In acoustical measurements, sound pressure level is often measured in octave bands, and the centre frequencies of these bands are defined by ISO - 31.5 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz and 16 kHz to divide the audio spectrum into 10 equal parts.

The sound pressure level of sound that has been passed through an octave band pass filter is termed the *octave* band sound pressure level.

● Note 1 : the octave is used as a unit of logarithmic frequency interval.

1/3 : one-third octave bands are also in common use and 1/6, 1/12 or 1/24 octaves are used in narrow band analysis.

See also • constant bandwidth • constant percentage bandwidths • continuous spectrum • fast fourier transform • line spectrum • narrowband spectra •

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

**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 • anti-resonance • coupled modes • damped natural frequency • free oscillation • forced oscillation • fundamental mode of oscillation • modal numbers • mode of oscillation • natural frequency • normal mode of oscillation • resonance • self-induced oscillation • steady-state oscillation • transient oscillation • uncoupled mode • undamped 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. 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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