Glossary of Terms - H

A glossary of terms covering lighting, audio, vision and staging in the event and theatrical industry.

There can be a number of meanings for some terms which can vary from country to country and manufacturer to manufacturer. Information within this guide is indicative and every situation can require a different approach and solution. All care has been taken, however, Image Group NZ accepts no responsibility for inaccuracies. If you have any suggestions, comments or queries regarding this list please contact us.

H

H.320
An international standard designed to bring interoperability to videoconferencing.

HALF DUPLEX
A system capable of transmitting and receiving signals in one direction at a time.

HALOGEN CYCLE
Chemical process occurring in Tungsten Halogen lamps which makes them possible. During the lamps life, Tungsten evaporates from the filament, and would normally deposit itself on the glass wall of a Tungsten lamp, causing it to blacken, and causing the output of the lamp to reduce until it finally blew. In a Tungsten Halogen lamp, the Tungsten combines with the Halogen gas elements present in the lamp envelope and is re-deposited back onto the filament. This process needs a very high temperature to operate, so Tungsten Halogen lamps are able to be a lot smaller, and run a lot hotter, than their Tungsten equivalents. See also Tungsten Halogen.

HALOGEN LAMPS
Used in some low priced data projectors, and most Ohp's, these lamps last about 40 hours, with consistent output throughout their life. Although halogens look very white compared to a normal incandescent lamp, they are not as white as metal halide units.

HAAS EFFECT
Refers to the condition of the human auditory system that permits a listener to merge all the information arriving in the first 20 milliseconds as a single event. This is sometimes called the precedent effect.

HARMONIC
One of a series of sounds, each of which has a frequency which is an integral multiple of some fundamental frequency.

HARD WIRED REMOTE
Generally a remote control is wireless, and uses infra-red transmitter. There are situations where this is not practical: including large rooms where the speaker is 35 ft or more from the projector; Rear projection, where the screen will pass some signal, but normally has the presenter pretty much tied down. Also, the presenter has to point the remote "at" the projector which often means turning away from the audience. A couple of projectors offer wireless remotes that will accept a cable (hard wiring) back to the projector, assuring range and signal getting though.

HAZER
See SMOKE MACHINE.

HDTV
The new High-Definition Television standard displays and broadcasts signals that use many more scan lines than normal television and a 16:9 aspect ratio rather than the old 4:3 aspect ratio of a standard TV set. HDTV also refers to the televisions that pick up these signals. HDTV does not degrade through many generations of editing; the final broadcast is crystal clear. The resulting picture is up to five times sharper than that of today's sets, with CD-quality sound.

HEADS ON STAGE
A shouted warning (often just "Heads !") for staff to be aware of activity above them. Also used when an object is being dropped from above.

HEADSET

    General term for theatre communication equipment.
    A headphone and microphone combination used in such communications systems with a beltpack.
    See also CANS.

HEADROOM
The difference between the average operating power level of an amplifier circuit and the point at which clipping or severe distortion occurs.

HEARING
The human hearing system is very well designed. It has a dynamic range of over 120 dB. Contemporary digital recording techniques can only achieve a dynamic range of about 90 dB. The typical threshold of pain is around 140 dB, with discomfort starting around a sound level of 118 dB. The normal hearing range is considered to be 15 Hz to 20 kHz. The typical Hi-Fi specification range is 20 Hz to 20 kHz. Typically, however, the average person cannot hear 20Hz. Sound reinforcement specifications reflect 50 Hz to 15 kHz (sometimes 40 Hz). Interestingly enough, this just happens to be the FCC limits on FM radio. The typical telephone has a frequency response of 400 Hz to 4 kHz. The human ear does not hear all frequencies at the same intensity. It's less sensitive at both the lower and upper ends of the frequency spectrum, and this characteristic varies with both age and sex. The amount of sensitivity is also a function of sound pressure level. The greatest intensity variations occur at very low sound pressure levels. The curve is relatively flat at sound pressures of 90 dB or so (Fletcher-Munson). The decibel is used in acoustic measurements because the human ear responds to the intensity of sound in approximately a logarithmic manner.
Sensitivity: Only 5% of people can hear a 1 dB difference in level (60 dB, 1 kHz); about 50% of people can hear a 2 dB change; and almost everyone can hear a 3 dB change. This means that when looking at equipment specifications, 1 dB frequency response specs are good; 3 dB specs are fair.
Relative level: 50% of people say about a 7.5 dB increase in level seems twice as loud . . . some as low as 5 dB, and some as high as 10 dB. This test is very level and frequency sensitive. Higher sound levels produce lower numbers and frequencies below 1 kHz and above 5 kHz yield higher numbers. The rule of thumb is 10dB. Applied to sound systems, to achieve output levels that appear to be twice as loud the system must produce over eight times more output power!

HEATSHIELD
Made by Rosco, Heat Shield is a special clear gel which when placed between a lamp and a coloured gel, dissipates a large amount of heat to give the gel a longer life. There must be an air gap between the Heat Shield and the gel, or it will not be effective.

HERTZ (HZ)
A unit of measurement, previously referred to as cycles per second used to indicate the frequency of sound or electrical wave. A unit of motion referenced to a time period of one second. The frequency of a vibration or oscillation in units per second.

HIGH GAIN SCREEN
A screen that uses one of many methods to collect light and reflect it back to the audience. This dramatically increase the brightness of the image over a white wall or semi-matte screen. Technologies used include curved screens, special metal foil screens (some polarized), and certain glass beaded screens. Prices and performance vary tremendously, but attention to the screen can make a big difference, particularly in "tough" environments such as trade shows.

HIGH HAT
See TOP HAT.

HIGH PASS
All signals above a given crossover frequency.

HIGH Z OR HIGH IMPEDANCE
Any resistance to AC voltage or current generally greater than 2,000 Ohms.

HIGHEST TAKES PRECEDENCE*
Abbreviated to HTP, this is the standard by which some lighting desks operate. If there is more than one control on the desk affecting a particular channel, then the highest level of the controls will take priority and affect the output of the desk and the dimmers. This system is universal on manual lighting desks, but there are problems with the control of moving lights, scrollers etc.

HISS
Audio-frequency noise having subjective characteristics analogous to prolonged sibilant sounds.

HMI (HYDRAGYRUM MEDIUM ARC-LENGTH IODIDE)
A mercury-halide discharge lamp with a colour temperature of 5600K (daylight).

HOOK CLAMP
A clamp with a wing bolt for hanging a lantern on a horizontal lighting bar. Introduced in the UK in 1959 by Strand, replacing the 2-bolt and 2-nut L Clamp. See SAFETY CHAIN and BOOM ARM.

HOOK UP
A Hook Up is paperwork generated by the Lighting Designer for a show. It lists connections or layouts between number systems. For example, a Channel Hook Up lists the channel numbers used on the lighting plan alongside the dimmer numbers into which they're connected, and a brief text description of that channels function.

HORIZONTAL FREQUENCY
This indicates how long it takes to scan each of the horizontal lines that make up the display. The unit of measurement is kilohertz (kHz). It is directly related to the number of lines and the vertical refresh (frequency) so that the higher the vertical refresh or the number of lines, the higher the horizontal frequency required. Illuminance

HOUSE LIGHTS
The auditorium lighting which is commonly faded out when the performance starts.

HTI
See DISCHARGE LAMP.

HTP
See HIGHEST TAKES PRECEDENCE.

HUM
An electrical disturbance that can occur in sound equipment due to the frequency of the power distribution system or any number of its harmonics. Our power line frequency in the U.S. is 60 Hz. Hum can occur at 60 Hz, 120 Hz, 180 Hz, 240 Hz, etc.

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