# Noise & Vibration Calculations

**Acoustic Cavity**- The acoustics of enclosed volumes are important when considering sound propagation within the volume.
**Angular Frequency**- The frequency of a steady recurring phenomenum in radians per second.
**Articulation Index**- A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.
**A Weighting**- This is the most generally used filter when making overall noise measurements.
**B**- An abbreviation of bel, the logarithm of the ratio of two powers. The decibel is one tenth of a bel.
**Bandwidth Time Product**- Used in calculating the confidence limit of a measurement.
**Bark**- The Bark is the standard unit corresponding to one critical band width of human hearing.
**bel**- The bel is the logarithm of the ratio of two powers, and the decibel is one tenth of a bel.
**B Weighting**- The B-weigthing curve approximately follows the 70 phon curve.
**Cavity Acoustics**- The acoustics associated with enclosed volumes.
**Circular Frequency**- The frequency of a steady recurring phenomenum in radians per second.
**Coincidence**- The sound radiation efficiency of a plate is dependant upon the coupling of sound waves in the air and flexural waves (vibration) in the plate. Optimum efficiency (maximum energy transfer from vibration to sound or visa versa) is achieved when the plate is vibrated such that the wavelength of flexural waves in the plate is equal to the wavelength of acoustic waves in the air.
**C Weighting**- The C-weighting curve approximately follows the 100 phon curve.
**Decibel**- The human ear responds logarithmically and it is convenient to deal in logarithmic units in audio systems.
**Engine Radiated Noise**- Some empirical noise prediction models have been derived from a sample of 1m radiated engine noise measurements.
**Expansion Chamber**- The expansion chamber can be considered as a simple low pass filter.
**Octave**- Two frequencies are an octave apart if the ratio of the higher frequency to the lower frequency is two.
**Quarter Wave Tube**- A resonator used most commonly on automotive air intake systems to reduce resonance.
**Room Modes**- Frequencies at which sound waves in a room resonate (in the form of standing waves), based on the room dimensions.
**Single Acoustic Barrier**- A single layer acoustic barrier.
**Sound Power Level**- Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.
**Sound Pressure Level**- Ten times the logarithm of the ratio of the time-mean-square pressure of a sound to the square of the reference sound pressure.
**Sound Transmission Class**- In acoustics, a single number rating for describing sound transmission loss of a wall or partition.
**Speed of Sound**- Depends on the elastic and inertial properties of the particular medium.
**Strouhal Number**- A dimensionless unit.
**Wavelength**- The shortest repetition length for a periodic wave.
**Wavenumber**- Wavenumber describes the spatial variation of waves, phase change per unit distance.