Modern Recording Techniques 7th Edition Ch 2 Sound and Hearing - Posted on 2013-04-15 by Malecia Bynum

• Sound is actually a concept that describes the brain's perception and interpretation of a physical auditory stimulus
• Sound can be divided into 4 areas:
  • The basics of sound
  • The characteristics of the ear
  • How the ear is stimulated by sound
  • The psychoacoustics of hearing
• Sound arrives at the ear in the form of periodic variations in atmospheric pressure called sound-pressure waves
• An area that has greater than normal atmospheric pressure is called compression
• An area with lower than normal atmospheric pressure is called rarefaction 
• Only the sound wave itself moves through the atmosphere in the form of high-pressure compression waves that continue to push against areas of lower pressure (in an outward direction). This outward pressure motion is known as wave propagation.
• A waveform is essentially the graphic representation of a sound-pressure level or voltage level as it moves through a medium over time. 
• A waveform will generally have the following characteristics:
  • Amplitude
  • Frequency
  • Velocity
  • Wavelength 
  • Phase
  • Harmonic content
  • Envelope
• (Above) Amplitude and Frequency are the most fundamental 
• The distance above or below the centerline of a waveform represents the amplitude level of that signal
• The measurement of of either the maximum positive or negative signal level of a wave is called its peak amplitude value (peak value)
• The total measurement of the positive and negative peak signal levels is called the peak-to-peak value 
• The root-mean-square (rms) value was developed to determine a meaningful average level of a waveform over time 
• The rms value of a perfect sine wave is equal to 0.707 times its instantaneous peak amplitude level 
• rms voltage = 0.707 x peak voltage 
• peak voltage = 1.414 x rms voltage 
• The rate at which an acoustic generator, electrical signal or vibrating mass repeats within a cycle of positive and negative amplitude is known as the frequency of that signal
• One completed excursion of a wave (which is plotted over the 360º axis of a circle) is known as a cycle 
• The number of cycles that occur within a second (frequency) is measured in Hertz (Hz)
• The velocity of a sound wave as it travels through air at 68ºF (20ºC) is approximately 1130 feet per second (ft/sec) or 344 meters per second (m/sec). This speed is temperature dependent and increases at a rate of 1.1 ft/sec for each Farenheit degree increase in temperature (2 ft/sec per Celsius degree).
• The wavelength of a waveform (frequently represented by the Greek letters lambda, 
• The physical length of a wave can be calculating using Wavelength = Velocity divided by Frequency
• The time it takes to complete 1 cycle is called the period of the wave. The period of the wave is expressed by using "T." Period of the Wave (T) = 1 divided by Frequency
• Much like  a light wave, sound reflects off a surface boundary at an angle that is equal to (and in the opposite direction of) its initial angle of incidence
• Sound can diffract (bend) around an object in a manner that reconstructs the signal back to its original form in both frequency and amplitude
• The charted output of an audio device is known as its frequency response curve. This curve is used to graphically represent how a device will respond to the audio spectrum and how it will affect a signal's overall sound.
• If the measured signal is the same level at all frequencies, the curve will be drawn as a flat, straight line from left to right (known as a flat frequency response curve). This indicates that the device passes all frequencies equally (with no frequency being emphasized or de-emphasized).
• Waveforms that are out of phase have equal amplitudes and frequency but start their cycle periods at different times
• Variations in phase, which are measured in degrees (º), can be described as a time delay between 2  or more waveforms
• Whenever 2 waveforms having the same frequency, shape and peak amplitude are completely in phase the newly combined waveform will have the same frequency, phase and shape but will double in amplitude. If the same 2 waves are combined completely out of phase they will cancel each other out when added which results in a straight line of 0 amplitude. If the 2nd wave is only partially out of phase the levels will be added at points where the combined amplitudes are positive and reduced in level where the combined result is negative.
• Phase shift is a term that describes one waveform's lead or lag in time with respect to another. Basically, it results from a time delay between 2 or more waveforms.
• Acoustic leakage between microphones and reflections from nearby boundaries should be kept to a minimum whenever possible
• The factor that helps us differentiate betwenn instrumental "voicings" is the presence of frequencies (called partials) that exist in addition to the fundamental pitch that's being played. 
• Partials that are higher than the fundamental frequency are called upper partials or overtones
• Overtone frrequencies that are whole number mulitples of the fundamental frequnecy are called harmonics
• The ear perceives frequencies that are whole, doubled multiples of the fundamental as being related in a special way (a phenomenon known as the musical octave)
• If frequencies are even mulitples of the harmonics they are known as even harmonics (pleasing tone)
• Frequencies with odd multiples of the fundmental are called odd harmonics (harsher tone)
• Musical waveforms are divided into 2 categories simple and complex
• Square, triangle, and sawtooth waves are examples of simple waves that contain a consistent harmonic structure
• Complex waves don't necessarily repeat and often are not symmetrical about the zero line 
• The harmonics and their relative intensities (determine's an instrument's sound) are called the timbre of an instrument 
• Each instrument produces a sonic envelope that works in combination with timbre to determine its unique and subjective sound. The envelope of a waveform can be described as characteristic variations in level that occur in time over the duration of a played note. The envelope of an acoustic or electronically generated signal is composed of 4 sections that vary in amplitude over time:
  • Attack refers to the time taken for a sound to build up to its full volume when a note is initially sounded
  • Decay refers to how quickly the sound levels off to a sustain level after the intial attack peak
  • Sustain refers to the duration of the ongoing sound that's generated following the intial decay
  • Release relates to how quick the sound will decay once the note is released 
• The unit used for measuring sound-pressure level (SPL) signal level and relative changes in signal level is the decibel (dB), a term that means 1/10th of a Bell (a telephone transmission measurement unit)
• The logartithm (log) is a mathematical function that reduces large numeric values into smaller, more manageable numbers
• In audio logarithmic values are used to express the differences in intensities between two levels
  • The log of the number 2 is 0.3 
  • When a number is an integral power of 10 the log can be found simply by adding up the number of zeros 
  • Numbers that are greater than 1 will have a positive log value, while those less than 1 will have a negative log value 
• The decibel is a logarithmic value that "expresses differences in intensities between 2 levels." These levels are expressed by several units of measure, the most common being sound-pressure level (SPL), voltage (V) and power (wattage, or W).
• SPL is the acoustic pressure that's built up within a defined atmoshpheric area (uaually a square centimeter, or cm²). The higher the SPL the louder the sound. 
• SPL levels change with the square of the distance
• (Voltage can be thought of as the pressure behind electrons within a wire).
• Power is usually a measure of wattage or current and can be thought of as the flow of electrons through a wire over time. Power id generally associated with audio signals that are carried throughout an audio production system. 
• A 1-dB change is barely noticeable to most ears
• Turning something up by 3 dB will double the signal's level (believe it or not, doubling the signal level won't increase the perceived loudness as much as you might think)
• Turning something down by 3 dB will have the signal's level (likewise, halving the signal level won't decrease the perceived loudness as much as you might think)
• The log of an exponent  of 10 can be easily figured by simply counting the zeros
• Sound waves are changed into mechanical vibrations which are transferred to the inner ear by way of 3 bones known as the hammer, anvil and stirrup (they act as an amplifier and protection device)
• A convenient pressue-level reference is the threshold of hearing which is the minimum sound pressure that produces the phenomenon of hearing in most people and is equal to 0.0002 microbar. One microbar is equal to 1 millionth of normal atmospheric pressure.
• An SPL that causes discomfort in a listener 50% of the time is called the threshold of feeling (118 dB)
• The SPL that causes pain in a listener 50% of the time is called the threshold of pain (140 dB)
• The average tolerable volumes are 85 dB to 95 dB
• There are 3 different types of hearing loss:
  • Acoustic trauma
  • Temporary threshold shift
  • Permanent threshold shift
• The area of psychoacoustics deals with how and why the brain interprets a particular sound stimulus in a certain way
• The Fletcher-Munson equal-loudness contour curves indicate the ear's average sensitivity to different frequencies at various levels 
• In the ear's response 3 types of interaction effects can occur:
  • Beats
  • Combination tones
  • Masking 
• 2 tones that differ only slightly in frequency and have aproximately the same amplitude will produce an effect known as beats
• Combination tones result when 2 loud tones differ by more than 50 Hz 
• Masking is the phenomenon by which loud signals prevent the ear from hearing softer sounds. The greatest masking effect occurs when the frequency of the sound and the frequency of the masking noise are close to each other. 
• The capability of 2 ears to localize a sound source within an acoustic space is called spatial or binaural localization. This effect is the result of 3 acoustic cues that are received by the ears:
  • Interaural intensity differences
  • Interaural arrival-time differences
  • The effects of the pinnae (outer ears)
• Interaural arrival-time differences (a different method of localization) is employed at lower frequencies 
• Changing the the relative interaural intensity differences thus creates the illusion of physical positioning  between the speakers known as panning
• 3 types of reflections are generated within an enclosed space:
  • Direct Sound
  • Early Reflections
  • Reverb
• In air, sound travels at a constant speed of about 1130 feet per second, so a wave that travels from the source to the listener will follow the shortest path and arrive at the listener's ear first; this is known as direct sound
• Waves that bounce off of surrouding surfaces in a room must travel further than direct sound to reach the listener and therefore arrive after the direct sound and from a multitude of directions; these waves form what is called early reflectios
• Another aspect that occurs with the early reflections is called temporal fusion. Early reflections arriving at the listener within 30 msec of the direct sound are not only audibly suppressed, but are also fused with the direct sound.
• Whenever room reflections continue to bounce off of room boundaries, a randomly decaying set of sounds can often be heard after the source stops in the form of reveberation
• The time it takes for a reverberant sound to decrease to 60 dB below its original level is called its decay time or reverb time and is determined by the room's absorption characteristics
• By repeating a signal using a short delay of 4 to 20 msec (or so), the brain can be fooled into thinking that the apparent number of instruments being played is doubled; this process is called doubling
• If a longer delay is chosen the repeat will be heard as discrete echoes, causing the delay to create a slap echo or slap back