Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 62489-2:2014 — Audio-Frequency Induction Loop Systems for Assisted Hearing — EMF Exposure Assessment
Electroacoustics — Audio-frequency induction loop systems for assisted hearing — Part 2: Methods of calculating and measuring the low-frequency magnetic field emissions from the loop for assessing conformity with guidelines on limits for human exposure
Why This Matters
The shift from current density to induced electric field as the basic restriction metric means that loop installers now have a more straightforward path to compliance, reducing the need for complex tissue modeling.
The shift from current density to induced electric field as the basic restriction metric means that loop installers now have a more straightforward path to compliance, reducing the need for complex tissue modeling.
Design Consideration
When installing large area loops, the magnetic field at the reference point (typically 1.2 m above floor level) must be calculated considering the loop perimeter, number of turns, and the drive current. The standard provides explicit formulae for rectangular and circular loop geometries.
When installing large area loops, the magnetic field at the reference point (typically 1.2 m above floor level) must be calculated considering the loop perimeter, number of turns, and the drive current. The standard provides explicit formulae for rectangular and circular loop geometries.
Engineering Insight
For system designers, the most practical takeaway is that the induced electric field metric (used in ICNIRP 2010) is significantly easier to evaluate than the previous current density metric. The reference levels for magnetic flux density have increased, meaning that most properly designed induction loop systems will comply without additional shielding.
For system designers, the most practical takeaway is that the induced electric field metric (used in ICNIRP 2010) is significantly easier to evaluate than the previous current density metric. The reference levels for magnetic flux density have increased, meaning that most properly designed induction loop systems will comply without additional shielding.
Introduction to IEC 62489-2 and Induction Loop Systems
Audio-frequency induction loop systems have become a cornerstone of assistive hearing technology in public venues, auditoriums, places of worship, and service counters worldwide. These systems operate by generating a magnetic field within a loop of wire, which is then picked up by the telecoil (T-coil) inside hearing aids or cochlear implant processors. IEC 62489-2, published in 2014 as the second edition, specifically addresses the methods for calculating and measuring the low-frequency magnetic field emissions from these loops to assess conformity with human exposure guidelines.
The 2014 revision was necessitated by significant changes in the ICNIRP (International Commission on Non-Ionizing Radiation Protection) guidelines, which shifted the underlying metric from tissue current density to induced electric field. This change fundamentally altered how exposure limits are calculated and applied, resulting in a considerable simplification of the assessment process for induction loop installations.
Loop Configurations and Measurement Methods
IEC 62489-2 categorizes induction loops into four main configuration types:
- Large area loops — smallest dimension larger than 1 m, typically installed at floor level in rooms and auditoriums
- Medium-area loops — dimensions on the order of 1 m, often vertically oriented at service desks
- Small area loops — largest dimension less than 1 m, including portable systems and clipboard loops
- Solenoid antennas — including ear-hook and neck-loop designs
The standard provides detailed calculation methods for the magnetic field at a reference point, taking into account the loop geometry, drive current, and operating frequency. The frequency range of interest spans from 100 Hz to 5 kHz with a wide relative bandwidth. A key characteristic of induction loop signals is their high peak-to-average ratio (at least 4:1), which necessitates special consideration in exposure assessment.
Compliance Assessment and Practical Implications
The compliance assessment method in IEC 62489-2 consists of two parallel approaches: calculation-based prediction and in-situ measurement. The calculation method uses the rated loop current and geometry to predict the magnetic field at the reference position, while the measurement method uses a calibrated field probe to verify actual emissions.
A critical aspect addressed in the standard is the difference between the signals produced by induction loops and those typically considered in general EMF exposure standards. Induction loop signals exhibit: wide relative bandwidth (5 kHz to 100 Hz ratio), no predominant frequency within the band, rapidly varying amplitude, and a high peak-to-average amplitude ratio. These characteristics mean that standard EMF measurement techniques may not be directly applicable without the modifications specified in this standard.
| Loop Type | Typical Dimension | Common Application | EMF Assessment Method |
|---|---|---|---|
| Large area | >1 m | Auditorium, theater | Calculation + measurement |
| Medium area | ~1 m | Service desk, counter | Calculation preferred |
| Small area | <1 m | Portable, clipboard | Measurement preferred |
| Solenoid | Varies | Neck-loop, ear-hook | Measurement required |
Frequently Asked Questions
Q: What is the main change in the 2014 edition of IEC 62489-2 compared to the earlier version?
The fundamental change is the shift from tissue current density to induced electric field as the basic restriction metric for human exposure assessment, aligning with the ICNIRP 2010 guidelines. This simplifies compliance evaluation for induction loop systems.
Q: What frequency range do audio-frequency induction loop systems operate in?
These systems operate in the range from approximately 100 Hz to 5 kHz, with a wide relative bandwidth. The standard addresses the unique characteristics of these signals compared to typical EMF exposure scenarios.
Q: Do I need to perform both calculation and measurement for compliance?
Not necessarily. The standard provides both methods as parallel options. For many installations, particularly large area loops with well-defined geometry, calculation alone may suffice. For complex installations or verification purposes, in-situ measurement is recommended.
Q: How does the peak-to-average ratio of induction loop signals affect EMF assessment?
Induction loop signals typically have a peak-to-average ratio of at least 4:1. This means that using simple RMS-based measurements may underestimate peak exposure, and the standard provides specific guidance for handling this characteristic.
