This figure, which I call “the Speech Cues Audiogram,” shows the estimated portion of speech information that is contained within each octave band for varying levels of hearing loss. These values, which I call “speech cues,” represent the relative contribution of each octave band to speech understanding. Notice how the speech cues become fewer as hearing loss worsens.
The pattern (loss of information with increasing hearing loss) is similar across frequencies, except in the 4000-Hz zone where the values begin declining sooner (at a better hearing level). The unique way in which information is lost in the 4000-Hz band was pointed out to me by Harvey Dillon, PhD, director of research at Australia's National Acoustic Labs. He and his colleagues at NAL have conducted extensive research in this area.
When a patient's hearing is close to normal, the octave bands centered at 250, 500, 1000, 2000, and 4000 Hz contribute 8%, 14%, 22%, 33%, and 23%, respectively, of the information that the person uses to understand words.1 In other words, the five common octave bands, from 250 to 4000 Hz, contain 8, 14, 22, 33, and 23 speech cues, respectively. The low-frequency octave bands contribute much less information to word understanding than the high-frequency zones.
As hearing loss becomes more severe, the damage in the inner ear limits how much speech information can pass through the ear. People with profound sensorineural hearing loss have great difficulty understanding speech even when they are fitted correctly with hearing aids. The values in the Speech Cues Audiogram reflect my estimates of the loss of speech information at various frequencies for varying degrees of hearing loss in people whose hearing loss follows the typical pattern (more below). Let me emphasize, the data in Figure 1 are estimates of general tendencies and are not intended to be taken as precise guides for specific patients.
USING THE SPEECH CUES AUDIOGRAM
Let's consider a few patients to get an idea on how to use this graphic.
For a mild loss
Patient #1, “Judy” has a mild high-frequency hearing loss. Her pure-tone thresholds are: 10 dB at 250 Hz, 15 at 500, 20 at 1000, 35 at 2000, and 50 dB at 4000 Hz in both ears. When her thresholds are superimposed on Figure 1, we see that Judy is a good candidate for amplification and will probably have access to speech information in all frequency zones. Amplification in the 4000-Hz octave band (from 3000–6000 Hz) is recommended because most of the speech cues will be available in that zone, if Judy receives well-fitted amplification. Remember, she has 50-dB thresholds at 4000 Hz.
Please note: Most hearing aid fittings do a poor job amplifying the octave band at 4000 Hz. Real-ear tests often show little useful amplification in this zone. As a profession, we are in the habit of rolling off the high frequencies to avoid feedback. In this case, the patient can benefit significantly from properly fitted instruments that amplify all zones, especially the octave band at 4000 Hz.
Now, let's suppose that Judy says, “My hearing seems okay. Why do I need hearing aids?” Using Figure 1, we would point to her threshold at 4000 Hz and say something like, “It's true that you hear well in the lower frequencies, but you are missing many speech cues in the higher frequencies. Almost a quarter of all speech information (23%) is contained in the octave band at 4000 Hz. You probably notice your hearing loss when you are in noisy listening situations.” Judy concedes that she does have difficulty understanding people when it's noisy.
For high-frequency loss
Next consider patient #2, “Joe,” who has a severely sloping hearing loss with very limited hearing in the higher frequencies. His pure-tone thresholds are: 25 dB at 250 Hz, 40 at 500, 50 at 1000, 60 at 2000, and 80 dB at 4000 Hz. When Joe's hearing thresholds are superimposed on Figure 1, we see that it is probably unwise to give him much amplification in the 4000-Hz zone. If his hearing loss is typical, i.e., if it follows the general average of people with this type of loss, the number of speech cues available to him is very limited in the 4000-Hz octave band. The damage in Joe's inner ear is so severe that little speech information can pass through this zone.
The new real-ear targets published by the National Acoustic Labs (NAL NL1) incorporate this concept and limit amplification in zones where significant damage precludes the decoding and effective use of speech information. In a case like Joe's, it's better to focus our efforts (amplification) in the zones where considerable speech information is available. Notice that the Speech Cues Audiogram suggests that most of the speech information at the 60-dB threshold level at 2000 Hz is available in that zone. It seems prudent to give Joe adequate amplification in the 2000-Hz zone, but not 4000.
A reverse curve
Next, consider a third patient, “Sarah,” who has a reverse-curve hearing loss. Her thresholds are: 60 dB at 250 Hz, 50 at 500, 40 at 1000, 30 at 2000, and 25 at 4000. It is obvious that Sarah has considerable hearing loss in the lower frequencies, yet she reports little problems associated with her hearing.
When Sarah's thresholds are plotted on Figure 1, notice that she has good hearing in the zones most important to word understanding. Her “good” hearing sensitivity at 2000 and 4000 Hz allows her to receive more than 60% of the speech information (33% from the octave band at 2000 Hz, 23% from the octave band at 4000 Hz, and some information from the upper portion of the 1000-Hz octave band).
In the old days, we would have attempted to help Sarah by fitting her with flat amplification, moderate-length ear canals, and small vents. However, these fittings created more problems than most patients could tolerate. Today, if Sarah wants to hear better, we would be careful not to occlude her ears and would not get too ambitious in putting amplification in the lower frequencies. Sarah would be a good candidate for an open fitting. Her residual hearing could be easily improved by 10 to 20 dB in the higher frequencies and she would experience no negative effects from occluding her ear.
“TYPICAL” HEARING LOSS
The data in Figure 1 apply to people with sensorineural hearing loss, not mixed or conductive hearing loss, and they represent averages, general tendencies, the median not the mean. The distortion (dysfunction) in many patients' ears is often significantly better or worse than shown in Figure 1.
Many patients do not follow the pattern outlined in Figure 1. Some can successfully use ultra-high-intensity amplification, others cannot. This graphic is intended as a teaching tool, a way to show patients and new practitioners how the availability of speech information changes at different hearing levels and across frequency.
NAL has done much meaningful research in this area and is one of the best authorities on the topic. For more precise information on this subject, I suggest you contact NAL.
When being fitted with hearing aids, the patient should receive a complete audiometric assessment, including SRTs and word-understanding tests, and a hearing-in-noise test such as the QuickSIN from Etymotic Research. Most patients follow general tendencies. However, exceptions are not unusual, so you need to know if a patient deviates markedly from the norm. A complete picture of the patient's hearing is only seen when all data are considered.
Many patients have unreasonable expectations of hearing aids. Sometimes they are too optimistic, sometime too pessimistic.
In cases where the hearing loss is not so severe, the quality of aided hearing can be very good. When the hearing loss is nearer the bottom of the audiogram in the higher frequencies, the quality of the aided hearing is markedly reduced. Figure 1 is intended as a counseling tool for use in helping patients see how many speech cues they may receive from properly fitted amplification.
© 2006 Lippincott Williams & Wilkins, Inc.
1. American National Standards Institute: ANSI S3.5–1969, Methods for the Articulation Index. New York: ANSI.