IEC 60118-4 Has Changed:Part 2 Counter Loops (2)

                                  Guidance For Commissioning  Counter Loops

Go to the end of this article to download your free Commisioning Certificate


For optimum field strength uniformity the ideal  counter loop  is approximately 2m x 2m and  installed in front of the counter in the floor or ceiling.   

 

The field strength variation in the vertical plane is approx. 6db from 1.2m to 1.7m.  In the horizontal plane the variation is 5db from the


centre to the edge of the loop.

However, this approach is not suitable for multiple adjacent counters and can be impractical to install.  Typically, smaller preformed ‘Counter Loops’ are used instead.

The latest edition of IEC 60118-4:2014   provides  new guidelines for the performance of one to one  loop systems such as counter loops. 

 

It specifies a volume of space over which the field strength should be tested.

 

 

The field strength should be measured at a height of 1.2m, 1.45m and 1.7m at locations A, B and C as shown in figure 1.  At these 9 points the field strength should be within a range of  -6dB to +6dB.

 

Note:

1.The horizontal distances specified are from the counter edge (the nearest position to the loop that the user may stand) not the loop.  The distance  to the loop from the counter edge is  the offset, d.

2.The field strength should not exceed +8dB  in the area where users are expected to stand (Red shaded area).  However, due to the nature of the loop it is recognised that this is often unavoidable and the user is expected to adjust there position accordingly

 3. 0dB is  defined as 400mA/m rms

 4. Microphones should be  fixed in place as close as  possible to the  staff speaking position.

 5. The microphone gain should be carefully adjusted  to avoid over sensitivity to background noise

 6 The operator should be trained on the systems use and on how to check the system using a loop listener

 7. Clear signage to indicate the presence of the system must be visible

 

 Click to Download the Full ArticleCounter Loop Tests Certificate Booklet Format Format


 Loop Orientation  

Loop Mounted Vertically

With the loop mounted vertically in the counter there is good field strength uniformity in the horizontal plane

 Test Conditions

The top front edge of the loop is at 0.77m from the floor.  The loop current = 1.4A. ms .  There is no horizontal offset. Loop Type:- Univox preformed 30cm x 30cm multi-turn loop.

 

Loop Folded At Right Angle

With the loop folded at right angles there is good uniformity in the horizontal  and vertical plane. 

With a small horizontal offset the uniformity in the vertical plane can be improved

                

TestConditions

The top front edge of the loop is at 0.77m from the floor.  The loop current = 1.4A. ms .  There is no horizontal offset. Loop Type:- Univox preformed 30cm x 30cm multi-turn loop.

 

Loop Mounted Horizontally

With the loop mounted horizontally in the counter good uniformity in the vertical plane in close proximity to the loop edge is achieved  However,  this loop orientation exhibits a null field that may intrude in the user defined space. See

 

Test Conditions

The top front edge of the loop is at 0.77m from the floor.  The loop current = 1.4A. ms .  There is no horizontal offset. Loop Type:- Univox preformed 30cm x 30cm multi-turn loop

 


 

Counter Loop Kits And Test Equipment To Make Compliance Easy

The FSM 2.0 Field Strength Meter

The FSM 2.0 is a microprocessor controlled instrument using multi-tone signals to make advanced measurements of induction loop systems quick, accurate and easy. It is the only FSM to fully satisfy the demands of the IEC60118-4 standard by being capable of measuring noise levels down to -47dB.

And since it is FSM 2programmable, its mode of operation can be updated to reflect changes to the standard as they occur. The instrument has a clear, backlit, LCD display and steps through the required measurements in a logical sequence, matching the test certificate to simplify the certification process.

With sharp, narrow band filtering and a noise spectrum display function, spill control and background noise assessments are also made easy.

With the dedicated 1.6kHz test tone available for this meter, the maximum power bandwidth  is also easy to assess.

 

Loop Listener

 Loop Listener

The Univox® Loop Listener has an inbuilt speaker for the assessment of audio quality and power level LED's to check field strength. It is an essential tool for any facility serious in maintaining good operational loop systems.

 

CTC-Counter Loop Kits

The CLS-1 Compact loop amplifier is at the heart of our counter loop kits.

 

It has the highest output power in its class and features  Univox ® Dual Action AGC technology for superior intelligibility.  The amplifier has 3 inputs with volume, base and treble (Metal Loss Compensation) controls on the front panel.  Once set, the unit can be dropped into the mounting bracket supplied restricting further access.

CTC-120 Counter Loop Kit

 

CTC-121 Counter Loop Kit

 


 


 

Counter Loop Test Certificate

 

The Counter Loop Test Certificate has been designed specifically for the purpose of commissioning Counter Loop Systems.

To get your copy Download Here

Counter Loop Tests Certificate Booklet Format FormatCounter Loop Tests Certificate A4 Format
Booklet Format       A4 Format

 


 

 

 

 

 

 

IEC 60118-4 Has Changed:Part 1 Maximum Power Bandwidth

 

Why is full power bandwidth to 5kHz not necessary?

Normal hearing (and hearing aids) is less sensitive to low frequencies.

This means for the perception of equivalent loudness, we need less power at high frequencies compared to what is required at low frequencies. 

 

The blue curve on the graph above is described as A weighted and is an approximation to the sensitivity of normal hearing .  The curve below shows the sensitivity of a typical hearing aid.

Both graphs show a similar response.

ITU speech shown below and other test signals used to simulate speech are opposite in characteristic to the response curves shown above.  More power is required at the lower frequencies

 

At 5kHz the signal level is approx.  2 4dB down on that at 1 kHz.  For every 12 dB reduction in field strength the loop current is halved.  This implies that at 5kHz , a ¼ of the current is required compared to that at 1kHz which is 1/16 th of the power ( P=I2R) so the system does not need to be capable of delivering full power all the way to 5kHz

Why do we measure at 5kHz if the Maximum power bandwidth is 1.6kHz

When commissioning  hearing loop systems in accordance with the IEC standard, we check the magnetic field strength at  5kHz (and 100Hz) to ensure it is no lower than what we have measured at 1kHz.  This test is only  to determine whether the constructional metal is absorbing the higher frequencies. It is conducted with the output of the amplifier turned down;  -12dB in the case of  Univox, other manufacturers choose to use less demanding test signals such as pink noise.

With the output set to deliver -12dB (small signal) and assuming the loop design  is within our guidelines, the amplifier will be delivering a constant current over the full frequency range. **So if the field strength is not  the same at 5kHz as measured at 1kHz,

the difference is due to magnetic absorption.  We can compensate for this by using narrow loops or  by using metal loss compensation control.

The FSM 2.0 Field Strength Meter

The FSM 2.0 is a microprocessor controlled instrument using multi-tone signals to make advanced measurements of induction loop systems quick, accurate and easy. It is the only FSM to fully satisfy the demands of the IEC60118-4 standard by being capable of measuring noise levels down to -47dB. And since it is FSM 2programmable, its mode of operation can be updated to reflect changes to the standard as they occur. The instrument has a clear, backlit, LCD display and steps through the required measurements in a logical sequence, matching the test certificate to simplify the certification process. With sharp, narrow band filtering and a noise spectrum display function, spill control and background noise assessments are also made easy.

With the dedicated 1.6kHz test tone available for this meter, the maximum power bandwidth  is easy to assess

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The 2015 edition of the IEC  standard, IEC 60118-4:2015 includes an additional  performance test, Amplifier Overload at 1.6kHz (Maximum Power Bandwidth)

The test should be carried out on the induction loop system during the commissioning process.  It is aimed at checking that the amplifier is capable of delivering  *full current into the load presented by the loop up to 1.6kHz.

If the amplifier is not capable of maintaining this output current, there is a possibility that voltage clipping will occur on some signal peaks which may cause audible signal distortion.  (As loop systems rely on the loop current and not voltage a small amount of voltage clipping is not an issue and will not affect the perceived audio quality even though it is visible on an oscilloscope or a clip detector.  So any tests on the maximum power bandwidth should be done in conjunction with a listening test.

*full current in the context of this article is  the rms current at which the field strength of 0dB ±3dB is achieved using a 1kHz sine wave tone or equivalent and where metal loss correction is correctly adjusted.

 

** The loop cable is inductive It has both a d.c and an a.c resistance  (reactance). The a.c component is frequency dependent .  The higher the frequency, the higher the reactance and therefore the higher the voltage output from the amplifier required to maintain a constant current.

The Univox design software defaults to designing for Speech, 1.6kHz.  So providing you follow the design and use the cable type and length specified, including feed cables, your system should automatically meet this new requirement (assuming you set the same levels as you chose in the software and all other assumptions are correct)

 Testing for maximum power bandwidth to 1.6kHz

There are several methods that can be used to check the maximum power bandwidth of the system.  The most accurate requires the use of an oscilloscope .  This method is not covered in this document as we do not expect many installers will have access to an oscilloscope in the field.

 1.) For loop amplifiers with a ‘peak voltage clip’ LED

 The most recent models in the Univox range include a ‘peak voltage clip ’LED.

Method A

With the system setup to deliver the ***correct field strength (0dB±3dB) using a 1kHz tone or equivalent apply the live signal to the system and monitor the LED. It should remain off although the occasional flicker on the peaks of the signal are acceptable.

 A listening test is required to complete the assessment.

 

 This is probably the least stringent of acceptable test methods.  Although the input signal is not analysed and there is no confirmation that the live signal includes full power frequency components >=1.6kHz.  Either the system passes because it has adequate power bandwidth or  based on actual inputs, the system does not need it. 

 Method B

Using a 1kHz sine wave tone or equivalent set the system up to deliver the ***correct field strength (0dB ±3dB).  Now apply a 1.6kHz sine wave tone.  If the ‘Voltage clip’ LED remains off, the maximum power bandwidth is=> 1.6kHz.

A listening test is required to complete the assessment.

 2.) For loop amplifiers without a ‘peak voltage clip’ LED

 Using a 1kHz sine wave tone or equivalent set the system up to deliver the ***correct field strength (0dB ±3dB).  Now apply a 1.6kHz  sine wave tone and measure the field strength.  It should not have changed. Any drop in field strength indicates that the amplifier is voltage clipping and is not able to deliver the required current at this frequency.

('Squaring' of the current waveform as observed on an Oscilloscope is a more precise method)

A listening test is required to complete the assessment

 

***The standard does allow the field strength at 1kHz to be reduced by 1dB before applying the 1.6kHz tone, but metal loss compensation must be applied

 

The Univox Log Book and Commissioning Certificate now include the 1.6kHz Maximum Power Bandwidth Check.

Both documents are available as PDF Forms and can be downloaded from the knowledge centre on our website.

http://www.univoxaudio.co.uk/log-book

To access you will need to create an account and log in  

Positive feedback is not always good news

 

Understanding Positive Feedback

We all like to  receive some positive feedback, a pat on the back, a compliment, some recognition for the work we have done or for something we have achieved.  However, In the audio world, things are quite different, positive feedback can lead to instability, and too much positive feedback can cause catastrophic system failure; consider pushing a child on a swing, each push is in synch with the motion of the swing requiring relatively little effort yet the child swings higher and higher .  If the positive feedback continues, the system becomes unstable and eventually it breaks down ending in tears.

 

Audio systems are similar and grown men do cry!  With an open microphone near a speaker, the output from the speaker is fed back into the system pushing the output higher and higher eventually the system self oscillates and a loud screech emanates from the speaker.  If corrective action is not taken, the amplifier and possibly the speaker will self destruct.

 

To recover from this situation, the operator can move the microphone further away from the speaker, turn the sensitivity down on the microphone, turn the volume down on the  amplifier driving the speaker, filter the oscillating frequency, a combination of these or switch the system off and start again.

 

Every audio system will have a point at which it becomes unstable and self oscillates.  A systems integrator needs to understand where this point is and to try and maximise the dynamic range  to give the best system performance.

 

Induction loop systems are no different except when they self oscillate it all happens in silence and invariably goes undetected until it is too late since the feedback mechanism is magnetic.

 

Finding the stability threshold for the loop system

 

The combined input and output settings at which the system begins to self oscillate is the stability threshold.  To determine these points either the input level or the output level should be set to the desired position with the other control being adjusted from zero until the threshold is reached.

 

We suggest you set up the input level as specified in the amplifier documentation and then adjust the loop current from zero, observing the field strength generated.  Increasing the current should cause the field strength to increase. If the field strength suddenly drops by 6dB the amplifier is self oscillating and the 2nd stage of the AGC has been activated.  You will need to back the controls off or turn the system off to reset .  An oscilloscope connected across the loop terminals (it must not be grounded) can be used to observe and measure the oscillation frequencies. 

 

For a Super Loop system, you will need to adjust both master and slave.  Start with both outputs set to zero, adjust the master whilst monitoring for feedback and if it is not reached, adjust the slave.  You will need to monitor both master and slave outputs as either could begin to self oscillate

 

                      Is the system self oscillating?

 

It is often very difficult to tell whether the system is self-oscillating until it is too late.

To check a system:

 

Set the field strength generated by the system to 0dB at the desired height using test signals from an independent source.  Now connect the live feed and check the output level.

 

  • Has the field strength dropped by approximately 6dB? 

 

Keep quiet so there is no input to the microphone.

 

  • Does the audio input LED stay on? 

 

  • Do the drive current LED’s stay on?

 

The above are all indications that the system is unstable and self oscillating.  The system should be turned off to avoid damage and remedial action to prevent or reduce susceptibility to self oscillation as described  should be taken.

 

Loop listenerThe Univox loop listener uses a peak detector so it can easily be used to check the field strength level of live signals.  With test signals you would expect to see the green led lit indicating that the field strength is at least 0dB.  When you connect the live signal you would expect the green LED to flicker on the peaks of the signal with the orange LED flickering occasionally too.  If neither LED flickers, the field strength has dropped to below -6dB. 

 

 

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Causes of  Positive Feedback

1 ) A dynamic microphone is being used inside the induction loop

            

 

Dynamic microphones use a magnetic pickup.

 

 

   To reduce the susceptibility to feedback:

  • Use a dynamic microphone with better shielding.
  • Only use the microphone outside the loop and if necessary add a cancelation loop to this section.
  • Use a Super Loop system.
  • Reduce the output of the loop system (turn the current down).
  • Reduce the sensitivity of the microphone (turn the input gain down).
  • Limit the bandwidth of the loop input signal using DSP or other filtering techniques.
  • Use a combination of the above.

  To avoid the Issue of feedback:

  • Do not use a dynamic microphone.
  • Use a condenser microphone or other type of  microphone that does not use a magnetic transducer.

 

2 ) An electric guitar is being used inside an induction loop

 An electric guitar uses a magnetic pick -up coil to detect the vibrating strings.

 

 

 

 

  To reduce the susceptibility to feedback:

  • Only use the electric guitar outside the loop and if necessary add a cancelation loop to this section.
  • Use an SLS system.
  • Reduce the output of the loop system (turn the current down).
  • Reduce the sensitivity of the guitar pick up coil
  • Limit the bandwidth of the loop input signal using DSP or other filtering techniques.
  • Use a combination of the above.

  To avoid the Issue of feedback:

  • Do not use an electric guitar where an induction loop system is being operated.

 

3 ) The Loop or loop feed cable is parallel and in close proximity to the low signal input cables

The high current in the loop and feed cable creates a magnetic field (as intended).  This magnetic field will induce currents into adjacent conductors. 

If the conductors are carrying input signals eg. microphone signals then the   output is being coupled back into the input;  positive feedback

To reduce the susceptibility to feedback:

  • Increase the separation distance between the cables (at least 30cm separation between cables should be maintained).
  • Reduce the distance over which the cables run parallel to each other.
  • Use twisted pair loop feed cables (star quad configured  loop feed cables are best).
  • Use balanced connections for high common mode rejection.
  • Reduce the output of the loop system (turn the current down).
  • Reduce the level of the input signal (turn the input down).
  • Limit the bandwidth of the loop input signal using DSP or other filtering techniques. (5kHz knee point or 4kHz if the system is only being used for speech) .
  • Modify the loop Amplifier.  The manufacturer my be able to suggest some modifications to the amplifier to reduce feedback sensitivity.  These modifications may cause other instability issues and therefore must be applied with caution.
  • Use a combination of the above.

  To avoid the Issue of feedback:

  • Do not run loop cables in parallel with input cables. (They may cross at right angles).

 


 

Damage to induction loop amplifiers caused by positive feedback is not covered under warranty.  It is the responsibility of the installer to ensure that the system is stable.


 

 

Loop Configurations Explained

Why One Size Does Not Fit All

Universal Access

Loops and telecoils have the potential to  provide truly universal access for hard of hearing people here in the UK and throughout the world , but only if they all deliver the same high performance.

To realise this potential,  international agreement on the performance of a loop system has been reached and is embodied in the International performance standard for Audio Frequency Induction Loop Systems , IEC60118-4:2006. 

This standard defines the magnetic  field strength, signal uniformity, frequency response and background magnetic noise for a system. It means that a hearing aid user  using  a system in a bank, at a train station , at church, in a theatre etc... here or any where else in the world can simply select the T -programme on their hearing aid to relax, participate and enjoy the moment.

An induction loop system that does not deliver standard compliant performance is a missed opportunity.  It will not delight customers encouraging them to return  and may render the facility in breech of its duty to provide an equal level of access to everyone as set out in the Equalities Act 2010.

Where there is a need for audio communication to inform entertain or communicate, there is  most likely a need for assistive listening.  By nature of the environment of each application, the demands on  the induction loop system may be very different, requiring a different solution.

Large area coverage, spill control or compensation for signal loss due to metal structures  can rarely  be achieved with the humble perimeter loop.  A  knowledgeable, experienced loop designer will consider these performance requirements with installation and budget constraints to determine the most appropriate solution, always mindful that the system must comply with the international  performance standard IEC 60118-4:2006. The typical options are :

  • Perimeter Loop
  • Cancellation Loop
  • Figure 8 Loop
  • Super 8 Loop
  • SuperLoop

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Loops In Theatres Are Causing A Buzz

And so they should.

Theatre Managers- Create the right buzz:

1. Imagine what it’s like to be a hard of hearing person

  • Do you want to declare your disability, not enjoy your experience because you couldn’t hear, have a theatre pay lip service to you? No.
  • 1 in 6 people in the UK have hearing problems. By creating a truly inclusive environment, more will visit with family and friends. And that’s good for business

2. Use only professional audio experts to install your loop

  • An induction loop is like any other audio system. It needs designing and installing by experienced professional audio specialists.
  • By working with experts you will be assured of a job well done, no compromise for users and no poor installations to rectify.

3. Be fully aware of all the costs of using a loop alternative

  • FM and Infrared systems may appear to be an easy low cost alternative to an induction loop, but aside from poor performance and no anonymity for users, they do not offer long term value for money.
  • Ongoing costs include managing, cleaning, charging, repairing, and replacing receivers.

Installers - Eliminate the Buzz with 5 simple steps

1. Prevent Ground Loops

  • Only install equipment with balanced input/output connections
  • Connect input signals to the loop amplifier using twisted pair screened cables.
  • Connect the chassis of each piece of equipment to the same outlet supply ground point, keeping the conductors as short as possible using a star ground configuration.

2. If A Ground Loop Exists, Rectify It By:-

  • Lifting the ‘ground lift’ switch on either the loop amplifier or the audio source equipment if fitted.
  • Disconnecting the ground connection in the interconnecting signal lead at the input end to the induction loop amplifier.
  • Using an isolation transformer on all inputs to the induction loop amplifier.

3. Avoid Cross Talk

  • Do not run any signal cables parallel and in close proximity to the output (loop) cables.
  • Use an SLS system; this has an average field strength of almost zero and is much more efficient than a standard perimeter loop reducing the magnetic field and risk of cross talk by 40 to 60dB within the looped area.

4. Prevent Acoustic/Inductive Feedback

  • Do not use dynamic microphones or electric guitars within the looped area.
  • Install a SuperLoopTM system with spill control to the stage and to the mixing console.

5. Set The Input Level Correctly

  • Adjust the input sensitivity of the loop amplifier using actual signals in accordance with the manufacturer’s instructions. The higher the gain, the less stable the system may become

Not only are induction loop systems the preferred assistive listening technology of hard of hearing people, they also offer the best value for money, hassle free, no compromise solution for theatre managers and installers.

But sadly, it’s not the buzz of enjoyment that the hard of hearing person gets from using the system, the buzz of delight from simply switching their hearing aid to T and sensing the sounds so clearly they think they are in their head. The buzz of gratitude they have when they arrive at the theatre knowing they will not need to hunt down a receiver to hear, or the buzz they get out of using the assistive listening system without anyone else knowing.

It’s not even the buzz the theatre manager gets, delighted with the money and time saved from not having receivers to manage, clean, charge, repair and replace. And there’s certainly no buzz for the Installer, confident in the knowledge that he is getting it right, avoiding frustration from post installation problems, and delighting his customers.

No. It‘s the buzz, the hum and the screech that the installed loop system will, according to the theatre manager, inevitably cause. In fact, touring companies are so paranoid about the buzz from loop systems that they will insist they are turned off during performances, making them redundant and forcing the theatre to use a more discriminatory technology. Ordinarily, this would provoke a louder buzz of dissatisfied, excluded customers but, hard of hearing people are so used to a second rate service, they will rarely complain.

But it does not have to be like this. There are thousands of induction loop installations in theatres throughout the world working tirelessly to assist hard of hearing people without causing the slightest perturbation in the audio system. And it’s easy to achieve. Copenhagen Opera House, the Stockholm Concert Hall and the Danish broadcasting City are just a few examples.

Induction loops are no different to any other audio system. A well designed system, correctly installed will provide years of buzz free, hum free, screech-less service.

The same techniques used by professional audio technicians and equipment manufacturers to mitigate feedback, eliminate buzz and squash hum in an audio system apply to induction loop systems. Ground loops, feedback paths and cross signal coupling are well understood by sound technicians.

By using a common star ground, isolation transformers, balanced line feeds and a few other simple techniques they can cope with them all.

So why does it sometimes go wrong with induction loop installations?

Either professional audio technicians forget all they have learnt when installing loop systems or more likely, induction loop systems are being installed by well-meaning electricians and other contractors who do not understand the issues.


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