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Objective TestA sound test analysis that is carried out without a jury but is based on user-selected metrics.
Occluded Ear SimulatorA device simulating the acoustic properties of the tympanic membrane and its attached structures.
OctaveA 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 and ANSI. The sound pressure level of sound that has been passed through an octave band pass filter is termed the octave band sound pressure level. Similarly, for one-third octave bands, there being three such bands in each octave band.
Octave Band AnalyzerAn instrument that measures sound levels in octave bands.
Octave Band LevelThe integrated sound pressure level of only those sine-wave components in a specified octave band.
Octave FilterA filter whose upper-to-lower passband limits bear a ratio of 2. Is preferably centred at one of the preferred frequencies given in ISO R266 and should meet the attenuation characteristic of IEC R255 and ANSI S1.11-1966 Class II. Octave filters are used to make a real-time frequency analysis. These filters are called constant percentage bandwidth (CPB) filters. Octave filters can be divided into 1/3-octave filters, 1/12- or 1/24-octave filters, depending on the required analysis resolution.
OffsetDistance between rotational centre lines at any given normal plane, usually measured at the coupling midpoint. Usually measured in mils in the US, and mm or microns in the rest of the world.
Omnidirectional SourceA source that emits equal amounts of energy in all directions and generates spherical waves.
Operating ForcesIn structure-borne contribution measurements, the forces in action during operation. Also referred to as path input strength.
Operational DataIn structure-borne contribution measurements, refers to operational conditions, such as operational speed, road conditions, etc. The data comes from the following points:
Operational Deflection Shapes (ODS)The deformation patterns of a structure when it is forced to vibrate at a particular frequency under a particular stationary operating condition. They can be regarded as a visualisation of a particular dynamic behaviour. The main application areas are in the automotive and aerospace industries, manufacturers of process machinery and generators in power stations.
Operational Deflection Shapes SimulationIn system analysis, the inherent structural dynamic properties of a structure are determined based on Frequency Response Function measurements. When measuring the frequency response function between a number of predefined Degrees of Freedom on the structure under test, a modal model can be constructed when applying modal analysis. This modal model can then be loaded with a forcing function whereby the operational deflection shape at a chosen frequency can be determined.
OrderAn expression of frequency that relates a frequency (sub-synchronous, synchronous or non-synchronous) to shaft turning speed (TS). It is calculated using the simple formula: Order = f/TS. In order analysis, the frequency axis of the spectrum is expressed in orders of shaft TS (that is, peaks may be referred to as 1 TS, 2 TS or 0.43 TS, 6.77 TS, etc.).
Order AnalysisA form of frequency analysis, used with rotating machines where the amplitude of signal frequency components is plotted as a function of multiples of the rotating frequency, that is in orders of rpm rather than in Hz or rpm. This implies that tracking is used.
Order DomainHarmonic and sub-harmonic decomposition of a signal based on a tachometer.
When a recurring frequency component can be identified as being related to a characteristic of the object under test.
Order TrackingOrder tracking is a special case of FFT analysis applied to variable-speed rotating machines where the sampling frequency of the analyzer 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. In this way, the running speed and its harmonics will always occur at the same frequencies, or orders, in the spectrum regardless of the machine speed. Other vibration components not related to running speed, such as line frequency effects will not be synchronous with running speed, and will show up as curves on the waterfall plot. A tachometer pulse from the machine is needed to determine the FFT analyzer’s sampling frequency. Some analyzers have the order tracking function built in, but others need an external frequency multiplier to derive the sampling frequency from the tachometer signal.
OscillationThe variation with time, alternately increasing and decreasing, of (a) some feature of an audible sound, such as the sound pressure; or (b) some feature of a vibrating solid object, such as the displacement of its surface.
OscilloscopeA cathode-ray type of indicating instrument.
OSHAThe Occupational Safety and Health Administration.
OssiclesA linkage of three tiny bones providing the mechanical coupling between the eardrum and the oval window of the cochlea consisting of the hammer, anvil, and stirrup.
Out-of-band NoiseNoise occurring outside the bandwidth that is being measured.
Out-of-phaseThe offset in time of two related signals.
Output AmplifierA power amplifier designed to drive a loudspeaker or other load.
Oval WindowA tiny membranous window on the cochlea to which the footplate of the stirrup ossicle is attached. The sound from the eardrum is transmitted to the fluid of the inner ear through the oval window.
Overall RMS LevelA measure of the total RMS magnitude within a specified frequency range.
Overlap ProcessingIn the FFT analyzer, the time signal is stored in a buffer before being processed to form the spectrum. The FFT algorithm only processes the data when the time buffer is full, and after the widowing function, that is, Hanning, is applied to it. This windowing causes data at the beginning and end of the time records to be represented at the wrong amplitude values, creating errors in the spectral amplitude levels. If two time buffers are used, and if the FFT algorithm is allowed to process the signal alternately from each buffer at a rate faster than the time it takes to fill the buffers, overlap processing is said to be the result. Overlap processing is desirable when using a Hanning Window because it ensures against loss of data for parts of the signal that occur near the beginning and end of the window. Most FFT-type data collectors use 50% overlap processing as a default. An overlap of 66.7% will completely correct for amplitude errors caused by the Hanning Window.
OverlayA type of graph that is equivalent to viewing a waterfall graph straight on so that you only see the X- and Y-axis, but you can flick through each of the spectra along the Z-axis.
OvertoneA component of a complex tone having a frequency higher than the fundamental.Brüel & Kjær Sound & Vibration Dictionary End User Agreement