4 Selection of hearing protectors

  4.1 Selection of suitable hearing protectors should be carried out according to the HML-method described in ISO 4869-2:1994. In order to give guidance to ship operators and seafarers in choosing proper hearing protection, a short description of the HML-method and its use is given below.

  4.2 The HML-method is a rating which is calculated in accordance with ISO 4869-2:1994, "Estimation of effective A-weighted sound pressure levels when hearing protectors are worn". Using the H, M, and L ratings requires both A-weighted (L_Aeq) and C-weighted (L_Ceq) sound pressure levels of the noise and the HML values for the hearing protector in question, which will be provided by the manufacturer.

  4.2.1 The HML values for a hearing protector are related to the attenuation that the protector offers in noise of high, medium and low frequencies. These H and M values are used in the calculation of the protected exposure level for noises which have primary energy in the middle and high frequencies. This is considered the case if the measured L_Ceq and L_Aeq levels differ by 2 dB or less.

  4.2.2 The M and L values for the hearing protector are used in the calculation of the protected exposure level for noises which have appreciable low-frequency components and for which the measured L_Ceq and L_Aeq levels differ by more than 2 dB in those spaces where the protector is intended to be used.

  4.3 An example of simple use of the HML method:

 On a given ship, the measured sound level in the machinery room is 110 dB(A), 115 dB(C). The chosen hearing protectors have the following attenuation according to the manufacturer: H= 35 dB, M=30 dB, L=20 dB.

• .1 Mark the hearing protectors' L and M values on the vertical line starting at the actual noise level (110 dB(A)).

• .2 Settle if the noise has low or high/medium frequency. If the difference L_Ceq- L_Aeq is more than 2 dB the noise has low frequency (L) and if L_ceq-L_Aeqis less than 2 dB the noise has high or medium frequency (M).

• .3 If the sound is of high/medium frequency (L_Ceq-L_Aeq≤2), follow the diagonal line from the M-value and take a reading of the noise level inside the hearing protectors. In this case the noise level inside the hearing protectors is 80 dB(A) which means that the attenuation of the hearing protectors are sufficient for work over 8 hours a day.

• .4 If the sound has low frequency (L_Ceq-L_Aeq>2), follow the diagonal lines from the L-value and take a reading of the noise level inside the hearing protectors. In this case, the noise level inside the hearing protectors is >85 dB(A) which means that the hearing protectors are not good enough even for a working day of 8 hours. Choose a hearing protector that has an L-value above 25 dB instead.

  4.4 Calculation by the HML-method – Principle and example

 Determination of feasibility of a particular protector in a specific noise environment can also be calculated. The values H, M and L may be used to estimate L'A (total A-weighted noise level at the ear) for a particular protector in specific noise situation.

• .1 Calculate L_Ceq-L_Aeq (This requires measurements of L_Aeq and L_Ceq. All class 1 sound level meter can apply A-weighted or C-weighted.)

• .2 If L_Ceq-L_Aeq is ≤2 dB, the Predicted Noise Reduction level (PNR) is calculated using the equation:

 If L_Ceq-L_Aeq is >2 dB, PNR is calculated using the equation:

  .3 The PNR is then subtracted from the total A-weighted noise level to give the effective A-weighted level at the ear under the protector L'A:

 L'A = L_Aeq - PNR

 In this case, the noise level inside the hearing protectors is below 80 dB(A) which means that the attenuation of the hearing protectors is sufficient for work over eight hours a day.