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Designing and Installing a HEARING LOOP For the deaf Part 2: By JOHN CLARKE Last month we introduced the subject of hearing aid inductive loops and explained how they were designed. We also mentioned that most amplifiers could be used to drive hearing loops, albeit with a bit of tweaking in most cases. Now we move on to some of the commercial equipment designed specifically for driving hearing loops. A s we explained last month, the vast majority of build-it-yourself and commercial (hi fi and PA) amplifiers are voltage amplifiers, whereas hearing loop amplifiers are current-operated devices. That’s not to say you can’t use a voltage amplifier on a hearing loop – you can, with appropriate treble boost to compensate for rolloff in signal strength due to loop inductance. But typical amplifier treble controls are not suitable because they do not operate at the correct frequency. There is a better way, and that is to “pre-condition” the audio feed to the amplifier – and we’ll shortly be describing such a device. It’s quite simple and relatively cheap (especially if that means you don’t have to buy a new hearing loop amplifier). This month we’re going to look at some of the commercial hearing loop amplifiers often found in public buildings. These are the ones often installed by professional organisations who are these days fitting out most new buildings and retro-fitting olders ones, as we also explained last month. Auditec hearing loop amplifiers The Auditec (www.auditec.com.au) range of hearing loop amplifiers is an example of what is available commercially. This Australian company designs and manufactured its range locally and offer a five year warranty. The Auditec 1077 amplifier shown here is in a 2-unit rack mount case. Lower powered amplifiers are built into a smaller instrument-style case. They are available from Bavas Music City, (www.bavasmusic.com.au), Delsound, (www.delsound.com.au) and Moore Hearing, (www.moorehearing.com.au). The amplifiers include signal compression (to maintain a more constant signal level), a bargraph loop level display, and loop disconnect indicators. Frequency response of the amplifiers is from 100Hz to 5kHz. The table below shows the amplifiers that are available and the size of the loop that each amplifier can drive. The 1077 amplifier for example can drive a loop that has a maximum perimeter of 150m. This equates to a maximum loop size of 15 x 60m or 20 x 55m or similar but note that the smaller dimension must not exceed 20m. So you cannot use a 37.5m square loop. Minimum loop size is 10 x 10m and that equals the minimum loop perimeter of 40m. The wire used is 2 x 24/0.2mm figure-8 wire connected in series to form an effective two turns around the loop. Auditec’s model 1077 transconductance amplifier (another way of saying current amplifier!) designed specifically for hearing loop use. It can drive a loop between 40m and 150m long. 74  Silicon Chip siliconchip.com.au Auditec’s model 1077 – at first glance, there is not much to distinguish it from a conventional (voltage) PA amplifier. The 1088 and 1099 models also include a separate 10W amplifier for loop monitoring via a local speaker. The table below shows various loop sizes and lengths for Auditec hearing loop amplifiers. Each amplifier includes the bargraph level display to enable the loop level to be set correctly. The level is set so that the orange LED just lights on loud levels but without the red LED lighting. It appears that the amplifier displays the signal level based on the total resistance of the loop and that the listening height above the loop is assumed to be in the seated position above a floor mounted loop. As explained last month, loop power is dependent upon loop size and height above the loop to produce the necessary field level. You may require a lower loop current if the length of wire used for the loop does not make up the total length. So, for example, if you have a loop that is 10m x 10m but the total wire length is not 40m as you would expect for a 10 x 10m square loop but is, say, 60m. This extra wire length is used to reach the amplifier that is not Model Power (VA) Maximum total loop length (m) Minimum total loop length (m) 1044 1055 1077 1088 1099 20 60 120 120 300 40 80 150 150 400 20 20 40 40 40 located nearby the loop. For this setup, the signal level display may differ from the true level. Ideally for any hearing loop setup, the field strength should be monitored using a field strength meter to ensure Vout 9k SIGNAL Vin 1k Maximum width across the narrow side (m) 5 15 20 20 30 Vout R R L LOAD (INDUCTION LOOP) SIGNAL Vin L LOAD (INDUCTION LOOP) R/10 A VOLTAGE AMPLIFIER B CURRENT AMPLIFIER Fig.15a (left): repeated from last issue, a voltage amplifier driving a hearing aid loop load will produce less current in the loop with rising load impedance. Fig.15b (right) : a current amplifier driving a hearing aid loop load will maintain current in the loop with rising loop impedance. siliconchip.com.au October 2010  75 Last month we described a build-it-yourself hearing loop receiver but if you aren’t inclined to build your own, here’s a commercially available receiver for use with headphones (available from Moore Hearing, www.moorehearing.com.au). Even if you don’t have a hearing loss, a hearing loop receiver is handy when you’re setting up the loop, to monitor for sound quality without the need for assistance from a person with a T-coil-fitted hearing aid. the level is correct. We will be publishing a suitable level meter in a future issue. You may require a higher powered amplifier if the height above (or below) the loop is significant compared to the loop size. More detail about extra power requirements for height above or below the loop is in the Voltage amplifier section (see last month’s article). Voltage and current amplifiers Fig.15a shows the configuration of a voltage amplifier with a gain of 10. It is based around a high gain amplifier with negative feedback between the output and inverting input. The output voltage is divided by 1kΩ/(1kΩ +9kΩ) and so overall the division is by 10. This divided signal is applied to the inverting input and output is adjusted so that the inverting input is at the same voltage as the noninverting (+) input. Gain of the overall signal from input to output is 10. Gain can also be calculated by the equation 1+ (9k/1k). The output drives the load between VOUT and ground. Voltage output is independent of the amplifier load assuming the amplifier can drive the load. If this amplifier drives a hearing loop, then for a fixed A practical PA system incorporating a hearing loop. This church setup consists of a Redback (Altronics) diversity UHF wireless microphone receiver, a foldback amplifier sitting on top of the Auditec hearing loop amplifier, while underneath are two 120W front-of-house amplifiers with a small audio patch box just visible on top of them. signal level, the current through the hearing loop will vary with the load impedance. Since the load comprises a resistance and an inductance, the effect of the inductance will be to increase load impedance with frequency. See the section ‘Inductance of the loop’ for more detail. The current through the loop will therefore fall with increasing frequency. For example if the overall load impedance doubles to become 2R, the load current will be halved compared to if the load is just R. To maintain a constant current in the load with rising load impedance, the applied input signal needs to rise with frequency. With the current amplifier configuration shown in Fig.15b, the load is a part of the feedback network. At low frequencies, the impedance of the load is just the resistance [R] and so the division of the output voltage applied to the inverting input is (R/10)/(R +R/10). Gain between the input and output is therefore 11. Note, however, that the signal across the load is less than the full VOUT. This is because the load is not between VOUT and ground but is via the R/10 resistance. Therefore 76  Silicon Chip siliconchip.com.au Fig.16 shows the magnetic field strength for a sine wave 100mA/m at 100mA/m. MAGNETIC Program FIELD material is set STRENGTH for the same level using a long time measurement. Peak levels for the program material will 1kHz SINE WAVE reach 400mA/m measured SHORT TERM LEVEL = 100mA/m with a 125ms LONG TERM LEVEL = 100mA/m weighting. only 10/11ths of the signal at Vout is across the load. The remaining 1/11th of the signal is across the R/10 resistor and this represents a small power loss. Overall gain as far as the load is concerned is therefore 10, the same as the voltage amplifier. Another way of looking at this is to note is that the voltage signal at the input (VIN) will be the same as the voltage across the R/10 resistor. When the load is just R at low frequencies, the signal through R is 10 times the Vin signal. Because the load impedance is a part of the feedback for the amplifier, any changes in the load impedance will alter the gain. So for example, if the overall load impedance is doubled to become 2R, amplifier output is VIN x (2R +R/10)/R/10 and that simplifies to 21. Signal across the load is 20 times the input (2R/R/10). Therefore the voltage across the load doubles when the load resistance doubles. This maintains a constant load current regardless of the load impedance. So a current amplifier automatically increases voltage across the load as the load impedance increases. With any amplifier the output must be able to maintain the voltage swing required to provide the gain of the amplifier. This depends on the power and voltage swing available from the amplifier. Ampetronic design guide As mentioned last month, details on suitable loop designs with steel buildings can be obtained from Ampetronic (www.ampetronic.com). They provide a design guide for induction loops that includes information on spill control and loop arrays. The image at left is taken from this guide. One thing to note is that this design guide misinterprets the field strength requirement for the hearing loop. The Ampetronic design guide incorrectly states that: “the magnetic field strength must be 400mA/m ±3dB across the volume of use. This is the reading with 125ms RMS measurement with a 1kHz sine wave applied to the system.” The standards specifically state that the field strength should be 100mA/m (within 3dB) as created by a 1kHz sinusoidal signal. It is only with normal program matesiliconchip.com.au 400mA/m MAGNETIC FIELD STRENGTH PROGRAM MATERIAL (SPEECH) SHORT TERM LEVEL = 400mA/m (125ms TIME WEIGHTING) LONG TERM LEVEL = 100mA/m (LONG TERM AVERAGE) rial (such as in speech) where the 400mA/m level will be reached and this is during peaks in level using a timeweighted measurement of 125ms. Long time measurement of the program material should equal the sine wave level. For more detail see the section below headed ‘Hearing Loop Standard’. Setting the long time average field strength level to 400mA/m may provide better results in induction loop receivers because the signal to noise level is improved by 12dB. However, this is not the standard level and at this level it is likely to cause a hearing aid to overload particularly during signal peaks. Hearing loop standard The hearing loop and designs in this article conform to the current specifications for Hearing Aids titled: Magnetic field strength in audio frequency induction loops for hearing aid purposes. Details are available in European standard IEC 60118-4 Ed. 1.0 (1981) and the Australian and New Zealand standard AS60118.4-2007. Both European and Australian/ NZ standards have the same specifications. Hearing loop magnetic field strength levels are recommended to be at 100mA/m. This is for a 1kHz sine wave signal. The level for program material when measured over a long time period should equal this sine wave level. The program material is expected to vary by 12dB in level using a 125ms time weighting. Measured peaks will therefore rise to 400mA/m. The same 125ms time weighting for the sinewave signal will remain at 100mA/m. Fig.16 shows the magnetic field strength for a sine wave at 100mA/m. Program material is set for the same sine wave level using a long time measurement. Peak levels measured with a 125ms weighting will reach 400mA/m. Note that the sinewave level will remain at 100mA/m with either time weighting measurement over 125ms or long time. Values for maximum background environmental field strength and loop frequency response are also provided in the AS60118.4-2007 standard. Standards are available from SAI Global at http://infostore. saiglobal.com/store/ SC October 2010  77