Terminology of Sound

Terms, terms, and more terms. Good thing I want to know what they mean hahah! Here we go!

2.13.11

• Sound-Pressure Waves - When sound arrives at the ear in the form of periodic variations in sound pressure.
• Compression - When a vibrating mass moves outward from its normal resting state, such as the strings of a guitar or the vocal cords, it squeezes air molecules into a compressed area, away from the sound source. This causes the area to be acted on to have a greater than normal atmospheric pressure. 
• Rarefraction - When the vibration mass moves inward from its normal resting state and an area with less-than-normal atmospheric pressure is created. 
• Wave Propegation - Tying compression and rarefraction together, wave propegation is the movement of the soundwaves outward from places of high pressure to places of low pressure like the way molecules will puss outward from a baloon when popped.
• Waveform - It is essentially the graphic representation of a sound-pressure level or voltage level as it moves through a medium over time. It lets us see and explain wave propegation in our physical enviornment. 
• Amplitude - The distance above or below the centerline of a waveform, such as a pure sine wave. The greater the distance or displacement from the centerline, the great the intensity.
• Frequency - The rate at which an acoustic generator, electrical signal, or vibrating mass repeats within a cycle of positve and negative amplitudes. 
• Velocity - The velocity of a sound wave as it travels though air at 68°F is approximately 1130 ftt. per second. This speed is tempurature dependent and increases at a rate of 1.1 ft. per second for each degree increase in tempurature. 
• Wavelength - The physical distance in a medium between the beginning and the end of a cycle. 
• Phase - When two or more waveforms are involved in producing a sound, their realtive amplitudes can, and often are, different at some point in time. Phase varies in degrees from 0 to 360. When out of phase waveforms are anything but 180° out of phase, they add together increasing the amplitude. 
• Harmonic Content - The pressence of other frequencies relative to the fundamental pitch. 
• Phase Shift - A term that describes one waveform's lead or lag in time with respect to another. It results from a time delay between two or more waveforms. 
• Patials - The reason that not every instrument sounds the same like a one frequency sine wave, is because of other frequencies, or partials, that exsist in addition to the fundamental pitch that's being played.  
• Overtone - Upper partials in the higher frequencies above the fundamental pitch.
• Harmonics - Overtone frequencies that are whole number multiples of the fundamental.
• Sine Wave - Named so because its amplitude corresponds to the trigonometric sine function, is considered to begin at 0° with an aplitude of 0. The waveform increases gradually to 90°, 180°, 270°, and back to where it started at 360° completing the circle.  
• Cycle - One completed excursion of a wave, which is plotted over a 360° axis of a circle. 
• Hertz - The number of cycles that occur within a second in mesured in hertz. 
• Wavelength = V/F - The mathmatical equation to calculate the physical length of a wave. 
• Reflection of Sound - Just as light reflects off surfaces, sound waves reflect off surface boundaries at an angle that is equal to, and in the opposite direction of, its initial angle of incidence. 
• Diffraction of Sound - When sound bends around an object in a manner that reconstructs the signal back to its original form in both frequency and amplitude.
• Frequency Response Curve - The charted output of an audio device. This is used to depict how a device will respond to the audio spectrum and how it will affect a signal's overall sound. 
• Timbre - The harmonics and their relative intensities which determine an instruments characteristic sound. 
• Envelope - This is another characteristic of an instrument that differentiates it from others. An envelope of a waveform is characteristic variations in level that occur in time over the duration of a note.
• Attack/Decay/Sustain/Release - Each envelope has these 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 started. Decay is how quickly the sound levels off to a sustain level after the attack peak. Sustain is the duration of the ongoing sound that is generated after the attack decay, and release is how quickly the sound will ultimately decay when the note is released. 
• Decible - The unit used to measure sound-pressure levels, and relative changes in signal levels. It is a logarithmic value that expresses differences in intensities between 2 levels.
• SPL - SPL stands for sound-pressure level. It is the acoustic pressue that's built up within a defined atmospheric area. Basically, the higher the SPL, the louder the sound.
• Fletcher-Munson Curve of Equal Loudness - The Fletcher-Munson curve shows an equalloudness contour for pure tones as percieved by humans having an average hearing acuity. The curves indicate the average sensitivity to different frequencies at various levels. 
• Beats - Two tones that differ only slightly in frequency and have approximately the same amplitude will produce an effect known as beats. This effect sounds like repetitive volume surges that are equal in frequency to the difference between these two tones. 
• Panning - A placement tecnique acheieved when the engineer changes the relative interaural intensity differences and thus creates the illusion of physical positioning between the speakers by by changing the proportion that is sent to each speaker. 
• Direct Sound - Because sound travels at a constant speed, sound will take the shortest path to a listeners ear, thus, direct sounds detwermine our perception of a sound source's location and size and timbre. 
• Early Reflections - Waves that bounce off the surrounding surfaces of the room have to travel further to get to the listeners ear and will arrive later than direct sound. Because of this, early refelctions give us clues as to the reflectivity, size, and general nature of an acoustic space.
• Acoustics - a science dealing with the production, effects, and transmition of sound waves; the transmission of sound waves through various mediums, including reflection, refraction, diffraction, absorption, and interference; the characteristics of auditoriums, theaters, and studios, as well as their design. 
• Acoustic Isolation - The prevention of external noise (bleed) transmitting into the studio enviornment through the air, ground, or building structure. Basically, controlling the sound from leaking out to a place you don't want it to go.
• Vocal Booths - Also called Isolation Booths, or iso-booths, a vocal booth is a small version of an iso-room and are perfect for isolating vocals and a single instrument from the larger studio. 
• Frequency Balance - The frequency components of a room should not adversely affect the acoustic balance of instruments and/or speakers. The acoustic enviornment should not alter the sound quality of the original or recorded performance. The room should exhibit a relatively flat frequency response over the entire audio range without adding its own particular sound coloration.
• Near Field Monitors - Monitors that deliver clean sound at high SPLs. These are being used more often because they are more acurate in representing the sound that would ne reproduced by the average home speaker system. These also cost a lot less than farfield monitors. 
• Far Field Monitors - Often involve large, multidriver loudspeakers that are capable of delivering relatively accurate sound at moxerate to high volume levels. Usually these monitors are built into the control room wall to reduce reflections arouns and behind the enclosure and to increase overall speaker efficiency. 
• Angle of Incidence/Reflection - A refelcted sound will be equal to and opposite from the angle of incidence. When a sound hits a flat surface at a 45° angle, the reflected sound will be 90° from the angle of the first sound path. 
• Standing Waves - Also known as Room Modes, occur when sound is reflected off of parallel surfaces and travels back on its own path, thereby causing phase differences to interfere with a room's amplitude response.
• Room Modes - Expressed as integer multiples of the length, width, and depth of the room and indicate which multiple is being referred to for a particular reflection. 
• Flutter (Slap) Echo - A condition that occurs when parallel boundaries are spaced far enough apart that the listener is able to discern a numner of discrete echoes. It will often produce a "boingy" hollow sound that greatly affects a room's sound character as well as its frequency response. 
• Diffusers - Acoustical boundaries that reflect the sound wave back at various anfles that are wider than the original inident angle, which breaks up the energy-destructive standing waves. 
• Direct Signal - Signal made up of the original, incident sound that travels from the source to the listener. 
• Reverb - The persistence of a signal in the form of reflected waves within an acoustic space, that continues after the original sound has ceased. 

Source: Huber, David Miles., and Robert E. Runstein. Modern Recording Techniques. 7th ed. Boston: Focal, 2001. Print.

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