The SNR is a single number rating system determined according to international Standard ISO 4869. SNRs are expressed in decibels (dB) and are used as a guide for comparing the potential noise reduction capability of different hearing protection devices. Generally the higher the SNR number, the better the performance of the ear defender across a range of noise frequencies, for example an SNR of 30 may not give a 30 decibel attenuation across all frequencies. The figures associated with H, M and L indicate the level of protection within the range of high, Medium or low frequency noise, for example a wearer of an ear defender where H=34, a high frequency noise of 100 decibels will drop to 66 decibels. Wearing the same ear defender where M=28, a medium frequency noise of 100 decibels will drop to 72 decibels.