The Federal Communications Commission (FCC) initiated work on developing standards for compatibility between wireless communications devices and hearing aids in 1995. An early standard, ANSI C63.19-2001, entitled “American National Standard for Methods of Measurement of Compatibility between Wireless Communications Devices and Hearing Aids,” was published by an FCC committee in 2001, and several versions have followed.
The ANSI C63.19 standards apply to both wireless communications devices and hearing aids, and set forth standardized methods for measurement and parametric requirements for ensuring operational compatibility and accessibility of hearing aids used with wireless communications devices, including cordless, cellular, personal communications service (PCS), and Voice over Internet Protocol (VoIP) phones, operating in frequency bands ranging from 800 MHz to 3 GHz.
In particular, the standards describe operating parameters for hearing aids operating in either an acoustic coupling mode or a telecoil (T-Coil) coupling mode mode. The standards includes techniques for measuring the following for wireless communications devices: RF electric-field emissions, RF magnetic-field emissions, T-coil mode magnetic-signal strength in the audio band, T-coil mode magnetic-signal frequency response through the audio band, and T-coil mode magnetic-signal and noise articulation index.
Electromagnetic emissions from wireless communication devices can interfere with hearing aid reception of a telecoil signal. A telecoil signal is typically transmitted via inductive coupling from a loop transmitter or a speaker to a telecoil (T-coil) designed to receive such inductive signals in an audio frequency band (e.g., a frequency band ranging from 300 to 3400 Hz). The standards also provide tests that can be used to assess the electromagnetic characteristics of hearing aids and wireless communications devices.
In telecoil mode, a hearing aid or other device detects a varying magnetic-field emitted by a speaker coil or specialized telecoil transmitter coupled to a wireless communications device. Using inductive coupling to transmit a signal is in contrast to earlier hearing aid devices, which simply amplify a received acoustic signal. Hearing aids operating in acoustic mode have performance issues, especially when operating in loud, crowded environments. Some hearing aids are able to operate in both an acoustic coupling mode and a T-coil mode, where the mode used is selected by a user (e.g., by pressing a switch or button on the hearing aid) or selected automatically (by detection of a suitable signal with a T-coil). By using a T-coil receiver, reception issues associated with amplifying an acoustic signal received using a microphone (e.g., audio feedback) can be minimized or avoided.
Several parameters can vary the amount of current, signal quality, and transmission distance achieved using a T-coil transmitter. For example, length of the coil, diameter of the coil, and the number of windings affect these performance parameters. For designs incorporating a speaker voice coil as the T-coil transmitter, aspects of the design of the nearby speaker magnet (e.g., magnet geometry or iron content) impact the shielding of the T-coil transmitter by the speaker magnet. In addition, the distance between a T-coil transmitter and receiver, as well as interference from nearby electrical systems (e.g., noise from internal signals in a smartphone or transmitted signals in an RF band, such as TDMA or GSM bands), also have an effect on the intensity and quality of the received electromagnetic field strength.
FIGS. 1A and 1B are a top view 100 and side view 130 of a wireless communication device (a cell phone 102) indicating various locations of telecoil receivers during experimental testing according to the ANSI C63.19-2007 standard. The cell phone 102 includes a liquid crystal display (LCD) 105, a microphone 109, and several holes in the cell phone housing that form an acoustic speaker port 108. An ear coupling cup 109 surrounds the speaker port 108. Not visible inside the housing of the cell phone 102 are a speaker 110 and a telecoil transmitter 112, whose locations are indicated by dashed circles. Magnetic field intensity (H-field) measurements are made by a telecoil receiver at the measurement reference point 120 indicated by an “x.” Also shown are several test positions for a telecoil receiver, including an axial position 122, and two radial test positions 124 and 125. A telecoil receiver axis 140 used while the telecoil is in the radial test positions 124 and 125 is also shown. The measurement reference point 120 is at the intersection of a reference plane 142, which is located 10 mm above an outermost portion of the ear coupling cup 109 and the telecoil receiver axis 140. Notably, the telecoil transmitter 112 is placed at a distance above the LCD 105, in order to avoid the LCD from interfering with the inductive signal transmitted by the transmitter along the telecoil receiver axis 140.
FIGS. 2A and 2B are charts 200 and 250 that show the telecoil frequency response specified by ANSI C63.19-2007. The chart 200 in FIG. 2A is for wireless devices having a field strength attenuation of less than −15 dB A/m at 1 kHz. The X-axis is a log scale axis, showing frequency response over a range from 100 Hz to 10 kHz. The Y-axis indicates the magnetic field frequency response, in dB relative to the value at 1 kHz, ranging from −20 to +20 dB. A series of line segments 210 indicates the upper bound of the acceptable range, and the second series of line segments 211 indicates the lower bound of the acceptable range. Thus, the field intensity of the desired signal is measured at the center of an audio band (1 kHz), and subsequent measurements of the received field intensity are desirably located within the upper and lower bounds indicated across an audio band of 300 to 3400 Hz.
Similarly, FIG. 2B is a chart 250 of the frequency response specified by ANSI C63.19 for wireless devices having a field strength attenuation of greater than −15 dB A/m at 1 kHz. As shown, a first series of line segments 260 indicates the upper bound of the acceptable region, and a second series of line segments 261 indicates the lower bound of the acceptable region. The scales of the X- and Y-axes are the same as those in FIG. 2A. Similarly as in FIG. 2A, the field intensity of the desired signal is measured at the center of an audio band (1 kHz), and subsequent measurements of the received field intensity are desirably located within the upper and lower bounds indicated across an audio band of 300 through 3400 Hz.
FIGS. 3A and 3B are charts 300 and 350 for the cellphone 102 shown in FIGS. 1A and 1B. The measured magnetic field strength is plotted along the Y-axis over the corresponding applied transducer voltage, which is shown along the X-axis. The chart 300 in FIG. 3A indicates measured magnetic field strength for a telecoil receiver oriented in a radial position as shown in FIGS. 1A-1B. A line 310 indicates the minimum measured field strength as defined by the HAC ANSI C63.19-2007 specification. As shown, a line 320 corresponds to a telecoil receiver in a radial position (e.g., at radial position 124) at a distance of 11 mm from the reference plane 142. A line 321 corresponds to a radially-positioned telecoil at a distance of 13 mm. Finally, a line 322 corresponds a radially-positioned telecoil at a distance of 13 mm. As shown, the measured magnetic field strength decreases as the distance between the telecoil receiver and the telecoil transmitter 112 increases.
The chart 350 in FIG. 3B indicates a measured magnetic field strength for a telecoil receiver oriented in an axial position 122 as shown in FIGS. 1A-1B. A line 360 indicates the minimum measured field strength as defined by the HAC ANSI C63.19-2007 specification. As shown, a line 370 corresponds to a telecoil receiver in an axial position 122 at a distance of 11 mm from the reference plane 142. A line 371 corresponds to a radially-positioned telecoil at a distance of 13 mm. Finally, a line 372 corresponds a radially-positioned telecoil at a distance of 13 mm. As shown, the measured magnetic field strength decreases as the distance between the axial-positioned telecoil receiver and the telecoil transmitter 112 increases.
As wireless communications device form factors shrink and the number of features and functions incorporated in these devices increases, developing wireless devices that deliver a desired form factor and functionality while also complying with hearing aid compatibility standards is increasingly difficult. Therefore, there exists ample opportunity for improvement in technologies to realize wireless devices having desirable form factors, functionality, and HAC compatibility.