Serial Frequency Analysis : the measurement of octave or third octave bands of noise where a single filter is stepped across the different bands one at a time, suitable mainly for steady noise signals only. Superseded, in the main, by Real Time Analysis.
Sine Wave : or pure tone is characterized by it's frequency (number of cycles per second) or it's Wavelength (distance it travels within a period) and the Amplitude .
Single Event Noise Exposure Level : SENEL : there are two variations of this term:-
1:- the dB(A) level which if it lasted for one second would produce the same
A-weightedSound Energy as the actual event.
2:- similar except the start and end of the measurement is defined, usually as 10 dB below the Lmax : see also T10
Both are similar on the Sound Exposure Level : SEL but we believe the second was developed to take account of single events like aircraft noise where the
Lmax is important but the duration should also be taken factored in.
An event with a higher Lmax can have a lower SEL than a longer event.
Single Number Rating : SNR : a single number rating system for hearing protectors - BS EN ISO 4869
Sound : any pressure variation that the human ear can detect. Depending on the medium, sound extends and affects a greater area (propagates) at different speeds. In air, sound propagates at a speed of approximately 343 m/s. In liquids and solids, the propagation velocity is greater - 1480 m/s in water and 5120 m/s in steel, for example.
Sound Source : a simple sound source emits sound uniformly in all directions - under Free-Field conditions. A complex sound source is composed of various sources, multiple frequencies and directivity patterns.
Specific Volume : v : the number of cubic metres occupied by one kilogram of the substance : m3/kg.
Spectra : is the plural of spectrum
Spectral Density : the spectral density of the wave, when multiplied by an appropriate factor, will give the power carried by the wave, per unit frequency, known as the
Power Spectral Density (PSD) of the signal. Power spectral density is commonly expressed in Watts per
Spectrum Analyser : an instrument to analyse a sound or vibration wave into it's frequency components. A spectrum analyser converts a signal from the time domain into the frequency domain,. The FFT, Octave and 1/3-octave analysers are the most common type today, but there are many other types.
Spectrum Averaging : a short term spectrum analysis may include information due to external sources, for example background noise. Repeating the measurements over a longer period and averaging the spectra will cause any random signals to be 'discarded' and your confidence in the measurement will improve.
Speed of Sound : c : ≈ 331.5 + 0.60 T(°C), at 20 °C, the speed of sound in air is approximately 343 m/s and the decrease of speed with temperature is referred to as a negative Sound Speed Gradient. The speed of sound is also dependent, to a minor extent, on Atmospheric Pressure and relative humidity.
Sound travels faster in liquids and solids. For example the speed of sound in water is 1,480 m/s and for iron 5,120 m/s, these values are also temperature dependent, also giving rise to Sound Speed Gradients.
Standard Atmospheric Pressure : atm : atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface. Standard Atmospheric Pressure is equal to 101.325 kPa the preferred SI units or 8760 mmHg and 1013.25 millibars.
Standardized : measurement in accordance with a Standard or 'Norm'.
Static Pressure : a pressure at a point in a medium that would exist at that point in the absence of sound waves, symbol p0, unit Pascal.
Stationary Signal : a stationary signal is a signal whose average statistical properties over a time interval of interest are constant. In general, the vibration signatures of rotating machines are stationary.