An academic in-depth look into the physics behind sound and audio, and how the science comes into play in professional audio recording.

1. By Fred Ginsburg CAS PhD

2. • Vertical axis = AMPLITUDE (aka loudness)
• Horizontal axis = FREQUENCY
• Positive and Negative components of the wave
• Expressed as Hz or kHz. Example: 22 kHz (or 22k for
short)

3. • Digital audio is a graphic “snapshot of the sine wave”
• The “pixel grid” is divided into “vertical resolution” and
“horizontal resolution”.
• Vertical res are BITS, as in 16 bit, 24 bit, 32 bit floating.
• BITS actually refer to the size/detail of the data packet
• Horizontal res is SAMPLING RATE, slices per second, as
in 44.1 kHz, 48 kHz

4. • Sampling rates commonly range from 8 to 192.
• Low fidelity rates: 8, 11, 16, 22, 32. Poor freq response.
• Used for radio comms, telephone, cheap recorders,
along with lower bit rates.
• Higher sampling rates: 44.1, 48, 88.2, 96, 192
• 44.1 = consumer audio CD (.CDA)
• 48 = professional audio files, digital cinema/DCP
• 96 = uber quality (music) recording intended for future
editing and down conversion to 48 or 44.1
• 192 = NASA grade instrumentation recording of vibration

5. • “In order to get a clear “snapshot” of the freq response,
we need to sample it twice, and then subtract a portion
for housekeeping.
• “Twice, has something to do with the fact that each sound
wave has two parts to it, a positive curve and a negative
curve.
• “Housekeeping requires 2k.”
• Sampling rate, divide by two, subtract another 2k = Freq
Response. (Sample/2) minus 2 = Freq.
• *note: Prof Ginsburg’s words, not the actual
mathematical (geek speak) theorem.

6. • Example. 48k sampling rate
• 48/2 = 24
• 24 minus 2 = 22
• Freq response of 48k is only 22 kHz, as in 20-22 kHz
• Example. 44.1 sampling rate
• 44/2 = 22
• 22 minus 2 = 20
• Freq response of 44.1 k is only 20 kHz, as in 40-20 kHz

7. • Noise purposely added to the digital track to mask the
sterile, “on/off” listening experience of pure digital (1-0-1-
0) sampling.
• Kind of a way of smoothing off the rough edges, so to
speak.
• Think of it as adding a touch of diffusion to make a high
resolution “facial portrait” more flattering!

8. • Freq response approx
20-22kHz, but reduces as
we get older to just 12 k or
14 k.
• Pain threshold approx 120
dB spl (sound pressure
level)
• Hearing damage from 90
dB
• Avoid loud earphones,
monitor speakers!!!
• Hearing damage is
permanent.

10. • A way to measure the volume (gain) of an audio signal
• Doubling the sound pressure (voltage) corresponds to a
measured level change of 6 dB. Doubling of sound
intensity (acoustic energy) belongs to a calculated level
change of 3 dB.
• Think of 3 dB as one F-stop of sound
• We perceive equal (voltage) levels of sound as different
“loudness” based on their frequencies, referred to as
Fletcher-Munson curves.
• We use weighted meters (such as A-weighted) to
compensate.
• Engineers use variations of the dB term to refer to actual
voltage levels of the audio signal. Too geeky to get into.

11. • 20 Hz to120 Hz = Bass.
• Most mics roll off around 80 Hz to filter out low freq noise such as
handling, wind, rumble/vibration, distant traffic, ventilation
systems.
• 100 Hz to 5 kHz = Mid-range. average range of human
voice
• 5 kHz to 6 kHz = Upper mid-range to lower High freqs.
• Sometimes we roll off sibilance (de-Essing) of human voice
• 6 kHz to 22 kHz = High frequencies. Harmonics.

12. • Reverb is characterized as random, blended repetitions
of a sound occurring within thirty milliseconds after the
sound is made. This is all the sound that immediately
bounces off any nearby surfaces before it gets back to
your ears.
• Echo is defined by distinct repetitions of a sound
occurring after 30 milliseconds. This is when you can
unquestionably hear a distinct... well, echo of a sound
coming back to you.
• Echo often is a diminished quality of the signal, due to
greater disproportional frequency loss and delay.

13. • Measurement of resistance. Means different things
depending on the context.
• Mic Level output, 250 Ω, a low level output.
• Line Level output, 600 Ω, a much louder output.
• Mic Level input, 250 Ω, expects a low level input.
• Line Level input, 600 Ω, wants a much louder input.
• Approx 50 dB difference in volume between mic v line
level
• Headphones, 40 to 100 Ω is ideal. Less than 35 > too
loud and easily distorted. More than 100 < provides not
enough volume from field recorders.

14. • Mic out to Mic in. Sounds OK, but weaker signal more
prone to interference.
• Line out to Line in. Sounds OK, a stronger signal less
prone to interference. Best settings to use if you can.
• Mic out to Line in. Very low and feint volume. Think of
sending 12v into a 100v light bulb.
• Line out to Mic in. Too powerful a signal for that input.
Audio will be loud and distorted. Think of 100v going into
a 12v light bulb.
• Line level is normal level for most devices. Mic level
requires pre-amp to raise level up to Line level for mixing
or recording.

15. • Why actors tell each other to “break a leg”
• Has nothing to do with fracturing one’s bones!
• During Medieval era, theater actors only got paid if they
appeared on stage that day.
• To “break a leg” meant that your leg cleared (aka broke)
the edge of the stage curtains and you would be visible in
the performance…
• So you were entitled to wages.