Acoustic Glossary


O : Sound and Vibration Definitions, Terms, Units, Measurements ...

Octave 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.

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, 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.

One-third Octave Bands are also in common use and 1/6, 1/12 or 1/24 octaves are used in narrow band analysis.

Related Terms : Narrow Band NoisePink NoiseWhite NoiseWideband Noise
See also : Constant Bandwidth and Constant Percentage BandwidthsOctave BandsFFT- Fast Fourier Transform

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.

Fundamental Mode of Oscillation Definition

IEC 801-2415, mode of oscillation of a system having the lowest natural frequency

Modal Numbers Definition

IEC 801-24-15, set of integers by which the normal modes of oscillation of a system are ordered as to frequency

Mode of Oscillation Definition

IEC 801-24-13, characteristic pattern assumed by a vibrating system, in which the motion of every particle is simple harmonic with the same frequency
Note : two or more modes may exist concurrently in a multiple-degree-of-freedom system.

Normal Mode of Oscillation Definition

IEC 801-24-14, mode of free oscillation of an undamped system
Note : In general, any composite motion of the system is analysable into a summation of its normal modes, each of which may oscillate entirely independently of the others.

Steady-state Oscillation Definition

IEC 801-24-24, oscillation that continues without change

Uncoupled Mode Definition

IEC 801-24-18, normal mode oscillating independently of other modes

Undamped Natural Frequency Definition

IEC 801-24-18, frequency of free oscillation resulting from only elastic and inertial forces of the system

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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