H : Sound and Vibration • definitions • terms • units • measurements ...
, if the sound from a specific source arrives at one ear a few milliseconds later than the other ear, then our hearing mechanism will judge the sound to be coming from the side of the head where the earliest sound arrived. This is true for arriving sounds up to about 25 milliseconds of delay, after which it will begin to sound like two distinct sounds.
This effect is very useful in sound reinforcement systems. Listeners at the back of an auditorium often experience difficulty and simply adding a local loudspeaker increases the volume but the sound now appears from the wrong direction. However if the sound from the reinforcement loudspeaker is delayed by 10 to 20 ms and is up to 10 dB louder than the direct sound, then the listener still localises all sound from the direction of the stage but benefits from the higher sound level enhanced by the speakers
Hand Arm Vibration is defined by the HSE, as 'mechanical vibration which is transmitted into the hands and arms during a work activity'.
The Vibration at Work Regulations state
Hand-arm vibration exposure assessment
The exposure action value is 2.5 m/s2 A(8)
The exposure limit value is 5 m/s2 A(8)
The assessment of the level of exposure to hand-arm vibration is based on the calculation of the daily exposure value normalized to an eight-hour reference period A(8), expressed as the square root of the sum of the squares (rms) (total value) of the frequency-weighted acceleration values, determined on the orthogonal axes ahwx, ahwy, ahwz as defined in Chapters 4 and 5 and Annex A to ISO Standard 5349-1
Hand Arm Vibration is also known as vibration white finger.
See also •
acceleration equivalent level, Aeq •
hand arm vibration measurements.
Hand Arm Vibration Syndrome (HAVS)
, the generic terms used to describe a variety of injuries incurred to the hands and arms caused through excessive exposure to vibrating tools.
HAVS, also known as the industrial injury, vibration white finger which describes the main symptom displayed in sufferers. The blanching appearance in the fingers and hands is created by poor blood circulation which destroys blood vessels and tissue. It is also known as secondary raynaud's syndrome.
See also • carpal tunnel syndrome
Hand Arm Weightings
built into hand arm vibration white finger meters.
, a smooth amplitude weighting of a time signal with zero at the beginning and the end of the time record - 'bell shaped'. Commonly used in
fast fourier analysis
when analysing gated continuous signals and long
to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum. The selectivity of the hanning window
is good, and the maximum picket fence error
is 1.42 dB.
Related Terms •
rectangular window •
, middle C on a piano, for example, sounds different to middle C on a guitar due to the harmonics. The fundamental frequency
= 256 Hz is the same but the harmonics
2f, 3f, 4f, etc., produced by the two instruments are different.
Harmonics in a series are also known as partials
Harmonic Definition IEC 801-30-03, sinusoidal component of a complex sound wave whose frequency is an integral multiple of the frequency of the fundamental.
Harmonic Series, set of frequencies which are integer multiples of the fundamental. For example if the fundamental frequency is 100 Hz then the second harmonic is 200 Hz, the 3rd 300 Hz, etc.
Harmonic Series of Sounds IEC 801-30-04, series of sounds within which the fundamental frequency of each of them is an integral multiple of the lowest fundamental frequency.
Subharmonic Response Definition IEC 801-24-25, periodic response of a system at a frequency that is a submultiple of the excitation frequency
HAV • hand arm vibration
HAVS • hand arm vibration syndrome
Head and Torso Simulator
Health and Safety at Work
noise at work regulations
vibration at work regulations
, noise induced hearing loss may be temporary or permanent.
, should be issued to employees, according to the HSE : UK Government Health and Safety Executive
, where extra protection is needed above what has been achieved using noise control or as a short-term measure while other methods of controlling noise are being developed.
In the UK there a dozens of hearing protectors suppliers who offer a wide range of products. There are three methods of calculating the degree of protection, knowing the noise levels measured in each 'noisy' area, the HML method •
Octave band method •
The HSE provide a spreadsheet for all three calculations.
See also •
BS EN ISO 4869 •
NRR (noise reduction rating) •
Real world +4 dB
Helmholtz Resonator, a cavity type resonator designed to vibrate at one particular frequency. A common example is an empty bottle: the air inside vibrates when you blow across the top. Adding water to reduce the volume results in frequency changes.
Helmholtz Resonator Definition IEC 801-31-28, resonator consisting of a relatively large volume and a small orifice
, the unit of frequency
of a sound. One hertz equals one cycle per second.
1 kHz = 1000 Hz, 2 kHz = 2000 Hz, etc.
High Pass Filter
, signals below the cut-off frequency are attenuated, the attenuation slope is called the roll-off
HML (High, Medium, Low)
is the attenuation in decibels provided by the hearing protector manufacturers. High = 2000 to 8000 Hz, Medium = 1000 to 2000 Hz and Low = 63 to 1000Hz. To assess protectors for use in specific environments A-weighted and C-weighted Leq sound levels
are also required to establish the PNR (Predicted Noise Reduction).
See also other hearing protector procedures
, a foot pounds per second unit of power
, 1 hp equal to 550 fps = 746 watts
HSE : UK Government Health and Safety Executive
hand arm vibration syndrome
whole body vibration
Hz • Hertz