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

**S/N : Signal-to-Noise ratio** : the difference between the nominal or maximum operating level and the noise floor in dB

**Sampling Frequency**: the rate at which a continuous waveform is digitised, given in Hz.

**Sampling Interval** : how long the data is measured at each sample point.

**Sampling Theorem** : says that ideally a signal should be sampled at a rate twice its highest frequency component.

**Scalar Quantity** : a quantity fully described by a magnitude or numerical value, for example Density, Mass and Speed.
As opposed to a Vector Quantity which has both magnitude and direction, for example Acceleration,
Force and Velocity

## Scalar Quantities may be added, subtracted or multiplied like ordinary numbers,
Vector Quantities can not.

**Schroeder** : method for calculating the Reverberation Time from the impulse noise decay curve. Backward Curve Integration algorithm developed by Manfred Schroeder at Bell Labs in the nineteen-sixties.

**second : s** : the second, symbol s, is the name of the SI base unit of time.

## 0.001 s = 1 ms millisecond : 0.000001 s = 1 μs microsecond

**Seismic** : relating to earthquakes or other vibration in the earth

**Seismic Reflection** : the Reflection of waves at boundaries between different rock formations

**Seismic Refraction** : the Refraction of waves passing through formations of 'earth' having different seismic velocities

**Seismic Velocity** : the Velocity of wave propagation in particular ground or rock formation

**Seismogram** : a record produced by a seismograph

**Seismograph** : A measuring instrument for detecting and measuring the intensity and direction and duration of movements of the ground (as in ground-borne vibration) - Certified Seismographs

**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 Superseded, in the main, by Real Time Analysis.

**Shock** : rapid transient transmission of mechanical energy.

**SI Units** : is the world's most widely system of units devised around the convenience of the number 10.

## There are 7 base units from which other units are derived and known as **SI derived units**.

SI Unit prefixes:

Factor | Name | Symbol | Multiplying Factor |

10^{12} | tera | T | 1,000,000,000,000 |

10^{9} | giga | G | 1,000,000,000 |

10^{6} | mega | M | 1,000,000 |

10^{3} | kilo | K | 1,000 |

10^{-3} | milli | m | 0.001 |

10^{-6} | micro | μ | 0.000.001 |

10^{-9} | nano | n | 0.000.000.001 |

10^{-12} | pico | p | 0.000.000.000.001 |

Example 10^{-6} g = 1 μg = 1 microgram or one millionth of a gram.

**Sideband** : in Frequency Domain functions, pairs of frequencies with similar amplitude that appear equally spaced on either side of a centre frequency - produced by Modulation.

**Signal-to-Noise ratio** : the difference between the nominal or maximum operating level and the noise floor in dB

**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-weighted
Sound 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

**Single Number Rating : SNR** : a rating system for duct silencers.

### Slow Time Weighting - also known as **Slow Response** and **Slow Time Constant**.

**SNR** : see Single Number Rating above

**Sones** : a unit to compare the loudness of two sounds.

## By definition one sone = 40 phons and also equals 40 dB on the Equal Loudness Contours.

A 10dB increase, from 40 to 50 phons sounds twice as loud, so 50 phons = 2 sones and the following table applies:-

40 phon = 1 sone

50 phon = 2 sones

60 phon = 4 sones

70 phon = 8 sones

80 phon = 16 sones

90 phon = 32 sones ... and so on

**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 Field Quantities** : that follow

**Sound Power through a Surface Element** : also known as Sound Energy Flux.

**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.

**Sound Spectrum** : representation of the magnitudes (and sometimes of the phases) of the components of a complex sound as a function of frequency.

**Sound Speed Gradient** : The Speed of Sound decreases with decreasing temperature and creates a negative sound speed gradient. An increase in temperature results in a positive sound speed gradient

**Sound Transmission** : passage of a sound wave through a medium or series of media.

**Sound Velocity** : is usually taken to mean the Speed Of Sound, **Should not be confused with** Sound Particle Velocity, which is the velocity of the individual particles.

**Spatial Averaging** : taking measurements at various positions and averaging the results. Mandatory in Sound Insulation measurements and recommended anywhere multiple reflections are present.

**Specific Noise Level** is the A-weighted, Leq Level produced by a noise source during a specified period of time.

**Specific Volume : v** : the number of cubic metres occupied by one kilogram of the substance : m^{3}/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
Hertz (W/Hz).

**Spectral Density Limit** : as the bandwidth approaches zero, of the mean square value of a field quantity divided by bandwidth.

**Spectral Lines** : the number of constant bandwidth lines used in the measurement of spectra.

**Spectrum** : the description of a sound wave's resolution into its components of frequency and amplitude.

**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.

**The Speed of Sound should not to be confused with the** Sound Particle Velocity.

**Spreading Loss** : that part of the transmission loss due to the divergence, i.e. spreading, of sound waves in accordance with the configuration of the system, also known as Divergence Loss.

**Note** : Divergence Loss exists, for example, for spherical waves emitted by a point source.

**square metre : m**^{2} : Area.

**square root : √** : the square root of a number is a smaller number that, when multiplied by itself equals the original number.

**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 *p*_{0}, 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.

**Statistical Analysis** : a calculation performed by a Sound Level Meter on the noise levels measured during the measurement period to describe the Statistical Levels Ln of the noise.

**Steady State Oscillation** : Oscillation that continues without change.

**Stochastic** : details of individual events may be unpredictable but the overall character of the sound is. For example rain falling, sound of insects, birds, etc.

**Structure-borne Noise** : a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through the air.