Acoustic Glossary


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

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 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

Mass : m the 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.

Mass Density kilogram per cubic metre - kg/m3 - see also the 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

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 that 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

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.

Measurement Microphones are 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

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

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

Mechanical Impedance at a point IEC Definition,

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 IEC Definition,

imaginary part of a Mechanical Impedance


Mechanical Resistance IEC Definition,

real part of a Mechanical Impedance

Medium (plural media) The intervening substance through which physical forces are transmitted

Mel 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.

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 per second : m/s : Speed of Sound or the Velocity

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.

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

MLS : Maximum Length Sequence - an 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

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 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 = 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.

Monopole Sound Source IEC Definition,

source that radiates sound equally in all directions in a Free Field.

ms : millisecond 1 ms = 0.001 Second

MSHA : Mine Safety and Health Administration : USA.

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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