2. Intro to Acoustics Flashcards
Absorption
When sound waves are absorbed by a surface, as opposed to bouncing off it (see reflection). Absorptive materials tend to “deaden” the sound of a room as they absorb sound energy.
Reflection
When sound waves bounce off of a surface, as opposed to being absorbed by it (see absorption). Most materials absorb certain frequencies and reflect others.
Slap Echo
(Also called slapback.) A single, distinct echo, which usually results from higher frequencies reflecting off a wall. Slap echo creates a “live” sounding effect similar to what you hear in an arena. It can be artificially produced using a delay or reverb plugin.
Echo
The repetition of a sound when that sound reflects off one or more distant surfaces. We recognize a sound as an “echo” (as opposed to “reverb”) when the distance is far enough to detect the delay between the initial sound and the reflected sound.
Early Reflections
The first reflections to reach our ears in an acoustically reflective environment. Most reverb plugins let you adjust the early reflection’s delay time and volume.
Reverb
Short for “reverberation” - a combination of many echoes occurring too frequently to hear each one individually. Reverb is the accurate term for the hollow, trailing off sound we are used to hearing in “echoey” spaces.
Constructive interference
When two sound waves line up and their peaks and valleys reinforce each other. Remember - peaks are areas of high pressure and valleys are areas of low pressure.
Acoustic amplifier
The part of a musical instrument that vibrates along with the initial vibration of the instrument, causing the surrounding air to move more, making the sound louder. For example: the body of an acoustic guitar, the bell of a horn, a drum’s shell, a person’s mouth, and the wooden soundboard of a piano.
Standing wave
Also called “room modes” - an unwanted interference pattern that occurs when sound bounces between two parallel surfaces in a room and gets amplified to an unwanted level in certain parts of the room while being nearly absent in others. Standing waves are most common in square or rectangular rooms with parallel walls, so acoustic designers try to prevent these waves by installing absorptive materials or introducing other items to offset the parallel surfaces.
Destructive interference
When two sound waves line up so that peaks meet valleys and cancel each other. Remember - peaks are areas of high pressure and valleys are areas of low pressure.
Phase
A measurement of the time alignment of two similar waveforms. If you have two nearly identical waveforms (e.g. from two microphones recording a single source), and they are slightly off in time, they are said to be “out of phase.” If they are in time with each other, they are said to be “in phase.”
Phase cancelation
A hollow sound created when two sound waves are out of phase.
Feedback
An unwanted whine caused when specific frequencies are amplified and re-amplified through a sound system. Feedback can be avoided by careful microphone positioning, and can be reduced by using equalization (EQ) to reduce the level of the frequencies causing the feedback.
Bass trap
A special type of acoustic absorber that causes a low frequency wave to reflect back on itself, partly canceling out the wave.
Ringing out a room
The process of identifying and compensating for the problem frequencies in a room for the purpose of better audio in that space. This is typically done by sending pink noise through the speakers, turning up the microphones to the point of feedback, and using EQ to notch out the offending frequencies.
