I think you will make speech less easy to understand if you allow only frequencies of 85hz to 255hz to pass. Those are the frequencies listed on https://en.wikipedia.org/wiki/Voice_frequency, but those are the fundamental frequencies and are best for vowels. Most consonants need higher frequencies than those to really understand what is being said. Using that range will give a sound that is like mumbling. I don't hear as well as I used to, and it is definitely the higher ones that I need.