Sound & Hearing - Posted on 2013-08-06 by Tinamarie Linares

Sound is made possible through the vibrations of an object displacing the air  molecules around it.

Compression is vibrations pushing air molecules forward into air molecules. Space left behind in compression is the rare-faction. The inversesquare law dictates the amount of energy lost per unit of distance. Atmospheric pressure is the natural air pressure around us. A waveform is the graphic representation of the amplitude (volume) of a soundpressure wave over a period of time.  Microphone converts acoustic (sound pressure level SPL) to voltage (electrical level).  Peak-to-peak amplitude level is value between peak (highest) and trough (lowest).  Optimal mixing level is 85-95 dB SPL.  Frequency is defined as the number of occurences of a repeating event per unit of time.  Frequency directly relates to pitch, measured in cycles per second (hertz, hz).  Humans can hear between 20Hz and 20kHz. Timbre is the harmonic content and frequency fingerprint that differentiates the sound of 1 instrument from another.  The fundamental frequency is the primary / most present frequency heard when an instrument plays a note. Timbre depends on what the instruments'  built with and how it's played.  When 2 instruments share a frequency range, increasing amplitude of 1 could make other disappear.  Velocity (speed of sound) is dependent upon temperature. The wavelength of a waveform is the distance of beginning to end of 1 cycle.  Phase is a waveform's position in time.  The sine wave is created by an oscillator - converts direct current to alternating current. Sound travels roughly at 740-770mph.

Pinna (Outer Ear) is latin for feather.  It helps localize sound & filters out certain frequencies cuz of its shape. Tympanic membrane (Ear drum) vibrates along with sound wave vibrations.  Attached to the back are the smallest bones in the body - the malleus (hammer), the incus (anvil), & the stapes (stirrup)  - which amplify sound.  Cochlea is snail shaped organ in the middle ear.  It's lined with reed-like fibers connected to hair follicles which vibrate and send impulses through nerve endings into the brain. Ringing in your ear is the sound of 1 of the follicles dying.  Raindrops are 40dB, Fireworks are 140dB.  Safe exposure limit is 85dB at 8hrs a day.  Hairdryers are dangerous after 30mins.  Anything over 85dB for an extended exposure is dangerous.  Whispers are 30dB. Normal conversation is 60dB.  The appropriate level for mixing is between 85-95dB.  Auditory masking occurs when the perception of 1 sound is affected by the presence of another. Our brains focus on the loudest sound present for survival.  Acoustic beats occurs when 2 sounds close in frequency are played together, the result is an audible phase interaction.

1st reproductions of sound were 1860 of a french woman singing & 1887 by Edison with a phonograph. Mono remained standard til late 1950s.