Introduction

• In this study of sound basics, we will try to visualize sound

Frequency

• Origins of the Hertz
  • Goal: Visualize "frequency"
  • "Frequency" is number of complete cycles per second
  • Hz, kHz
• Oscillations
  • Goal: the part we are interested in is the vertical distance of the rotating arm
  • As it moves up and down ±1 note how the movement is super smooth because it's based on a circle
• From the Circle to the Wave
  • Goal: visualize a wave by stretching out the up and down movement over 1 second in slow motion
  • Wave is called sinusoid and is the purest single tone - super smooth, super pure
  • Takeaway: This sin wave can represent
    • Air pressure variation as tone transmitted to ears
    • Voltage in equipment representing the tone
    • Numbers representing the tone on CD
• Turn Faster
  • Goal: to see and hear the wave as frequency changes
  • Change Frequency from 1 to 50 - this illustrates how you can see frequency is the number of sine waves in 1s
  • We hear from 20Hz to 20kHz
    • Now listen to 10 pure tones tuned to A key that span this range
    • Note how our hearing perceive when the frequency is doubled
    • Think how they relate to the equalizer on mixers
    • How the frequency relates to voice, and music
  • Play example sine tone 22Hz, this is pure A, 22 waves per sec
  • Play 55Hz, next octave
  • Play 110Hz, beginning of vocal range, and is "Low" knob
  • Play 220Hz, beginning of "Mid" knob
  • Play 440Hz, favourite tone of musicians
  • Play 880Hz, 1760Hz, 3520Hz
  • Play 7040Hz, end of important vocal range, end of "Mid" knob
  • Play 14080Hz, "High" knob
  • Important takeaway - 100Hz to 6kHz vocal, 20Hz to 20kHz music

Volume

• Amplification
  • Goal: to see and hear the sine wave as our amplification changes, and relate to loudness perceived
  • Amplification is to change amplitude of a signal
  • This is the 440Hz tone we heard before
    • But with amplitude times 2, which is really a bigger circle
    • Notice sine wave is much bigger - really double
    • How much louder? Twice as loud?
    • Play 440+6db - louder, but not 2 times louder
    • "2" times amplitude is +6 db, one yellow on level meter
  • Change A to 1
    • In this case A = 1 means no changes made to the tone
    • A = 1 can also mean the signal is at the nominal level, which the equipment is built to handle
    • Also the level marked 0db on mixers
    • Play 440 at 0dB
    • Zero db, level meter shows full scale green
  • Change A to 1/2
    • Notice sine wave is halved - really cut a lot
    • How much softer? Half as loud?
    • Play 440-6db - not much softer, certainly not 1/2 times the original
    • 1/2 is -6db - level meter only one notch down
  • Change A to 1/4 to 1/16
    • Means sine is halved again
    • How much softer?
    • Play 440-x db
  • Notice sine wave amplitude is halved every -6dB
    • dB seems consistent with what we hear - step wise reduction, and level meter only one notch down
    • We don't hear drastice changes although tone amplitude change is drastic
  • Takeaway 1 - How we hear - not quite proportional to amplitude
    • Very sensitive to small volume, and not so sensitive to large amplitude
  • Takeaway 2 - Practical implications on settings: "gain" and "faders"
    • -6db seems a small step but signal is halved - really big
    • What we reduce here have to be made up for by the next equipment gain
    • But amplifiers also add noise - Play pink.mp3 to learn what noise sounds like
    • This is why setting the overall system gain structure is so important
• Signal Levels
  Goal: to see the different nominal amplitudes use in different audio equipment
  0 dBu - 0.775V, reference standard for dBu, why dBu? This is the reference level for professional mixers, note 0.775/0.775 is 1, so log 1 is zero
  Change+4 dBu - 1.23V, nominal i/o from mixer, aka professional line level, different from consumer
  Change-8 dBu - 0.32V, nominal i/o from consumer products like computers and recording devices, note if you want to output from mixer to consumer equipment there is a need to drop -12 dB on the mixer, why?
  Change-40 dBu - 0.01V, microphone level, very weak, vulnerable to interference
  How to know what is on your wires? Check the manual, check the level meter
  Takeaway - know your equipment, know your decibels, retain the signal quality as it goes through your equipment

Sound

• What is a sound?
  • Goal: to understand all sounds can be made from adding sine waves of different frequencies together
  • So far, we have heard only pure tones
  • All sounds made of mixture of sine waves - observe the picture of sine 440 + 523
  • Does it looks like a complex wave? - Surprisingly, it can be separated out to individual sine waves again
  • Even our ears can separate the individual sine waves - Play 440 + 523 - what are the tones?
• Example of a sound
  • Goal: to see evidence that even the most unlikely wave is made from sin waves
  • What is a square wave? Also periodic, but switches between 1 and -1 abruptly. Sharp corners...
  • Can a square wave be made up of sine waves? Super sharp vs super smooth
  • Let's start with a sine wave of the same frequency
  • Add sine waves, notice odd harmonics, odd fractional amplitudes
  • Play sine 440
  • Play square 440 - notice the difference?
  • Practical takeaway - square waves have lots of sine waves in them, infinite, tapering off slowly
  • Practical takeaway - all sounds made up of sum of sine waves - add them together ... and can be separated
• Unwanted sounds
  • Goal: to see and hear the consequences that square waves are still a mix of sin waves
  • Any practical use knowing square waves made of sine waves?
  • In the graph here we start with a pure sine wave
  • Imagine our equipment clips at 1 - e.g. power supply rails
  • Increase our volume until it goes past 1, level meter red
  • When it clips, what shape does it resemble? Square wave
  • And what happens to the original pure tone? A lot of unwanted sounds added
  • Practical take away
    • Clipping - bad sound, rough
    • Clip happens at speakers, at power amp inputs, at mixer - red LED
    • Digital clip now a common mishap - for example, at the input of a digital video mixer
    • VLC - setting volume greater than 100% can cause digital clipping

Phase

• Advance the Wave
  • Goal: to see "phase"
  • Two waves said to be in phase when phase angle between is 0
  • What is "phase angle?"
  • Advance from 0° to 360°
  • Phase angle is angle between the two radius (or hands) rotating
  • What happens at 180? - sound is sum of sines, so cancels
  • Important takeaway - when speaker cables connected in reverse
    • Know why we keep speaker cable colors consistent
    • Otherwise sounds softer - canceled out

Over the Air

• Sound pressure decay
  • Goal: to see sound dissipate
  • Sound pressure decays 1/r, illustrated in graph, at 1m, pressure is 1, rapidly drops until at 10m, it is only 1/10
  • If measure sound 10m away compare to 1m away, 20 dB drop
  • Try talking in open space to feel the lack of reverberation
  • Why do our voices sound louder in a small room?
  • Reverberation is reflection all over the room, and re-reflection, prevents the 1/r dying away
  • Reverberation can be a friend or an enemy
    • Friend, when it increase loudness
    • But when reflection continues for a long time, muddy sound results
    • When muddy sound is louder than the loudspeaker - speech is not clear/intelligible
    • Takeaway - distributed speakers assist in overcoming unwanted reverberation
    • Takeaway - some amount of reverberance is useful