The Speech Cue Train Game (2002)

FigureProbably the most important part of hearing aid fitting is ensuring that the important cues of speech are heard by the wearer at the levels that make them easiest to understand. We'll give some examples with a fun teaching analogy that makes the concept of speech cues easy to understand and use. Relax and enjoy. OCTAVE BANDS AND BOXCARS We are going to divide the speech signal into five octave bands, and represent each band as the cargo in one boxcar in a train. In case you don't have a train handy, you can cut out five squares (roughly 2x2 inches) from a blank sheet of paper. Write “ob 250 Hz” on one paper; this stands for the octave-band centered at 250 Hz. Write: ob 500 Hz, ob1000 Hz, ob 2000 Hz, and ob 4000 Hz on the other sheets. Each box represents the entire octave band, not just the single frequency at its center. Also, find eight dimes, two nickels and a penny. We are going to put coins on top of each paper to represent the speech cues available within each octave band. You would like to get all the coins to their destination (the ear and brain). However, that is usually impossible, so your goal is to complete the journey with as much money as possible. Now, arrange your five paper squares (boxcars) in a row, end-to-end, like cars on a train. Put a nickel on top of the first square, a dime on the second, two dimes on the third, three dimes on the fourth, and two dimes on the last square (see Figure 1).Figure 1.: The five train boxcars represent the five octave bands 250 Hz to 4000 Hz. The coins inside the cars represent the amount of speech information contained within each octave band.The octave bands 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz do not contribute equally to word understanding. According to ANSI, the amount of speech information (speech cues) contained within those octave bands for normal listeners—and also for people with mild and moderate hearing loss—is 8%, 14%, 22%, 33%, 23% in each band, respectively. Because it's easier to work with nickels and dimes than with a lot of pennies, we rounded off the ANSI values and made simple weighting values of 5-10-20-30-20. In this analogy, there is nickel's worth of speech cues in the first box, a dime's worth in the second box, etc. If a coin falls out of a box (a paper square) or if you lose the entire box, you lose that amount of money. We attempt to get the speech cues in each octave band into the patient's range of hearing. If amplification is eliminated in a zone, the patient doesn't get any speech cues in that zone. Now, let's play the Speech Cue Train Game. DELIVERING THE SPEECH CARGO (1) You start with 5-10-20-30-20 (all the speech cues), but box number five (octave band at 4000 Hz) squeaks too much, so you throw the box away! Question: What did you lose? Answer: two dimes (23% of speech cues), since the high frequencies are reduced to avoid feedback. If the fitting has feedback and we cut the gain in the higher frequencies, we may eliminate 23% of the speech information. (2) You start with 5-10-20-30-20, but the axles on the first two boxcars (250 Hz and 500 Hz) are plugged up and are slowing down the train. You remove these boxcars. Question: How much did you lose? Answer: You lost 15 cents (low frequencies lost through the vent). If the patient complains of being “plugged up,” we may ask the factory to change the venting to an IROS vent not realizing the loss of gain and hence speech information in the lower frequencies. If we use an IROS vent and if the patient has sufficient low-frequency loss to need an insertion gain of more than 0 dB, we must compensate for the loss of low-frequency gain by adding a lot of gain into the 2-cc response curve. (There's no need to make any compensation if the target low-frequency gain is only 0 dB gain, as this will be achieved by unamplified sound flowing inwards through the IROS vent.) (3) You start with 5-10-20-30-20. Along the way you find four extra number 1 boxes with their cargos, so you hitch them to your train, and congratulate yourself on having more money than you started with. At the first steep hill, the train slows, then stalls, as it is now too heavy. The guard comes along and throws out a coin from one of the number one boxes and one coin from each of the number two, three, four and five boxes. The train successfully starts up again. Question: How much money reaches the destination? Answer: Twenty-five cents less than you started with since, because sound is too loud at some frequencies, volume is reduced everywhere. If more than the optimal amount of gain is provided at any frequency, the wearer will decrease the volume (or ask you to if the hearing aid has no control). When this happens, speech information can be lost at all frequencies. Losing speech information from the higher frequencies very quickly leads to a poor fitting. (4) You start with 5-10-20-30-20, but as your boxes move along, a dime falls out of the third, fourth, and fifth boxes. Question: How much did you lose? Answer: You lost 30 cents in the high frequencies. If real-ear amplification is a little weak in the higher frequencies, you lose more than a third of the speech cues. (5) You start with 5-10-20-30-20. The coins stay in the boxes when the track is smooth (no noise), but every time the track gets rough (background noise) the coin in the first box falls out. It may even fall under the wheels of the following boxcars, in which case the whole train runs off the track! Question: What do you do? Answer. If you think you're going to lose the coin in the first box (low frequency), throw it far away from the train. It's only a nickel. Just be sure the coins in the other boxes are safe. If the patient lives near a busy road and is exposed to a lot of low-frequency background noise, reduce the amplification in the 250-Hz zone. It has limited value for word understanding and may mask the useful speech information in the higher frequencies. Of course, hearing aids with adaptive noise suppression will reduce the low-frequency gain automatically in situations where the signal-to-noise ratio is especially poor in the low-frequency region. Very important note: In this case, the octave band at 250 Hz is worth even less than its maximum potential of a nickel, while the octave band at 2000 Hz may still be worth its full 30 cents if the noise is very low-frequency dominated. Very important question: If you anticipate a lot of low-frequency background noise, which is the most important box? Now, you might believe we are completely crazy asking you which has more value. But, many of the smartest people in this profession believe in restoring the ear to its normal loudness function regardless of the listening condition. When you ask them how much gain to give a person with a flat 50-dB hearing loss, they answer, “Equal gain in each zone.” This means they would put 25 dB in all zones (working with the half-gain rule), including the octave band at 250 Hz. In a situation where the patient is constantly exposed to low-frequency noise, we would put little or no gain in the octave band at 250 -Hz because we are looking at the value of the coins, not the boxes. Even if there is no noise, the 250-Hz and 500-Hz bands do not need as much amplification because the level of these low-frequency speech bands is naturally greater than the level of higher frequency bands. Looking at the big picture This analogy gives you a different perspective on fitting hearing aids. If you think only about amplification (boxes), you miss the big picture: helping the patient understand words. If you look at the coins (speech cues), you pay attention to the total value of the money (speech information) and less attention to boxes (amount of gain in each frequency). To get this perspective, put your five little sheets of white paper on top of a blank sheet. Then, put the coins on top of the little sheets. Stand back a few feet. You see only the coins; the white boxes disappear on the large white sheet. THREE STRATEGIES Three basic strategies are commonly used in fitting hearing aids. Some practitioners seek primarily to make the patient happy. Other people primarily try to restore the ear to its normal condition by recreating the normal perception of loudness. The third group—including us—wants to optimize the patient's word-understanding ability by “equalizing” the cues in the speech spectrum. These three strategies can be referred to as “happy,” “normalize,” and “equalize.” Figures 2 and 3 help illustrate the last two approaches. The normal speech envelope is shown in Figure 2. Speech cues are indicated by dots, with each dot being equally important. Speech cues (dots) show you each zone's contribution to word understanding. The greater the density of the dots, the more important the zone is for word understanding.Figure 2: . The intensity (strength) and cues (information contained within) of normal speech. Intensity is illustrated by two dark curved lines that represent the upper and lower limits of the speech envelope. These curved lines, which demarcate the vertical height of the speech envelope, are 30 dB apart at all frequencies. Speech cues (information used for word understanding) are indicated by dots. Each dot is equally important. Note the variation in density of the dots in the lows, middle, and high regions.Figure 3.: The normal speech envelope has been cut into five parts. Each segment has been adjusted so they all have the same intensity.In Figure 3, we have cut the speech envelope into five “packages.” Each package (or box) has been adjusted in intensity so that the “boxes” are equally loud. Practitioners using the “normalize” strategy deliver speech cues to the ear as shown in Figure 2. Those following the “equalize” strategy deliver speech cues to the ear as in Figure 3. There's more information about normalizing and equalizing to come. But, for now, let's get back to the game. (6) You start with 5-10-20-30-20. But you are using the “happy” approach, so you let your child arrange the coins for you. Question: Where did the coins end up? Answer: Who knows? The first time my (RLM) little boy arranged the coins, he put them all on top of each other. This is analogous to a hearing aid fitting with gain in only one octave band. Goofy, I admit, but so are many actual hearing aid fittings if you ask patients to tell you what they want without first having them experience an amplification scheme designed to give them close to optimal speech understanding. Normalize vs. equalize The maximum word-understanding strategy can also be called the “equalize” strategy because the five octave bands have been adjusted so they are presented to the patient equally loudly (Figure 2). The advantage of this strategy is that it places more emphasis (amplification) in the high-frequency zones where the importance function values are the greatest and where speech energy is the least. Less emphasis (amplification) is placed in the low-frequency zones where the importance function values are the lowest. If your goal is to give patients the best possible word understanding, the sound the hearing aid adds to the ear (real-ear insertion response-REIR) should result in approximately equal loudness at all frequencies. This is the principle on which the NAL-R prescription, for linear hearing aids fitted to patients with mild to moderate losses, is based. The NAL-NL1 prescription for non-linear hearing aids is actually derived by maximizing calculated speech intelligibility, but the usual consequence for patients with mild to moderate loss is that the various bands of speech are amplified to equal loudness. SUMMARY Many situations reduce the effectiveness of amplification and eliminate the speech cues. If you use the “normalize” strategy, you put a lot of gain in the lower frequencies, which will probably cause the patient to reduce the volume level when he perceives excessive noise. It helps to consider the fitting from a speech cue point of view, as you do in using the equalize strategy. When you look at the coins (speech cues), you intuitively work to maximize speech information. This helps to reduce the familiar complaint, “I hear, but I don't understand the words.” We are aware that many knowledgeable audiologists advocate the normalize approach. However, based on extensive clinical experience and research, we are convinced that the equalize strategy does the best job of maximizing the hearing aid patient's ability to understand speech. We welcome comments from readers.