Hearing Loss and Speech Intelligibility

In the audiogram below, you can see where our basic speech sounds lie when engaging in normal conversation.

Two things are important to recognize. For the most part:

1. consonants are higher pitched than vowels (they lie more to the right on the chart).
2. consonants are spoken more softly than vowels (they lie higher on the chart, in the lower decibel ranges).

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.)

Most vowels and consonants lie in the following regions:

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, and it's a doozy:

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. Unless you're psychic, it's pretty much impossible to reconstruct the original text.

If you or someone you know has ever said, "I can hear the words, I just can't understand them," this is probably why.

(One additional characteristic of consonants: they act as breakpoints, separating syllables and words from one another. If we can't hear the consonants clearly, the words seem to run together and people sound like they are mumbling. And since women and children have higher-pitched voices than men, it seems like they mumble more.)

Putting it together:

1. Consonants are more important than vowels in understanding speech.
2. Consonants are spoken more softly than vowels, and they tend to get drowned out in noisy environments.
3. Consonants are higher-pitched than vowels and and most hearing loss occurs in the higher frequencies.

It's harder for people with hearing loss to hear the consonants in the first place, and noisy environments just makes it worse. No matter how hard they try, they are just not getting it all. The result is fatigue, frustration, and an increasing reluctance to engage in socially frustrating situations.

To illustrate how a hearing loss affects speech comprehension, we will now superimpose the audiogram of the person with moderate/severe hearing loss over the speech zone. The screened area lies below the subject's threshold of hearing, i.e., what he can't hear.

As you can see, in a normal conversation this person will simply not be able to hear many of the consonants. The other person will have to speak considerably louder to be understood. That is, he will have to push the loudness of his speech above the subject's threshold of hearing. This obviously puts a strain on everybody involved.

My audiograms