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

Oblique Modes

### Octave 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.

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 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 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.

**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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