introduction to fmea and fmeca in design

DGEA QT035_01(c) Edworthy Audio Engineering Consultants 1

“Buzz – is it a big deal?”

Why misunderstandings about environmental magnetic noise mustn’t prevent telecoil use

Revision 04

for: 2nd International Hearing Loop Conferenceby: Edworthy Audio Engineering Consultants

Tel: +44(0)1435 830195E-mail: [email protected]: www.edworthyaudio.com


Presentation structure

1. Categories of noise occasionally caused in hearing aids

2. How much noise is too much?3. What can I do about the noise?


What is this noise in the hearing aid?

3 categories of noise: -

Environmental radio-frequencyinterference (details in addendum)

Interference from the loop system itself (details in addendum)

Environmental magnetic interference


Is noise really a problem?

Many loop systems have no noise problemWhere there is noise, it can often be solved satisfactorily

The possibility of noise must not prevent audiologists dispensing telecoil-activated hearing aids


Environmental radio-frequencyinterference

What does it sound like?Tones, warbles, static (higher frequencies)Heard with both T and microphone position/programsHeard when loop system is not switched onFor more information on RF interference, please see addendum


Interference generated by the loop system itself

What does it sound like?Hum, Buzz, Crackles, WhistlesOnly heard with a hearing aid in the T position/programStops when loop system is switched offFor more information on interference generated by the loop itself, please see addendum


Environmental magneticinterference

What does it sound like?Hum, Buzzing, CracklesOnly heard with a hearing aid in the T position/programHeard even when loop system is switched off



How is it caused?Any alternating electric current creates an alternating magnetic fieldThe same way an audio induction (hearing) loop system works



Where does it come from?Badly installed AC power cablingAC power transformersElectric motorsElectric heating systemsCRT (tube) televisions/display monitorsFluorescent (strip) lightingLighting dimmersOther nearby loop systems


How much noise is too much?

Very subjective!For critical listening the noise level must be very lowWhere short-term communication is a priority, the noise level can be higherGenerally: If the noise spoils enjoyment or makes it difficult to understand speech, it is too noisy



For those not using hearing-aids, how noisy is this venue?Demonstration of environmental magnetic

noise in this room



International Standard IEC 60118-4 has technical requirements for environmental magnetic noise

Recommends maximum noise levels for different loop uses (critical listening and communication only)Requires measurements with ‘A’-weighted filteringMore technical details available in the addendum to this presentation


Over to Richard for the next slides


Case Studies (1)CRT (tube) display near ticket window causing buzz and hum

Re-positioned on desk, moved away from window = much reduced noise!Replaced with flat screen LCD/LED type = no noise!

Buzz and crackles from laser printerSwitched-off printer while loop is in use = no noise!


Case Studies (2)Buzz and hum from lighting power circuits because line & neutral (source & return) conductors take separate routes

AC power cabling re-installed with line & neutral conductors in same sheath = no noise!

Buzz and crackles from air conditioning plant equipment

Replaced thermostat devices with modern, low interference, types = no noise!


What can I do about the noise?Use hearing aid or receiver to locate source if possibleSwitch-off electrical equipment until noise stopsContact venue management

Alert them to Disability LegislationAlert them to potential loss of hearing aid users’ business

Contact owner of interference-generating equipment (e.g. power company)

Disability Legislation, Advocacy organisations?


What if noise level is too high and hearing assistance is still needed?Infrared or FM systems

Neck loops and silhouettes (ear hooks)They need telecoil in operation, so may pick up noiseSilhouette can overcome noise if hearing aid has volume control

Headphones must be suitable for hearing aid wearers

Direct audio inputAudio ‘shoe’, ‘boot’, etc.Bluetooth or other wireless technology if available for hearing aid


Review1. We’ve looked at causes of environmental magnetic

interference which can often be assessed and resolved quickly

2. We’ve described processes by which one can remedy magnetic interference present in the environment and who will need to be involved in that process.

3. We’ve described how compliance with the international standard for audio frequency induction loop systems addresses environmental magnetic interference

I hope we’ve convinced you that if noise is a problem, it can be overcome!


In conclusion

Thank you for listening!Was it a good presentation for you?

What went well?What could have gone better?What else do you want to know?


Contact details

Feel free to contact us at [email protected]@contactainc.com

Jensen Transformers’ websitewww.jensen-transformers.com/apps_wp.html

Look for Application Papers Jensen System Troubleshooting Guide AN003 AN004


Addendum

The following slides provide more information about the causes and cures for

Environmental radio-frequency interferenceInterference generated by the loop system itself



How is it caused?Radio signals from cell-phones, radio and television transmitters, etc.Picked up by the internal circuitry of a hearing aid (not usually by the telecoil)



Always likely to be some interference when very close to a transmitter (e.g. cell phone)Good design of hearing aid electronics can reduce sensitivity to RF interferenceRF interference not covered by the international loop standard


What can I do about this noise?

Simple:Move hearing aid away from transmitterMove transmitter away from hearing aid

Complex/expensive:Build metal ‘Faraday Cage’ around hearing aid user – not practical!Some RF shielding materials available that can be placed between cell phone and ear

Don’t wrap cell phone, that stops the cell phone working!


Interference generated by the loop system itself

How is it caused?Electrical noise picked up by amplifier input wiringCircuit noise from the amplifier itself (usually negligible)Self-oscillation, caused by some dynamic microphones and magnetic musical instrument pickupsGround (hum) loops and poor grounding practices


Noise generated by the loop system itself

Electrical noise pick up in amplifier input wiringUse balanced wiring interconnections (see Jensen Transformers website)Re-route/reconfigure cabling and connections

Circuit noise from the amplifier itselfBetter quality amplifier




Noise generated by the loop system itself

Self-oscillationRe-position dynamic microphones and magnetic musical instrument pickups away from loop

Ground (hum) loopsCure ground loop (see Jensen Transformers website)NEVER DISCONNECT SAFETY GROUNDGood grounding practices


How much noise is permitted by the International Standard?

IEC 60118-4: 2006Noise level measured in deciBels (dB)

DeciBel = tenth of a Bel1 Bel = log10(H1/H2)

H1 and H2 are two magnetic field strengths



Measured with reference to 400 mA/mOften called ‘dB L’

E.g.0 dB L = 400 mA/m (reference audio level)-22 dB L = 32 mA/m-32 dB L = 10 mA/m-47 dB L = 1.8 mA/m (equivalent to typical low acoustic background noise)



A-weighted noise level of -47 dB L recommended for quality audio-32 dB L and worse needs listening assessment

Soft hum can be acceptable, but harsh buzzing or loud tones are usually not acceptable

But no worse than -22 dB L

0 dBL

-47 dBL

-32 dBL

-22 dBL


How do we measure environmental magnetic interference?

Meter/receiver must use ‘A’-weighted filterMimics the ear’s changing sensitivity with frequencyLess sensitive at lower and higher frequencies

Must use meter with certain meter characteristics

True RMS reading/0.125 sec averaging time or PPMCalibrated in dB ref. 400 mA/m


How do we measure environmental magnetic interference?

Must orient the pick-up coil the same way as hearing aid telecoilsCheck the full 3-dimensional space in which hearing aids will be usedFrom

4 feet from floor for children and seated people,up to

5 feet 7 inches from floor for standing adults


Minimizing power system interference

The following 4 slides were prepared by John Woodgate of J.M.Woodgate and AssociatesThey help you understand why power system noise occursThey provide the rules that should be followed to minimize interference


Hum, buzz or‘power system interference’

Poorly-installed power wiring can form loops carrying mains current, generating magnetic fields that sound as hum or buzz in a hearing aid in T mode.

Appliance

Live

Neutral

Current ➔

Magnetic field

Separated Live and Neutral wires

© J.M.Woodgate and Associates


Ground loops – another source of hum

ApplianceLive and Neutral close

Ground

Magnetic fieldWrong!

Current splits



Live wires: black, red

Neutral wires: white (grey)

Ground wires: green (green/yellow)

RULESMr. Black and Mr. Red always go with Mr. White in the same cable;

Mr. White and Mrs. Green must meet only once, at the service entrance.

Minimising magnetic field emissions from power wiring – American rules



Minimising magnetic field emissions from power wiring – European rulesLive wires: brown

Neutral wires: blue

Earth wires: green-yellow

RULESMr. Brown always goes with Mr. Blue in the same cable;

Mr. Blue and Mrs. Green-Yellow must meet only once, in the supplier’s wiring.


introduction to fmea and fmeca in design

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