Calculating the residual level at the ear: the HML check

What the calculation is for

In Germany no more than 85 dB(A) and 137 dB(C) may reach the ear (§ 8(2) LärmVibrationsArbSchV); EU-wide, the exposure limit values of 87 dB(A) and 140 dB(C) apply under hearing protection (Directive 2003/10/EC). DGUV Rule 112-194 recommends a residual level of 70 to 80 dB(A). Below that, there is a risk of overprotection.

Assessment of protection according to DGUV Rule 112-194 (2024), annex 2, table 5
Residual sound pressure level	Residual peak sound pressure level	Assessment
above 85 dB(A)	above 137 dB(C)	not permitted
above 80 dB(A)	above 135 dB(C)	not recommended
70 to 80 dB(A)	up to 135 dB(C)	recommended
below 70 dB(A)	-	check communication and feeling of isolation

The calculation in five steps

Determine the level at the workplace. What is needed is the daily noise exposure level LEX,8h from measurement or a database (calculating the daily value).
Set the noise class. If the difference between the C-weighted and the A-weighted level is at most 5 dB (LC − LA ≤ 5 dB), the noise is mid- to high-frequency (HM). If it is larger than 5 dB, it is predominantly low-frequency (L). Examples are in SNR and H/M/L.
Take the attenuation value. From the manufacturer’s information: the M value for HM noise, the L value for low-frequency noise (attenuation values).
Subtract the real-world deduction Ks (table below).
Calculate and assess the residual level: L'EX,8h = LEX,8h − (M − Ks) for HM noise, L'EX,8h = LEX,8h − (L − Ks) for low-frequency noise. Compare the result with table 5.

Real-world deductions Ks

Annex 1, table 4 of DGUV Rule 112-194, for untrained users
Type of hearing protector	Ks
earplugs that are shaped before use	9 dB
pre-moulded earplugs	5 dB
banded earplugs	5 dB
earmuffs	5 dB
custom-moulded earplugs with functional check (before first use and at least every three years)	3 dB

For very high levels with a combination of earplugs and earmuffs, 9 dB or 5 dB applies, depending on the type of earplug. With qualified use the deduction does not apply (Ks = 0 dB); see Qualified use. In the UK, the HSE recommends a 4 dB real-world deduction for hearing protection (see the HSE source).

Worked examples with EarProtect values

Worked examples with the HML check; M and L from the test documents (EP1227: M 35 dB; EP904: M 25, L 18 dB; EP906: L 20 dB). They show the calculation; they do not replace a check at the workplace.
Case	Calculation	Residual level	Assessment
100 dB(A), HM, foam earplug EP1227 (Ks 9)	100 − (35 − 9)	74 dB(A)	recommended
90 dB(A), HM, foam earplug EP1227 (Ks 9)	90 − (35 − 9)	64 dB(A)	below 70: check communication
90 dB(A), HM, earmuffs EP904 (Ks 5)	90 − (25 − 5)	70 dB(A)	recommended
95 dB(A), low-frequency, earmuffs EP906 (Ks 5)	95 − (20 − 5)	80 dB(A)	recommended
95 dB(A), low-frequency, earmuffs EP904 (Ks 5)	95 − (18 − 5)	82 dB(A)	not recommended
100 dB(A), HM, foam earplug EP1227, qualified use (Ks 0)	100 − 35	65 dB(A)	below 70: check communication

The fourth and fifth examples show why the noise class matters: with low-frequency noise the smaller L value applies, and two earmuffs with a similar SNR then differ by 2 dB. Which EarProtect products fall in the recommended range at which level is calculated by the hearing protection finder.

Checking the peak sound pressure level

L'pC,peak = LpC,peak − (M − Ks) for HM noise; L'pC,peak = LpC,peak − (L − Ks − 5 dB) for low-frequency noise. The result must be at most 137 dB(C).

According to annex 2, protection is rated as well suited if L'pC,peak is at most 135 dB(C) and the residual level L'EX,8h is between 70 and 80 dB(A). More on bangs and impacts is in Hearing protection for impulse noise.

More accurate methods

Selection methods according to DGUV Rule 112-194, annex 2
Method	Inputs	Note
HML check	LEX,8h (A-weighted), noise class, M or L, Ks	For everyday practice; with a real-world deduction unless there is training for qualified use
HML method	A- and C-weighted level, manufacturer’s H, M, L (predicted noise level reduction PNR)	L'A = LA − PNR; only with qualified use
Octave band method	Level per octave band, frequency weighting, assumed protection value APV per band	Laborious, for individual cases, for example with hearing loss; requires qualified use
SNR method	C-weighted level, SNR	Less accurate; never subtract the SNR from the A-weighted level

Frequently asked questions

How do you calculate the residual level under hearing protection?

With the HML check: residual level = noise level − (M − real-world deduction) for mid- to high-frequency noise, noise level − (L − real-world deduction) for low-frequency noise. The result should be 70 to 80 dB(A).

What is the real-world deduction Ks?

A deduction from the M or L value because hearing protection attenuates less in practice than in the laboratory: 9 dB for foam earplugs that are shaped, 5 dB for pre-moulded earplugs, banded earplugs and earmuffs, 3 dB for custom-moulded earplugs with a functional check.

When does the real-world deduction not apply?

With qualified use: practical training at least four times a year, documented (DGUV Rule 112-194, section 6.2.5 and annex 1).

How can I tell whether noise is low-frequency?

By the difference between the C- and A-weighted levels: if LC − LA is greater than 5 dB, calculate with the L value, otherwise with the M value.

Is the SNR enough for the calculation?

Not for the HML check, which uses M or L. The SNR method requires the C-weighted level and is less accurate; simply subtracting the SNR from the A-weighted level is wrong.