THX standard level 85dB SPL at -20dB full scale at the listening position
National Institute for Occupational Safety and Health NIOSH
Recommended Exposure Limit REL 85dB SPL 8 hours
NIOSH REL 105dB SPL 4 minutes 43 seconds
NIOSH REL 116dB SPL 22 seconds
NIOSH REL 119dB SPL 11 seconds
NIOSH "Ocupational Noise Exposure" https://www.cdc.gov/niosh/docs/98-126/default.html
geometric mean 20 Hz to 20 kHz = 632 Hz = center of the audio spectrum
octave = log2 (frequency 2 / frequency 1) = log10 (frequency 2 / frequency 1) / log10 2
12 semitones = 1 octave equal temperament
4186.009 Hz = C8 piano key 88
698.4565 Hz = F5 top line treble clef
659.2551 Hz = E5
440.0000 Hz = A4 standard pitch
349.2282 Hz = F4
329.6276 Hz = E4 guitar open high E
261.6256 Hz = C4 middle C between bass and treble clefs
97.99886 Hz = G2 bass guitar open G, bottom line bass clef
82.40689 Hz = E2 guitar open low E
41.20344 Hz = E1 bass guitar open E
32.70320 Hz = C1 16’ organ pipe
27.50000 Hz = A0 piano key 1
Wikipedia, "Piano key frequencies" https://en.wikipedia.org/wiki/Piano_key_frequencies
Rod Elliott, Elliott Sound Products ESP, "Benefits of Biamplification" https://sound-au.com/bi-amp.htm https://sound-au.com/bi-amp2.htm
Rod Elliott, Elliott Sound Products ESP, “Why Do Tweeters Blow?” https://sound-au.com/tweeters.htm
Rod Elliott, Elliott Sound Products ESP, "Baffle Step Compensation" https://sound-au.com/bafflestep.htm
Siegfried Linkwitz, Linkwitz Lab, "Crossovers" https://www.linkwitzlab.com/crossovers.htm
Sublime Acoustic K231 active crossover https://sublimeacoustic.com/products/k231-stereo-3-way-active-crossover
Audiophile Heretic, July 3, 2026 Menu