Help Topics - M

Masking

The process by which the threshold of audibility for a sound is raised by the presence of another masking sound.

- -

Masking noise

A noise that is intense enough to render inaudible or un-intelligible another sound that is also present. Typical uses of masking noise are between doctor’s surgeries and waiting rooms to prevent those in the waiting room from hearing the intimate conversations of the patient currently talking to the doctor. Water fountains or background music are examples of masking noises for this purpose.

- -

Mass law

The amount of sound insulation afforded by a wall is proportional to the mass of the material in the wall. Broadly speaking doubling the mass (per unit area) increases the sound attenuation by 6 dB and a doubling of the frequency increases the attenuation by 6 dB. These figures are theoretical limits and in practice a 5 dB reduction is more often found.

- -

Maximum exposure limit

These are the statutory limits that should not normally ever be exceeded. MELs are particularly applied to substances known to be carcinogenic.

- -

Maximum noise level

The highest instantaneous sound pressure level over a given interval of time with a selected frequency weighting and specified time weighting. For example, an A weighted fast maximum level of 96 dB in a workshop.

- -

Medium

Any solid substance carrying a sound wave. Sound will not travel through a vacuum since there are no particles to transfer the energy from one point to another.

- -

Micron

A unit of length. One millionth of a meter or one thousandth of a millimeter.  One micron equals 0.00004 of an inch.

- -

Microphone

A transducer that changes the physical motion of air molecules into an equivalent electrical signal that can be processed by a sound measurement system such as a sound level meter. Different devices have different frequency responses and sensitivities. All other things being equal a smaller diameter microphone diaphragm produces a lower electrical output signal for a given sound pressure. The sensitivity of a microphone is given as the number of Volts obtained for a reference sound pressure. Typical examples are 10 mV/Pa for a general-purpose microphone capsule.

- -

Microprocessor

An electrical component that is able to rapidly process samples of an input signal obtained from an analog to digital converter and produce an output signal proportional to the time average level, the highest level or other computed values. Typically these calculations are performed many times a second in order that none of the input signal is missed. The microprocessor is the heart of the modern digital sound level meter and controls the main functions that are performed by the instrument

- -

Middle ear

Part of the hearing mechanism comprising the three smallest bones in the human body. The “hammer, anvil and stirrup” bones form a mechanical lever with a mechanical advantage of about 1.5 between the ear drum and the oval window of the cochlea.

- -

Minimum noise level

The lowest instantaneous sound pressure level over a given interval of time with a selected frequency weighting and specified time weighting. For example, an A weighted slow minimum level of 36 dB in an office.

- -

Monitoring

Periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and/or pollutant levels in various media or in humans, plants and animals.

- -

MSHA

The Mine Safety and Health Administration in the USA.

- -