COMPRESSION: Historical Development & Use Today

Transcription

1 COMPRESSION: Historical Development & Use Today Take the Continuing Education quiz on page 59. By Ted Venema, PhD Many professionals struggle with or are intimidated by the concept of compression. This excellent article provides simple but very useful explanations. It is also an excellent resource for anyone who is training new hearing aid specialists. Sandy Hubbard, BS, ACA, BC-HIS IHS Editorial Advisory Committee Member The word sensori-neural has two parts. Sensory refers to mild-moderate sensori-neural hearing loss (SNHL) caused by mostly outer hair cell (OHC) damage. Neural refers more to SNHL caused by inner hair cell (IHC) damage. In most cases, OHC damage usually occurs before IHC damage. This is why neural SNHL is also a more severe degree of SNHL. Here, we specifically discuss the technical aspects of compression used in hearing aids, in order to accommodate both types of SNHL. Compression per se is a gain issue. Inasmuch as input + gain = output, compression also then affects the output. Always remember that providing gain and output, however, is only half the challenge of hearing aid fittings. The other half is a matter of increasing the signal-to-noise ratio for the client, making speech (the signal) louder and hence more distinct from the background noise. Today s approach to this endeavor involves the usage of digital noise reduction and directional microphones, but these topics will not be examined in this article. Here, we will examine compression types for mild-tomoderate sensory hearing loss versus compression for severe neural hearing loss. We will also discuss the dynamic aspects of compression, which consist of attack and release times. Readers should not think that compression began with today s digital hearing aids. 48

2 Compression began and flourished way back in the analog era of hearing aids several decades ago. The decade of the 1990s at the tail end of the reign of analog hearing aids, was actually the golden age of compression. Hearing aids then used one specific type of compression or another. Changing to another type of compression meant choosing an entirely different hearing aid! Wide dynamic range compression (WDRC) was beginning to emerge, and seasoned clinicians had to learn it as a new type of compression. Most types of compression developed in the analog era have simply been adapted and implemented in the software of today s digital hearing aids. Input/Output Functions Input/output (I/O) functions are the language of compression, and so it may be a good idea to make them your friends. On an I/O function, the horizontal or X axis show input sound pressure level (SPLs) and the vertical or Y axis show output SPLs. The diagonal lines show the difference between corresponding input and output levels. In other words, they show the gain of a hearing aid that takes place for different input SPLs. Before compression emerged on to the scene, hearing aids only provided what was called linear gain. This kind of gain is represented by a 45 diagonal line (Figure 1). The point at which that line suddenly takes a bend is called the Linear Amplification Figure 1. In this example, the gain is linear (shown by the 45 line); eg; for 20-dB inputs, the output is 80 db SPL; for 60-dB SPL inputs, the output is 120 db SPL. For 100-dB SPL inputs, the output would theoretically be 160 db SPL. Due to limiting by means of peak clipping, however, the output is maintained at a maximum here of 120 db SPL. Compression Amplification Figure 2. The output shown here is linear up until the knee-point. Beyond (to the right of) the knee-point, the output still increases with input increases, but this increase is no longer at a corresponding 1:1 rate. For example, a 20-dB input increase from 60 db SPL to 80 db SPL results in only about a 5-dB output increase, making the compression ratio 20:5 or 4:1. Here, the MPO is limited by compression. knee-point, and it shows the input where compression begins. The gain shown in Figure 1 is linear to the left of (or below) the knee-point because for any increase of input SPL, there is a correspondingly equal increase of output SPL. This is a 1:1 input/output ratio. In other words, for every one db input increase there is a corresponding 1 db output increase. For example, if the hearing aid has a gain of 60 db, then a 10-dB SPL input will result in a 70-dB SPL output, a 20-dB SPL input results in an 80-dB SPL output, and so on, up until an input level of 60 db SPL. Past this, something called peak clipping was utilized to ensure that the maximum power output (MPO) never exceeds a certain amount; for example, 120 db SPL. The MPO could be raised or lowered to accommodate the client s loudness discomfort levels. The main problem with limiting the MPO with peak clipping was that when the output sound exceeded the set MPO, the hearing aid became saturated, thus distorting the sound. With peak clipping, the receiver diaphragm (like the cone of a speaker) is literally restricted in its back-andforth movements by the walls of the receiver. When this happens, sine waves of sounds are literally clipped or turned into square waves, which themselves are complex sounds. In short, simple sinusoids containing single Continued on page 50 49

3 frequencies are converted into complex sounds containing more than one frequency. This is how harmonic distortion (as measured by ANSI procedures) is produced. Linear hearing aids were the state of the art in hearing aid technology well into the 1980s and they still lingered about in the early 1990s. Now compare and contrast compression shown in Figure 2 to the linear gain shown in Figure 1. For the sake of clarity in explanation, both are shown to have a knee-point at an input level of 60 db SPL, and both provide 60 db of linear gain up until this input level. The differences appear past or to the right of the knee-point. In Figure 2, once the input level exceeds 60 db SPL, compression serves to limit the MPO. Note how the MPO does not take a straight horizontal direction to the right; instead, it rises but with a shallower slope. The MPO is limited, but in a slight giving kind of way. Like peak clipping did for linear hearing aids, compression also determines the MPO of the hearing aid. As in Figure 1, the MPO is shown by the general height of any line that is to the right of the knee-point. Compression ratios are the amount of compression provided by the hearing aid once compression begins at the knee-point. It can be visualized on an I/O function by the slant of the line after (or to the right of) the kneepoint. A 10:1 compression ratio means that for every 10-dB increase of input SPL, there is only a 1-dB corresponding increase to the output SPL. A 2:1 compression ratio means that for every 10-dB increase of input SPL, there is a corresponding 5-dB increase to the output SPL of the hearing aid. Higher compression ratios indicate more compression; lower compression ratios indicate less compression. In general, one can think of the knee-point as the when of compression and the ratio as the how much of compression. Always remember too that I/O functions display only inputs and outputs. To find the gain for some specific input, one must always look at the corresponding output and then subtract the input from that output. Readers are also advised that the length of any gain function (showing either linear gain or compression) has precious little (read nothing ) to do with the amount of gain. It is only the position of the gain functions themselves right or left along the horizontal axis that shows an increase or a decrease in gain. Figure 2 shows that a right-ward shift in gain functions actually shows a decrease in gain, while a left-ward shift would show an increase in gain. See the vertical line going down from the knee-point to the input of 60 db SPL. Note also the horizontal line going left from the knee-point to the output axis. This shows that an input of 60 gives an output of 120. Now look at the dotted parallel 450 line from the input of 20 until it meets with the darker line showing compression. On this line, it becomes clear that an input of 60 gives an output of only 100. The gain has now decreased. Output Limiting Compression We now come to a major fork in the road concerning compression namely, output limiting compression (OLC) and WDRC. These are essentially two different compression schemes, and they refer to separate ranges of compression threshold kneepoints and compression ratios. Since OLC emerged first, it will be described first, followed by WDRC. Output Limiting Compression and MPO Adjustment Figure 3. An I/O function (left) and MPO adjustment (right) are shown for OLC. Note the relatively high knee-point and high compression ratio. Maximum (linear) gain is provided for soft and average input levels. Past the knee-point, however, a high compression ratio dramatically limits the MPO. Note that lowering the knee-point also lowers the MPO. The salient features of OLC are shown in the I/O function in Figure 3 (left). OLC has relatively high compression knee-points (e.g., 60 db SPL or more) and high compression ratios (greater than 3 or 4:1). A high knee-point means that the hearing aid begins to use compression only at high input SPLs. For soft and moderate inputs, below or to the left of the knee-point, the OLC hearing aid provides linear gain. 50

4 Figure 3 shows a 10:1 compression ratio, resulting in an almost completely horizontal line to the right of the kneepoint. Compare this to the linear gain shown in Figure 1. The two figures look very similar. OLC can be thus considered to be a close cousin to linear gain. With linear gain, the MPO is limited by means of hard peak clipping. With OLC, the MPO is limited with a bit of give, in other words, by means of a high compression ratio. The numerical values on the figures here are kept the same, mainly for illustration and comparison. Linear hearing aids actually came in all kinds of strengths, with greater and lesser amounts of gain. The thing they all did, however, was to limit the MPO by means of peak clipping. OLC hearing aids simply utilized a high degree of compression to accomplish the same thing. This is how the first compression (OLC) emerged in the 1980s; it was a method of limiting the MPO without the distortion caused by peak clipping! OLC is especially useful for clients with neural (severe) SNHL who would benefit from high-power hearing aids. These clients have a very narrow dynamic range. They tend to prefer a strong, linear gain over a wide range of input SPLs, at least until the output SPL becomes close to their loudness tolerance or uncomfortable loudness levels. High-power OLC hearing aids gave lots of gain for soft sounds and the same lots of gain for average input sounds, making average conversational speech quite audible. Figure 3 (left) shows that OLC provides a strong degree of compression over a narrow range of intense inputs. In other words, it waits for a fairly high-input SPL to go into compression, but once it goes into compression, it really goes into compression. In this way, OLC could be said to focus on the ceiling of a client s dynamic range. Figure 3 (right panel) shows a similar MPO adjustment to that of linear hearing aids. Again, the purpose is to best address the client s loudness tolerance levels. Some clinicians opt to set the MPO (measured in db SPL) to be about 15 db higher than the client s reported loudness tolerance levels (measured in db HL). The rationale here is that the difference in db HL versus db SPL over various intensity levels and across the speech frequencies is close to an average of about 15 db (with db SPL showing the greater db values). It should be added here that another hearing loss that might be mentioned as a candidate for OLC is conductive hearing loss. Note that unlike severe SNHL, the dynamic range of conductive hearing loss is not normally diminished by much (conductive hearing loss is much like a plug in the ear). Since the thresholds as well as loudness tolerance levels for this clinical population would both be elevated from normal, they too would also benefit from linear gain along with a high MPO. WDRC per se would not be the best fit for conductive hearing loss. Wide Dynamic Range Compression (WDRC) WDRC hearing aids became extremely popular during the 1990s. As such, it was a newcomer to the world of compression. Many clinicians who were used to OLC and its adjustments were initially quite confused by WDRC and especially how it was adjusted. Wide Dynamic Range Compression and Gain Adjustment Figure 4. An I/O function (left) and MPO adjustment (right) are shown for WDRC. Note the relatively low knee-point and low compression ratio, and also, that the linear gain here is only 40 db. Maximum (linear) gain is thus provided only for soft input levels. Past the knee-point, a weak (2:1) ratio of compression gradually limits the MPO. Note that lowering the knee-point increases the gain for soft inputs. A typical I/O function for WDRC is shown in Figure 4 (left). In contrast to OLC, WDRC is associated with low threshold knee-points (below 60 db SPL) and low compression ratios (less than 4:1). In fact, WDRC most commonly utilizes a 2:1 compression ratio. A look at Figure 4 (left) shows that due to its low knee-point, the WDRC hearing aid is in compression over a relatively wide range of inputs. As such, it is almost always in compression. Look at the slope of WDRC as shown in Figure 4 (left panel) Continued on page 52 51

5 and compare that to the slope of OLC, as shown in the left panel of Figure 3. It is evident that WDRC provides a weak degree of compression over a wide range of inputs. Unlike linear or OLC, WDRC gradually reduces the output (and hence the gain) for a wide range of moderate to intense input SPLs. Only for very soft inputs is linear (maximum) gain provided; all other inputs are given compression (less gain). In contrast to OLC, WDRC can be seen as having a focus on the floor of hearing sensitivity. In analog hearing aids with WDRC, a threshold kneepoint (TK) control adjusted the amount of linear gain given for very soft input sounds (Figure 4, right panel). The TK adjustment shifts the 450 linear gain function to the right or left. Note how this is so very different from the adjustment of the MPO with OLC (Figure 3, right panel). The left-most 450 linear gain line shows the greatest gain for soft inputs. The right-most 450 linear gain line shows the least amount of gain for soft inputs. In summary, as the knee-point with the TK control is lowered, the gain for low-intensity input sounds is increased. As the kneepoint is raised, the gain for low-intensity input sounds is decreased. On today s digital hearing aids, the TK adjustment works the same way, but it may not always be called a TK control. It may be seen on digital software simply as the left or right adjustment of the left-most, knee-point on a rather complex-looking I/O function (to be described in the next section). There is no real rule for adjusting the TK control; the main reason it was adjustable in the first place was because a TK setting for maximum gain in quiet environments could result in the client being able to hear the internal amplifier and microphone noise of the hearing aid itself. The audible hiss can be annoying, especially for the client who has excellent low-frequency hearing. In today s digital hearing aids, expansion (also to be discussed later) is commonly used along with WDRC, in order to reduce the audibility of the hissing sounds in quiet. WDRC is normally used for the client who has OHC damage and consequently, sensory SNHL, which presents with a mild-to-moderate SNHL along with fair speech discrimination. The OHCs amplify soft sounds (approximately less than db SPL) so that the IHCs can sense them. WDRC was intellectually construed as an attempt to electro-acoustically imitate the function of the OHCs of the cochlea. Here is some food for clinical thought: It is no coincidence that oto-acoustic emissions and the knowledge of the OHCs, as well as the KAmp and WDRC became clinically popular at around the same time, namely, the late 1980s and early 1990s. WDRC was commonly associated with hearing aids using the KAmp circuit. The goal of amplification for this population is to restore normal loudness growth. To accomplish this goal, we need to amplify soft sounds by a lot and loud sounds by little or nothing at all. The low knee-point and low compression ratio serve to reduce a normally large dynamic range into the smaller one associated with mild-to-moderate SNHL. For example, a low compression ratio of 2:1 will compress a dynamic range of 100 db into one of 50 db. This is literally why this type of compression was called wide dynamic range compression. Output Limiting vs WDRC: Displayed as Frequency Responses Figure 5. OLC (left) provides the same linear gain for both soft (40) and average (60) input levels (the lines are shown slightly apart only to make the results for 40- and 60-dB inputs both visible). Once the input level exceeds the knee-point, there is a dramatic reduction in gain. The knee-point for WDRC (right) is much lower, and the compression ratio is also comparatively lower. WDRC thus provides very different amounts of gain for the different input intensities of 40, 60, and 80 db SPL. Both WDRC and OLC can also be described in terms of their effects upon the frequency response of a hearing aid (Figure 5). The left panel of Figure 5 shows that OLC provides its maximum gain for both soft and average input levels and then suddenly reduces its gain once the input level becomes more intense than its relatively high knee-point. The right panel of Figure 5 shows that WDRC gradually reduces its gain over a wide range of increasing input sound levels above its low knee-point. 52

6 Summary A Clinical Spectrum of Compression A Multi-Kneepoint Input/Output Function Figure 6. For linear, OLC, and WDRC, two sets of horizontal lines and three arrows are shown. The left horizontal lines represent inputs, the arrows represent gain, and the right horizontal lines represent output. The dotted horizontal line represents the loudness tolerance level for some particular client. Note how for linear gain and OLC, the gain is the same for soft and average input levels. Linear hearing aids used peak clipping to limit the MPO, resulting in distortion of sound quality. OLC uses compression to limit the MPO, shown by the lines squeezed together. Note how for WDRC, both the input and output lines are evenly spread apart. This is because the gain is gradually reduced as the input intensity increases. A large dynamic range is more evenly shrunk into a smaller one. Figure 6 summarizes where we have come so far. In the beginning, the signal processing in hearing aids involved simple linear gain along with peak clipping. Next came OLC, followed in time by WDRC. The left-most panel shows linear gain, where equal amounts of gain are applied to all input. Peak clipping is employed when the output would be excessive, although this caused distortion. The middle panel shows OLC, a very similar type of signal processing to linear gain except that compression instead of using peak clipping is used to limit the MPO. The right panel shows WDRC, where progressively less and less gain is applied to increasingly more intense inputs. In this manner, a wide dynamic range is neatly shrunk into a smaller one. Compression in Digital Hearing Aids Digital hearing aids simply combine all sorts of compression types that were found separately on yesterday s analog hearing aids. In any one of its channels, the fitting software for a digital hearing aid may very well show I/O functions that have two or more knee-points (Figure 7). As with all previous I/O functions, the greatest amount of gain is seen below or to the left of the left-most knee-point. Here, the gain is linear. In any channel, the knee-points can typically be Figure 7. The software for fitting many digital hearing aids often shows I/O functions that have more than two knee-points; each can be adjusted. Below the left-most knee-point, either linear gain or expansion can be selected. WDRC is found to the right of the left-most knee-point. Note how linear gain reappears to the right of the middle knee-point. moved either horizontally or vertically, thus completely affecting the compression characteristics. In all truth however, most clinicians do not adjust compression in this manner. Instead, the frequency response (not I/O functions) is usually the main focus. This is largely because frequency response is the most readily understood display by most clinicians. In most fitting software, adjustments in hearing aid gain/output across the channels actually changes the underlying compression; that is, the I/O function for each channel. Back to Figure 7; linear gain and expansion (to be described in the next section) appear below the first or left-most knee-point, which is shown at soft input. WDRC appears next, between the first and second knee-points (for moderate inputs). From 65 to about 80 db SPL, however, the gain becomes once again linear! Past 80-dB SPL inputs, the compression ratio is then dramatically increased, in order to limit the MPO. Let s look more closely at that second use of linear gain here. This has been utilized by various hearing aid manufacturers over the past decade. It is a means whereby to provide extra gain for average to slightly greater Continued on page 54 53

7 than average inputs, such as speech in a somewhat noisy environment. The reasoning here is at these levels, speech and noise are commonly mixed together. Most people generally prefer increased gain for these levels, so as to hear speech better in these more difficult listening situations. It is one solution to address the complaint given by many clients who wear WDRC hearing aids, such as, I can hear people at other tables better than the person sitting right across from me! Expansion Expansion is the opposite of compression. On the basis of everything discussed so far especially when considering compression, and the reduced dynamic range that results from SNHL one might wonder when this would ever be of use. Basically, expansion is a technique whereby to reduce internal microphone and amplifier noise that sometimes becomes audible to the listener in quiet. This is especially noticeable by those who have good low-frequency hearing. +'(2&$"#(4(5'(3"#$"#(6702&(4(8'9 :'(2&$"#(4(,:(3"#$"#(6702&(4(8:9 *'(2&$"#(4(,'(3"#$"#(6702&(4(*'9 8'(2&$"#(4(+'(3"#$"#(6702&(4(8'9 )'(2&$"#(4(*'(3"#$"#(6702&(4()'9 -'(2&$"#(4()'(3"#$"#(6702&(4(-'9 '(2&$"#(4((('(3"#$"#((6702&(4((('9 ;C3&7(D2#?(EFGHI(=/$0&123&(B=0&1( 7<=0#=1#(702&( 0#(60&J(3&CK(0#9(#?=(L&==$32&# Expansion./$0&123&!"#$"# -'',' +' *' )' ' '((((((((()'((((((((((*'((((((((((+'((((((((((,'(((((((((-'' Figure 8. Expansion provides greater than linear gain for very soft inputs below the left-most knee-point of compression. The vertical output axis is extended below the horizontal input axis to show how, with expansion, the gain increases as the inputs increase from 0 db SPL up to the compression knee-point. For increasing inputs beyond the knee-point, the gain once again decreases, because of the use of WDRC. %&$"# ;(7<=0#=<>#?0&( -@-(A3B$<=1123&(<0#23 Here s how and why it works: Figure 8 shows expansion superimposed on an I/O function showing typical WDRC, as offered by some fictitious hearing aid. In this example, straight WDRC without the use of expansion would provide 40 db of linear gain for all inputs below the knee-point. Now look at the function for expansion. The vertical output axis is extended downward on this figure to show where the function of expansion would terminate. At this point, the output for a 0-dB SPL input is 0 db SPL, and so the gain is 0 db! The gain dramatically increases, however, as the inputs increase, up until the knee-point shown. To accomplish this, expansion provides greater than 1:1 linear gain. In the case here it has a 1:2 input/output ratio; that is, for each added decibel of input, there are two decibels of added output! When used along with WDRC, expansion thus provides maximum gain at (and only at) the kneepoint. In an I/O function with multiple knee-points, this would be the left-most knee-point. The idea is to have this left-most knee-point set at an input level typical to very soft conversational speech, because then this soft speech is provided with the greatest amount of gain for the listener. The reasoning behind expansion is quite simple. Without it, WDRC supplies maximum gain for all soft inputs, and any and all internal microphone and amplifier noise is also given maximum amplification. This results in an unwanted audibility of internal hearing aid noise. By providing less gain below the knee-point, expansion thus acts like an internal noise squelch feature. Dynamic Aspects of Compression Until now, compression has been discussed in terms of threshold knee-points and compression ratios. These are sometimes known as the static aspects of compression. Sound in the environment, however, is constantly changing in intensity over time, and compression has to respond to these changes in intensity over time. The dynamic aspects of compression concerning reaction times of compression are known as the attack and the release times (Figure 9). When the input SPL exceeds the kneepoint of compression, the hearing aid attacks the sound by going into compression and reducing the gain. Once the input sound falls below the knee-point of compression, the hearing aid releases from compression and restores the linear gain. Hearing aids are not the only electrical devices that use compression, nor are they the first to have attack/release times. Audiovisual equipment has used OLC and WDRC, along with various schemes of attack/release times for many years. We have all heard the effects too. Recall, for example, television broadcasts where the sports 54

8 Dynamic Compression Characteristics Figure 9. The top shows input sound changing in intensity (vertical dimension) over time (horizontal dimension). The bottom shows the response of a compression hearing aid to the changes in sound input intensity over time (top). Compression circuitry takes some amount of time to respond to these changes. announcer is talking; when a score is made and the audience suddenly cheers, listeners may notice a slight lag in time for the audiovisual equipment to reduce its gain for the noise. Similarly, with sudden drops in intensity, it may again take some time for the system to release from compression. Most attack and release times have been set to achieve a best compromise between two undesirable extremes. Times that are too fast will cause the gain to fluctuate rapidly, and this may cause a jarring acoustical perception by the listener. Times that are too slow may make the compression act too slowly and cause a real lagging perception on the part of the listener. Poor management of attack/release times can cause a fluttering perception on the part of the listener. Most analog compression hearing aids initially used a technique called peak detection to track the peak intensity of input sound and thereby, signal the hearing aid to go into or out of compression. With peak detection, the attack and release times were constant and fixed for any incoming sound intensity patterns. Most peak detection systems in hearing aids were adjusted to provide quick (50 ms) attack times and longer, slower (150 ms ) release times. This was seen as the best compromise between objective effectiveness of compression and subjective listening comfort. In comparison to peak detection, automatic volume control (AVC) has a relatively long attack and long release times of several seconds. It thus does not respond to rapid fluctuations of sound input. The long attack/release times were actually intended to imitate the length of time it takes for a listener to react to sudden noise increases by physically raising a hand to manually adjusting the VC on a hearing aid; hence, its name! Widex promoted the use of AVC on its first digital hearing aid (the Senso); the reason was because field trial subjects who first tried the Senso liked it best. Syllabic compression refers to the exact opposite of AVC namely, relatively short attack and release times, shorter than the duration of the typical speech syllable. This enables the hearing aid to compress or reduce the gain for the peaks of more intense speech (usually the vowel sounds), thus providing more uniformity in the intensity of ongoing speech syllables. The main premise of syllabic compression is to allow a hearing aid to make the softer sounds of speech more audible without simultaneously making the normally louder parts of speech from becoming too loud. Adaptive compression had fixed, quick attack times, and release times that varied with the length of time it took for a loud sound to become quiet again. For short (transient) intense sound inputs like a door slam, the attack and release times were short. For sound inputs that took longer to become quiet again, the attack time remained quick but the release times were longer. The desired result was a reduction of compression pumping heard by the listener. Adaptive compression was originally patented by a now long-gone company called Telex; this company was also associated with FM systems of the day. Later on, adaptive compression became most commonly associated with the KAmp circuit, which was utilized by many of the hearing aid manufacturers. Average detection was a further development beyond adaptive compression. Unlike the peak detection method that tracked the peak amplitude of incoming sound waves, the average detection method employed both long average amplitudes and short average amplitudes of sound inputs over a given length of time. When the slow Continued on page 57 55

9 average SPL exceeded the knee-point of compression, then the gain was reduced. When the fast average exceeded the slow average by several db, it also signaled the circuit to go into compression. The main advantage here is that both the attack and release times varied with the length of the incoming intense sounds. Today s digital hearing aids mostly use syllabic compression and average detection. Most often, syllabic compression is the default for the low-frequency channels, and average detection is the default for the higher-frequency channels. It should also be noted that an additional focus over the past 10 or so years has become the instant compression of sudden, very loud transient sounds. Regarding dynamic compression characteristics, the jury is out ; that is, there does not seem to be a round consensus as to the best set of attack/release times. The complexity of the matter is complicated further when considering what set of attack/release times might be best for different types and degrees of hearing loss. Most clinicians do not tend to adjust or make changes to the default dynamic compression characteristics. Clinicians are often strongly ill-advised by the manufacturer to tamper with the default dynamic compression characteristics provided. n Ted Venema, earned a BA in Philosophy at Calvin College (1977), an MA in Audiology at Western Washington University (1988), and a PhD in Audiology at the University of Oklahoma (1993). He has worked as a clinical audiologist, and also in the hearing aid manufacturing sector (Unitron). He has also taught audiology at Auburn University in Alabama ( ) and also at Western University in Ontario Canada ( ). In 2006 he initiated, developed and implemented the HIS program at Conestoga College in Kitchener Ontario. As of September 2015, Ted is associated with the online HIS program at Ozarks Technical Community College in Springfield Missouri. Ted is the author of a textbook, Compression for Clinicians, which is now being rewritten as a 3 rd edition. Remember to take the IHS Continuing Education test on page

10 IHS Continuing Education Test Compression: Historical Development & Use Today article on page 48 (Sketching I/O functions for Questions 4 to 10 may be very helpful.) 1. On an I/O function, longer 45 lines represent more linear gain. a. true b. false 2. On an I/O function, moving a 45 line to the right represents more linear gain. a. true b. false 3. With WDRC, lowering the knee-point increases linear gain for soft inputs. a. true b. false 4. A compression hearing aid provides 90 db SPL output with 40 db SPL input; the gain here is db. a. 25 db b. 45 db c. 50 db d. 52 db 5. Same hearing aid: knee-point at 50 db SPL, compression ratio of 2:1; the output for a 60-dB SPL input is: a. 55 db SPL. b. 90 db SPL. c. 105 db SPL. d. 120 db SPL. 6. Same hearing aid: knee-point at 50 db SPL, compression ratio of 2:1; the gain for a 60-dB SPL input is: a. 45 db. b. 50 db. c. 52 db. d. 70 db. 7. A compression hearing aid provides 120 db SPL output with 50 db SPL input; the gain here is db. a. 45 db. b. 50 db. c. 52 db. d. 70 db. 8. Same hearing aid: knee-point at 70 db SPL, compression ratio of 10:1; the output for an 80-dB SPL input is: a. 45 db SPL. b. 78 db SPL. c. 141 db SPL. d. 142 db SPL. 9. Same hearing aid: knee-point at 70 db SPL, compression ratio of 10:1; the output for a 90-dB SPL input is: a. 45 db SPL. b. 78 db SPL. c. 141 db SPL. d. 142 db SPL. 10. Same hearing aid: knee-point at 70 db SPL, compression ratio of 10:1; the gain for a 90-dB SPL input is: a. 25 db. b. 35 db. c. 42 db. d. 52 db. For continuing education credit, complete this test and send the answer section to: International Hearing Society Middlebelt Rd., Ste. 4 Livonia, MI After your test has been graded, you will receive a certificate of completion. All questions regarding the examination must be in writing and directed to IHS. Credit: IHS designates this professional development activity for one (1) continuing education credit. Fees: $29.00 IHS member, $59.00 non-member. (Payment in U.S. funds only.) Name Address City State/Province Zip/Postal Code Office Telephone Last Four Digits of SS/SI # Professional and /or Academic Credentials Please check one: o $29.00 (IHS member) o $59.00 (non-member) Payment: Charge to: o Check Enclosed (payable to IHS) o American Express o Visa o MasterCard o Discover COMPRESSION: HISTORICAL DEVELOPMENT & USE TODAY (PHOTOCOPY THIS FORM AS NEEDED.) Card Holder Name Card Number Exp Date Signature Answer Section (Circle the correct response from the test questions above.) 1. a b 2. a b 3. a b 4. a b c d 5. a b c d 6. a b c d 7. a b c d 8. a b c d 9. a b c d 10. a b c d 59