Magnetic tape recorder: a device for capturing, storing and replaying analogue signals onto a tape medium containing ferromagnetic metal oxide particles.

Malleus: or hammer; one of the three bones of the middle ear, connected to the eardrum.

Masking: the process whereby the threshold of hearing for one sound is raised due to the presence of another thus rendering the first sound inaudible.

Mass law: an approximate relationship for predicting the sound reduction index of panels and partitions, based only on the surface density of the panel and the frequency of the sound.

Mean free path: a term used in the statistical treatment of sound in rooms, relating to the average distance between reflections.

Measurement time interval: a term used in standards on the measurements and rating environmental noise (BS 4142 and BS 7445) to indicate the total time over which measurements should be taken or predicted, for assessment purposes. Note that this may consist of the sum of a number of non-contiguous, short-term measurement time intervals.

Measurement uncertainty: see under uncertainty.

Mechanical filter: a resilient pad or layer, which prevents the transmission of high frequency vibration acts as a low velocity at a point in a vibrating system.

Mel: a unit of pitch; the pitch of any sound judged by listeners to be n times that of a 1 mel tone is n mels; 1000 mels is the pitch of a 1000 Hz tone at a sensation level of 40 dB.

Micron (μm): one thousandth of a millimetre or one millionth of a metre.

Microphone: a transducer, which converts acoustic signals into electrical (voltage) signals.

Middle ear: an air-filled space which connects the eardrum of the outer ear to the oval window of the inner ear by three small bones, called ossicles.

Milli-: a standard metric prefix meaning one thousandth.

MLS (maximum length sequence): a method for measuring room acoustics parameters featuring the use of maximum length sequence, pseudo-random but deterministic signals.

Modal analysis: a method for investigating models of vibration of structures involving mapping of the amplitude and phase of the vibration when the structure is excited into vibration.

Mode of vibration: a pattern of vibration of a vibrating system, characterised by a series of nodes and antinodes.

Mode shape: the shape of a particular mode of vibration is usually represented as the maximum displacement of the system from its mean or equilibrium position.

Modulus of elasticity: the stress divided by the strain for an elastic medium; an important factor in determining the speed of elastic waves in the medium; there are different types of elastic modulus, e.g. shear modulus, compression or bulk modulus and torsional modulus, for the different types of elastic deformation.

Monopole: a model or idealised point source of sound, which radiates spherical waves.

Music noise level (MNL): the A-weighted continuous equivalent sound level of the music noise measured at a particular location. Defined in the 1995 Noise Council Code of Practice on Environmental Noise Control at Concerts.