Acoustic Glossary


O : Sound and Vibration Terms and Definitions ...

Oblique Modes

Octave Frequency Bands divide the audio spectrum into 10 equal parts. The centre frequencies of these bands are defined by ISO as 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz and 16 kHz. Sound levels that have passed through an octave band pass filter are termed octave band sound levels.

Each Octave or octave band includes a range of frequencies whose upper frequency limit is twice that of its lower frequency limit. For example, the 1000 Hertz octave band contains noise energy at all frequencies from 707 to 1414 Hertz, rounded to 710 Hz and 1410 Hz for practical reasons.

Octave Definition (IEC 801-30-09) logarithmic frequency interval between two sounds whose fundamental frequency ratio is two
Note 1 : the octave is used as a unit of logarithmic frequency interval.

One-third (1/3) octave bands are in common use and 1/6, 1/12 or 1/24 octaves are used in narrow band analysis.

See also constant bandwidthconstant percentage bandwidthscontinuous spectrumfast fourier transformline spectrumnarrowband spectra

Octave Band Analyser an instrument that measures sound levels in octave bands.

Octave Band Method estimates the performance of hearing protection devices based on the dB attenuation in each octave provided by the hearing protector manufacturers. To assess protectors for use in specific environments the measured average sound levels are also required.

See also other hearing protector procedures

Octave Filter, a filter whose upper-to-lower passband limits bear a ratio of 2 and conforms to the preferred frequencies given in ISO R266 and should meet the attenuation characteristic of IEC R255. Octave filters are used to make a real-time frequency analysis. These filters are called constant percentage bandwidth (CPB) filters.

Octave filters may also be divided into one-third octave filters, 1/12- or 1/24-octave filters, depending on the required analysis resolution.

Omnidirectional Microphone measures sound energy arriving from all directions equally. In order to comply with the standard requirements measurement microphones are omnidirectional unless otherwise stated. To achieve an omnidirectional response at all frequencies requires a special and therefore expensive microphone. Also known as free-field microphones.

See also our measurement microphones basics

Omnidirectional Source a source that emits equal amounts of energy in all directions and generates spherical waves.

Order Analysis the analysis of harmonics related to rotational speed. The application of harmonic analysis to rotating machinery.

Order Tracking order tracking is a special case of FFT analysis applied to variable-speed rotating machines where the sampling frequency of the analyser is varied to be an exact multiple of the running speed of the machine while a series of spectra are recorded. The spectra are usually shown on top of one another on the page, and this is sometimes called a waterfall plot.

Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation.

See also, acoustic oscillationantiresonancebending oscillationcoupled modesdamped natural frequencyfree oscillationforced oscillationfundamental mode of oscillationmodal numbersmode of oscillationnatural frequencynormal mode of oscillationresonanceself-induced oscillationsteady-state oscillationtransient oscillationuncoupled modeundamped natural frequency

OSHA : Occupational Safety and Health Administration : USA

Overload the input to the sound level meter is too high for the current measurement range, so change the range.

Overload % percentage of time an overload occurred.

Over-range, the input to the Sound Level Meter is too high for the current measurement range. Change the range.

Over-range % percentage of time an over range occurred.

Overtones are the harmonics or partials of a discrete frequency component. The first overtone corresponds to the second harmonic in a harmonic series.

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