Acoustic Glossary

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

m (metre) • length.

m² (square metre) • area.

m³ (cubic metre) • volume.

m³/s (cubic metre per second) • volumetric flow.

m/s (metre per second) • speed of sound • velocity of sound

m/s² (metre per second squared) • acceleration.

m/s³ (metre per second cube) • jerk.

MAF • minimum audible field

Magnetic Field, a region around a magnetic material or a moving electric charge within which the force of magnetism acts.

Magnetic Flux a measure of the strength of a magnetic field over a given area perpendicular to it, equal to the product of the area and the magnetic flux density through it

Magnetic Flux Density measure of the strength of a magnetic field at a given point, expressed by the force per unit length on a conductor carrying unit current at that point B Also called: magnetic induction. The SI unit of magnetic flux density is the tesla.

Magnetic Induction
1 : magnetic flux or flux density
2 : the process by which an object or material is magnetised by an external magnetic field.

Masked Threshold, threshold of audibility for a specified sound in the presence of another (masking) sound.

Masking Noise a noise that is intense enough to render inaudible or unintelligible another sound that is also present, also known as auditory masking

See also • critical bands.

Mass (m), the scalar quantity of matter which a body contains, a fundamental property of an object. One of the seven SI base units, mass is measured in kilograms and the symbol is kg :
1 kg = 1000 g = 10^-3 tonne

Mass and weight are related, but not the same thing. An object with a mass of 1 kg would weigh 6 times more on Earth than on the moon, due to gravity.

See also • gravitational force.

Mass Density, kilogram per cubic metre - kg/m³, see also • density of air.

Mass Flow Rate is the mass of a substance which passes through a given surface per unit of time.

SI units • kilogram per second - kg/s

Mass Law, a doubling in mass or frequency results in a 6 dB increase in the sound insulation of a single leaf partition over a defined frequency range.

Mass Law provides a good working rule to predict the airborne sound insulation of a partition up to the region of the critical frequency and the coincidence effect

Mastoid Simulator Definition IEC 801-28-08, device which simulates the mechanical impedance of the average human mastoid where a bone vibrator may be applied to permit calibration of the vibrator. Also know as an artificial mastoid

Matter in the 'material' sense, is that which occupies space and possesses mass.

Maximum Length Sequence (MLS)

Maximum Level (Lmax) the maximum noise or vibration level during a measurement period or a noise event.

Lmax sound levels should also include other descriptors i.e. A, C, L or Z frequency weightings and F, S or I time constants.

Sometimes written as Max dB(A), not to be confused with Peak

Maximum Peak, treat with caution, by definition there can only be one peak value and this requires a special detector circuit that many meters do not have - see peak.

Maximum Perceived Noise Level (PNLmax) the maximum sound pressure level during aircraft flyover.

See also • perceived noise level.

Maximum Transient Vibration Value (MTVV) the highest maximum vibration level during a measurement period recorded in 1 second intervals to account for transient motion and short acceleration peaks, according to BS ISO 2631-1 Evaluation of human exposure to whole-body vibration. The SI units are m/s²

mb (millibar) a common metric unit of atmospheric pressure 1 mb = 0.001 Bar, 100 Pascals.

Mean is the average of the numbers, i.e. add up all the numbers and divide by how many numbers there are.

See also • arithmetic mean • geometric mean.

Mean Free Path Definition IEC 801-31-15, in an enclosure, distance travelled by sound waves between two successive reflections, on the average of a large number of reflections and for all initial directions of propagation.

Mean Square is the mean or average of the squared values of a time-varying quantity.

See also • RMS (root mean square) - widely used in acoustics

Measurement Microphones widely used in noise measurements because they offer the best linearity, frequency range and high stability.

Microphones are critical in acoustics so we have a full page on measurement microphones

See also • free-field microphones • pressure microphones • random incidence microphones

Measurement Time ideally the duration of a measurement should always be stated.

For example : LAeq (60min) = A-weighted, Leq, measured for 1 hour.

Mechanical Coupler Definition IEC 801-28-04, device for calibrating bone-conduction vibrators, designed to present a specified mechanical impedance to a vibrator applied with a specified static force, and equipped with an electromechanical transducer to enable the vibratory force level at the surface of contact between vibrator and mechanical coupler to be determined

Mechanical Impedance Definition IEC 801-25-26, at a point in a linear mechanical system, quotient of a force applied at a point, by the resulting component of velocity in the direction of the force
● Note : in the case of torsional mechanical impedance, the words force and velocity are replaced by torque and angular velocity.

Mechanical Reactance Definition IEC 801-25-28, imaginary part of a mechanical impedance

Mechanical Resistance Definition IEC 801-25-27, real part of a mechanical impedance

Medium (plural media) the intervening substance through which physical forces are transmitted.
See also • elastic medium

mega (M), SI prefix = 10⁶, see other SI Units

Mel Definition IEC 801-29-02, unit of pitch. A pure tone frontally presented, having a frequency of 1 000 Hz and a sound pressure level of 40 dB, causes a pitch of 1 000 mels
● Note : the pitch of a sound that is judged by the listener to be n times that of a 1 000 mels tone is n thousand mels.

metre (m), the basic unit of length in the metric system and in the International System of Units (SI).

1 metre = 39.370 inches = 3.2808 ft.
1000 m = 1 km kilometre : 0.01 m = 1 cm centimetre : 0.001 m = 1 mm millimetre.

metre cubed (m³) • volume

metre kilogram second • MKS

metre per second (m/s) • speed of sound • velocity

metre per second cubed (m/s³) • jerk

metre per second squared (m/s²) • acceleration

metre squared (m²) • area

micro (μ) • the SI prefix = 10^-6, see other SI prefixes

microbar (μbar) • 0.000.001 bar = 0.001 millibar = 0.1 pascal = 1 dyne per square centimetre : a CGS unit of pressure.

micrometre (μm) • 0.000.001 metre

micron (μ) • alternative name for the micrometre = 0.000.001 metre

microsecond (μs) • 0.000.001 second

Mid-band Frequency the centre frequency of a band pass filter, defined as the geometric mean of the upper and lower frequencies.

mil • a unit of distance = 0.001 inch = thousandths of an inch. 1 mil = 25.4 microns.

milli (m) • SI prefix = 10^-3 see other SI units

millibar (mb), a common metric unit of atmospheric pressure, one hundredth of a Bar.

1 mb = 0.001 bar = 100 Pa, pascals. A pascal is one newton per square metre.

millimetre (mm) • 0.001 metre

milliwatt (mW) • 0.001 watt

millisecond (ms) • 0.001 second

Minimum Audible Field (MAF), the zero phon curve on the equal loudness contours.

Minimum Level (Lmin), the minimum noise or vibration level during a measurement period or a noise event.

Sound levels should also include other descriptors i.e. A, C, L or Z weightings and F, S or I time constants.

Sometimes written as Min dB(A).

MKS (metre-kilogram-second), system of measurement using the metre, the kilogram and the second as basic units of length, mass and time.

See also • SI units.

MLS (maximum length sequence), electronically generated test signal that has a flat energy vs frequency curve over a wide frequency range, similar to white noise but is actually periodic, with a relatively long period, or slow repetition rate. The signal looks and sounds like random noise and is often referred to a pseudo-random noise. MLS measurements are widely used in acoustics for example measuring the frequency response of loudspeakers. Measurements are also possible in noisy environments as the correlation technique used reduces all the background noise uncorrelated with the MLS and the use of averaging can further increase the S/N ratio.

mm (millimetre) • 0.001 metre

Mode see room modes.

Mode of Oscillation Definition IEC 801-24-13, characteristic pattern assumed by a vibrating system, in which the motion of every particle is simple harmonic with the same frequency
● Note : two or more modes may exist concurrently in a multiple-degree-of-freedom system.

See also other oscillation terms

Modulation, a process by which a spectral component is modified by another component and sidebands result.

Modulation Transfer Function (MTF), the extent to which the fluctuations in the original signal are preserved in the signal reaching the listener. In the field of speech intelligibility, the difference between the original spectrum and the reverberant spectrum.

Modulus of Elasticity, the ratio of the stress applied to a body to the strain that results in the body in response to it. The modulus of elasticity of a material is a measure of its stiffness and for most materials remains constant over a range of stress.

Moment, a turning effect produced by a force acting at a distance on an object.

Moment of Force see torque

Moment of Inertia, a quantity expressing a body's tendency to resist angular acceleration.

Moment of Momentum measures and objects tendency to spin, units - kilogram metre squared per second - kg·m²/s

Momentum (p) a vector quantity, is the motion of a moving body, the product of it's mass and velocity. The units are N·s = kg·m/s.

See also • angular momentum

Monaural Sound Recording, a sound recording in which only one channel is used. If two channels (stereo or binaural sound) are available but only one channel is chosen, then the left channel is usually chosen by default.

Monopole Definition IEC 801-21-33, source that radiates sound equally in all directions in a free field, also known as a simple sound source

Strength of a Monopole Definition IEC 801-21-35, maximum instantaneous volume velocity produced by a simple source small compared with wavelength, emitting a wave with sinusoidal variation with time. Also known the strength of a simple sound source

Mouth Simulator Definition IEC 801-28-06, device consisting of a loudspeaker unit mounted in a baffle or an enclosure so shaped as to have a radiation pattern similar to that of the average human mouth. Also known as an artificial mouth.

ms (millisecond), 1 ms = 0.001 second

MSHA : Mine Safety and Health Administration : USA.

MTF • modulation transfer function

MTVV • maximum transient vibration value

Multiple Echo IEC 801-31-22, a succession of separate echoes originating from a single sound source.

Multi-spectrum, a one or two dimensional array of spectra, consisting of two or more spectra that were recorded during the same measurement - more information

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