Acoustic Glossary


H :  Sound and Vibration Terms, Definitions, Units, Measurements ..

Haas Effect : 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 : the Control of Vibration at Work Regulations specify an Exposure Action Value of 2.5 m/s² A(8) at which level employers should introduce technical and organisational measures to reduce exposure. An Exposure Limit Value of 5 m/s² A(8) should not be exceeded, see the Acceleration Equivalent Value : Aeq.

Hand Arm Vibration is defined by the HSE as 'mechanical vibration which is transmitted into the hands and arms during a work activity'.

see also : 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.

Hand Arm Weightings : Weighting 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 FFT 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

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Harmonic : middle C on a piano, for example, sounds different to middle C on a guitar due to the harmonics. The Fundamental Frequency f = 256 Hz is the same but the harmonics 2f, 3f, 4f, etc., produced by the two instruments are different.

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. The harmonics in a series are also known as Partials.

Harmonic Series of Sounds : series of sounds within which the fundamental frequency of each of them is an integral multiple of the lowest fundamental frequency.

HAV : Hand Arm Vibration
HAVS : Hand Arm Vibration Syndrome

Head and Torso Simulator

Health and Safety at Work : Noise at Work Regulations or Vibration at Work Regulations.

Hearing Loss : noise induced hearing loss may be temporary or permanent.

Hearing Protectors : should be issued to employees, according to HSE 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. You should not use hearing protection as an alternative to controlling noise by technical and organisational means.

see also : the 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. Add water and the frequency changes.

Hertz : Hz : the unit of Frequency or Pitch 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 : 3-values related to the attenuation provided by hearing protectors at High, Medium and Low frequencies - supplied by the manufacturer. BS EN ISO 4869

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Horsepower : HP : a foot pounds per second unit of Power, 1 hp equal to 550 fps = 746 Watts

HSE : Health and Safety Executive.

Human Vibration : see Hand Arm Vibration Syndrome - HAVS  and  Whole Body Vibration - WBV.

Hz : Hertz