Safety people love acronyms as it makes us feel special, special like our mums' always told us we were. So HML, or 'High, Medium and Low'. This is the middle way, a bit better than the SNR route but not as good as the Octave band route. HML breaks the sound into three frequency ranges and gives you a level of noise reduction for each.
H (high) - 2000 to 8000Hz
M (medium) - 1000 to 2000Hz
L (low) - 63 to 1000Hz
(Out of interest, 'bass' in music is around 90 to 200Hz in modern recordings so into the L part of the scale. Sub-bass is, as the name suggests, below this and is around 40 to 60Hz but at this level is more felt than heard).
To use HML to assess hearing protectors you need a couple of measurements for the noise in question, the A weighted noise level and the C weighted noise level, dB(A) and dB(C). Note, this is C weighting, not C peak - a subtle but important difference. C Weighting is the C noise level over time rather than the instantaneous peak noise. So, to get the measurement for this you use a good noise meter that gives you average noise levels for the job in question in both dB(A) and dB(C).
Next you look at your desired hearing protector and somewhere on the packaging will probably be HML data, or maybe in the leaflet which comes inside it. If it is not there then check the Hearing Protection Data on this site were HML is listed for 200+ types of protector.
As a worked example, this here jobby to the right is the Peltor Optime II ear muff and 'tis indeed a good style of muff and is the one I used when I did noise assessments and needed a good muff I could wear comfortably almost every day. The HML figures on the packaging for this are:
H-34dB, M-29d, L-20dB
So you have your noise measurement and you have the HML data for the muff, so now you need to do some magic calculations to determine if the muff is good enough, after all it is clearly very powerful at the high frequencies (the H of 34dB) but less so at the lower ones (the 20dB). This is normal for most hearing protection as the lower frequencies are harder to stop, and most noise sources are higher up the scale. What you don't do is go and do this calculation yourself and as with the other two methods (SNR and Octave band) there are free resources online to do this for you.
This is how the calculation looks using the Noisemeters.co.uk free calculator, and using example noise data of 91dB(A) and 104 dB(C).
As you can see, the calculator is nice and easy and tells you that the result under the ear muff is 78dB allowing for small real-world wearer-errors such as hair or glasses interfering with the seal.
78dB is just about perfect for under-the-protector levels so the muff is suitable for this job, indeed is nigh-on ideal.
If you don't have a noise meter which can do octave band measurements then HML is the next best and helps you decide if the hearing protection you are looking at is up to the job. And direct links to two free resources which can do the calculation for you are: