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Selecting hearing devices: features and parameters Contents Key to resources for activities 2 Compression 3 Other types of signal processing 6 Glossary 11 Feedback to activities 13 1 © NSW DET 2008 Key to resources for activities Note: Print this Key to resources for activities and refer to it as you work through the task guide. Resource number Resource title 1 Dillon, H (2001) Hearing Aids, Boomerang Press, Sydney Chapter 6: Compression systems in hearing aids 2 Synopsis Section 6.1: Compression’s major role: Reducing the signal’s dynamic range Dillon, H (2001) Hearing Aids, Boomerang Press, Sydney Chapter 7: Advanced signal processing schemes for hearing aids Now go to Your case studies on Miss Bennett beginning on slide 4. 2 © NSW DET 2008 Compression The most important and valuable means of sound modification, apart from amplification and filtering, is compression. As you may remember in your study of how we hear, a sensorineural hearing loss doesn’t just make it more difficult for a person to hear soft sounds, but also makes it more difficult for the person to tolerate loud sounds. The term for this is recruitment, and it describes the reduced dynamic range people with sensorineural losses usually experience. A hearing aid (device) without compression will amplify all incoming sounds by the same amount. This means that a soft sound will be made audible, which is lovely, but a loud sound may become uncomfortably loud, or even unbearable, which is not so lovely. Amplification like this (otherwise known as linear amplification) depends on the quick action of the person wearing the aid (device) to reach up for their volume control and turn the volume down. There are two problems with this. Some people with a particularly small dynamic range will need to be adjusting their volume control very frequently. Also, some loud sounds are very sudden, and the hand to the volume control just isn’t fast enough. A better option is for the hearing aid (device) to adjust the amount of amplification it gives to the sound before it sends it into the unsuspecting person’s ear. The way it does this is to squash, or compress, the amount of amplification it gives to louder signals. This is the basis of the concept of compression. Compression can be performed in a number of ways. Go to Resource 1 Activity 1 Write down the three types of compression described. 1. 2. 3. 3 © NSW DET 2008 Go to Resource 2 Activity 2 Write definitions for the following words and phrases as you read through Resource 2. 1. Attack time: 2. Release time: 3. Adaptive release time: 4. Compression threshold: 5. Compression ratio: 6. Input-controlled compression: 7. Output-controlled compression: 8. Write down the two main reasons why we might want to compress different frequency regions by different amounts. Go to Resource 1 4 © NSW DET 2008 Activity 3 As you read Resource 1, write down the key advantages of compression over linear amplification for each of the following sound levels, and note the most useful form of compression and compression threshold. For example: For very loud sounds: High thresholds, output controlled (also known as output limiting compression) reduces distortion; preferable for all hearing losses except those who will benefit from the additional SPL generated by a peak clipping aid. Now answer the following: For loud sounds: For medium sounds: For soft sounds: In Section 6.5.2, Benefits of multichannel relative to single-channel compression, Dillon cautions against using high compression ratios (greater than 3:1) in multichannel devices because there can be a loss of intelligibility. However, most modern hearing aids (devices) do have more than one channel, and many have more than 10. Activity 4 Write down three reasons for having multiple channels (other than compression). 5 © NSW DET 2008 An important note here is that the terms ‘multiple channels’ and ‘multiple bands’ can have two definitions. In most cases, the terms ‘channel’ and ‘band’ are synonymous – that is, a channel or band is a frequency range in which all processing is performed independently of other channels or bands. Less frequently, ‘channel’ and ‘band’ are used to mean different things: a ‘channel’ refers to a frequency range in which all processing is performed independently of other channels, and a ‘band’ is a frequency range within, or independent of, a channel. This second distinction can be rather confusing; fortunately most manufacturers use the first definition. So compression’s main role for people with sensorineural hearing loss is to ‘squash’ all incoming signals into their reduced dynamic range. The opposite of compression is expansion, where the amplification of soft sounds is less than the amplification of mid-level sounds, and the amplification of loud sounds is greater than that of mid-range sounds. There is no application of expansion for louder sounds for people with hearing loss, however expansion is useful for the very soft sounds, such as the hearing aid (device) internal noise. Expansion, which is also known as ‘soft sound squelch’ and ‘microphone noise reduction’ can also be used to reduce the amplification of quiet environmental humming noises such as the fridge or computer. Other types of signal processing Now we move on to Chapter 7, which looks at other ways the electroacoustics of the hearing aid (device) can be used to improve the sound signal for particular clients. We will focus on three features: directional microphones noise reduction technology feedback suppression. Directional microphones Go to Resource 2 6 © NSW DET 2008 You may have heard clients say, “The main problem with my hearing is that I can’t seem to understand what people are saying in background noise”. This is probably the most common difficulty for people with sensorineural hearing losses. Unfortunately, it is also one of the things that hearing aids (devices) are notoriously bad at alleviating. The directional microphone is the most effective of the various features that hearing aid (device) manufacturers can use to improve the signal-to-noise ratio for the people who are wearing their products. Activity 5 Which of the following statements is false? Circle the incorrect one. (a) A directional microphone system is a single microphone with an opening to the front and the rear, in which the signal from the rear is delayed slightly and subtracted from the signal from the front. (b) Directional microphones consist of an array of two or more microphones, in which the signal from the rear microphone is delayed slightly and subtracted from the signal from the front microphone. (c) Directional microphones need to be separated by a large enough distance. (d) Adaptive directional microphones change from omnidirectional to directional when the noise level reaches a predetermined level. (e) An adaptive directional microphone is a directional microphone array in which the directivity pattern varies depending on where an unwanted noise is coming from. Noise reduction As mentioned, the use of directional microphones is the single most effective way of dealing with the problem of background noise. There are other ways, however. Go to Resource 2 7 © NSW DET 2008 Activity 6 1. Which style of hearing aid (device) would be most helped by single microphone noise reduction? 2. Write down the names of the two key means of noise reduction. 3. What effect do both key means of noise reduction have on a noisy signal? Noise reduction can be fixed, user-selected or adaptive. With adaptive noise reduction, the amount of noise reduction increases as the level of ambient sound increases. When used with adaptive directional microphones, particularly automatic adaptive directional microphones, the hearing aid (device) can maintain optimum comfort and intelligibility in most situations, without any user adjustments. When noise reduction and/or directional microphones are not automatic, the use of two or more separate programs, or memories, may be necessary. In this case the hearing aid (device) wearer manually chooses the level of noise reduction and/or directionality by selecting one of two or more preset programs or memories. Multiple-program hearing aids (devices) usually have a switch or button which allows the user to change program in much the same way that you might change from a sport channel to a movie channel on your TV. The programs can also be accessed via a remote control where fitted. The hearing aid (device) programs are usually set by the audiometrist or audiologist (with recommendations by the hearing aid (device) manufacturer), and allow the hearing aid (device) to provide optimum sound quality for different listening situations. For example, one program may be set without noise reduction and without directional microphones to provide optimum sound quality for a quiet situation; another program might be set with noise reduction and directional microphones to provide optimum signal-to-noise advantage, for use in noisy situations. Another program might be allocated for the telecoil. Multiple-memory/program hearing aids (devices) became increasingly common in the mid to late 1990s, as digitally programmable analogue 8 © NSW DET 2008 hearing aids (devices) increased in prevalence. As digital hearing aids (devices) have made possible more adaptive and automatic adjustments, the need for multiple manually controlled programs is reducing. Multiple programs/memories are still necessary for selection of the telecoil, although many hearing aids (devices) can be set to select telecoil when a phone is brought up to the aid. Feedback reduction Have you ever sat with someone whose hearing aid (device) is whistling? This annoying and very embarrassing problem can be alleviated by clever electronics. Go to Resource 2 Activity 7 Describe the three methods of feedback management currently used with hearing aids (devices). Electro-acoustic methods of feedback such as these, and particularly the last method, have made possible the development of the Open Fitting BTE, also known as the Thin-Tube or Slim-Tube BTE. With this hearing aid (device), the friction hook and standard 2mm tubing have been replaced by very thin (0.9mm or 1.3mm) replaceable tubing, and the standard ear mould has been replaced by a tiny, floppy dome which holds the thin tubing in place without blocking the ear canal. An alternative to the dome is a custom-made shell which acts as an anchor for the tubing, but is otherwise a hollow cylinder shaped like the ear canal. 9 © NSW DET 2008 Open Fitting BTEs greatly reduce the incidence of occlusion effect, and so can be used for people with normal and near-normal low-frequency hearing. Go to Resource 2 Although modern hearing aids (devices) really are a far cry from the hearing aids (devices) of only a decade ago, it’s useful to remember that they are still hearing aids (devices). People with sensorineural losses have lost some of their hearing and it cannot be replaced with electronics. At this point, go back to the task guide: Identify different types of hearing devices in this unit and reread the descriptions of the two creatively named hearing aids (devices). You should now have a pretty good idea just how similar they are. In the real world it is unlikely that two hearing aids (devices) with such similar features will have greatly different prices. However it is quite common for two hearing aids (devices) with similar prices to have different combinations of features. Your job, as you help your client decide which hearing aid (device) options best meet his or her hearing loss and hearing needs, is to match frequency response and features with audiogram and needs. This is a balancing act, to be sure, and one which is not helped much by the highly creative and evocative terms different manufacturers use to describe the same things. But once you’re able to translate those terms so that the true hearing options can be compared, it’s a task which is quite possible – and even fun. A very good place to start the comparison is the measurement of the sound output of a hearing aid (device). The measurement of a hearing aid (device) allows you to estimate how suitable its gain is, relative to your client’s requirements. It also allows you to evaluate how well it is doing its job once your client is wearing it. 10 © NSW DET 2008 Glossary Acoustic hearing aid (device) effects Anything non-electrical that affects the sound reaching the eardrum (pinna, microphone tubing, ear hook, ear mould tubing, ear mould/ear shell, sound bore, venting, ear canal) ALD Assistive listening device BTE Behind-The-Ear hearing aid (device) CIC Completely-In-the-Canal hearing aid (device) Digitally programmable The settings of a digitally programmable hearing aid (device) are adjusted by a computer or programming device. This allows for much greater flexibility in adjustment than is possible using trimpots. Note that a digitally programmable hearing aid (device) is an analogue aid (device) Electro-acoustic hearing aid (device) effects Anything that pertains to the hearing aid (device) transducers (microphone, receiver and everything between them) ITC In-The-Canal hearing aid (device) ITE In-The-Ear hearing aid (device) MC Mini-Canal hearing aid (device) MPO Maximum power output : this is usually used to mean OSPL90 NRS National Relay Service, an Australian Government service which allows a person using a TTY to communicate with someone who does not have a TTY. See <www.relayservice.com.au> for more information OSPL90 The (output) SPL (sound pressure level) produced by a hearing aid (device) when it receives a 90dB input Peak clipping A method of maximum output limiting in which the peaks of the signal that would otherwise be greater than the maximum level are ‘clipped’. As a result there is no reduction in gain, but there is significant distortion REIR Real Ear Insertion Response. REIG stands for Real Ear Insertion Gain, while IG stands for Insertion Gain. Note that REIR, REIG and IG are generally used interchangeably REUG Real Ear Unaided Gain; note that REUR and REUG are generally used interchangeably REUR Real Ear Unaided Response Signal-to-noise ratio (S/N or SNR) A measure of the level of the desired signal, relative to the level of undesired signal (background noise). The larger the SNR, the 11 © NSW DET 2008 less the undesired signal masks the desired signal Trimpot A tiny potentiometer on a hearing aid (device) which allows the settings of the hearing aid (device) to be adjusted (using a screwdriver) TTY Telephone typewriter: a device which allows a hearing-impaired person to send text via an ordinary phone. The text can only be read by someone else with a TTY 12 © NSW DET 2008 Feedback to activities Activity 1 feedback Write down the three types of compression described. 1. high-level compression, of which one type is compression limiting 2. wide dynamic range compression (WDRC) 3. low-level compression Activity 2 feedback Write definitions for the following words and phrases as you read through Resource 2. 1. Attack time: The time taken for the compressor to react to an increase in signal level 2. Release time: The time taken for the compressor to react to a decrease in input level 3. Adaptive release time: The release time is short for brief intense sounds, but becomes longer as the duration of an intense sound increases 4. Compression threshold: The SPL above which the hearing aid (device) begins compressing 5. Compression ratio: The change in input level needed to produce a 1dB change in output level; note that the compression ratio describes how much the gain decreases with input increase 6. Input-controlled compression: Compression controlled from a point on the input side of the volume control (the compression threshold is independent of the volume control) 7. Output-controlled compression: Compression controlled from a point on the output side of the volume control (the compression threshold is independent of the volume control setting) 8. Write down the two main reasons why we might want to compress different frequency regions by different amounts. Hearing loss usually varies with frequency. Signals and noises in the environment have more energy in some frequency regions than in others. 13 © NSW DET 2008 Activity 3 feedback As you read Resource, write down the key advantages of compression over linear amplification for each of the following sound levels, and note the most useful form of compression and compression threshold. For loud sounds: (medium or low compression threshold – inputcontrolled) increases listening comfort For medium sounds: (low compression threshold – input-controlled) reduces the need for the volume control For soft sounds: (low compression threshold – input-controlled) increases audibility Activity 4 feedback Write down three reasons for having multiple channels (other than compression). Multiple channels enable the gain-frequency response to be most easily and flexibly controlled. Noise suppression is more effective. Feedback suppression can be more effective. Activity 5 feedback Which of the following statements is false? Circle the incorrect one. (a) A directional microphone system is a single microphone with an opening to the front and the rear, in which the signal from the rear is delayed slightly and subtracted from the signal from the front. (b) Directional microphones consist of an array of two or more microphones, in which the signal from the rear microphone is delayed slightly and subtracted from the signal from the front microphone. (c) Directional microphones need to be separated by a large enough distance. (d) Adaptive directional microphones change from omnidirectional to directional when the noise level reaches a predetermined level. (e) An adaptive directional microphone is a directional microphone array in which the directivity pattern varies depending on where an unwanted noise is coming from. (d) is false: this is an automatic directional microphone, not an adaptive directional microphone (note that you can have both, ie an automatic adaptive directional microphone) 14 © NSW DET 2008 Activity 6 feedback 1. Which style of hearing aid (device) would be most helped by single microphone noise reduction? The CIC, which does not have enough space for a directional microphone. 2. 3. Write down the names of the two key means of noise reduction. Wiener filtering spectral subtraction What effect do both key means of noise reduction have on a noisy signal? They decrease the gain most at those frequencies where the signal-tonoise ratio is worst. Activity 7 feedback Describe the three methods of feedback management currently used with hearing aids (devices). reduction of gain at a frequency or frequencies where feedback is a possibility variation of the phase response of the gain at any frequency where feedback is a possibility production of a second, intentional feedback path with just the right gain and phase response to cancel the external leakage path. 15 © NSW DET 2008