - at 90-100 phon bass needs far less boost than at 40 phon.
phon = perceived-loudness-matched dB; sone = linear loudness scale where 1 sone = 40 phon.
+10 phon (10 dB) is judged roughly TWICE as loud = +1 sone-doubling step.
+10 dB SPL = 10x the sound INTENSITY (power); it only feels ~2x louder.
Speed of sound in air = 343 m/s at 20 C; +6 dB = double pressure, -3 dB = half power.
How it works
Pick a reference: a 1 kHz tone at a set dB SPL (e.g. 40 dB).
Play a second tone at another pitch; raise/lower its level until it FEELS equally loud.
Record the dB SPL needed - that's one point on the curve.
Repeat across 20 Hz-20 kHz to trace one full contour.
Redo at 0, 20, 40, 60, 80, 100 phon - each is a separate hill on the map.
Result: low and high ends sag (need more dB), the 3 kHz region dips (needs less).
Real examples
Quiet home hi-fi: bass vanishes, so the 'Loudness' button boosts lows + highs to fake a flat curve.
Mixing a track LOUD then playing it soft: the bass and air seem to disappear - the curves predict it.
A 20 Hz sub tone and a 1 kHz beep at the SAME dB SPL: the 1 kHz sounds way louder.
Cinema reference level (85 dB SPL, Dolby) is chosen so mixes land on flatter, more reliable contours.
How it helps in live sound
Tune the system at SHOW SPL, not quietly - low-level tuning over-boosts bass that turns to mud when loud.
Soundcheck at performance level; band + crowd raise the operating point ~80-100 phon where curves flatten.
If you must mix quieter, expect to add a little low + high shelf to keep it feeling full.
Use a calibrated SPL meter (C-weighting for bass-heavy gigs) so you KNOW your phon operating point.
Don't chase a flat RTA at low volume - the ear isn't flat; trust the contours and your ears at gig level.
Subs that 'felt right' at 75 dB will be overcooked at 100 dB - pull 4-8 dB of sub when you push the rig.
Everyday analogy
It's like a hilly map where bass and 'air' sit in valleys - you must shout (add level) for them to be heard as loud as the easy mid-range peak.
Watch out
Myth: bass needs the same EQ at any volume. Truth: at low SPL bass needs big boost, but the curves flatten when loud - over-EQ'd lows turn to mud at show level.
Fun fact
Your ear canal is a ~2.5 cm tube that resonates near 3 kHz, giving a free +10 dB acoustic boost right where speech consonants live - which is exactly why those frequencies feel loudest.
Key takeaways
Equal-loudness contours = how many dB each pitch needs to FEEL equally loud (ISO 226).
Bass and high 'air' sag - they need MORE level; 2-5 kHz needs LESS.
phon = loudness label tied to dB at 1 kHz; +10 phon ~ twice as loud.
Curves FLATTEN at high SPL - bass boost matters far less when loud.
Tune and mix at SHOW level so the ear's curve matches the audience's.
Low-level tuning lies to you: it over-boosts bass that muds up at full volume.