HON95-wiki/media/audio/basics.md

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title: Audio Basics breadcrumbs:

• title: Media

• title: Audio

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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.

• 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).

• 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.

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