Transcription
Design GuideINDUCTION LOOP SYSTEMSTCH 007F
Induction Loop Design GuideTable of ContentsDesign Process 3Basic Concepts 3Do you need a loop system? 4The area of use 4Site Survey 5Field of coverage (also referred to as “Volume of Use”) 5Is there more than one area that needs coverage? 5Wire Considerations 5Amplifier Location 5Audio Source 5Noise-free Source Audio 5Overspill, Interference and Confidentiality 6Metal structures and magnetic absorption (metal loss) 7Design Requirements and Best Practices 8Types of Loops 8Single Array 10Phased Array (low overspill) 10Loop Design 11Cancellation loop design 12Single array design 13Phased array (low overspill) design 13Ultra-low overspill loop design 14What is an induction loop system? 3Testing for Electromagnetic Background Noise 5Perimeter Loop 9Perimeter loop design 11Cable Choices and Selection 15Installation Considerations 16Length Limits 16Field Strength Meter 17Tone Generator 17Testing and Maintaining Induction Loop Systems 18Site Survey 18Commissioning 18Regular System Checks 182
Induction Loop Design GuideDesign ProcessDesigning a loop system is a process with many steps. A successful outcome depends on how carefully each step is performed. Thesesteps are explained in detail in this guide. The order of these steps generally follows the order of the table of contents.Basic ConceptsWhat is an induction loop system?An induction loop system consists of one or more loops of wire, driven by an amplifier, to produce a magnetic field. When audio is inputinto the amplifier, this magnetic field fluctuates. These fluctuations, or magnetic waves, are picked up by t-coil equipped devices (such as ahearing aid or loop receiver), which amplify the signal and turn it back into sound waves that the ear can hear.Below is one example of a loop system called a perimeter loop, because it is usually installed around the inside perimeter of a room. Hereis how it works:Audio from a microphone (or an audio system) is input into the loop amplifier. The audio is amplified and sent through the loop in the formof current that fluctuates (goes in one direction then the other direction). This alternating current, when flowing through the wire, producesan electromagnetic field that is radiated into the listening area. Devices with t-coils pick up this magnetic energy, amplify it, and turn it intosound waves that the ear can hear.If a person requires hearing assistance and they do not have a t-coil equipped hearing aid, they can use a induction loop receiver to hearthe audio.ne lionMioaetmpphcroTLoop AmplifierNOTE: The illustration above is shown as an example of a perimeter loop, illustrating the principles of how an induction loop systemworks. Perimeter loops work well on older traditional buildings with wood floors on wood beams. However, modern constructionmethods often use concrete floors, incorporating the use of re-bar or steel-pan decking onto which the concrete is poured. The presence of steel in these floors results in metal loss, where the magnetic field is absorbed by the field-groundingA493effect the steel creates.These types of floors will require a phased-array design, as perimeter loops will not provide system performance to the IEC 60118-4Specification.3
Induction Loop Design GuideDo you need a loop system?A loop system is one method, or technology, for providing assistive listening. There are other methods too, such as FM, Digital, Infrared,and Wi-Fi. So, choosing an effective technology that is appropriate for the venue and the audience is very important. There are severaladvantages to using a loop system for assistive listening:1. People with t-coil equipped devices including hearing aids or cochlear implants) are able to hear the audio program without additionalequipment such as a separate receiver, headphones or earphones. These people not only hear the audio at the level they need, but itis processed through the DSP in the hearing aid with their corrected EQ for their individual hearing loss.2. Some people are sensitive to being “singled-out” for being hearing impaired. With a loop system, all they have to do is switch on thet-coil in their hearing device, making it a very discreet form of assistive listening.3. Since the audio program is sent directly into the amplifier, the audio is isolated (background noise is reduced or virtually eliminated),drastically improving the intelligibility of the audio. For this to work correctly, the source audio itself must be free of noise. If usinga microphone, the type of microphone and the direction of pickup are important. An omnidirectional microphone will not work wellbecause it will pick up background noise along with the speaker’s voice thus defeating the purpose of an assistive listening system.4. By sending program audio content (microphones or other sources) from the loop to the t-coil-equipped device, the listeningexperience is improved by bypassing the effects of the acoustical space of the venue. Multi-flutter echo and reverberation can reducespeech intelligibility and negatively affect the overall listening experience.The area of useThe loop system has to deliver a tight range of signal strength and frequency response to the hearing devices in the listening area. Thelistening area is often called the listening plane or “area of use”. To determine the area of use, the following information must be gathered:1. Range of heightsThe following table gives typical distances from the floor to the ear. Use this table to determine the area of use (listening plane). Theloop wire can be at ceiling height or on the floor, but should never be within the area of use. The t-coil requires a perpendicular (vertical in this case) orientation to the loop wire or it will not pick up the loop signal.AdultsChildrenStanding4’-7” to 5’-7”3’-11” to 5’-11”Seated3’-11” to 4’-7”2’-11” to 3’-11”Warning: The wire cannot be placed at ear height.2. Listeners in fixed positions vs movingIf listeners are in fixed positions, the field strength and frequency response must meet IEC 60118-4 Specifications at the ear height locations. Aisles or empty area between rows may have less signal or frequency response, as long as the ear height positions meet theIEC Specifications.If listeners will be moving around the room, consistent coverage is required to reduce nulls (drop-out) and changes in frequency response as the listeners move.3. Where is the system requiredIt is important to evaluate where the listeners need to use the system, and where it is not required. Hallways, entry/exit locations, oreven sections of certain seating areas may not require hearing assistance, depending on the venue. The loop system is only requiredto meet specifications where hearing assistance is required.NOTE: Be aware of state and local requirements as well. For example, the California Building Code (CBC) requires 100% coverage ofthe venue. These situations are known as ‘Authority Having Jurisdiction” (AHJ), as California laws stipulate that their CBC supersedesthe authority of the Americans with Disabilities Act (ADA).4
Induction Loop Design GuideSite SurveyA site survey is performed in an existing structure, prior to installing the loop system. Site surveys should take many things intoconsideration.Reminder: Any equipment that will likely be operating when the loop system is on, should be operating during the testing. This includesthings like lighting, dimmers, audio, video, projection systems, etc.Testing for Electromagnetic Background NoiseUsing a field strength meter (Williams AV model PLM FSMP), hold the device at 5 feet above the finished floor, in a vertical position, to alignthe telecoil with the component of the magnetic field you are testing. On the meter, press the top switch in and the bottom switch out (whichsets the device to A weighted and -20dB).An acceptable reading is less than -32dB on the -20dB scale of the FSM.Readings should be taken throughout the area the loop will cover to pinpoint any localized magnetic background noise. Again, anyequipment likely to be operating when the loop system is on should be operating during the testing.Field of coverage (also referred to as Volume of Use)The site survey should consider the desired area of coverage. Where does the customer and venue need and allow coverage? This willhelp the designer recommend a loop layout that will satisfactorily cover the desired area. Considerations must be made to tiered seating,overspill control, types of floor or surfaces where the loop wire is to be installed, and size of area to be covered. Aisles generally do notneed to have coverage, so the center of aisles are often a good area to place the wire.Is there more than one area that needs coverage?If installing more than one looped area at the venue, low-spill phased array designs may need to be developed.Wire ConsiderationsWhat type of wire will be utilized? Select from flat wire, single conductor or Direct Burial Cable (DBC). Determine whether this wire needs tobe buried in the concrete floor or placed under carpet, wood or tile.Williams AV does not provide Direct Burial Cable (DBC). We recommend using Belden’s 14 AWG stranded Hook-up/Lead Wire Direct Burialcable (9438).All amplifiers have a specification for recommended load. The length and gauge of the wire determine the load on the amplifier output.Longer wire runs may require thicker wire to stay within the amplifier load specification. The lead wire from the amplifier to the loop is alsopart of the total load (impedance) on the amplifier.When installing loops into large venues, consider the DC resistance of the selected wire in your assessment as to how many amplifiers willbe required. Williams AV amplifiers require a DC resistance of 0.5 - 3.0Ω.Amplifier LocationWhere will the amplifier(s) reside? The amplifier(s) often are not located in the same room as the listeners, because amplifiers often havefans that create noise. Please refer to each amplifier specification sheet to find out the correct heat ratings (BTUs), as this will assist withfactoring in the needed airflow to the rack.Audio SourceHow and where will I access the audio source? What types of connections do I need? Is it digital, line level, microphones, 70V distributedaudio?Noise-free Source AudioThere are many different types of sources that may be used with a loop system. Music or a pre-recorded program may be used, or aperson using a microphone. The loop system is only as good as the source audio. If the source has noise, this noise will be broadcast tothe loop. In the case of a person speaking, different choices should be made for the microphone, depending on if the person is stationaryor moving. Some microphones may pick up too many surrounding sounds, and therefore would not be a good choice for the loop system.Below is a chart to assist in choosing the correct microphone and/or audio source that should yield the best results for a loop system.5
Induction Loop Design GuideFigure 2: Typical source audio requirementsType of Input AudioTypical Input RequirementsFixed Talker(Examples: Podium, courtroom microphone)Directional MicrophoneA directional microphone is designed to pick up voice and reduce oreliminate background noise. Omnidirectional microphones are notrecommended, as they pick up too much surrounding noise.Moving Talker(Examples: Standing/walking presenter on a stage, classroom teacher)Mobile microphoneThese microphones typically include a radio transmitter/receiver.Some examples include lapel microphone, headset microphone, orhandheld microphone. These microphones should be as directionalas possible, and the transmitter/receiver system should not introduceadditional noise.Audio System(Examples: CD, DVD or Blu-Ray Player or Computer fed through anAnalog or Digital Mixing board, or Audio Switcher/Preamplifier)Audio system with clean outputThe original source material should be as clean (free of noise) aspossible. The system by which the audio is input into the loop amplifiershould not introduce additional noise including hum, background noise,pops/clicks or any other form of noise.Multiple Fixed Talkers(Examples: Conference Room, Courtroom with multiple mics)Directional boundary microphones. Special mixing.Multiple speakers at fixed positions require directional microphonesthat should reduce background noise and not pick up nearby speakers.Each microphone should present an individual’s voice as isolatedand clearly as possible. Special mixing may be required to improvethe intelligibility of each individual speaker when blended with otherspeakers (such as compression, equalization, gating, etc).Multiple Moving Talkers(Examples: Large Conference, Live Theater)Multiple mobile microphones. Special mixing.Multiple speakers in moving positions require directional mobilemicrophones with radio transmitter/receiver systems. All microphonesmust reduce background noise and not pick up nearby speakers. Theradio transmitter/receiver system must not introduce any additionalnoise. Special mixing may be required to enhance or reduce theintelligibility of each speaker when blended with other speakers (suchas compression, equalization, gating, etc).If doing a site survey of an existing venue, find out what existing sources will be used. Microphones, audio sources, mixing systems, audioswitchers, preamplifiers, and even connectors, wire, and adapters all need to be reviewed. Any of these items can be a potential source ofnoise or distortion. Noise checks or simple listening tests can help to identify potential problems and solutions before a loop system designis started. Contact Williams AV TechBlue (technical support) at 800.328.6190 (Ask for TechBlue) or [email protected] if you wouldlike advice on conducting a site survey.Overspill, Interference and ConfidentialityOverspill occurs when the magnetic signal from a loop travels beyond the listening area into other areas where the signal is not intended orrequired. A perimeter loop can radiate as much as 4 times the loop width away from the loop itself.This can cause problems if multiple rooms have active programs and listeners are on the border of a listening area. They may hear twoprograms at once. If a program is not intended to be heard outside of a room for confidentiality reasons (such as a courtroom case), the spillof the signal can allow any user with a t-coil or loop receiver to eavesdrop on the program.In some cases, this may not be a concern, as in the case of a church service or city council meeting. The use of the space will aid indetermining the appropriate loop design. Following is a diagram of what the overspill looks like from a simple perimeter loop system.6
Induction Loop Design GuideFigure 3: Perimeter loops spill signal outside the loop (white strongest, black none)OverviewSingle ArrayThe following needs to be determined:Perimeter Loop1. Is there or will there be another loop system to the side, above or below this loop, within 4x the width of this loop?Phased Array (Low Ov2. Are there confidentiality concerns that would require the overspill of the loop to be contained?If yes in either case, a perimeter loop cannot be used. Loop designs that help contain spill are called phased arrays (or more specificallylow-spill phased arrays) which use a series of interwoven loops to reduce spill from the loop. With a well-designed phased-array system,spill can be reduced to up to 4 ft. (1.2 m) from the loop edge and 12 ft. (3.7 m) above and below the loop plane.Metal structures and magnetic absorption (metal loss)Most commercial buildings contain metal as part of the structure (I-beams, steel re-bar and/or steel pan decking in floors and ceilings, andmetal ceiling grids). These fixed metal structures have a grounding-effect to magnetic signals. They absorb the magnetic energy, reducingit (sometimes greatly) as the signal travels past the structure. Most of this effect is seen at higher frequencies (5 kHz and above). This iscommonly referred to as “Metal Loss”.In an existing building, an approximate amount of metal loss can be determined through a site survey (see the section on site surveys).By laying down a figure-8 test loop, then sending test tones through the loop and measuring the field strength with an FSM at differentfrequencies, you determine where the metal loss is occurring, at what frequencies, and how much field strength is being lost.Simply providing more power from the loop amplifier will not solve the problem of metal loss. In fact, supplying more power to theloop can make the problem worse by causing low-frequency distortion (due to too much power at these frequencies), while mid andhigh frequencies will then suffer from a lack of power. Williams AV amplifiers can correct for metal loss and low frequency adjusting theequalization built into the signal processing.Ultra Low OverspillCancellation LoopProviding more power at the frequencies where metal loss occurs can help, but it is only part of the solution. In terms of the loop designitself, tighter (narrower width) loops help control metal loss by lowering the variation of the magnetic signal, providing a more consistentmagnetic field throughout the loop area. More power from the amplifier will be required in all cases where there is metal loss, but the effectof metal loss to the listener, and the demand on the amplifier, can be reduced by a well designed loop array.One of the nice features of the PLA DL210NET is the Metal Loss Compensation setting. This parametric EQ provides attenuation/gainadjustment from 0 dB to 12 dB in the frequency range between 500 Hz to 9 kHz to aid in achieving flat frequency response in thepresence of metal."Table 2: AWG Chart" on page 14 shows maximum allowable loop wire lengths for different wire gauges.Contact Williams AV TechBlue (technical support) at 800.328.6190 (Ask for TechBlue) or [email protected] if you would like assistance oradvice on conducting a site survey.7
Induction Loop Design GuideDesign Requirements and Best PracticesThe loop system must meet performance requirements set by the international standard for loop systems IEC 60118-4. The system mustmeet these requirements for the loop system to be certified, and ensuring the loop system provides a benefit.NOTE: We recommend that all field strength meter measurements are taken at five (5) feet above the finished floor to account forboth seated and standing height(s).1. Background Noise 32 dB (reference 400 mA/m with a calibrated field strength meter, A-weighted, -20 dB scale)Background noise here refers to electromagnetic interference (EMI), or unwanted magnetic signal (noise), from sources of magnetic energywithin the room. Florescent lights, motors, incandescent lighting, dimmers, audio, video and projection systems and anything with a highamount of current running through it, can generate magnetic noise.With the loop amplifier turned off, and all possible EMI sources turned on (this includes all lighting, dimmers, audio, video and projectionsystems, etc.), EMI is then measured with a field strength meter relative to the 400 mA/m reference point. This energy should not be higherthan -32 dB from reference, anywhere in the area of use.Since EMI is usually generated by the environment, rather than the loop system itself, this can often be measured before a loop systemis ever installed. Part of the site survey includes the determination of how much magnetic noise exists in the room. Field strengthmeasurements are taken with the lights on, and all other typical equipment running as it would normally when the loop system would be inuse.Sometimes the high levels of EMI can be caused by bad electrical wiring, improper grounding, or motors (like a vacuum cleaner, etc.). Ifthe EMI source can be determined, and the cause is related to improper wiring/grounding, contact the venue owner and have them hire alicensed electrician to correct the problem. If the EMI is being generated by older lighting dimmer packs, or bad ballasts, the venue ownermay need to have these replaced prior to having the loop wire installed. When the loop area has been tested to be free of EMI, the loopwire installation can commence.NOTE: Testing for EMI should be done during the initial site survey and also on the day of installation of the loop wire, to ensure thatEMI levels are still acceptable.2. Field Strength 400 mA/m 3 dB @ 1 kHzAt this point the loop wire has been installed. Using the internal 1 kHz test tone in the loop amplifier, the magnetic field strength must be400 mA/m 3 dB throughout the area of use. The Digi-Loop product line has a built-in test tone generator for this purpose.3. Frequency Response 3 dB at 100 Hz, 1 kHz and 5 kHz (1 kHz Reference)Using the 400 mA/m at 1 kHz as the reference point, the field strength must not vary more than 3 dB when measuring at 100 Hz and 5 kHzwithin the area of use. In order for the human voice to be intelligible, the system must have flat frequency response between 100 Hz - 5 kHz(even though harmonics of the human voice can be outside of this range).Types of LoopsThere are five basic types of loop layouts for an induction loop system. The type of loop chosen depends on system requirements, metalloss (environment), and a thorough site survey that includes electromagnetic interference (EMI) testing.8
Induction Loop Design GuidePerimeter LoopPerimeter LoopPhased Array (LowPerimeter LoopPhased ArrayThis is one loop of wire around the perimeter of the room or listening area.A perimeter loop can be used in rooms/areas from 65 - 82 ft. (19.8 - 25 m) wide with no metal present, or rooms/areas 6 - 16 ft. (1.8 - 4.9 m)wide with minimal metal present. This type of loop is usually only possible for smaller rooms in modern buildings. If overspill is a concern, aLow OverspiCancellationLoopare typically installed on wood floors with wood beams, brick, or Ultraperimeter loop is generally not recommended.Perimeter loopsstone (nometal) in the supporting structure.Cancellation LoopUltra Low OvCancellation LoopThis is single run of wire overlaid in an offset figure-8 pattern, with one large loop, and a small loop on the side where spill needs to berestricted.This type of loop is used when spill needs to be controlled in one direction only.NOTE: Like a Perimeter Loop, the cancellation loop should be used on wood floors with wood beams, brick, or stone (no metal) in thesupporting structure.9
Induction Loop Design GuideSingleSingle ArrayArraySingle ArrayThis is a single run of wire, overlaid back and forth to create equal-width segments.This type of loop is often used when there are minimal to no metal losses in the room, as it provides more even coverage than a simpleperimeter or cancellation loop. This loop works well for fixed-seating applications. It is generally not recommended if listeners will bemoving.Phased Array (low overspill)PhasedPhased ArrayArray (Low(Low Overspill)Overspill)This is similar to a single array except there are 2 runs of wire, forming a primary array and a secondary array. These two arrays are overlaidin an offset pattern, each driven by separate amplifiers 90 degrees out of phase with each other.A phased array can be used in rooms/areas of various sizes where metal loss is present. This type of array controls overspill more tightlythan a single array. It also provides more Ultraeven coveragewithin the listening area. If people will be moving, a phased array may be sufficient,Ultra LowLow OverspillOverspilldepending upon the room/area.10
Induction Loop Design GuideUltra-low overspill phased arrayUltra Low OverspillThis is a phased-array with narrower segments.This type of array provides the greatest possible spill control and most consistent coverage within a room. An ultra-low spill phased arraycan be used in rooms/areas of various sizes where metal loss is present. This type of array also controls overspill around the entire looparea more tightly than a phased array - as close as 5 ft. (1.5 m) from the loop edge, and 12 ft. (3.7 m) above and below. This design isexcellent for confidentiality and adjacent loop systems. It is also excellent for metal loss.A570Loop DesignThe following design techniques are crucial to designing a successful induction loop system.Perimeter loop designThe following must be considered when designing a perimeter loop:LocationThe loop should be installed below the user. As long as there is no metal that may interfere with the loop, it can be installed on top the floor,as well as under carpet, linoleum or tile. Installations can also be done within cement flooring, with special considerations. Installing the loopabove the user should be avoided. The loop should never be installed at a user’s height.Metal InterferenceMetal can affect the sound quality of the loop. Metal structures, such as reinforced concrete or rebar, must be considered when placingthe loop. Metal loss is not always predictable, and a thorough site survey should be performed to ensure the loop will be efficiently placed.Different loop patterns, such as a phased array, may also help avoid metal interference. Contact Williams AV TechBlue (technical support) at800.328.6190 (Ask for TechBlue) or [email protected] for advice on avoiding metal interference or performing a site survey.Audio LeaksAudio from a perimeter loop will e able to be detected a notable distance from above, below, and around the looped room. This can affectaudio quality or other loops in surrounding rooms and hallways. For less audio leaks, a different pattern of loop should be used to containthe audio within a room.11
Induction Loop Design GuideFloor installCeiling insLoop AmplifierLoop AmplifierPerimeter LoopLoop DisplacementThe distance between the loop height and the listening (hearing aid) height is called Loop Displacement. The loop should be placedbetween 8 to 25 % of the width of the room, usually 2 ft. (0.6 m) and 8 ft. (2.4 m) below head height (about 4 ft.[1.2 m]). The loop could beplaced below the floor, LooprangingAmplifier1 ft. (0.3 m) to 4 ft. (1.2 m) beneath the flooring.Cancellation loop designCancellation loops are a normal perimeter loop with a smaller loop at one end, making an uneven ‘figure-8’ at one end. These loopsreduce loop audio leaks in one direction, which may be needed where there are adjacent rooms.Sizing the smaller loop is not an exact science, and will require trying several different sizes or a computer simulation. Williams AV TechBlue(technical support) at 800.328.6190 (Ask for TechBlue) or [email protected] will help you to design a cancellation loop.Cancellation LoopFigure 4: Cancellation LoopMainLoopCancellationLoopLoop AmplifierLimits overspillin this directionSingle ArrayLoop Amplifier12
LoopLoopInduction Loop Design GuideLoop AmplifierLimits overspillin this directionSingle array designSingle arrays are multiple smaller figure-8 shaped loops made from the same loop cable. In a single arrays, there must be Loop segments that are all the same width. A minimal amount of metal interference. Current flowing in the opposite directions to the adjacent loops.Figure 5: Single ArraySingle ArrayLoop AmplifierPhased array (low overspill) designLow loss arrays are best used for an even audio reception across a large area, or in an area with metal interference, such as concrete oversteel pan decking.Two loops with several figure-8 shaped segments that are driven by two amplifiers. A phase shifter sets the amplifiers to be 90 degreesout of phase with one another. The first array has segments of equal size that are separated by a gap. The second array’s equally-sizedsegments are placed in the gaps of the first array.The length of the longest side of the room, and the desired width of the loop segments are needed to design a phased array. A 4 in. - 8in. (10.2 - 20.3 cm) gap around the edge of a room is needed for proper quality. Segments can be between 6 ft. (1.8 m) to 12 ft. (3.7 m) wide,although metal interference can require that the segments be smaller.Call Williams AV TechBlue (technical support) at 800.328.6190 (Ask for TechBlue) or [email protected] to assist with the design work.Phased Array (Low Overspill)Figure 6: Phased ArrayPrimary LoopSecondary LoopPrimary Loop Secondary Loop Phased ArrayLoop Amplifierverspillrection13
Induction Loop Design GuideLimits overspillin this directionUltra-low overspill loop designUltra low-spill arrays require thoughtful design. Audio leaks can be restricted to 5 ft. (1.5 m) of the loop edge, and 12 ft. (3.7 m) above andbelow with the proper design tools and experience. This still may not be sufficient for adjacent rooms or maintaining confidentiality.You must provide dimensioned drawings and some basic information about the installation and Williams AV will return a fully marked upset of drawings (loop design) and comprehensive notes at no charge for our dealers, consultants and customers. Contact Williams AVTechBlue (technical support) at 800.328.6190 (ask for TechBlue) or [email protected] for assistance with the design.Figure 7: Ultra Low OverspillUltra Low OverspillTighter loop elements more spill controlCable Choices and SelectionLength LimitsA492Determine the total length of cable that needs to be driven by the amplifier in your system. For an array system, use the longest ofthe twoarray cable lengths. This typically will be the Primary Loop.Compare the length required against the maximum length for your Williams AV amplifier. For a maximum cable length to be used, theamplifier must be outputting close to its maximum current. Reference "Table 2: AWG Chart" on page 14.If you need help optimizing your amplifier’s output, contact Williams AV TechBlue (technical support) at 800.328.6190 (Ask for TechBlue) [email protected] for a more accurate calculation of amplifier capability.To select the proper wire for th
3 Induction Loop Design Guide Design Process Designing a loop system is a process with many steps. A successful outcome depends on how carefully each step is performed.
