HML Method

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Inside the HML Method


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IntroductionUnlike most occupational injuries, there is no visible evidence of noise-induced hearing loss (NIHL). It

is not traumatic and often goes unnoticed when it first occurs. Noise-induced hearing loss accumulates

over time, its effects realized long after the damage has been done. NIHL is permanent and irreversible.

With proper education, motivation and protection, however, it is also 100% preventable.

Hearing conservation is about more than supplying your employees with earplugs or earmuffs that block

the most noise. It is about finding the solution thats right for your people. At Howard Leight, we realize

that the people who depend on our products to protect their hearing are as diverse as you can imagine.

And the ways people select the right hearing protection are just as diverse.

This guide provides you with insights and tactics to select appropriate earplugs and earmuffs for your

employees based on HML attenuation values. While there are several factors that should be considered

in the selection of hearing protectors, including comfort, convenience and communication, the HML

Method is an effective determinant of protection by High, Medium and Low frequencies of noise.


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Establishing Noise MeasurementsAs a component of European Union Directive 2003/10/EN, which outlines the

protocols for occupational Hearing Conservation Programmes (HCP), workers

may not be exposed to noise exposure limits of 87 dBA or higher (protected level).

In achieving this endeavor, HCP managers are required to determine and assess

noise risk in the workplace; reduce exposures through engineering or administrative

controls; provide training and appropriate hearing protectors to noise-exposed

employees; perform annual audiometry on employees; and assess the programme

on a regular basis.

In establishing noise risks in the workplace, HCP managers are required to

collect representative noise samples through area and personal dosimetry.

A sound level meter is used when measuring area noise. It provides a mean

value of ambient noise level across all frequencies,

and uses a unit called decibel (dB) to measure

sound pressure level or volume.

The human ear is more advanced in how it

registers sound than a sound level meter. In

fact, the ear does not register sound in the

same way as a sound level meter the ear

is more sensitive to the frequencies that are

most important to us, such as voices, signals

and alarms.

To translate the sound measured by the sound

level meter into the way the ear registers and is

affected by the sound, a filter is applied (A-filter).

This A-filtered sound uses the measurement unit dBA. The dBA value is important to know when

determining a noise environment, because it tells us the extent to which the noise affects our hearing.

It is also important to know what frequencies are most prominent in the noise environment. This helps

us to select the most appropriate earplugs or earmuffs many hearing protectors attenuate, or blocknoise more than others at different frequencies.

To determine the most prominent frequencies, an additional filter is applied (C-filter). The C-filtered

sound level uses the unit dBC. If the noise environment consists of mostly low-frequency noise,

the C-filtered sound level becomes much higher than the A-filtered sound level.

By comparing the dBA-value with the dBC-value, you can determine if the area noise consists of

mostly high, medium or low frequency noise. Most sound level meters can be set to measure both

dBA and dBC.

A, B and C WEIGHTING CURVES-10

0

10

20

30

40

50

10 100 1000 10000 100000

Frequency in Hertz

SoundLe

velin

Decibels

A-Weighting

B-Weighting

C-Weighting

A sound level meter measures

area noise levels in decibels (dB)

Photo courtesy of Larson Davisa PCB Piezotronics division


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How to Apply the HML MethodIn selecting the appropriate hearing protectors for your noise environments, you will need to know:

Noise Levels perform measurements with a sound level meter to determine area noise levels

HML Attenuation Data this information is printed on the hearing protectors packaging and in its

user instructions

HML Table table of attenuation values across High, Medium and Low frequencies

Step Example

Determine the noise level by measuring

dBA and dBC with a sound level meter.

If you do not have access to the dBC

value on a worksite, you can estimate the

frequency range by listening. You can also

check the type of machines that are used,

and determine if they are typically low or

high frequency machines. Or you may refer

to the chart that displays some examples of

machines that have mostly medium to high

frequency noise and some that produce

typically low frequency noise.

Subtract dBA from dBC to determine

what type of noise environment you have.

If the difference is < 5 Your noise

environment is mostly medium to high

frequency. Check the H and M value

when choosing hearing protection.

If the difference is 5 Your noise

environment consists of mostly low

frequencies. Check the M and L valuewhen selecting hearing protection.

Locate the dBC dBA value on the bottom

of the HML table.

dBC - dBA = 5

dBC = 117

dBA = 112


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How to Apply the HML Method Continued

Step Example

Identify the attenuation data of the

hearing protector.

Start with making a line between the H

and M values, then continueto theL value

and throughout the whole table.

You will find the HML values on the

hearing protectors packaging or in its

user instructions.

Make a line from your dBC dBA

value straight up until it meets the

attenuation line.

This is how much the hearing protector

attenuates in your sound environment.

It is called the Predicted Noise Level

Reduction (PNR) value, which is the

estimated attenuation of the hearing

protector in dB, when the protector is

used properly.

To estimate the attenuation offered by a

specific hearing protector in your specific

environment, subtract the PNR (Predicted

Noise Level Reduction) from the dBA noise

level measured in your area.

Leightning L3 Earmuff,SNR 34, H:33 M:32 L:27

PNR = 30 dB

dBA - PNR = 112 - 30 = 72dB


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Table 1. HML Table


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Using the HML Method Instead of SNRThough the SNR is a laboratory estimate of protection that a hearing protector is expected to provide in

one fixed number, the actual attenuation of the protector at a frequency may be much higher or much

lower than the SNR value especially in the low frequencies. The SNR value can make you believe

you are protected even if you are not. In table 2, below, we demonstrate the attenuation of four different

hearing protectors. All protectors have at least SNR 33 dB. As demonstrated in table 2, it is only in the

high frequencies that the attenuation reaches levels similar to the SNR value.

Table 2. HML Table with Attenuation Data of Earmuffs with High SNR Value

Howard Leight Clarity C3earmu(SNR 33)

Howard Leight Thunder T3earmu(SNR 36)

Sordin Left/Right largeearmu(SNR 33)

Peltor Optime III earmu(SNR 35)


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Comparing Hearing ProtectorsIn comparing the attenuation values of different hearing protectors, the HML Method makes it is easy

to determine how hearing protectors will attenuate in different sound environments. Table 3 compares

three different earmuffs with low SNR values with the HML Method.

Table 3. HML Table with Attenuation Data of Earmuffs with Low SNR Values

Note that while the Howard Leight Clarity C1 earmuff varies in attenuation between 24 dB and 19 dB,

the Peltor Optime I earmuff varies between 32 dB and 12 dB. When you have a small range in variation

of attenuation over the HML frequencies, you can be sure that you are protected even if you do not

know the frequency of your noise environment.

Howard Leight Clarity C1

earmuSNR 25, Weight 194g.

Peltor Optime I earmu

SNR 27, Weight 180g.

Sordin Left/Right low earmu

SNR 24, Weight 200g.


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Table 4. HML Table with Attenuation Data of Earmuffs with Medium SNR Values

Howard Leight Leighthing L1


Peltor Optime II earmu

SNR 31, Weight 210 g.

Sordin Left/Right medium



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Avoiding OverprotectionBalancing the need to protect your employees without compromising their overall protection from

hazardous noise is a challenge for any safety manager. When employees utilize high attenuating

earplugs or earmuffs in marginal noise environments, they may be at an additional workplace risk.

Too much protection, or overprotection, may isolate employees from important communications,

including co-workers voices, machine sounds, alarms and signals. The risks of overprotection are

very real when dealing with hearing conservation, and the consequences can be catastrophic. A

worker who cannot hear the warning signal of a truck or piece of heavy equipment backing up can

be in serious danger.

But the consequences are more common and more far-reaching than safety. Workers who cannot hear

on the job are much more likely to make mistakes than those who can communicate naturally with

their supervisors and co-workers. Also, studies have shown that workers who cannot communicate

clearly or effectively with their fellows tend to feel more isolated on the job, and are less likely to be

happy or productive.

The International Standards Organization recommends that protected noise levels that is, the noise

level under the earplug or earmuff should fall within a manageable 70-85 dB range (ISO Guideline

EN-458). Protected noise levels over 85 dB indicate exposures that put the worker at risk for hearing

damage. Protected noise levels under 70 dB may indicate overprotection, and workers may feel isolated

from their work environment.

Table 5. Worker Exposure at the Ear with Hearing Protection, Based on ISO Guideline

When studying an HML table, speech and warning signals are most likely high frequencies. High

attenuation of high frequency sounds and moderate attenuation of low frequency sounds can

overprotect workers against important sounds. Be careful.


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Your Turn: Applythe HML MethodMake your own analysis of hearing protectors

using the HML method. We have provided you

with a copy of the HML table and a worksheet

for your own analysis.

HML WorksheetCompany

Department

Equipment

dBA dBC dBC dBA

dBA

Predicted Noise Level Reduction (PNR) Value

Estimated Noise Level Under Hearing Protector


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GlossaryA-Weighting

A filter applied by noise measurement devices, intended to replicate the frequency sensitivity of the

human ear. Sound level meters set to the A-weighting will filter out much of the low-frequency noise

they measure, similar to the response of the human ear. In contrast, the C-weighting is a flatter filter,

allowing more low frequencies to be measured.

Attenuation

A reduction in noise level. Hearing protectors are rated for their attenuation; protectors with higherattenuation reduce more noise.

C-Weighting

A filter applied to noise measurements. In contrast to the A-weighting, the C-weighting is a flatter filter,

and allows more low frequencies to be measured. The C-weighting was originally conceived to be the

best predictor of the ears sensitivity to tones at high noise levels. But the ears risk to damage from

noise has since been found to be predicted much better by the A-weighting scale. Noise measurements

made with theC-weightingscaleare designateddBC.

Daily Noise Exposure LevelAn 8-hour time-weighted average measurement of noise exposure. This measurement includes both

continuous and impulsive noise.

Exposure Limit

The maximum allowable daily noise exposure level, taking account of attenuation provided by hearing

protectors worn by a worker.

Frequency

The physical measurement of the oscillations in a sound wave (measured in units called Hertz).

Subjectively, we hear frequency as pitch of a sound. The frequency range that can be perceived byhuman hearing generally extends from 20 20,000 Hertz, but the sounds that are most useful to us

(in the speech and conversation range) are in the narrower range from 300 3,000 Hertz. Audiometric

tests administered in industry generally test hearing at six or seven different standardized frequencies:

500, 1000, 2000, 3000, 4000, 6000 and sometimes 8000 Hertz. In noise monitoring and audiometric

testing, frequency is often measured in thousands of Hertz, or kilohertz (kHz).


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Glossary ContinuedHML

The High-Medium-Low (HML) calculation method is most commonly used in Europe. The HML system

is based on eight representative noise spectra. In practice, the HML method provides a sufficiently

accurate measure of hearing protector performance. When using this three number system, the first

number (H) represents the attenuation afforded in predominately high-frequency environments, the

middle number (M) represents the protection afforded in noise environments dominated by medium

frequencies and the third number (L) represents theprotectionafforded in noiseenvironmentsdominated

by low-frequency sounds. The great advantage of HML calculations is that only dB(A) and dB(C) are

required as noise data input.

H High frequency attenuation value

Represents the attenuation of a hearing protector in noise environments dominated by high

frequency sounds.

M Medium frequency attenuation value

Represents the attenuation of a hearing protector in noise environments dominated by medium

frequency sounds.

L Low frequency attenuation valueRepresents the attenuation of a hearing protector in noise environments dominated by low

frequency sounds.

Lower Action Value (80 dBA)

The daily noise exposure level at which precautionary measures must be in place. If noise levels

exceed 80 dBA, then hearing protectors must be available to exposed workers (usage is voluntary)

and training must be provided to exposed workers.

PNR (Predicted Noise Level Reduction)

The expected attenuation of the hearing protector calculated by the HML Method.

Peak Sound Pressure

The maximum instantaneous value of a C-weighted noise measurement.


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Howard Leight

7828 Waterville Road, San Diego, CA 92154

ph. 800/430-5490 fax 401/232-3110

www.howardleight.com

Glossary ContinuedSingle Number Rating (SNR)

Thehearing protector ratingused bytheEU.Attenuation testsare conductedat independent laboratories,

using human subjects to determine the average attenuation achieved by the protector. The SNR is found

on the packaging of all hearing protectors.

Sound Level Meter

A noise monitoring device that measures instant area noise levels. Since noise monitoring with a

sound level meter is specific to the immediate area where the measurement is being taken, these

measurements are also referred to as area sampling. The input to a sound level meter can be filteredthrough different weightings (see A-weighting and C-weighting) to mimic the reception of the human

ear. Optional attachments, such as Octave Band filters, can further restrict the noise measurement

only to specific frequency bands. Sound level meters used for regulatory compliance must meet

specifications in ANSI Standard S1.4-1971, Specifications for Sound Level Meters.

Upper Action Value (85 dBA)

The daily noise exposure level at which protective measures must be in place. If noise levels exceed

85 dBA, the employer must ensure the use of hearing protectors among exposed workers (usage

is mandatory).

HML Method

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