Acoustic Glossary

Definitions, Terms, Units and Parameters

M

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m2 : square metre - area.
 
 
m3 : cubic metre - volume.
 
 
m3/s : cubic metre per second - volumetric flow.
 
 

m/s : metre per second

: speed of sound and/or velocity
 
 
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

: a 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 magnetized by an external magnetic field.
 
 

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 quantity of matter which a body contains as measured by it's acceleration under a given force or on the force exerted on it by gravity. In the SI system of units mass is measured in kilograms and the symbol is kg

1 kg = 1000 g = 10-3 tonne
 
 

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 sound insulation for a single leaf partition over a defined frequency range.
 
 

Maximum Length Sequence

: MLS - is basically a pseudo-random sequence of pulses. MLS measurements are quite standard in many different application fields, one of them acoustics. For more details click here.
 
 

Maximum Peak

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

Maximum Perceived Noise Level : PNLmax

: during aircraft flyover, used in Effective Perceived Noise Level calculations.
 
 

Maximum Sound Level : Lmax

: the maximum noise level during a measurement period or a noise event.

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

Sometimes written as Max dB(A). Do not confuse with PEAK
 
 

mb : millibar

: a common metric unit of atmospheric pressure 1 mb = 0.001 bar, 100 pascals.
 
 

Measurement Microphones

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

Because of their importance 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.
 
 

mega : M

: SI prefix = 106 see other SI Units
 
 

metre : m

: is 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-kilogram-second : MKS

: a system of measurement using the metre, the kilogram and the second as basic units of length, mass and time

See also the SI Units.
 
 

metre per second : m/s

: unit of speed of sound and/or velocity
 
 
metre per second cube : m/s3 : jerk.
 
 
metre per second squared : m/s2 : acceleration.
 
 

micro : μ

: 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
 
 
Microphones
 
 
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 Sound Level : Lmin

: the minimum noise level during a measurement period or a noise event.

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

: a system of measurement using the metre, the kilogram and the second as basic units of length, mass and time

See also the SI Units.
 
 

MLS : Maximum Length Sequence

: MLS - is basically a pseudo-random sequence of pulses. MLS measurements are quite standard in many different application fields, one of them acoustics. For more details click here.
 
 

mm : millimetre

: 0.001 metre
 
 

Modal Analysis

: is a process of determining the natural frequencies, damping factors, and mode shapes for a structure. This is usually done either experimentally through frequency response testing or mathematically using finite element analysis.
 
 

Mode

: a room resonance. Axial modes in rectangular rooms are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.
 
 

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 spectrum of the reverberated signal.
 
 

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

: = m·v = the motion of a moving body, measured as a product of it's mass and velocity - units: N·s = kg·m/s
 
 

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.
 
 

ms : millisecond

: 1 ms = 0.001 second
 
 

MSHA

: Mine Safety and Health Administration : USA.
 
 

Multi-spectrum

: a one or two dimensional array of spectra. A multi-spectrum consists of two or more spectra that were recorded during the same measurement - more information

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