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Introduction to Audio Measurement

Environmental Noise Measurement

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Environmental noise comes in many forms, and is an increasing problem in the modern age, causing stress, ill health, and in some cases hearing damage. While some standards are in place relating to particular types of noise, they are applied to varying degrees by different councils across the UK, and in other countries. The basis of the standards is also open to question, and the subject of ongoing research projects in some cases.

Occupational Noise Exposure

This is usually measured A-weighted, using a personal exposure meter as defined in BS6402: 1985. This makes repeated measurements over a period of time, such as 8hrs, and calculates the Leq (equivalent level) using a root-mean-square calculation (rms).

A special form of occupational exposure arises with musicians who experience particularly high levels when playing, especially in amplified pop groups. Special earplugs are available which claim to give uniform attenuation across the frequency range, such that performers can still hear the proper musical balance. Alternatively, isolating earpieces fed from a mix-down of the entire band, can provide a better balance, and are increasingly used by some pop musicians, often with wireless feeds.

Aircraft Noise

A special weighting curve exists for the assessment of aircraft noise. This is the D-weighting defined in IEC179: 1965 (Sound Level Meters).

Motor Vehicle Noise Testing

BS3539 specififies "Sound Level Meters for the Measurement of Noise Emitted by Motor Vehicles". This uses A-weighting together with time weighting F and rms detection. The time weighting requires that a special repetitive tone burst signal consisting of 11 cycles of 2kHz at 108dB repeating 40 times per second shall read the same as a steady signal of the same rms value to within 0.5dB.

Residential or Community Noise and Traffic Noise

World Health Organisation guidlines exist for community noise, and suggest that the maximum level, measured A-wtd Leq should be around 30 to 35dBSPL.

In the UK, residential noise is considered in relation to planning, and for this purpose a special Planning Policy Guidance note was issued by the Departement for the Environment as PPG24: 1994. This lists Noise Exposure Categories (NEC) A to D, where, for example, NEC A requires an LAeq of less than 55dB SPL while NEC D signifies over 72dB SPL. These are daytime values, night values being around 10dB lower.

Noise from industrial development is to be assessed, according to PPG24, using BS4142: 1990, which applies corrections for tonal or impulsive noise. PPG24 then suggests that noise from a development is likely to bring complaints if it is 10dB higher than the background noise without contribution from the new source. 5dB is a marginal increase that may bring some complaints. BS8233 also gives guidance on acceptable noise levels within buildings.

Entertainment Noise

This is an increasing cause for complaint, usually from people living near to pubs and clubs. While A-weighted measurements are sometimes used, these greatly underestimate the level of low frequencies in the 20 to 160Hz region, which are found to be the main problem, since they penetrate walls and windows to a greater extent than other forms of noise, and also tend to be especially annoying because of the throbbing nature of the bass on modern pop and dance music. A report by Salford University (see link below)suggests that 1/3rd octave analysis of the 10 to 160Hz region in terms of Leq (equivalent level) for each band may be the most appropriate method for assessing the problem and provides a table of proposed acceptable levels which vary from 71dB SPL at 20Hz to 49dB at 40Hz and 34dB at 160Hz. Another useful report appears in the Journal of Environmental Health (JEHR) (see link).

Noise Isolation

This can be measured between rooms, or from a point outside a room at a specified distance.

JEHR Objective Noise Criteria for the Assessment of Entertainment Noise

Salford University Report Assessing Noise from Pubs and Clubs - See page 28 for table of recommended levels.

World Health Organisation - Guidelines for Community Noise - See page 16 for details of their recommendations

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