Acoustic Glossary


Sound and Vibration : Definitions, Units, Measurements, Terms starting with W

W : under Watt

Warble Tone : sound whose frequency varies periodically about a mean value.

Waterfall : a 3-D plot displaying the amplitude of spectral components as a function of both time and frequency. The frequency spectrum is displayed as a curve for each specified time instant. Several such curves, for different time instants, are displayed simultaneously.

Watt : W : the unit of Power.

The rate at which work is done, or the rate at which Energy is expended. 1 watt is the power which in one second gives rise to energy of 1 Joule.

1 watt = 1 J/s = 1 N·m/s : base unit m2 kg s-3

1 watt is equivalent to 1.341 x 10-3 Horsepower

A watt is also the power dissipated by a current of 1 Ampere flowing across a potential difference of 1 volt.

see also : Sound Power

Watt per square metre : W/m2 : a common metric unit of heat, flux, density, irradiance, base unit: kg s-3

Watt second : W·s : a common metric unit of Work or energy, representing the Energy delivered at a rate of 1 watt for a period of 1 second.

1 watt second = 1 J = 1 N·m

WAV : Waveform Audio File Format : is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs.

see also : PCM and ADPCM.

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Waveform : the waveform is the shape of a time domain signal as seen on an oscilloscope screen. It is a visual representation or graph of the instantaneous value of the signal plotted against time. Inspection of the waveform can sometimes reveal information about the signal that the spectrum of the signal does not show.

For instance a sharp spike or impulse and a randomly varying continuous signal can have spectra that look almost identical, while their waveforms are completely different.

In machine vibration, spikes are usually caused by mechanical impacting, while random noise can be caused by the advanced stages of bearing degradation

Wave Front

Wavelength : λ : for a sound wave the wavelength is c/f, where c is the Speed of Sound (343 m/s in dry air at 20 °C) and f is the Frequency - the units are metres.

Wavelength is usually defined as the distance between sequential crests of a wave.

At 20 Hz the wavelength ≈ 17 m and at 20 kHz ≈ 17 mm, in air at 20 °C.

The frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f

Wave Number : or wavenumber is the spatial frequency of a wave, either in cycles per distance or Radians per unit distance.

It can be envisaged as the number of waves that exist over a specified distance (analogous to Frequency being the number of cycles or radians per unit time).

Wave Velocity : the velocity of a wave is equal to the product of its wavelength and Frequency.

Weber : Wb : is the SI unit of magnetic flux, volt-seconds, causing the electromotive force of one volt in a circuit of one turn when generated or removed in 1 second.

Weekly Personal Noise Exposure : LEP,w : a measure of the total noise received by an employee during a working week.

Similar to the Daily Personal Noise Exposure but calculated for a 40-hour week (five 8-hour days) instead of an 8-hour day.

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see also : Leq - Equivalent Continuous Sound Level

Weight : is the Gravitational Force acting on a body Mass

W = m g : where W = weight, m = mass and g = acceleration (m/s2).

So Weight and Mass are related, but not the same thing. An object with a mass of 1 kg would weigh about 9.8 Newtons on Earth, but 6 times less on the moon, due to Gravity. In space an object still has mass but is weightless.

In everyday use the term weight nearly always means mass. In science weight is the Force due to Gravity and the units are Newtons.

Weighted Sound Pressure Level is the Frequency Weighted and exponential Time Weighted - Sound Pressure Level.

Weighted Sound Reduction Index : Rw
Weighted Apparent Sound Reduction Index : R'w
Weighted Standardized Impact Sound Pressure Level : LnT,w
Weighted Standardized Impact Sound Pressure Level : L'nT,w
Weighted Standardized Level Difference : DnT,w

Weighting Networks : an electronic filter in a sound level meter or a vibration meter that correlates the objective meter measurement to the human response.

For example the frequency response of the human ear, in which case the A-weighting Network is most commonly used with regard to noise control issues, regulations and environmental standards.

Similarly Hand arm and Whole Body measurements are frequency dependent. There are a range of human vibration networks or filters because measurements are made in 3-axis and the weightings for whole body measurements, seated, standing or lying down are different from the hand arm filters see the list below.

Weighting Network : Bb : for measuring whole body vibration in fixed-guide transport systems in seated or recumbent positions.
Weighting Network : Bc : for measuring horizontal whole body vibration in the X direction, perpendicular to the spinal column, at the seat back.
Weighting Network : Bd : for measuring horizontal whole body vibration in the X and Y directions, at right angles to the spinal column, in seated, standing or recumbent positions.
Weighting Network : Be : for measuring rotational whole body vibration in 3-axis.
Weighting Network : Bg : for evaluation of whole body vibration in the Z direction, with regard to hand control and visibility.
Weighting Network : Bh : for measuring hand arm vibration in the X / Y / Z axis.
Weighting Network : Bj : for measuring vibration acting on the head of a recumbent person in the vertical direction, perpendicular to the lying surface.
Weighting Network : Bk : for measuring whole body vibrations in the direction of the spinal column in seated and standing positions, for measurements in the vertical direction, perpendicular to the lying surface in recumbent position and for measurements in the X / Y / Z axis acting on the feet in seated position.
page up Weighting Network : Bm : for measuring whole body vibrations in buildings in the X / Y / Z axis.

Weighting Window : Fast Fourier Transform theory tells us that time and frequency are simply two alternative ways of observing a signal. By changing the nature of a signal in the Time Domain, we implicitly change the nature of the spectrum in the frequency domain. This is exactly what we do when we apply a weighting function or Time Window.

White Finger : disorder of the hands caused by using hand-held tools, such as chain saws and jack hammers. Results in reduction of the hand's ability to feel or to regulate its temperature. May also result in numbness and excessive sensitivity to low temperatures. Called Raynaud's disease.

White Noise : noise whose Power Spectral Density is essentially independent of frequency, so is characterless. However levels measurements using sound analysers with 1/1 or 1/3 octave (Constant Percentage Bandwidth) filters will increase by 3dB for each increasing frequency step. Pink noise on the other hand will produce constant levels across the spectrum.

Related Terms :
Broadband Noise
Narrowband Noise
Pink Noise
Random Noise
see also :
Constant Bandwidth l Constant Percentage Bandwidths l Octave Bands l FFT- Fast Fourier Transform.

Whole-body Vibration is defined as mechanical vibration which is transmitted into the body, when seated or standing, through the supporting surface, during a work activity.

Our bodies are exposed to vibration at work from many machines, such as construction machinery (bulldozers, tow motors, fork lifts and cranes), heavy equipment (grinders, jack hammers), and power hand tools. Vibration has been proven to result in disorders of both the hand and arm, the neck, and the back.

see also
Maximum Transient Vibration Value : MTVV
Vibration at Work Regulations
Vibration Dose Value : VDV.
see also the Daily Exposure Levels

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Whole Body Weightings

Wideband Noise also called Broadband Noise, whose energy is distributed over a wide section of the audible range.

Related Terms :
Broadband Noise
Narrowband Noise
Pink Noise
Random Noise
White Noise
see also :
Constant Bandwidth l Constant Percentage Bandwidths l Octave Bands l FFT- Fast Fourier Transform.

Windowing : a technique used to reduce Spectral Leakage by multiplying the Time Domain waveform by a window function. The process of windowing reduces the amplitudes of discontinuities at the edges of a waveform, thereby reducing the Spectral Leakage.

Related Terms :
Hanning Window
Rectangular Window

Windscreens : the 'foam' windscreens extensively used with sound level meters present some interesting problems. They reduce the wind noise by perhaps 15 dB or more on a good day, so in windy conditions how do you know you are measuring the noise source or the wind or a combination of the two? **

Some guidance is given in ISO 1996 (community noise survey noise standard) which recommends you measure downwind at wind speeds of between 1 and 5 m/s. This can result in wind 'generated' noise levels of around 30dB (A) with the windscreen on.

** a practical solution is to listen to the AC output of the sound level meter using earphones, or the audio recording feature included in some sound level meters like the B&K 2250

Work : = Force x Distance = Energy and is defined as the amount of Energy transferred by a force.

Work is a Scalar quantity and the SI unit is the Joule.

WBV : WBV : Whole Body Vibration