Why can I hear but have trouble understanding what people are saying?
Put another way, what is it that makes speech understandable that my ears don't give me?

1. Frequency:
   ... (related to pitch with low frequencies coming from the bass <the left side> of the piano keyboard, and the high pitches from the right side of the keyboard). The units of frequency are expressed in Hertz (Hz) or sometimes cycles per second (cps). Thus a 100 Hz tone sounds like a low pitch on the piano and represents a sound that pushes the eardrum membrane in and out 100 times a second. Similarly, a 4000 Hz tone would push the eardrum membrane in and out 4000 times a second.

2. Intensity: the force or power of the sound, related to loudness of the sound. The units are expressed in decibels (dB). These are NOT percentage units, and their nature as multiples of 10 will be clarified later in this paper. Thus a 40 decibel loss of hearing means your hearing organ needs 10,000 times more power than normal to be activated, and a 60 dB loss means your hearing organ needs 1,000,000 times more power than normal to be activated. It doesn't seem that bad because the ear has a huge range of sensitivity ( 1 to 10,000,000,000,000 or 1:1013) between the faintest sound it can detect and the ones that cause pain.
3. Time
   temporal factors relating to length of the sound, and how it starts, changes and finishes. The units are usually expressed in seconds or milleseconds <msecs>(thousandths of a second), or microseconds <usecs>. Thus, 1000 msecs and 1,000,000 usecs both equal 1 second.
   
   Speech is made up of many frequencies that change rapidly in time (between 10 and 200 msecs) from low to middle to high pitches. The parts that carry the most intensity and loudness are in the frequency range of 80 to 400 Hz which is represented by the lower portion of the keyboard below A=440 (the tone to which the symphony orchestra tunes). The portion of speech energy that carries much of the information that makes speech understandable is concentrated in the frequencies between 300 and about 4000 Hz.

Understanding the Impact of the Audiogram that is most important to you.

In the audiogram in Figure 1 we see only two of the three dimensions: Intensity expressed in the vertical axis and labeled dB Hearing Level or dBHL, and Frequency expressed on the horizontal axis in Hz. Look also at the shaded zone which has exactly 100 dots in it . Remember that each dot carries 1 per cent of the information that contributes to speech clarity and that the number of dots that are audible to you predict how well you will understand quiet speech from a six foot distance.

You should also see that the dots are unevenly distributed, with many more of them filling in the gray zone between 1000 and 3000 Hz than in the 250 to 500 Hz zone.

This shaded zone has many names. Pediatric audiologists sometimes call it the "Speech Banana" or the "Ling Zone" after Daniel Ling who popularized it as a criterion target for successful aiding of young deaf children so that they could learn language by ear. To others it is called the Articulation Index zone because the number of dots that are audible to you predict how well you will understand quiet speech from a six foot distance. If you know your unaided audiogram, copy it on to this chart and we will show you how to predict your strengths and weaknesses in speech understanding. If you do not have an audiogram or do not know how to interpret it, get one from your certified clinical audiologist and ask him or her to explain to you its meaning in this context.

Now count the dots that are below your audiometric curve. {I have drawn in a sample audiogram for a patient who hears only 40 percent of the dots.} The more dots that are below your curve the better you will be able to hear normal conversational speech. The fewer dots that are below your curve, more trouble you will have. And if you have fewer than 95 audible dots you will definitely have more trouble hearing in noise than in quiet, but keep in mind that everyone has trouble hearing in noise, some of us have more trouble than others.

Figure 1 Each of these 100 dots carries 1% of the clarity of speech. Note the density of dots/dB of hearing is greatest between 1000 and 4000 Hz. This system predicts your hearing ability when you are 6 feet from an average speaker.

Figure 2 (below) shows how to use the index of numbers of dots to predict your ability to hear individual words compared to sentences and words in context. Look at the index of 0.4 which represents the audiogram you saw in Figure 1. The general area is highlighted by the large "A". With an index of 0.4 the person who already knows English can understand over 95% of sentences with sensible context in quiet. In contrast the same person would hear much more poorly if the target being listened for was a single word, or a number or name or address with no context clues. Thus, someone with a hearing loss like the one in Figure 1 might reasonably conclude he/she had no real hearing impairment, only when people "mumble", meaning only when he can't figure out what they are saying from the context.

Figure 2. The Articulation Index and equivalent speech-to-noise ratios in dB. Note that the AI is made up of the sum of all the infromation available in 50 dB HL of speech to the average normal listener. Each one-half of 1% of the AI is represented by one dot in Figure 1. (Adapted from Webster J.C.: Interpretations of speec and noise characteristics of NTID learning centers. J. Acoust. Soc. Am. 66 (Suppl 1): S37, 1979; with permission.)

Look also at the bottom horizontal axis marked "speech -to- noise ratio". When people listen at 6 feet away from a moderate level speaker, they usually hear at a speech- to- noise ratio of 18 dB, meaning the speech is roughly 18 dB stronger than the background noise. With a hearing loss index of 0.4 however, the victim starts out at a disadvantage, listening in quiet with an equivalent speech-to-noise ratio of 0 dB (see Figure 2).

Summary:

The more dots you hear with or without your hearing aid at 6 feet from the speaker (50 dB HL to the audiologist) the better your comprehension of everyday speech is likely to be.

Where we are going next: Since most congenital deafness and much acquired deafness is traceable to genetic causes and/or damage, it is useful to know as much as possible about the new strides that are being made in genetics. Hence the research symposium on which we are about to embark will tell us what is new and exciting with respect to hearing impairment and genetics.

References:

Major facts in this presentation and some of the audio demonstrations are in a book published in 1996 by Singular Publishing Group called "Hair cells and Hearing Aids" Edited by C. Berlin. Further explanation of this diagram and its relationship to human genetic phenotyping can be found in a review entitled Genomics and Hearing Impairment by Keats, B., and Berlin, C., pp 7-16, in Genome Research, January 1999.