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Author
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Topic: HI/VI Induction neckloop recommendations?
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Harold Hallikainen
Jedi Master Film Handler
Posts: 906
From: Denver, CO, USA
Registered: Aug 2009
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posted 01-26-2019 11:32 AM
Here are my notes on neck loops:
Hearing Aid Compatibility
Wikipedia discusses the standards for hearing aid loops. Generally, over a 60 second period, the magnetic field should average 100mA/m. The instantaneous field should be about 12dB higher, or 400mA/m.
IRH-230 magnetic fields were measured with a FoSmeter+. The signal was full volume noise (no IR signal present). This is similar to the peak loudness obtained from these headphones. Note that the neckloops had a higher magnetic field than the meter could measure. Therefore, the receiver volume control was turned down to yield a field of 0dB with the first neckloop. With the volume all the way up, the field exceeded 16dB. The dB levels for the other neckloops are referenced to the first one. For example, the NoizFree read +3dB from this new reference. With the volume at full, the field would be greater than +19dB.
Headphone fields were measured with the meter oriented for the maximum reading. Neckloop fields were measured with the meter standing vertically on a table in the center of the loop with the loop also on the table.
An approximation of loop impedance was measured by driving the loop with 400Hz through 12 ohms. The voltage across the resistor was measured (VR), and the voltage across the loop was measured (VL). The loop impedance was calculated as Z = 12 * (VL/VR).
Description Field Strength dB ref 100mA/m Loop Impedance Ohms Center of headphone +12 . Behind headphone -3 . Unmarked neckloop >+16
Headphone output reduced to yield 0dB for neckloop reference 34
Comtek NTC-102 neckloop +3 dB ref unmarked >19dB ref 100mA/m Z=36 ohms
NoizFree +3 dB ref unmarked >19dB ref 100mA/m Z= 27 ohms
Sennheiser EZT1011 +3 dB ref unmarked >19dB ref 100mA/m Z= 33 ohms
Unmarked neckloop 0 dB ref unmarked >16 dB ref 100mA/m Z= 34 ohms
Other Comments
The neck loops and field strength meter were provided by Connections Unlimited. In a telephone conversation with them, they said the neckloop impedance may vary between 16 and 28 ohms. They also said that the loops are typically driven with 35mW.
An IRH-230 with no IR noise at full volume was wired to drive a 36 ohm resistor. 230mV was measured, giving an output power of 1.4mW. When driving a 12 ohm resistor, 170mV was measured for a power of 2.4mW.
We have done some hearing aid testing with MR. In previous tests, the T-coil in his hearing aid was activated with the IRH-230 over his ear or behind his ear. In testing of 7/23/12, his T-coil was not activated with the IRH-230 over the ear or behind the ear. It also was not activated with any of the neckloops driven by the IRH-230. It was activated by a telephone. Measurements on the HH office telephone showed a maximum field of 0dB referenced to 100mA/m. On 8/9/12, measurements were done on an IRH-230 using program material (DTS demo disk through JSD-60 and IRC-28C). Voltage was measured with the voltmeter function of the Leader LDM-171. Results are in the table below.
Description Measured Voltage Power Noise into 12 ohms 250mV 5.2mW Signal Peaks into 12 ohms 400mV 13.3mW Noise into 36 ohms 500mV 6.9mW Signal Peaks into 36 ohms 600mV 10mW
The noise voltage measurement is about twice the previous reading. The noise voltage was rechecked with the Fluke digital voltmeter. It still reads about 250mV. So, it appears the Fluke is not good at measuring audio.
Based on the latest readings, we are delivering a peak of 10mW into a 36 ohm loop. The 600mV peak is not substantially above the 500mV noise level (no carrier), about 1.6dB above the noise level. The magnetic field readings done previously would still be valid. Magnetic fields are supposed to average 100mA/m and peak at 400mA/m (+12dB).
Measured the unmarked neck loop field strength at 6 inches above the loop (loop flat on table) with an IRH-230 full volume no-carrier noise. Measured level was +3dB reference 100mA/m. Running program audio into the IRC-28C in the conference room resulted in a magnetic field of +12dB 6 inches outside the loop. hh 8/13/12
We've done testing with MR for hearing aid compatibility. He has a Nuear Imagine SV, which has automatic T-coil switching.
Automatic switching is based on the presence of a DC magnetic field which is present with a telephone and our headphones due to the permanent magnet in each. However, a neck loop will not generate a DC magnetic field, so his hearing aid will not be tripped. In tests on 8/14/12, his hearing aid went into T-coil mode when an IRH-230 was placed near his ear, whether the IRH-230 was powered on or not. When the T-coil was enabled, he did hear audio through it, but said that it sounds better using acoustic coupling instead of magnetic. This brochure from the Hearing Loss Association of America has a note saying "Automatic telecoils are available but work only with telephones, not hearing loops, so ask your audiologist or hearing instrument specialist to include a manually-operated telecoil in your hearing aid and ask for advice on how to use it." HH 8/14/12
HH talked with local audiologist KS. She said most hearing aids can be programmed to allow use with neck loops (not require a permanent magnet). She said that most of her customers just use the microphone input instead of the T-coil. I asked what the experience of her customers is at the movies. She said that they generally do not need their hearing aid since the sound is so loud (or too loud). But, headphones, neck loop, or closed captions could still be useful to understand the dialog.
We tested headphones driving a neck loop and listening with AudioLink-MI model 2 magnetic induction receiver. 6 inches above the loop, we got good audio quality and a field strength in the 100mA/m to 400mA/m area.
Harold
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