Sound Level Specifications

Sound Level Specifications: THE EVOLUTION OF QUIET

By John C. Dolehanty

September 2005

Q. - What is the easiest and most cost effective way to ensure quiet industrial facilities?

A. - Sound Level Specifications.

Many companies have suffered through the frustrating cycle of spending precious resources on quieting their facility only to have their efforts thwarted by the purchase and installation of noisy equipment and machines. The smart companies learn from this experience, and implement sound level specifications to ensure that machinery and equipment are designed and built to operate quietly.

The proper utilization of a sound level specification for the purchase of new equipment is essential for all noise control and hearing conservation programs. The sound level specification prevents noise problems in new facilities and helps decrease sound levels over time in older facilities as loud equipment is replaced with quieter equipment.

The intent of this article is to (1) explain the purpose of a machine and equipment sound level specification and why it is important, (2) highlight components of an effective specification, and (3) identify common mistakes in assessing compliance with specifications.

Purpose of a Sound Level Specification

An effective noise control program depends heavily upon a working sound level purchase specification. The primary purpose of a sound level purchase specification is to ensure that noise control measures are considered and implemented in the design and build process. This ensures that when the equipment is installed it meets its primary criteria of preventing/reducing employee Hearing Conservation Program (HCP) inclusion. Secondary equipment criteria may include meeting an environmental performance limit or meeting interior office limits that ensure proper speech communications (office HVAC, etc.)

The primary purpose of the sound level specification is to assist in the prevention of noise induced hearing loss by ensuring employee noise exposures do not exceed 85 dBA TWA_8hr. A full shift (8-hour) employee noise exposure of 85 dBA TWA_8-hour requires employers to create and maintain an on-going and effective hearing conservation program.

With a working sound level purchase specification in place, future noise control actions are typically limited to maintenance of equipment and controls currently in place. Without a working sound level purchase specification, current and past noise control efforts can be negated when noise generating equipment is located in proximity to areas/employees that have been quieted due to noise control efforts.

OSHA does not mandate equipment sound levels. If you purchase a piece of equipment that causes employee exposure levels to exceed 85 dBA TWA, you’ve also purchased the responsibility to maintain an on-going and effective hearing conservation program!

The scheduled replacement of noisy equipment is the ideal opportunity to reduce noise levels. The time and the resources are available to make it happen. It is essential that a sound level specification is part of the replacement process.

Uses Beyond Hearing Conservation Programs

An average sound level limit of 80 dBA is intended to prevent occupationally related noise induced hearing loss. A sound level purchase specification that references an A-weighted sound pressure level at 1 meter can not address environmental noise issues (property line / community noise) nor is it appropriate in addressing interior room acoustics and speech intelligibility issues.

The local noise ordinance, as appropriate, should be referenced for environmental concerns. Property line limits for Industrial sources typically vary from 55 dBA to 70 dBA. Interior room acoustics are typically addressed using NC and RC curve criteria. For example, open plan offices are typically designed to meet NC-40.

Essential Components of Effective Sound Level Specifications

Limits

It is important to consider factors such as regulatory limits, additive effects of additional noise sources, operator interactions, equipment types and available control technology when selecting a sound level limit for specifications.

Many sound level purchase specifications limit new equipment sound pressure levels to an average sound level of 80 dBA (or less) at 1 meter along the envelope of the equipment/system, and at any operator’s position(s). This limit has proven quite effective in preventing employee exposures from exceeding 85 dBA TWA. A limit of 80 dBA was chosen to allow for the introduction of an 80 dBA sound level into areas where the current sound level is as high as 83 dBA and still not exceed a cumulative sound level of 85 dBA. (A-weighted sound levels are specified because A-weighted levels are used to assess employee noise exposures.) It should also be noted that some specific pieces of equipment are traditionally quoted in sound power levels, specifically electric motors. Note that an electric motor’s sound power level (L_W) of 85 dBA is actually less than 80 dBA at 1 meter.

Equipment

A type I or II integrating sound level meter is the preferred instrument for determining equipment sound level. The sound level meter should be capable of measuring average, maximum, minimum and peak sound levels. A non-integrating sound level meter should only be used if the equipment in question produces a very steady-state (non-changing) sound level (this rarely occurs!). The meter should be configured so that there is no lower threshold cutoff (in order to capture the entire range of equipment sound levels. In addition, a 3 dB exchange rate (or doubling rate) is typically used. This setting more accurately reflects the true physics of sound propagation, as apposed to the more liberal 5 dB setting reference by OSHA for its employee exposure monitoring.

An octave band (frequency) sound level meter is not typically needed, but can be useful when measuring ultrasonic (high frequency) sources.

Measuring Procedures

Most sound level purchase specifications are based off of the National Machine Tool Builder’s Association’s (NMTBA) Noise Measurement Techniques, Second Edition, from January, 1976. NMTBA has been succeeded by Association for Manufacturing Technology (AMT) and they are in the finals steps of approving an updated ANSI Technical Reference Guideline: B11.TR5: Sound Level Measurement Guidelines, A guide for measuring, evaluating and documenting sound levels emitted by machinery to replace the NMTBA guideline.

In general, average (L_EQ) sound levels are measured at a distance of 1 meter along the perimeter of the equipment/system along with any operator position(s). The sound level meter should be positioned 1.5 meters above the floor (equivalent to an average person’s hearing zone), unless there is a defined operator’s position at some other elevations.

Sound level measurements should be made during typical operating conditions and should capture the worst-case operating condition. For most equipment this occurs under load but some systems actually produce higher sound levels while idling. In addition, the sound level should be averaged over multiple cycles of the equipment to ensure that the measured values are representative.

The background sound level (during testing) should always be as low as possible, preferably 10 decibels or more below the specified limit. Background levels that are 10 decibels or more below the equipment level are acoustically negligible. The background sound level should be measured prior to the equipment testing, and then subtracted from the equipment sound level.

Common Errors Made in Compliance Assessment

Many companies feel that because they have a sound level specification, they are protected from receiving noisy equipment. Unfortunately, due to a lack of acoustical expertise and sometimes improper equipment usage, it is not unusual to find incorrectly documented sound level data. These errors and oversights can prove costly to both suppliers and end-users of the equipment.

Equipment

Calibration - Measurement equipment must be field calibrated (with a hand held calibrator) before and after measurement sessions. If the meter does not calibrate to within ± 1 decibel after the measurement session, the equipment measurements should be repeated. In addition, measurement equipment must be re-calibrated annually at a certified testing laboratory. The integrating sound level meter and its associated field calibrator should always have a seal that indicates its laboratory calibration is National Institute of Standards and Technology (NIST) traceable. Documentation of meter calibration should always be included with any written report.

One error that often occurs is when the wrong measurement equipment is used. Whenever possible, an integrating sound level meter should be used. A non-integrating sound level meter should only be used when the sound levels are non-varying or steady state.

Configuration - The sound level meter needs to be configured correctly. Typically, this would entail the following: setting the exchange rate to 3 dB, setting the threshold to 0 dB (or the lowest possible setting), setting the dynamic range so that the sound levels fall essentially equally between the high and low levels, and setting the measurement metric to L_EQ (average level). Ideally, the meter should also concurrently measure the following metrics: sample duration, L_MAXimum, L_MINimum, and L_Peak. Often times a dosimeter is used for documenting equipment sound levels. This is not a problem in itself, if the meter is properly configured. Remember, the standard dosimeter setup is intended for employee exposure documentation, not equipment sound level evaluation.

Another error commonly encountered is when the wrong equipment conditions are measured. The sound level measurements must be representative of typical (or is some instances worst case) production/operating conditions. If these conditions are not possible, then a simulation/representation can be applied. For example, multiple sources can be measured separately and then combined mathematically to get the overall sound level. (Remember to remove the background sound from each individual source measurement.)

It is worth repeating that equipment measurements are very different from an employee noise exposure measurement. Using typical employee dosimetery equipment/techniques may yield incorrect information. Typical employee exposure measurements (OSHA) use an 80 dBA lower threshold cut-off with a 5 dB exchange rate. A threshold setting of 80 dBA will integrate all sound levels below 80 dBA as 0 dBA in this setup. This incorrect set up will usually yield artificially low sound levels.

Procedure

A common measurement mistake occurs when the meter is placed too close to the equipment, with the thinking that one can collect a good sample this way (e.g.. like an air sample). Since sound pressure is a function of distance from the sound source, the meter location is critical.

The background level should be subtracted from the measured equipment sound level. Optimally, the background should be at least 10 dBA below the equipment sound level. The variability of the background sound level should be steady if the background sound level approaches the equipment sound level.

The perimeter sound levels are never to be averaged; they should be treated as stand along data points. And remember, decibels are logarithmic values and cannot be manipulated using standard arithmetic means.

Keys To Successful Sound Specification Utilization

The key to successfully utilizing a sound level specification to minimize the harmful effects of noise at an industrial facility lies in creating a partnership between buyer and suppliers. Some of critical elements of this partnership include:

Timing - Addressing noise problems before the equipment is operational leads to more effective noise control efforts.
Training - Does your company provide training to suppliers to assist them in understanding your specification and assistance in conducting compliance assessments?
Documentation - Do you provide suppliers with guidelines and formats for documenting measurement locations, meter configuration, duration of each sample, as well as calibration history?
Evaluation & Feedback - Evaluating supplier capabilities and providing feedback on compliance efforts provides a basis for improvement on both meeting the specification and providing consistent data.

Remember: a sound level purchase specification can only be effective if it is consistently applied and enforced. A good rule of thumb to remember: the equipment suppliers’ efforts to meet the sound level purchase specifications will mirror the same commitment given to enforcing it!

Mr. Dolehanty is the President of Phase To, Inc., and has over 18 years of experience in hearing conservation and noise control. Mr. Dolehanty can be reached at (517) 886-9379 and at johndolehanty@phaseto.com.

Copyright © 2006 Phase To, Inc.