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Designing and Installing a HEARING LOOP For the deaf Many people have hearing impairment. Whether they are watching TV, listening to radio or music, attending a concert, meeting or religious service, they have difficulty hearing, or understanding, what is going on – and that may be in spite of using a hearing aid. Hearing loops, which inductively couple an audio signal to a hearing aid, are an increasingly common method of helping ease that difficulty. J ust because you have a hearing aid does not mean that ship which older people frequent. In fact, many modern your hearing problems are solved. When you have nor- buildings are so equipped these days. In the home, of course, the problem can be just as difmal hearing, your ears are very good at discriminating    between noise and the sounds you want to hear. Not so ficult, especially when shared with those without hearing with a hearing aid, particularly if you are wearing only one. impairment. But it is unusual for hearing loops to be inThe hearing aid is basically a microphone, amplifier and stalled in the home. Until now, that is: in this article we describe how to set earpiece. Unfortunately the microphone picks up all sounds and noise then amplifies all by the same amount. The wearer up a basic hearing loop for the home or for small to quite large meeting rooms, to Australian, New Zealand and IEC often has great difficulty discerning what is going on. In many situations this problem can be largely overcome (International Electrotechnical Commission) standards – by a hearing loop, fed by an audio amplifier. The loop is and how to drive it. This could be done using a commercially made ampliplaced around the room or hall and the radiated signal is then picked up by a hearing aid fitted with a T-coil (or fier specifically intended for hearing loop applications but equally could be a standard commercial amplifier or even Telecoil; see the sidebar, “The origin of the Telecoil”). Alternatively, the signal can be picked up via a Cochlea one of the many amplifier designs published by SILICON CHIP. Professional hearing loop installations can cost many implant or even a loop receiver, as described elsewhere thousands of dollars, especially when retro-fitted (most in this issue, driving conventional headphones/earbuds. new public buildings these days have Hearing loss increases with age so it them installed during construction in is common for hearing loops to be used, Part 1: By JOHN CLARKE appropriate areas as a matter of course). for example, in halls and places of wor22  Silicon Chip siliconchip.com.au LOOP RECEIVER & HEADPHONES HEARING AID WITH T-COIL SIGNAL SOURCE MICROPHONE AMPLIFIER AMPLIFIER M LOUDSPEAKER T SWITCH T-COIL VOLUME CONTROL, RESPONSE SHAPING AUDIO INDUCTION LOOP 1 Fig.1: the basic arrangement for a hearing loop. Signal from the room PA is amplified and coupled into the loop. The resulting magnetic field is detected by suitably equipped hearing aids or receivers. OUTPUT T-COIL However, a do-it-yourself installationVOLTAGE along the lines set out in this article can provide excellent results and save a heap of dollars. It is relatively easy to fit and can be made small or quite large, depending on the area needed to be covered. What’s a hearing loop? MAGNETIC In its simplest form, a hearing loop system comprises FIELD a signal source, an amplifier and a large loop of wire around the room or hall. As this loop forms a coil with an AC curAUDIO rent flowing through it, it radiates an electro-magnetic wave INDUCTION which is in sympathy with the signal source. LOOP 3 This radiated signal can be detected by a hearing aid equipped with a T-coil or indeed, a loop receiver (with headphones) designed for the purpose. Fig.1 shows the arrangement but we will explain just how this works shortly. If you want to set up a hearing loop in your home you should be able to get satisfactory results without any special equipment. For larger setups in halls, the magnetic field produced by the signal in the loop needs to be set to the required level, so that all hearing aids with T-coils will operate correctly. In a later article in this series we will show how to build and calibrate a signal level meter to measure signal levels from the installed loop. Our hearing loop is suitable for use in a home, office, hall, church or similar building. We include design graphs 2 Fig.2: a hearing aid equipped with both T-coil and microphone to cover both signal sources. Many hearing aids will have a switch to select both. Obviously, the loudspeaker is tiny enough to fit in the ear. and tables to make it easy to select the wire size and its L length, along with the amplifier power requirements for a particular installation. For large loops, say in a community hall or church, you H will need a signal pre-conditioner. In a later issue we will present a suitable design to allow a standard amplifier to be employed. The pre-conditioner provides stereo signal mixing, audio compression, treble boost to provide compensation for loop inductance and treble rolloff I above 5kHz. Other articles will provide circuit and construction details for an induction loop receiver (see p62 of this issue) and 5 a microphone loop driver. Now let’s describe the basics of a hearing aid. How does a hearing aid work? As we mentioned earlier, in its simplest form a hearing Pulpit Centre Aisle Steps Pew Pew Listening Area Archway pillars Archway pillars Sound Desk Centre Aisle HEARING LOOP FITTED Pews Service table To use this facility, sit within the listening area shown shaded and switch your hearing aid to the T-coil position. Kitchen A Hearing Loop is installed in this building. Front Entrance Plan View Where a hearing loop is fitted, it doesn’t usually cover the entire area. Hence a “map” is needed, such as this one in a church, to show deaf people with hearing aids where to sit. siliconchip.com.au The hearing loop (white figure-8) is laid out here for testing before permanent installation under the floor. September 2010  23 A commercial hearing loop amplifier, in this case the model 1077 from Auditec. It’s a current amplifier, which has some advantages in hearing loop use but standard voltage amplifiers are certainly usable as well. aid comprises a microphone, an amplifier and a miniature loudspeaker. In normal use the sound picked up by the microphone is amplified and processed, depending on the complexity of the hearing aid. The amplified signal is then reproduced via the loudspeaker which is closely coupled to the wearer’s eardrum at a level which compensates for the loss of hearing. Fig.2 shows the general internal arrangement. Better, modern hearing aids also include signal processing to try to present the clearest audio to the wearer. And the best also include a Telecoil (or T-coil), which comprises a coil of wire on a ferrite core. A switch on the hearing aid selects the T-coil or microphone as the input source. Originally used to couple the electromagnetic energy from a specially equipped telephone into the hearing aid (hence the name), their use has now expanded to be able to detect an electromagnetic signal from a hearing loop, where fitted. Not all hearing aids have a T-coil and obviously, without one, there is absolutely no advantage from either telephones or hearing loops. Fig.3 shows the magnetic field produced by the hearing loop (sometimes referred to as an audio induction loop) and how this couples into the T-coil. Normally the induction loop is horizontal and the T-coil is vertical (for a person who is sitting or standing). Any variation of the T-coil from its vertical position will reduce the received signal. There is nothing to stop the orientation of the hearing loop being vertical, allowing hearing aid wearers to use the Here’s a commercial hearing loop receiver which drives standard headphones. Or you can build your own: see the article on page 62! 24  Silicon Chip loop when lying horizontal. One disadvantage of the T-coil inductor is that it produces a signal which rises in level with increasing frequency. This is because the induced voltage is proportional to the rate of change of the magnetic field and so higher frequencies will give a higher voltage. This rising response is normally compensated for within the hearing aid to produce a flatter frequency response. So why would a person with a hearing aid prefer to listen via the T-coil instead of listening directly to the sound from a public address or similar sound system? After all, a hearing aid is designed to pick up sound, amplify it and tailor the frequency response to suit the individual user. As already noted, people with normal hearing have little trouble discriminating between unwanted noise and the sounds they want to hear. By contrast, the wearer of the hearing aid finds that in a room full of people or in a noisy environment, all they hear is a whole lot of noise and it prevents them from following any one sound or conversation. To that you can add natural reverberation in a large room, the noise of people moving about and maybe background music. The room, especially if it’s reasonably sized, may well have some form of public address system fitted. That’s fine for those with normal hearing but ironically, a PA can introduce more reverberation, cause hearing aid overload (distortion) and can raise bass levels to further muddy the sound clarity. The solution is to channel signal directly from the public address system into an audio induction loop to be picked by the hearing aid T-coil. The resulting sound is clearer because it only contains that broadcast by the sound system and extraneous sounds from other people and reverberation are absent. As good as it is, listening via a T-coil is not perfect: the hearing aid user can feel isolated from the rest of the group of people in the building because they do not hear the ambient sounds of the people around them. To overcome this, some hearing aids include switching to select three options: T-coil, T-coil plus microphone and microphone only. The T-coil plus microphone setting mixes the signals to allow ambient sounds and the broadcast (PA) signal to be heard but even this can be a compromise. There is no perfect electronic cure for deafness! Protect your hearing while you have it. As an aside, it is widely and reliably forecast that the siliconchip.com.au 1 2 T-COIL The origin of the Telecoil OUTPUT VOLTAGE L MAGNETIC FIELD 3Fig.3: AUDIO INDUCTION LOOP Current flowing in the hearing loop produces a magnetic field that couples into the T-coil. Voltage is produced across the T-coil terminals. next ten to twenty years or so will see an explosion in the number of younger people with irreversible hearing damage, caused (in particular) by years of exposure to loud rock music (why do bands have to play so loud?) and more importantly, the massive use of ear-buds at excessive volume from cassette players, then CD players and most recently MP3/MP4 players and mobile phones. Designing a hearing loop system Before embarking on designing and installing a hearing loop, you need to decide whether the building is suitable for installing a loop. For many buildings the loop can be installed beneath the floor, especially if it is timber construction and there is access to the underside of the flooring. Where there is a concrete floor, the loop could be placed around the floor under carpet or behind skirting boards. Alternatively, the loop could be placed in the ceiling, provided it is not too high above normal listening level. Installing a hearing loop in buildings made with steel frames or reinforced concrete is more difficult. This is because the steel tends to reduce the magnetic field strength. The solution may be to provide more current drive in the loop with a larger amplifier and/or by using more complex loop designs. For most installations, a single loop is all that is needed. Loop performance can be checked before it is permanently installed by simply running the loop wire temporarily around the area (eg, on the floor) where required. An important factor to consider when deciding on the positioning of a loop is interference from the mains power lines. In particular, phase-controlled light dimmers for stage and auditorium lighting often cause a buzzing sound, predominantly at 100Hz. The interference will be highest when the lamps are dimmed. Fluorescent lamps can cause interference when they are switching on but do not usually cause problems once lit. Another source of interference is close proximity to computers and monitors; in fact anything with a “switchmode” power supply. We’ll be describing a Hearing Loop Level Meter in a future article, which can be used to check the background interference levels down to 21dB below a 100mA/m reference. What level? According to the Australian standards (AS60118.4-2007), environmental audio frequency background field levels siliconchip.com.au 5 Hearing aids installed with a Telecoil or T-coil began as a solution to a problem that occurs when using a hearing aid with a telephone. The Hname Telecoil originates from the words telephone and coil. To understand the problem you need to be aware that there is coupling between the telephone mouthpiece and the telephone earpiece, so as you speak some of the sound is heard through the earpiece.IThe coupling is called side tone and is deliberately introduced to prevent the telephone sounding dead when speaking. This can cause a problem when using a hearing aid. When it is brought close to the earpiece of a telephone, the hearing aid often produces a loud-pitched squeal, or feedback. This is caused by the microphone on the hearing aid picking up sound that is amplified and reproduced by the hearing aid loudspeaker, which is then received by the telephone handpiece and then further re-amplified by the hearing aid and so on. To allow a hearing aid wearer to use a telephone, without this problem occuring, the telephone is modified to include a wire loop that is driven by the same signal as the telephone loudspeaker. The loop produces a small magnetic field that varies in sympathy with the signal. To utilise this feature, the hearing aid needs to include a Telecoil (T-coil) that detects signal from the phone’s magnetic field. When required to be used in this way, the hearing aid is switched to the “T-coil” position, disabling the hearing aid microphone and thus avoiding the audio feedback. Some telephones include a Telecoil already installed within the handpiece; some may need one fitted as an accessory. More information is available from your telephone supplier or via The Independent Living Centres Australia (www.ilcaustralia.com/home) Some hearing aids are designed to automatically switch over to the T-coil position in the presence of a strong DC magnetic field. The magnet in the telephone earpiece provides this field. Due to the success of the T-coil in hearing aids with telephones, its application has broadened to where hearing loops are now commonly used wherever sound needs to be available for the hearing impaired. A “behind the ear” hearing aid. The tube at the top feeds into the ear canal, fed by the miniature loudspeaker at the top of the unit. Controls on the back of the unit include a volume control, power switch and the allimportant T-coil/ microphone switch. September 2010  25 1 2 should be below –20dB ‘A-weighted’ with respect to a 100mA/m reference field (or –40dB below 1A/m) using a OUTPUT T-COIL slow (S) time weighting of 1 second. VOLTAGE We do have reservations about whether this level is sufficiently low for satisfactory hearing loop performance. The Hearing Loop Level Meter will also measure noise using a wider frequency response than the A-weighting provides. This can give a more realistic indication of whether noise will be intrusive. MAGNETIC Another consideration is whether the loop wire will be FIELD running close and parallel to signal wires in a public address system, such as for microphones. This has the potential to cause instability in the sound system although it INDUCTION isAUDIO usually LOOP no more wiring 3 severe than feedback caused by loudspeaker running close to the microphone cables. Further problems may occur with dynamic, electret and UHF radio microphones and guitars with magnetic pickups. It is wise to test for these problems with a temporary loop installation. Problems will be evident if the sound seems distorted or has a “metallic” quality. An oscilloscope can also be used to monitor the sound system signal for any instability. Note that an audio induction loop setup will not cause direct acoustic feedback, ie, the squeal associated with audio coupling of microphones and guitars to loudspeakers. Spill Generally, the area where a hearing aid will receive the signal is within the loop itself. Outside the loop, the signal level drops off. Fig.4 shows the measured field strength of a 10m x 10m square loop at a height of 1m above the loop. The signal is reasonably constant (to within 3dB) within the loop area but drops off just outside the loop. Any signal outside the loop is called the “spill”. Spill means that the signal is not secure and might be intercepted from outside the building, simply by using a T-coil-equipped hearing aid. If security is important, that is a consideration. Spill also means that if more than one Field strength over loop area for a 10m square loop <at> 1m loop is installed in a building above loop measures are required to prevent interference between them. 0 -5 -10 Field Strength (dB) H I Fig.5: for a magnetic 5 field strength (H) of 100mA/m at the centre of the square loop, the current required through the loop of side length L is I=L/9n amps, where n is the number of turns. More than one loop will be required where a very large area needs to be covered. If each loop broadcasts the same signal, then using out-of-phase adjacent loops can minimise signal loss at the loop junction. Where the signal in each loop is different (eg, in a multicinema theatre) the loop design must prevent any signal spill into adjacent loops. Special loop designs enable spill to be minimised. For more information on spill control, see Ampetronic’s website: www.ampetronic.com Coverage area In many cases it is only necessary to provide loop coverage for part of a room or hall rather than attempt to provide for the full area. For example, where a hall has seating for say 500 people, you may only need to provide hearing loop coverage for 50 seats or perhaps even less. This would mean that a map would be required to show potential users the designated listening area and/or any booking system would need to provide priority for hearing impaired within that area. A smaller loop also means that a lower-powered amplifier can be used. Amplifiers for Hearing Loops 5 ) B d ( h tg n re tS ld ie F L -15 -20 -25 -30 -35 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 Distance from centre (m) Distance from centre (m) Fig.4: field strength over loop area for a 10m square loop at a height of 1m above loop. 26  Silicon Chip As noted, an audio amplifier is required to “drive” the loop. You have three choices: using a commercial hearing loop amplifier, using a standard commercially-made amplifier . . . or you build your own! Most commercial amplifiers specifically made for hearing loop use are “current” amplifiers, whereas “ordinary” amplifiers, including ones you would build yourself, are “voltage” amplifiers. Current amplifiers have the advantage that the loop current does not vary with frequency, which would normally occur due to the inductance of the loop. However, standard voltage amplifiers can be used as well although it is true that they provide reduced current to the loop as the frequency rises. This is easily fixed, in most cases, with some judicious treble boost. And with our signal pre-conditioner for power amplifiers to be described in a future issue, using a voltage amplifier becomes very practical. Minimum load for a voltage amplifier One requirement when using a voltage amplifier is that the siliconchip.com.au Vout 9k SIGNAL Vin 1k Vout R R L LOAD (INDUCTION LOOP) SIGNAL Vin L LOAD (INDUCTION LOOP) R/10 B CURRENT AMPLIFIER A VOLTAGE AMPLIFIER Fig.6 (left): a voltage amplifier driving a hearing aid loop load will produce less current in the loop with rising load impedance. Fig.7 (right) : a current amplifier driving a hearing aid loop load will maintain current in the loop with rising loop impedance. More on this subject next month. loop must be designed to suit its minimum load, typically 4Ω. Hence, the design is based on the size of the loop and wire gauge required to provide a 4Ω DC resistance. Once you have decided on the hearing loop dimensions, you add up the length of wire sides (almost invariably the “loops” are rectangular or square) required to make up the loop (don’t forget the wire between the loop and the amplifier). Then the gauge of wire to provide a 4Ω load is selected from Table 1. But that is not the full story because the wire must be able to carry the current needed to produce the required magnetic field strength of 100mA/m (millamps/metre). This 100mA/m field strength is the standard level long term average signal level. With normal program material, peak signals can be 12dB higher or up to 400mA/m. To allow for this we have set a large factor of safety for the wire current rating by restricting average wire current to 5A/square mm when the wire could easily accept 8-10A continuously. Calculation of the current requirements to produce the 100mA/m field strength (H) at the centre of a square loop and along the same plane as the loop uses the equation: Current (A) = L(m)/9n, where L(m) is the length of the side in metres and n is the number of turns. For the purposes of loop design, a rectangular loop can use the same equation with L as the smaller of the rectangle sides. As an example, when using the equation for a single-turn 9m square loop, a current of 1A is required to produce the 100mA/m field. For a 2-turn loop the current requirement to produce that same field is halved, to 0.5A. How much amplifier power? The amplifier power needed must allow for the signal to be +12dB over the base signal level, without overload (ie, clipping). So the required amplifier power requirement will be (current required for 400mA/m field strength) squared multiplied by the 4Ω load. As an example, if the current required is 1A, the power will only be 4W. If it is 4A, the power required will be 64W. Listener’s height Another factor to consider is that the maximum field strength lies in the same plane as the loop and will be lower at a distance above (or below) the plane of the loop. So a design for monitoring signal in the same plane of the Table 1: Loop wire and current calculator Wire cross section area (mm2) Wire current capacity (based on 5A/mm2) (A) Ohms per metre (Ω/m) (based on 0.017241Ω mm2/m at 20°C) Wire length required for 4Ω (For figure-8 wire use half this length) Maximum square loop size (two turns) Current for 100mA/m for max. loop size (A) Current required for 1.7m above or below loop (A) 1 x 0.25mm 1 x 0.315mm 1 x 0.5mm 0.049 0.07793 0.1963 0.245 0.389 0.982 0.351 0.2212 0.0878 5.7m 18m 45m 0.7m square 2.25m square 5.63m square 0.078 0.25 0.63 1.50 1.01 14 x 0.14mm 14 x 0.18mm 14 x 0.20mm 19 x 0.18mm 20 x 0.18mm 24 x 0.20mm 41 x 0.20mm 0.21555 0.3626 0.43982 0.48349 0.50894 0.75398 1.28805 1.077 1.81 2.20 2.42 2.54 3.77 6.44 0.080 0.0484 0.039 0.03566 0.03388 0.02287 0.013387 50m 84m 104m 112m 118m 176m 298m 6.25m square 10.5m square 13m square 14m square 14.75m square 22m square 37.5m square 0.70 1.17 1.44 1.56 1.64 2.44 4.17 1.05 1.40 1.58 1.64 1.71 2.45 4.18 Wire size When you’ve decided on a loop dimension, use this to read off the nearest wire size and length required to make a 4Ω load. siliconchip.com.au September 2010  27 1400 Loop current and power multiplier versus height above loop 1300 That is because the current is directly proportional to field strength. If the listening height is changed so that more current is required in the loop to maintain field strength, then that means that the field strength will be lower at that height if the current is not increased to compensate. 25 24 1200 23 22 1100 21 20 Height comparison 1000 19 So let’s compare the variation in field strength between when a person is standing and when seated. We choose 1 Turn Current 1.7m as the expected highest listening point above the loop 2 Turns plane noting that hearing aids are at ear level rather than Power the height of the person. We choose 0.5m as the lowest expected listening height above the loop plane. For a 6.8m loop, a 1.7m height gives a 0.25 height to loop dimension ratio and the current multiplier is about 1.4. For the 0.5m height, the ratio against the loop dimension is very close to 0.1 and the multiplier is very close to 1. A 1.4 variation in field strength corresponds to a 3dB change. Taking the log of 1.4 and multiplying by 20 calculates this. So for the 6.8m square loop; if the loop current is set so the signal strength is correct at the 1.7m height, then the field strength will increase by 3dB at the 0.5m height due to the closer proximity to the loop. If the loop field strength is set for correct level at 0.5m, then the strength will drop by 3dB at 1.7m in height. The calculation shows that a 6.8m square loop is the smallest sized loop that will provide only a 3dB change in field strength level between the two expected minimum Height above (or below) loop/shortest side length and maximum heights above the loop. Smaller loops will have a wider variation while larger Fig.8: extra current and power are required for height offsets 1 2 3 5 7 10 15 20 25 30 35 40 45 loops will have less variation. If you are after minimal above or below the loop plane to maintain field strength. variation in field strength with height changes, use a larger square loop side dimension (m) loop. A 10m loop, for example, will show less than 3dB loop will not deliver that field strength at a higher level variation with a 2m change in listening height. above the plane. Note that the extra power requirements for the amplifier For most hearing loop installations the loop is either are very high when the listening height above or below the placed just below the floor, at floor level or in the ceiling. loop is significant compared to loop size. For example if Typically, this means that the listener’s hearing aid is about you are using a 2m loop and are 1m above the loop, the 1.7m above or below the plane of the loop. 0.5 height to loop size ratio shows a loop current requireFig.8 shows a graph of the extra current and power required for height offsets above or below the loop plane. To use the graph, divide the distance that the hearing aid will be above or below the loop plane by the shorter side length of the loop. So if the loop has a 5m shorter side and the height is 2m above the loop, the division gives us 0.4. Comparing 0.4 on the graph gives us a multiplier of FIGURE-8 about 2.1 times more current that must be applied to the CABLE loop to maintain the field strength at 2m above (or below) the loop plane. While the current needs to be 2.1 times greater, power requirements must be 4.4 times greater. This is where larger loops are better in this respect because the height above or below the loop plane is relatively small compared to the loop side dimension. This fact is important to consider because users of the induction loop are seldom all the same height, nor do they always remain at the same height. They might stand some of the time and sit for other times or they could be in a wheel chair. Ideally the loop should be sized so that the field strength does not vary by more than 3dB between the Fig.9: this shows how lowest and highest listening heights. to form a 2-turn loop The graph of Fig.8 can also be used to determine the using figure-8 wire. TO variation in field strength with changes in listening height. AMPLIFIER 18 900 17 16 800 700 600 15 Multiplier ) H m ( ec n tac u d In r ie l ip t l u M 14 13 Current 12 Power 11 10 9 500 8 7 400 6 5 300 4 3 200 2 1 100 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Height above (or below) loop shortest side length 28  Silicon Chip siliconchip.com.au 2-turns 4W Power requirements versus loop size 2-turns 4W siliconchip.com.au ) (W r e ) w o W ( P r e w o P Power (W) ment of 2.8 times higher compared to directly along the loop plane. Power requirements are eight times more. This also means that a 2m square loop is impractical because the listener must remain fixed at the one height otherwise the signal level will vary too much. When you have decided on a loop dimension, use Table.1 to read off the nearest wire size and length requirement to make a 4Ω load. You might require extra wire if the amplifier is not located close to the loop. Note that the table only shows figure-8 wire length. Figure-8 wire comprises two insulated and parallel running wires and when connected to make a single length of wire will form a 2-turn loop (see Fig.9). We show only figure-8 wire in the table because interestingly, a 2-turn loop is the only practical option for an induction loop that is driven using a voltage amplifier. It works out that a 2-turn loop that provides a 4Ω load will have the correct current rating to prevent overheating the loop wire. This applies even with the extra current requirement for loop monitoring at 1.7m above or below the loop. Using a single turn loop requires twice the current for the 100mA/m field strength and is likely to overheat the loop wire, making it impractical. Using more than two turns is not recommended because of loop inductance which increases by the square of the number of turns. So while two turns produces four times the inductance of a single turn loop, a four turn loop will have 16 times the inductance. Higher inductance means that the amplifier (whether a current or voltage type) needs to be able to provide much more voltage drive at higher frequencies. More details about this inductance effect are provided later. The table has values of wire resistance calculated based on copper resistance at 0.017241Ω mm2/m at 20°C. The cross sectional area is the radius of the wire squared times pi(). For wire with more than one strand, the area for one strand is multiplied by the number of strands. The ohms/ metre value was obtained by dividing the total cross sectional area into the 0.017241Ω mm2/m. Power requirements for a given loop size is calculated using the required current to produce the 100mA/m field and multiplying this by four to get the current for the 400mA peak. For a 2-turn loop, divide this value by two. Overall, this simplifies to multiplying the current for the 100mA/m field by two. The value is then squared and multiplied by the resistance (4Ω) to obtain the power requirement. Chances are that the loop you are using will not be exactly one of the loop sizes listed in the table. For an inbetween value loop size, use the next lowest listed loop size wire gauge. This will mean that the resistance will be higher than 4Ω due to the extra length for the larger loop. Amplifier power requirements may need to be higher if the rated power of the amplifier you are using is close to the amount of power required. To simplify calculations, Fig.10 shows amplifier power requirements for a 2-turn 4Ω loop of various sizes. One graph shows power required for directly at the loop plane and the second for 1.7m above (or below) the plane. The power requirements take into consideration the 400mA/m field strength produced during signal peaks. As mentioned if the loop is more than 4Ω, power requirements will need to be increased by the same ratio. So an 8Ω loop will require 400 390 380 400 370 390 360 380 350 370 340 360 330 350 320 340 310 330 300 320 290 310 280 300 270 290 260 280 250 270 240 260 230 250 220 240 210 230 200 220 190 210 180 200 170 190 160 180 150 170 140 160 130 150 120 140 110 130 100 120 90 110 80 100 70 90 60 80 50 70 40 60 30 50 20 40 10 30 0 20 1 10 0 Loop plane Loop plane 1.7m above (or be Loop plane loop 1.7m above (or belo 1.7m above loop (or below) loop 2 3 5 7 10 15 20 25 30 35 40 45 Square loop side dimensions (m) Square loop side dimension (m) 2 3 5 power 7 10 requirements 15 20 25 30 when 35 40 driving 45 Fig.10:1 amplifier a 2-turn 4Ω loop of various sizes. Power is shown for directly along side1.7m dimension (m) (or below) the plane. the loopSquare planeloop and above double the power. There is no problem using an amplifier that has more power than is required. For a loop of 15m and larger, the power requirements for along the plane and 1.7m are almost the same. This means that the field strength in the loop effectively does not vary over a 1.7m range. As a consequence any change in listening height above the plane of the loop will not be subject to variation in signal level. In practice 10m square loops also do not appear to have any noticeable signal level change with normal variations in height. What voltage amplifiers are suitable? As mentioned, a voltage amplifier for the loop designs described here needs to be able to drive a 4Ω load and it must be unconditionally stable. This is important because we do not want the amplifier oscillating at a very high frequency and radiating radio frequencies. In addition, the amplifier would produce lots of distortion if it is prone to oscillation. While many commercially made amplifiers could be used, Table 2 shows some of the more recent and suitable amplifiers that SILICON CHIP has published. The table September 2010  29 Loop Inductance We mentioned that loop inductance was a concern because it reduces the amount of current that is applied to the loop as frequency increases. Hence, treble boost is needed. Australian Standard AS60118.4-2007 recommends that the frequency response of the magnetic field be 100Hz to 5kHz within ±3dB. Naturally, the response can cover a wider range of frequencies. In practice though, having rolloff above 5kHz is ideal because it removes the need for excessive treble boost. We plotted loop inductance versus loop size and this can be seen in the graph of Fig.12. Inductance of a square, rectangular or circular loop can be calculated using an inductance calculator. We used the calculator at www.technick.net/public/ code/cp_dpage.php?aiocp_dp=util_inductance_calculator For the purpose of this exercise, inductance calculation was based on 1mm diameter wire (0.5mm radius). The µ value for air is 1. Inductance is shown for both a single turn loop and using figure-8 wire that forms two turns. Note how the inductance for two turns is four times that of one turn. The inductance values are based on a square loop shape. Rectangular loop inductance can be calculated using the rectangular shape option in the above mentioned inductance calculator. Typically, a rectangular loop will have the same inductance as a square loop that has the same wire length. For example a 10m square loop has the same inductance as a 15 x 5m rectangular loop. From the inductance we can calculate the 3dB down rolloff for a 4Ω loop. How this is calculated is described in the section entitled ‘Inductance of the loop’. A simplified calculation for 4Ω loops is that the -3dB frequency = 0.6366/inductance in Henries. Multiply the -3dB frequency by two for 8Ω loops. The graph in Fig.13 shows the –3dB rolloff frequency against loop side length. The graph reveals that for a 2-turn loop, the frequency response is no more than 3dB down at 5kHz for square loops up to almost 5m. Larger loops will require treble boost to compensate for the rolloff. Actual rolloff against frequency for various sized loops is shown in the Fig.14 graph. For the 5m square loop, rolloff is just over 3dB down at 5kHz, but for a 20m square loop i R V L Z XL R (4 ) 12 Fig.11: the total impedance of a series-connected resistor and inductor is calculated using a phasor diagram. Impedance of the resistor is R and reactance of the inductor is XL. Total impedance is Z. 30  Silicon Chip Inductance (H) indicates the recommended sized loop that could be used with each. The amplifier power requirement for the loop size takes into account the fact that the loop will be about 1.7m away from the listening position. See www.jaycar.com.au and www.altronics.com.au for kits. 390 Inductance versus loop size 380 370 1700 360 350 1600 340 330 1500 320 1400 310 300 1300 290 280 1200 270 1100 260 250 1000 240 ) H m ( 230 e c ) 900 n a 1T t 220 c W Loop plan ( u d r 1 turn n I 2T e 800 210 w o 200 P 700 190 1.7m abov 2 turns 180 loop 600 170 160 500 150 400 140 130 300 120 110 200 100 100 90 80 0 70 1 2 3 5 7 10 15 20 25 30 35 40 45 60 square loop sideloop dimension (m) Square side dimensions (m) 50 40 Fig.12: the plot of loop inductance versus loop size. The 30 graph shows inductance for both 1-turn and 2-turn loops. 20 Note how 10 inductance is four times greater in the 2-turn loop. Typically, a rectangular loop will have the same 0 inductance1 as2a square loop that has the same wire length. 3 5 7 10 15 20 25 30 35 40 45 Square loop side dimension (m) rolloff is –14dB down. The Hearing Loop amplifier signal pre-conditioner that we will describe in a later issue has treble boost compensation to correct for these rolloffs. Note that adding treble boost to an amplifier’s signal input might appear to mean that extra power will be required from the amplifier. However, extra amplifier power is not normally required because the power requirement for reproducing naturally occurring sounds becomes less at higher frequencies. Typically, natural sounds have the same energy per octave. And so while there are four octaves between 100Hz and 1600Hz there are less than two octaves between 1600Hz and 5kHz. Treble boost is only applied from about 1600Hz through to 6kHz. However for large loops (15m square and over), a fair degree of treble boost is necessary. In these cases it may be best to use a slightly higher powered amplifier than one selected from the design graph and tables, especially if the power available from the amplifier is only just sufficient for the size of the loop. It is not practical to compensate for treble loss for loops larger than 20m square. Impedance of the loop A hearing loop generally comprises a wire length in the shape of a rectangle or square. The impedance of the loop comprises the resistance of the wire plus the reactance due to the inductance of the loop. These two components are effectively in series. The loop resistance will remain siliconchip.com.au Loop Frequency Response (4W, 2 turns) (4W, 2 turns) -3dB upper rolloff frequency versus loop size based on a 4W 2-turn loop 0 20 0 19 -1 -1 18 -2 17 -2 16 -3 -3 -4 -4 -5 -5 -6 -6 15 14 LOOP SIZE 13 Frequency (kHz) 12 ) B d (l e v Le 11 10 ) B -7(d l e v e L -8 Level (dB) 3m square loop 3m square loo z) H k ( y c n e u q e rF 3m square 5m square loop 5m square loo 5m square 10 square loop 10 square loop -7 10m square 15m square15m loopsquare lo 15m square -8 9 20m square loop 20m square lo 20m square -9 8 7 -9 -10 -10 6 -11 -11 5 -12 4 -12 -13 3 -13 2 -14 -14 1 -15 0.25 0 1 2 3 5 7 10 15 length SideSide length (m) 20 25 30 35 40 45 0.5 -15 0.25 1 2 0.5 3 1 4 5 2 3 Frequency (kHz) 6 4 7 5 8 6 9 7 10 8 9 10 Frequ ency (kHz) (m) Frequency (kHz) Fig.13: this shows the –3dB rolloff frequency with various loop side lengths (4Ω, two turns). Frequency response varies by no more than 3dB up to 5kHz for loops no larger than 5m square. Larger loops will require treble boost to compensate for the rolloff before 5kHz. Fig.14: frequency response for various sized loops (4Ω, two turns). For a 5m square loop, rolloff is just over 3dB down at 5kHz but for a 20m square loop rolloff is –14dB down. Typically, a rectangular loop will have the same response and –3dB rolloff as a square loop with the same wire length. reasonably constant although it will vary with temperature. The main variation in the loop will be due to the reactance that rises with frequency. A pure resistance without inductance has a current that is in phase with the voltage. For a pure inductor, which has no resistance, the current lags the voltage by 90°. Its reactance is 2 x  x the frequency x the inductance (L). To find the total impedance effect of both the resistance and the reactance of the inductor we need to consider the two quantities as shown in the phasor diagram of Fig.11. Resistance is shown as R and the reactance (XL) is 90° difference in phase. To add the two values we square both the R value and the XL value, add the two squared values and then take the square root. This gives the value of the (Z) impedance. Mathematically, this is just using Pythagoras’ theorem to calculate the length of the hypotenuse in a right-angled triangle. Assuming the resistance R is 4Ω, at low frequencies the impedance of the inductor is low and so the overall impedance is close to 4Ω. As frequency rises, the impedance of the inductor rises and begins to have a greater effect on the overall impedance of the loop. Table 2: SILICON CHIP Amplifier Data Power into 4Ω Loop size Amplifier Name Silicon Chip publication date Kit supplier No. 20W 3-8m square Compact High Performance 12V Stereo Amplifier May 2010 Jaycar KC5495, Altronics K5136 30W 2.5-11m square Schoolies Amplifier December 2004 Altronics K5116 55W 2-16m square 50W Audio Amplifier Module March 1994 Jaycar KC5150, Altronics K5114 70W 2-18m square SC480 January 2003 Altronics K5120 200W 1.5-33m square Ultra-LD Mk2 August 2008 Jaycar KC5470, Altronics K5151 350W Up to 42m square Studio 350 Power Amplifier January 2004 Jaycar KC5372 This shows some of the more recent and suitable loop driving amplifiers published in SILICON CHIP, ranging from 20W through to 350W. The table also shows the recommended size of loop that could be used with each. siliconchip.com.au September 2010  31 The rising impedance has an effect on the current flow within the loop. So if an amplifier is fed with a constant voltage level, the current will reduce as frequency rises as the impedance increases. The loop current is the voltage divided by the impedance. At low frequencies, the reactance XL is close to zero and so the 4Ω resistance mainly sets loop current. As the frequency rises, the reactance increases, the total impedance rises and so current drops. The –3dB down frequency is when the resistance R is equal to the reactance XL. Then the current is 0.7071 of the DC current. As an example (and using simple numbers) lets say R is 1Ω and voltage is 1VAC. Current I at a low frequency is 1A. When the AC frequency is higher the reactance of the inductor will be 1Ω at a specific frequency depending on the inductance. The impedance Z becomes the square root of 2 or 1.414Ω. So the current is 1/1.414 or 0.7071 in value. This reduction to 0.7071A compared to the original 1A is the –3dB level. A hearing loop does not use radio! A common misconception with hearing loops is that they operate using radio waves. In other words, it is assumed that the loop acts as a radio antenna and the hearing aid includes a wireless receiver for reception. This is not true. The magnetic field from the loop is simply modulated at the audio signal frequency at up to around 5kHz. While the magnetic field produced by the loop is a part of the electromagnetic spectrum its properties are unlike radio waves: for example, the wavelength at 3kHz is so long at around 100km compared to radio waves that start at around 300m. In the same way, the electromagnetic fields produced by 50Hz power lines are not considered to be radio waves. Other examples of waves that are also part of the electromagnetic field spectrum include Infrared radiation (heat), visible light, ultra-violet light (UV) and X-rays. These too are not considered radio. Health effects using a hearing loop? While it is certain that some electromagnetic fields can cause detrimental health effects (eg, UV and X rays), it is unclear whether the low frequency and low level magnetic field from a hearing aid will have any detrimental effect. Most research concerning the effects on cells with electromagnetic radiation is concentrated on 50Hz power transmission along with the higher frequencies such as microwaves, X rays, ultra-violet radiation etc. Mobile phones come under the microwave category and operate at around 3GHz. The microwave energy from a mobile phone is vastly higher than that from a hearing loop and its frequency is at least 1 million times greater and with much higher energy. There is no correlation between the effects of microwave energy causing cell damage in the body and any effects caused by hearing loops. If we consider the 50Hz power line frequency as being the closest studied radiation compared to the hearing loop, the recommended maximum continuous exposure to magnetic field is 0.1mT (milliTesla). This data was obtained from the Australian Radiation Protection and Nuclear Safety Agency. (www.arpansa.gov.au/radiationprotection/ facsheets/is_emf.cfm). The recommended magnetic field strength in audiofrequency induction loops for hearing aid purposes is 100mA/m at 1kHz rising to 400mA/m during peaks, which equates to 0.126µT and 0.5µT respectively – more than 1000 times less than the 0.1mT level. Magnetic field strength For the hearing loop specifications, magnetic field strength is expressed using the units of A/m or amperes per meter. The letter H is used to label this field. The field represents the total amount of field strength provided by the loop. Another way of expressing a magnetic field is with the letter B, which is the magnetic field density and describes how the field is concentrated due to the medium within the field. Its units are in Tesla (T). The field medium can be free space (usually air) or it can be other material such as iron or ferrite. These latter mediums distort the magnetic field with higher concentrations found within the iron or ferrite. Where a hearing loop is installed and there is significant steel in the field, then available field strength in the free space (air) will be reduced because the field will be concentrated through the steel. The hearing loop needs to be driven with more power to counteract the loss within the steel. The B field strength values and the H magnetic field density values are easily converted from one to the other using the equation B=µH. B is the magnetic flux density (T) and µ is the permeability of the magnetic field medium. This is 4 x  x 10-7 for air and free space. For a hearing loop, the 100mA/m field strength produces a field density of 0.126µT. The 400mA/m level is 0.5µT. By the way, if you prefer to use Gauss (G) units instead of Tesla, the conversion is 0.1µT=1mG. So 0.126µT is 1.26mG. Next month An under-floor hearing loop installation. Unfortunately, under-floor access is rarely this good. Special considerations also apply if the floor is steel-reinforced concrete; indeed under-floor loops may not be possible. 32  Silicon Chip We’ll continue our look at Hearing Loops, examining at some of the commercial equipment available. SC siliconchip.com.au