To not forget

To not forget (Monday, 8 August, 2022)

• A-weighting vs C-weighting: To measure the amount of noise (to prevent ear damage) it is common to use A-weighting since this puts more weight on the frequency range for which our ears are most sensitive.
• Sound velocity in air is about 344 m/s.  Thus, a 100Hz tone travels with 3.44 m/s  (i.e., the wavelength of a 100Hz tone is 3.44 meter). Since f=1/T, the period T of this tone is 0.01 seconds.
• Sound requires 1/344 = 2.9ms to travel one meter
• First harmonic = root note; second harmonic = first overtone
• Transient: very first start of a sound.
• Our hearing ranges from approximately 20 to 20,000 Hz. The middle is, however, not 10,000 Hz but about 640. Our hearing is logarithmic.
• According to the Dutch Working Conditions Act (Arbo wet), you can stay for 8 hours in a room where 80 dB SPL prevails. Every 3 dB added means a halving of the time you are allowed to stay in that room. So: 80 dB = 8 hours, 83 dB = 4 hours, 86 dB = 2 hours, 89 dB = 1 hour, 92 dB = 30 minutes, 95 dB = 15 minutes, 98 dB = 7.5 minutes, 101 dB = 3 ,75 minute.
• Impedance. All audio electronics have combinations of resistors, capacitors and inductors connected in circuits, along with ‘active’ components like transistors or valves which provide amplification or act as switches. To make life slightly easier for ourselves, we often consider the total ‘resistance’ of a complex circuit involving resistors, capacitors and inductors as a composite lump, and that’s what we call the impedance. Impedance has the symbol Z — hence references to high-Z inputs, for example — and is still measured in Ohms. However, the actual value depends to some degree on the frequency of the signal voltages involved.  [see Sound On Sound for an excellent explanation].
• Acoustics
  • Comb filtering: result of short reflections (less than 10ms). Delays from about 20-30ms become audible for sounds with a fast attack, while delays of 40-50ms become audible for sounds/instruments with a slow attack. To avoid comb filtering we should have delays of more than 10ms. Thus reflections should travel at least distance of 10/2.9 = 3.5 meter.  Thus, nearest reflection surface should be at least 1.75 meter from sound source. 2.9ms is time for sound to travel 1 meter.
  • 1/4 wavelength law.  To absorb a particular frequency the thickness of the absorber should be at least 1/4 of the wavelength.  Thus to absorb a tone of 1000Hz (with a wavelength of 0.344 meter, we need at least 8.6cm of thickness.
  • Live End Dead End design. LEDE is a trademarked term for a particular acoustic design. In an LEDE studio, the area around the monitors is deadened, or made absorbent acoustically. The remainder of the room (behind the listener) is made “live” or reflective. The main principle is that the arrival of reflections at the console is in a specific order: 1. direct sound from the monitors; 2. First studio reflection (from the recording room, through the mics and monitors); 3. First control room reflection (off the back wall, assuming it is 10 feet or so behind the engineer). The idea is that by staggering these arrivals, the control room reflections don’t interfere with monitoring recorded studio acoustics.
  • Reflection Free Zone. RFZs are generally created through a combination of absorption and geometry, where the shape of the room focuses any reflections away from the “sweet spot” or desired RFZ.
  • Early Sound Scattering. ESS is a design for control rooms where the characteristic reflections (of all, including bass, frequencies) are so random that they have no character to impose on the listening space.
• Loudspeakers
  • Sensitivity. Volume (SPL) at a distance of 1 meter when the amplifier delivers 1 Watt of power.
  • Crossover. Piece of electronics to split full width frequency signal into 2 or more parts. Active crossover splits the signal before the amplifier. Passive crossover splits the signal after the amplifier.
  • Damping.  Time needed for the cone to come to rest
  • Compliance.  Measurement of stiffness. That is, how easy the cone starts moving. Compliance and Damping are opposing speaker requirements.
• Microphones (see also here)
  • Transducer. Converts one form of energy into an other form.
  • AKG D112 microphone is optimized for bass drum recording.
  • Neumann M149 microphone. Famous for vocal recording.
  • Proximity effect. This is the increase in bass response of a microphone when moving the sound source closer to the microphone.
  • Sensitivity. Output of microphone measured in mV / Pa at a signal of 1 kHz.
  • Self-Noise. Noise (dB) produced by microphone  itself. This is the signal level (Equivalent Input Noise) required to match the self-noise.
  • Polar patterns.
    • Omnidirectional microphones are equally sensitive to sound from all directions. Omni microphones don’t have proximity effect.
    • Cardioid microphones are most sensitive to sound coming from the front. Sound from the sides is picked up somewhat quieter, and sound from the rear is greatly rejected.
    • A microphone with a Figure-8 or bidirectional pickup pattern is sensitive to sound coming from the front and coming from the rear, but has a very high rejection for sound coming from the sides.
    • Supercardiod and hypercardioid microphones are even more directional than the standard cardioid. They’re slightly more sensitive from the back and have an even narrower range of sensitivity from the front.
  • Stereo setups
    • AB
      • Microphones in parallel
      • Omni or (hyper)cardioid microphones
      • Omni gives best stereo field. Stereo field result of volume and time differences between left and right microphone.
      • Low mono compatibility (comb filtering)
    • XY
      • Membranes above each other. Angle 90 degrees.  Don’t point at sound source (gives some coloring)
      • Directional microphone (hyper)cardioid)
      • Less convincing stereo field. Stereo field only result of volume differences between left and right microphone.
      • Good mono compatibility
    • ORTF
      • Membranes at distance of 17cm. Angle 110 degrees.
      • (hyper)cardioid microphones
      • Compromise between bad mono compatibility of AB and less convincing stereo field of XY.
    • NOS
      • Membranes at distance of 30cm. Angle 90 degrees.
      • (hyper)cardioid microphones
      • Compromise between bad mono compatibility of AB and less convincing stereo field of XY.
    • MS (middle-side)
      • Combination of bidirectional microphone (pointed side ways) and (hyper)cardioid (pointed to sound source).
      • Good stereo field and mono compatibility
      • Bidirectional microphone is connected to two mixer channels. The phase of one channel is reversed.
    • Blumlein
      • Two bidirectional microphones on top of each other. Angle 90 degrees.
      • Good stereo field and mono compatibility
    • Deca tree
      • Three Omni directional microphones. One pointed to the sound source. The others to the left and right.
      • Avoids a too large gap if microphones are placed at larger distances in AB setup.
• Mixers
  • Signal strength
    • dBu (unloaded). Professional audio standard. 0dBu = 0.775V; +4dBu=1.23V
    • 0 VU = +4dBu (although there are exceptions)
    • 0 dBV = 1 V. 0 dBu = -2.2 dBV
    • Consumer electronics works at -10 dBu (0.316 V)
  • Microphone level
    • Generally between -75 dBV (0.00018V) and -30 dBV (0.032).
    • The pre-amp (gain) will increase this signal to line level (1V)
  • After fader listening (AFL; Also called solo-in-place) and Pre-fader listening (PFL)
  • Q (quality) of parametric equalizer sets the bandwidth (bandwidth = f/Q)
  • Split monitor / inline monitor consoles.