Section 7S. The following words used in this section twenty-four A to twenty-four C, inclusive, unless the context otherwise requires shall have the following meanings:—

''A–weighted sound level'', the sound level in decibels as measured on a sound level meter using the A–weighting network. The level is designated dB(A).

''Decibel (dB)'', a unit for measuring the volume of a sound, equal to twenty times the logarithm to the base 10 of the ratio of the pressure of the sound measured to the reference pressure; which is 20 micropascals or 20 micronewtons per square meter.

''Motorcycle'', any motor vehicle having a seat or saddle for the use of the rider and designed to travel on not more than three wheels in contact with the ground, including any bicycle with a motor or driving wheel attached, except a tractor or a motor vehicle designed for carrying golf clubs and not more than four persons, an industrial three-wheel truck, or a motor vehicle on which the operator and passengers ride within an enclosed cab.

''Person'', any individual, association, partnership, or corporation, and includes any officer, employee, department, agency or instrumentality of the commonwealth or any political subdivision of the commonwealth.

''Registrar'', the registrar of motor vehicles.

''Sound level'', the weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B or C as specified in American National Standards Institute specifications for sound level meters (ANSI S1.4–1971). If the frequency weighting employed is not indicated, the A-weighting shall apply.