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64 short explainers on the background to the tools: levels, hearing, stereo, microphones, electronics and rooms.
The topics are inspired by the long-running educational material of Eberhard Sengpiel at sengpielaudio.com. All text here is original. Missing a topic? Request an article.
Sound waves, decibels, sound field quantities and level meters.
How decibels add for independent and related signals.
Why audio levels come in so many decibel flavours, and how they relate.
What a decibel is, why power and voltage use different factors, and numbers worth remembering.
The inverse distance law, common mistakes and where it fails.
Nominal levels, alignment and keeping signals between noise and clipping.
How wavelets lead to loudspeaker arrays that recreate a sound field.
How VU, PPM, true-peak and loudness meters differ.
What the colours of noise mean and when to use each one.
What happens when sound meets surfaces and obstacles.
Amplitude measures, crest factor and meter pitfalls.
The four sound field quantities and the acoustic Ohm's law.
How air temperature and distance translate into delay, and how to use it for PA and microphones.
Pressure waves, frequency, period, wavelength and the speed of sound.
How we hear direction, loudness, space and timbre.
How sensitivity depends on frequency and level.
Source and filter in the voice, the vowel triangle and the singer's formant.
The harmonic series, inharmonicity and the missing fundamental.
Why stereo sounds different on headphones and what helps.
ITD, ILD, pinna cues, the cone of confusion and directional bands.
The gap between direct sound and first reflection, and pre-delay in reverb.
Direction and frequency dependence of strings, brass, flute and reeds.
How noise damages hearing, exposure limits and protection.
Why music got louder, LUFS, and normalisation.
How the first arrival decides direction.
What weighting filters do and how to read dB(A) and dB(C).
Stereo techniques, phantom sources and how microphones work.
The Jecklin disc, the Decca tree and outriggers.
Level-difference stereo with microphones at one point.
Why delayed copies of a signal create notches, and how to avoid them.
Combining level and time differences in near-coincident pairs.
Direct sound, room sound and the critical distance for microphone placement.
Sensitivity, self-noise, maximum SPL and how to compare them.
How the transducer principle shapes the sound and use of a microphone.
How sum and difference signals give a width control after recording.
Why stereo can fail in mono and how to check it.
Constant-power, linear and compromise pan laws and their level numbers.
Summing localisation, the first wavefront and the sweet spot.
Omni plus figure-8: cardioid, super- and hypercardioid compared.
The difference between inverting polarity and shifting phase.
Why bass rises close to a directional microphone, and by how much.
What the stereophonic recording angle means and how to match it to the ensemble.
Time-difference stereo, spacing and recording angle.
The two cues behind phantom sources and how they combine.
XY, Blumlein, Mid/Side, AB, ORTF, NOS and DIN: how each encodes direction.
The 5.1 layout, the centre and LFE channels, and recording for surround.
Setups and workflows for pianos, choirs, orchestras and mixing.
Impedance, balanced lines, loudspeakers, filters and equalisation.
Voltage sources, nominal impedance, damping factor, bi-wiring and bi-amping.
How balanced lines reject noise, XLR and TRS wiring and phantom power.
THD and THD+N in percent and dB, intermodulation and audibility.
Filter types, tonal regions and why to cut rather than boost.
Why slope, order, bandwidth and Q are different things.
Constant-voltage distribution for many loudspeakers over long cables.
Ranges for microphones, lines, instruments, headphones and speakers.
RIAA, tape and FM emphasis expressed as time constants.
Why modern audio connects low to high impedance.
Standing waves, reverberation, absorption and treatment.
Three different jobs and the tools for each.
Reverberation-chamber measurement, edge effects and practical coefficients.
What does and does not work in room treatment.
How the two parts of the sound field add and the room radius.
Per-band RT60, reverberation level versus time and early decay.
Standing waves in rooms: how they form, why they matter and what to do about them.
Reverberation time, how it is calculated and typical targets for different rooms.
The common mathematics of string modes and room modes.
Glossaries and quick lookups.