Music Technology 220Audio RecordingSteve Turnidge, Instructor(Originally presented Fall 2001)Overview•An introduction to acoustics. •An analysis of sound. •Wave propagation theory as it relates to sound. •Waveform terminology including pitch vs. frequency and loudness vs. amplitude. •The threshold of hearing vs. the threshold of pain and the biology of the human hearing system.Acoustics: The Study of the Science of Sound Psychoacoustics: The Interaction of Sound With the Human Hearing MechanismAtmospheric (Air) Pressure•We are at the bottom of an ocean of air.•Air Pressure –the force exerted by the weight of the air above us. At sea level, it is 14.7 lbs per square inch, 1 kilogram per square centimeter. •Air pressure is measured by a barometer.Sound Pressure•Sound pressure –the (usually small) variation in air pressure. Not measured with a barometer.•The wave goes above the line –higher pressure, below the line, lower pressure.Atmospheric (Air) Pressure•Air has Mass and Elasticity.–You see the Mass of Air when the wind blows.–You experience the Elasticity of Air in tires and bicycle pumps –when you press on air it gets smaller. Air also expands when it can –as in a toy balloon.Propagation 1TimeImpactMassSpringOne-dimensional representation of wave motion in a medium.Propagation 2Two-dimensional representation of wave motion in water.Propagation 3Three-dimensional expanding shell of compression.TimeWaves in Water123546798123456789+-DisplacementTimeTimeTransverse Motion –Strings123546798123456789+-DisplacementTimeTransverse Motion –StringsTimeLongitudinal Motion -Air234567891+-DisplacementTime123546798+-DisplacementLongitudinal Motion -Air+-Sound PressureTimeAtmospheric PressureAtmospheric PressureVariationLongitudinal Motion -Air+-Sound PressureTimeAtmospheric PressureAtmospheric PressureVariationSpeaker movesOut -CompressionSpeaker moves In -RarefactionAir Pressure•Air molecules come together as air compresses -due to the speaker cone (or some other surface in motion) moving out.•The air, being elastic, pushes out, compressing in turn the next set of molecules. This is propagation.•When the speaker cone moves in, the air rarifies.•The Speed of Sound at 70º F in Air: 1,128 feet per second, 344 meters per second, or 769 miles per hour.An Analysis of SoundHow We Observe Sound 1•Waveform ViewHow We Observe Sound 2•Frequency CurveHow We Observe Sound 3•SonogramSeven Characteristics of a Wave7 Characteristics of a Wave•Amplitude•Frequency•Velocity•Wavelength•Phase•Harmonic Content•EnvelopeThe Important Ones:•Amplitude or Loudness•Frequency or Pitch•Harmonic Content or Timbre•Envelope•(Phase is important as well…)Amplitude+-Peak•Distance above or below zero crossing point•Peak level is from zero crossing to max + amplitude ormax -amplitudeAmplitude –Peak to Peak+-Peak to Peak•Peak to Peak -measured from Max + to Max -Amplitude –RMS+-RMS Level (.707 of Peakfor a sine wave)•RMS –Root Mean Square•SquareRootof the Mean(average) of the sum of the squares of each point on the waveformAmplitude•Amplitude -generally corresponds to loudness -measured in Sound Pressure Level with a Sound Pressure Level Meterin dB (decibels).•dBis a logarithmic scale –our hearing has such a wide range we measure it logarithmically to be able to express it manageably.Amplitude•Continuous exposure even to not-so-loud sound is dangerous –if your ears are ringing you’ve done yourself damage…•10dB –Rustle of a leaf•60dB –Conversation•80dB –Busy Street•90dB –Subway, city of Seattle Noise ordinance•110dB –Dance Club•118dB –Physical feeling•130dB –Jackhammer, Rock Concert•150dB –Jet take-offFrequency•Rate of occurrence of vibration in a system•In sound, the rate at which wave propagation occurs•A cycleis a complete occurrence of the waveform•A cycle passes through 360 degrees•Measured in cps (cycles per second) or Hertz (Hz), over 999 referred to as 1kHz (kilohertz)•Human hearing range is approximately 20Hz to 20kHz, roughly 11 octaves360º of a Cycle0º90º180º270º0º90º180º270º360º+-TimeTime360ºVelocity•Speed at which waveform travels through medium.•Affected by the medium and temperature.•Measured in feet per second.•V=d/t V=Velocity, d=distance, t=time.•The Speed of Sound at 70º F in air: 1,128 feet per second, 344 meters per second, or 769 miles per hour.Speed of Sound/MediaMediaMeters/SecondFeet/SecondAir: 70º F3441,128Fresh Water1,4804,860Salt Water1,5204,990Concrete3,40011,200Fir Timber3,80012,500Mild Steel5,05016,600Glass5,20017,100Wavelength•Distance between the beginning and end of a cycle, in an environment in which the wave propagates.•As frequency increases, wavelength decreases.•λ (Lambda) = V/f -λ is wavelength, V = velocity in a medium, f = frequency in Hz.Phase•The relationship between two similar waveforms that are offset from one another in time•Measured in degrees. Complete cancellation is 180, perfect phase is 0•Phase shift –describes amount of lead or lag from one wave to another•φ =Δt x f x 360 -φ is phase, Δt = delay time, f = frequency•The bane of dual mic setups, can cause extreme frequency degradation, also phasey, flangey soundsHarmonic Content 1•AKA Timbre(Tam-ber, not Tim-ber): tone color or texture of the sound.•Fundamental: Sound that corresponds to the note being played, i.e. first harmonic. A sine wave is a pure fundamental tone.•Other frequencies that make up a sound are overtones or upper harmonics or upper partials.•2ndharmonic is 2 times the fundamental frequency, 3rdharmonic 3 times, etc. Harmonic Content 2•Two kinds of musical waveforms: Simple and Complex.•Sine, Square, Triangle, and Sawtooth are simple; continuous and repetitive.•Non-repeating, varying waveforms are called complex.•Instruments have different amounts of different harmonics –this creates their tone color characteristic. •All sound can be broken down, and created from sine waves.Harmonic Content 3•To create a Square wave out of Sine waves: Add odd harmonics of a lower amplitude.•Two components: f + 3f Harmonic Content 4•Three components: •f + 3f + 5f•Gets closer to true square waveHarmonic Content 5•Seven components: •f + 3f + 5f + 7f + 9f +11f + 13f•Gets much closer to true square wave –Higher frequencies “square up” the cornersHarmonic Content 6•Some harmonics work well (consonant) with the fundamental frequency –some don’t (dissonant)•2ndharmonic –octave –fullness•3rd& 5thharmonics –odd –stopped/covered•2nd, 4th, and 6th–even –choral/singing•3rd–dullness•5th–annoying•7th& up –edge or bite•Harmonic Distortion –in the ear, when volume creates artificial harmonics.Envelope•Change in loudness over time in a sound•Huber defines with three variables –Attack, Internal dynamics, and decay•Typically found in synthesizers as ADSR –Attack, Decay, Sustain and Release.Other Properties and BehaviorsReflection•Reflection –Sound waves behave like light, or a ball –they reflect off of a surface at an angle equal to and in an opposite direction to the angle of attack•Flat surfaces bounce, convex disperse widely, concave focuses, corners reinforceDiffraction, Diffusion•Diffraction –Sound waves bend around barriers and through apertures•Diffusion –Uneven surfaces reflect waveforms at a variety of angles•Frequency response –the capability of a system to accurately represent a given range of frequenciesFrequency Response•Fletcher-Munson curve:•Below 85dB highs are emphasized•Above 85dB lows are emphasized•Flat: Accurate reproductionBeat Frequencies•Beats –as in tuning a guitar, two tones with slight difference in frequencies will pulsate in volume as they go in and out of phase.•The beat frequencies equal the difference between the two tones.Combination Tones•Combination tones –When 2 loud tones differ by more than 50Hz, the ear makes tones that are equal to the sum and the difference of the two tones.•Sum tone = f1+f2•Difference tone = f1 –f2Masking•Masking –Loud signals prevent the ear from hearing quieter signals. •This especially happens when the signals are close to the same frequency•This is the mechanism used in “perceptual coding” to compress audio data in MP3’s and Minidisks –discard audio content you “can’t hear”Perception of Direction 1•We determine direction of sound by our ears perceiving differences in time and amplitude between the two ears, as well as the way the pinnae (outer ears) reflect sound into our ear canals.•3D audio depend on these conceptsPerception of Direction 2•Interaural Intensity differences –Our head blocks sound, creating an acoustic shadow –we only hear reflected sound on the shadow side, of a lower amplitude than the direct sound. Especially significant at high frequencies.•Interaural arrival time differences –low frequencies take a while to wrap around your head to get to the shadowed ear, hence arriving slightly laterPerception of Direction 3•The pinnae of the ear reflect different frequencies from the different ridges, introducing specific delays of specific frequencies.Perception of Space 1•Sound reflects in any space not anechoic (literally, “without echoes”).•Depending on room surfaces, there is more or less reflection.•Direct sound is straight from the source•Early reflections are the first echoes off the walls and room surfaces•Reverb is the sound that persists in the room after the the source stopsPerception of Space 2•We use reverb to gauge the size of the room•Direct sound tell us source location, size and timbre.•Early reflections tell us size of the room•We can add reverb to audio to create virtual room spacesThe Threshold of Hearing Vs. The Threshold of Pain and the Biology of the Human Hearing SystemThreshold of Hearing•Minimum sound pressure audible recognized by average person 50% of the time•Equal to .0002 microbar –one millionth normal atmospheric pressure –0dB SPL Threshold of Feeling•Minimum SPL of discomfort recognized by average person 50% of the time between 200Hz and 10kHz •118dB SPLThreshold of Pain•Minimum SPL of pain recognized by average person 50% of the time between 200Hz and 10kHz•140dB SPLThe Ear 1•Sound pressure waves directed into ear canal by pinna•Waves go down ear canal to eardrum –the transducer that changes them to mechanical vibrationsThe Ear 2•Vibrations travel to inner ear by bones: Hammer, Anvil and Stirrup –these act as an amplifier and a limiter•Vibrations travel to cochlea –snail like spiral filled with fluidThe Ear 3•Tiny hairs in cochlea respond to different frequencies, stimulating different nerves•Hearing loss related to damage to cochlear hairs in specific areas