Immission: a measure of sound energy received at a particular location in the environment.

Impact noise: sound resulting from the impact between colliding bodies.

Impact sound insulation: the resistance of a floor to the transmission of impact sound; measured according to BS EN ISO 140-7.

Impedance: see under acoustic impedance.

Impedance matching: the use of a device to act as a buffer between a system, or component of a system, with a high output impedance and a system, or component system, with a low input impedance. An example of electrical impedance matching is the use of a preamplifier between a condenser microphone and the signal processing electronics in a sound level meter. An example of middle ear to match the very different acoustic impedances of air (in the outer ear) and cochlear fluid (in the inner ear): see also under input and output impedance.

Impulse: a transient signal of short duration; impulsive noise is often described by words such as bang, thump, clatter.

Impulse response: a sound pressure versus time measurement showing how a device or room responds to an impulse.

Incus: or anvil, the middle of the three bones in the middle ear.

Inductance: the property of an electrical coil, or inductor, associated with the rate of change of magnetic field; measured in henrys (H).

Inertia base: a concrete slab used under antivibration mounts to provide additional mass, rigidity and stability.

Infrasound: acoustic waves with frequencies below the audible range, i.e. below about 20 Hz.

Initial noise: ambient noise prevailing in an area before any modification of the existing situation (used in BS 7445).

Input and output impedance: output impedance is an important property of any device, which delivers a signal, e.g. a battery, a microphone, accelerometer or an amplifier. Devices with low output impedance can deliver higher electrical currents (and more energy) than those with high output impedance. Input impedance is an important property of any device, which receives a signal (i.e. of any ‘load’), such as a loudspeaker, a level recorder or an amplifier. Devices with low input impedance draw a higher current (and more energy) from the source than those with a high input impedance. If a source device with a very high output impedance is connected to a receiving device with a low input impedance the output voltage will be reduced because the source will be unable to deliver the current (or energy) demanded by the receiver. This situation may be improved by the use of an impedance matching device interposed between source and receiver: see also under impedance matching.

Insertion loss: a measure of the effectiveness of noise control devices such as silencers and enclosures; the insertion loss of a device is the difference, in dB, between the noise level with and without the device present.

Insulation: see under sound insulation.

Integrated impulse response method: a method for determining room acoustics parameters such as reverberation time, based on measurement of the impulse response of the room and defined in BS EN ISO 3382 and BS EN ISO 354.

Integrated Pollution Prevention and Control (IPPC): Regulations introduced to comply with European Commission Directive 96/61 whereby a permit system is used for controlling pollution from industrial activities.

Integrating circuit: an electrical circuit which converts an acceleration signal into a velocity or displacement signal.

Integrating sound level meter: a sound level meter, which electrically integrates sound pressure signals to measure the equivalent continuous sound level, LAeq.

Intelligibility: of speech signals; the degree to which each individual syllable of speech can be identified and understood.

Interference: (1) the principle of interference governs how waves interact; the combined wave disturbance is the algebraic sum of the individual wave disturbances, leading to the possibility of constructive and destructive interference; (2) the disturbing effect of unwanted signals on the wanted signal, often electrical in nature.

IPPC: see under Integrated Pollution Prevention and Control Regulations.

ISO: International Organisation for Standardisation.

Isolation: see under vibration isolation.

Isolation efficiency: a measure of the effectiveness of a vibration isolation; isolation efficiency = (1-T) √ó100%, where T is the transmissibility of the system; see also under transmissibility.