AuSIM: Glossary G-I

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Glossary [G-I] Terminology of Sound

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		Gain The amplification or attenuation of a sound source, usually measured in dB (decibels). 0 dB means no amplification and no attenuation. A positive value amplifies a source, a negative value attenuates it.
		Head Plane of Symmetry The mathematical plane that would divide a nose, face, or head into two equal symmetric parts.
		Head-Mounted Display A console display that can is physically mounted in front of the operator's eyes. The display can be enclosed or see-through, depending on the application.
		Head-Up Display A console display that can be monitored concurrently with a primary task. For example, a speedometer projected onto an automobile windshield, visual aircraft images projected onto an air traffic control tower, or 3D spatialized audio alerts being provided to a fighter pilot.
		Headphones A pair of listening devices joined by a band across the top of the head and worn in or over the ears; also called earphones. There are six combinations of physical headphone features: circumaural vs. supraural vs. intraaural, and closed vs. open. When comparing these aural head-mounted displays to visual head-mounted displays, open headphones are like "see-thru" HMDs (MicroVision's are a good example) while closed headphones equate to occluded HMDs such as Virtual Research's "V8". A distinction is made between headphones which are worn on the external ear (supraaural), those which surround the ear (circumaural), and "ear buds" which mount inside the ear canal (intraaural). Open headphones have foam ear pads that rest on the ears or ring pads that surround the ears. Closed headphones, on the other hand, nearly always have circumaural ear pads See also "Closed Headphones", "Open Headphones", "Circumaural", "Supraural", and "Intraaural"
		Headset Set of headphones or earphones: a pair of earphones, with a small microphone attached to enable two-way communication
		HRTF Head Related Transfer Functions (HRTFs) are sets of mathematical transformations that can be applied to a mono sound signal. The resulting left and right signals are the same as the signals that someone perceives when listening to a sound that is coming from a location in real-life 3D space. HRTFs are the core concept behind AuSIM3D, since they contain the information that is necessary to simulate a realistic sound space (see localization).® Once the HRTF of a generic person is captured, it can be used to create AuSIM3D sound for a large percentage of the population (most people's heads and ears, and therefore their HRTFs, are similar enough for the filters to be interchangeable).
		IID Interaural Intensity Difference, is similar to ITD. It says that if a sound is closer to one ear, the sound's intensity at that ear will be higher than the intensity at the other ear, which is not only further away, but usually receives a signal that has been shadowed by the listener's head. Sometimes referred to as the ILD or Interaural Level Difference. See also "3D Sound"
		ILD Interaural Level Difference, is similar to ITD. It says that if a sound is closer to one ear, the sound's intensity at that ear will be higher than the intensity at the other ear, which is not only further away, but usually receives a signal that has been shadowed by the listener's head. Sometimes referred to as the IID or Interaural Intensity Difference. See also "3D Sound".
		Intraaural The transducers mounts "in-ear". Ear-bud headphones and hearing aids are an example of such devices. They can be the best, but not always. Fitted intraaural devices (custom-molded) are very accurately and consistently positioned and thus the best 3D audio devices. Cheap "walkman" earbuds are not so consistent.
		Ipsilateral Ear Being on or affecting the same side of the body
		ITD Interaural Time Difference means that, unless a sound is located at exactly the same distance from each ear (e.g. directly in front), it will arrive at one ear before it arrives at the other. For example, if the sound arrives at the right ear before the left ear, the brain knows that the sound is coming from somewhere to the right. See also "3D Sound"

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