Understanding Audiograms, Hearing Loss, and Speech Intelligibility
"These factors play a big role in our ability to understand speech. For one thing, the great majority of people with hearing loss lose it in the higher frequencies, where the consonants lie. This is especially true of hearing loss due to aging. So a lot of older people hear the vowels but not the consonants. In addition, since consonants are spoken more softly, they tend to get drowned out in background noise."
(Tech note: Remember how we said earlier (Decibels) that every 10 dB increase or decrease doubles or halves the perceived loudness of the sound? Keeping that in mind, we can see that the "a" at 40 dB sounds twice as loud as the "p" (30 dB) and four times a loud as the "th" at 20 dB. The "e" (50 dB) sounds twice as loud as the "a", four times as loud as the "p", etc.)
So your typical person with hearing loss will have trouble hearing the consonants in the first place. He may be hanging on by a thread. Add a little background noise and he may lose them altogether. And since we're on the subject of speech intelligibility there is one more fact to consider:
3. Consonants convey most of the word information; they are much more important to speech intelligibility than vowels.
It is usually possible, for example, to figure out a word if you remove the vowels. But if you remove the consonants, you're lost. Try it yourself. Have a friend copy a line of text, leaving out the vowels. See if you can decipher the words. Then have him copy another line, this time leaving out the consonants. You'll find that it's pretty much impossible to reconstruct the original text.
Putting it together:
Consonants are more important than vowels in understanding speech.
Consonants are spoken more softly than vowels, and they tend to get drowned out in noisy environments.
Consonants are higher-pitched than vowels and and most hearing loss occurs in the higher frequencies.
As you can see, in a normal conversation this person will simply not be able to hear many of the consonants.