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Differences Between Type I & Type II Sound Level Meters

Too much noise can damage your health irreversibly. According to research published during 1993 in "Noise/News International" magazine, not only does noise pollution disrupt conversation, concentration and sleep, but it can also cause permanent hearing loss. The research highlights motorcycles, chain saws and motorboats as some of the worst offenders, but even riding the subway can expose you to unhealthy levels of noise.

  1. Regulation and Measurement

    • The 1972 Noise Control Act sets out policies for the control of noise in the United States, establishing standards for allowable noise emissions from commercial products of all types, from aircraft to zip lines. The NCA spawned regulation that set out how to measure noise emissions, and specified the use of sound level meters that comply with applicable standards. Precision noise measurement requires a Type I sound level meter (SLM), while general-purpose measurement requires a Type II SLM.

    Accuracy

    • Type I sound level meters are highly accurate.

      The unit of sound intensity is the decibel, a logarithmic measure of noise level referenced against the smallest sound a human can hear. According to the American National Standards Institute Standard S1.4-1971, a Type I meter must have an overall accuracy of plus or minus 1 dBA and a Type II meter an overall accuracy of plus or minus 2 dBA. The abbreviation "dBA" denotes A-weighted measurements, which take into account the frequency spectrum of the human hearing range.

    Sound Energy

    • Sound energy creates variations in air pressure as the sound waves propagate. The microphone in a sound level meter converts the pressure fluctuations into an electrical signal ready for processing. Two types of microphone remain in common usage as of 2011: condenser microphones, which are highly sensitive and have a wide dynamic range, and ceramic microphones, which are less sensitive but more robust. Type I SLMs require condenser microphones, while Type II SLMs can use either type.

    Frequency Response

    • Digital filters process a spectrum of audio signals.

      A digital filter network processes the signal from the microphone by applying weighting factors across the frequency spectrum of human hearing. The meter then calculates the root mean square value of the sound energy and provides visual and digital outputs. ANSI S1.4 requires that a Type I SLM have accuracy to within plus or minus 0.5 dB at any frequency between 22.4 and 11,200 hertz (Hz), while a Type II SLM is only required to achieve this level of accuracy between 63 Hz and 2,000 Hz.

    Weighting Network

    • A digital filter can have up to 4 different settings: A-, B-, C- and D-weighting. Switching out the filter network entirely gives the sound level meter a linear response. The most common setting is A-weighting, but technicians needing to measure loud sounds or even jet aircraft noise may require C-weighting or D-weighting for these applications. Type I instruments generally provide at least A- and C-weighted filtering and a linear response, while Type II instruments often feature only A-weighting.

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