audio-basics.md 4.9 KB
title: Audio Basics breadcrumbs:
title: Media
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Bands:
- Lows (ca. 20Hz-100Hz)
- Low midrange (ca. 100Hz-1kHz)
- High midrange (ca. 1kHz-10kHz)
- Highs (ca. 10kHz-20kHz)
Signal levels:
- (Note) This is the voltage (and somewhat impedance) inside cables/equipment.
- Mic level: Output from a microphone. Very weak, requires a preamp.
- Instrument level: Output from e.g. a guitar. Like mic level but slightly stronger.
- Line level (+4dBu): Professional equipment.
- Line level (-10dBV): Consumer equipment. Lower than +4dBu. Not to be confused with dBv.
- Speaker level: High-power signal going from an amplifier to a (passive) speaker.
- Phono: Old, for turntables etc. Much lower voltage than line level. Typically needs a phono preamp/stage with RIAA equalization.
Balance mode:
- Unbalanced: Ground and signal.
- Balanced: Ground and hot and cold signal with equal impedance. The cold signal is 0V but not (directly) connected to ground.
- Differential: Balanced but the cold signal is the opposite voltage of the hot signal instead of 0V.
- Balanced and unbalanced mono plugs/sockets can generally be connected together (with the loss of the balanced signal), but don't connect e.g. a stereo unbalanced TRS to a mono balanced TRS. It'll sound weird due to the signal mismatch.
Ground loops:
- When there exists physical loop in the ground wires. Typically when devices are connected to different grounded power outlets.
- Different potentials in the loop will cause undesired current flow.
- Can be heard as a 50Hz/60Hz hum in the audio signal.
- Solutions:
- Use balanced signals.
- Connect all equipment to a single grounding point, i.e. a single power outlet.
- Break the shielding on one cable to break the loop. Different boxes, like DI units, may have this as a feature known as a ground lift. However, make sure all shields are connected at one end (don't lift everything). Don't break the shielding/earthing on devices that needs it for safety reasons!
- Use a ground loop isolation transformer.
- Group the ground cables together so no currents get induced into the cables.
- Use a resistor and/or a ferrite bead to limit AC current.
Phantom power:
- Applies 48V to XLR3 (or similar) inputs, for powering mics and similar.
- Applying this to devices which aren't made for it can break them.
Impedance:
- Basically resistance but for AC.
Proximity effect:
- Increase of low frequency response when an audio source is close to a directional or cardioid microphone.
Optimal SPL for mixing and listening:
- Optimal SPL for normal music mixing and listening is around 75 dB SPL (conversational level) for small rooms and 85 dB SPL for large rooms.
- The reason a specific SPL is optimal is due to equal-loudness contours, where the music actually sounds different at different volumes.
- Mixing at high volume may help for getting the correct feel of the low end, however.
Equal-loudness contours:
- The perceived loudness for a given SPL depends on the frequency.
- This is typically visualized as equal-loudness contours, with frequency on the first axis, SPL on the second axis and a set of equal-loudness curves.
- Fletcher–Munson curves is an early version of equal-loudness contours, but is still sometimes used to refer to the same thing.
- This is why low-volume music sounds so bass-less and why e.g. car stereos typically provide a "loudness" setting to try to correct it for low volume levels (and make it sound terrible for normal volume levels).
SPL meter weightings:
- Type A is used for noise measurements and cuts off lower and higher frequencies compared to type C.
- Type C has flatter response than type A and is often used for audio calibration.
Feedback:
- Happens when sound is fed from speakers back into a microphone (accidentally), at a high enough "loop gain" that the feedback noise level quickly escalates to annoying/damaging levels.
- Generally only happens at certain resonating frequencies, depending on the venue/room.
- Preventing feedback:
- Avoid placing microphones in front of speakers.
- Use appropriate microphones, e.g. dynamic microphones pointing away from any (loud)speakers.
- Use an equalizer to reduce the level for feedback-inducing frequencies. To find the frequencies, test the setup at loud levels to try to induce it, then measure which frequency it's happening at.
- Don't use "feedback destroyers", they're crap.
White and pink noise:
- White noise has equal energy per frequency. The frequency-SPL curve is flat.
- Pink noise has equal energy per octave. Since each octave has double the frequency of the previous (
log2(freq) = octave), the frequency-SPL curve is decreasing. This makes pink noise appropriate for equalization for human hearing.
Pan law: TODO
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