H : Sound and Vibration Terms and Definitions ...
Delays longer than 25 milliseconds begin to sound like two distinct sound levels.
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 the mechanical vibration which is transmitted into the hands and arms during a work activity'.
The Vibration at Work Regulations state
The exposure action value is 2.5 m/s² A(8)
The exposure limit value is 5 m/s² A(8)
Hand-arm Vibration Exposure Assessment is based on the calculation of the daily exposure value normalised 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 (2001).
Hand Arm Vibration is also known as vibration white finger.See also acceleration equivalent level • 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 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 networks built into hand arm vibration white finger meters.
Hanning Window 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 transients 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 • windowing.
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 a 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 Definition (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
Health and Safety at Work • noise at work regulations • vibration at work regulations.
Hearing Loss, noise induced hearing loss may be temporary or permanent.
See also age-related threshold shift • noise induced permanent threshold shift • threshold of hearing
Hearing Protectors 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 • SNR 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
Hertz (Hz), the frequency or pitch of a sound.
1 Hz = 1 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
Horsepower (HP) a foot pounds per second unit of power, 1 hp equal to 550 fps = 746 watts.
HSE : UK Government Health and Safety Executive
Human Vibration : hand arm vibration syndrome • whole body vibration.
Hz : hertz
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