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Hearing Direct technical note:
SNR stands for Single Noise Rating and this is used within the European Union. The way in which it is calculated differs slightly due to different test standards and calculations from the NRR or Noise Reduction Rating system used in the USA.
In addition to an overall rating, the SNR further rates ear protectors in terms of the particular noise environments in which they will be used – H for high-frequency noise environments, M for mid-frequency, and L for low-frequency. So as an example, you may see the noise reduction information stated as: SNR 35, H=34, M=33, L=28. The estimated attenuation of sound changes according to the noise spectrum of the environment in which the protector is to be worn.