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IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 42, NO. 4, NOVEMBER 2000 347

Modeling the Electromagnetic Response of HearingAids to Digital Wireless Phones

Robert E. Schlegel and F. Hank Grant

Abstract—Interference produced in hearing aids by the pulsedRF signal from digital wireless phones has become an increasinglyimportant issue to wireless phone manufacturers and serviceproviders, hearing aid manufacturers and users, and governmentregulatory agencies. Development and validation of a comprehen-sive model of the interaction would greatly benefit the efforts toachieve mutual electromagnetic compatibility (EMC). In orderto develop reliable accurate methods to measure hearing aidimmunity, an exact mathematical relationship must be demon-strated between the interference generated in hearing aids usinga dipole with a standardized test signal [the input referencedinterference level (IRIL)] and that produced by actual wirelessphones with various signal formats [the overall input referencedinterference level (OIRIL)]. A set of theoretical conversion factorshas been developed and applied to predict OIRIL interferencefrom the standard IRIL measured immunity value. A square-lawrelationship exists within the linear response region of the hearingaid such that each 1 dB increase in RF power (or field strength indecibels V/m) results in a 2 dB sound pressure level (SPL) increasein acoustic power (or sound pressure level). Hence, the IRIL forany given field strength is obtained by doubling (in decibels) thechange in field strength and adding the result to the referenceIRIL (in decibels SPL). Subtracting 7.60 dB [for time-divisionmultiple access (TDMA)-50 Hz] or 10.68 dB [for TDMA-217 Hz orcode-division multiple access (CDMA)] from the IRIL predicts thecorresponding OIRIL. The lower and upper limits of the predictedOIRIL are constrained by the measured ambient sound level andthe amplifier saturation, respectively. The model predictions arevalid when comparable field strength gradients and distributions,separation distances and orientations are maintained between thehearing aid and the RF emitter.

Index Terms—Acoustic interference, digital wireless phones,hearing aids, input referenced interference spectrum.

NOMENCLATURE

AGC automatic gain control;BTE behind-the-ear hearing aid;CDMA code-division multiple access;FOG full-on gain setting of hearing aid;GSM global system for mobile communications;ILM interference level in microphone mode;IRIL input referenced interference level;IRIS input referenced interference spectrum;ITC in-the-canal hearing aid;ITE in-the-ear hearing aid;OIRIL overall input referenced interference level;RTG reference test gain of hearing aid;

Manuscript received July 27, 1999; revised May 9, 2000.The authors are with the Center for the Study of Wireless Electromagnetic

Compatibility, School of Industrial Engineering, University of Oklahoma,Norman, OK 73019 USA (e-mail: schlegel; [email protected]).

Publisher Item Identifier S 0018-9375(00)10204-2.

SPL sound pressure level;TDMA time-division multiple access.

I. INTRODUCTION

A. Review of the Problem

A PPROXIMATELY 20% of the 65 million wireless phonesubscribers in the United States currently use digital

or dual-mode (digital/analog) phones. Research studies haveestablished that digital wireless phones using a pulsed transmis-sion format may generate audible interference in hearing aids,creating a buzzing or static noise that may seriously degradethe intelligibility of speech and be annoying to the hearingaid wearer. The challenging issue of making digital wirelessphone technology accessible to those who wear hearing aidsis taken very seriously by wireless phone manufacturers andservice providers, hearing aid manufacturers and users, andgovernment regulatory agencies.

The authors have developed a comprehensive model of theimpact of digital wireless phones on hearing aid usability. Themodel identifies: 1) those elements that predict the level ofacoustic interference produced at the output of the hearing aidas a function of the wireless phone exposure and 2) the impactof that interference on system usability. Development andvalidation of the various components of the model will advancethe promulgation of effective test methods and performancestandards for hearing aid and wireless phone products thatachieve mutual compatibility. The focus of the current article ismodeling the acoustic response of hearing aids to electromag-netic (EM) fields. Fig. 1 presents a symbolic representation ofthis response to an actual wireless phone versus a standardizedtest signal.

II. -FIELD VERSUS -FIELD RESPONSE

Hearing aids may be susceptible to either the electric () ormagnetic ( ) components of RF fields. The short wire leadsconnecting hearing aid components (e.g., from the microphoneto the first amplification stage) are of lengths that often formgood receptors for the RF energy generated by a wireless phone.The voltages and currents induced by the-fields and -fieldsare applied to the various amplifier stages of the hearing aid.With analog phones, these fields are constant in amplitude andproduce only dc offsets in the circuitry. For digital phones,which generate a pulsed carrier with a fundamental frequencyin the audible region, the RF energy is demodulated at theamplifier input junction. The detected signal, which resemblesthe original digital pulse envelope, is then amplified and filtered

0018–9375/00$10.00 © 2000 IEEE

348 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 42, NO. 4, NOVEMBER 2000

Fig. 1. Symbolic representation of EM response comparison.

by the remaining stages of the hearing aid to produce audibleinterference.

The magnitude of the acoustic interference is affected by anumber of wireless phone and hearing aid factors [1]. Phonefactors of particular relevance for predicting interference levelsinclude the transmission power (measured in milliwatts or deci-bels re: 1 mW), the resulting -field strength (volts per meteror decibels re: 1 V/m) and -field strength (amps per meteror decibels re: 1 A/m), the RF carrier frequency (typically 800MHz versus 1900 MHz) and the digital signal format (pulserepetition rate and duty cycle). In general, these characteris-tics are dictated by the standard that governs the specific phonesystem.

Although hearing aid interference can occur for some low-im-munity aids when the aid is exposed in the far-field of a radi-ating source, the real issue is near-field interference which ex-ists when a hearing aid wearer wishes tousea digital wirelessphone. This use places the phone within a few centimeters of thehearing aid and exposes the aid to strong EM fields. Predictingnear-field interference is complicated by the complex nature ofthe -field and -field components in the near field. The fieldsare highly varying in the vicinity of the phone antenna and areaffected by the proximity of the human head and hand holdingthe phone. In addition, some hearing aids are placed partially(ITE aids) or completely (ITC aids) within the ear canal. Thetissues around the aid may alter the signal from the phone andaffect its influence.

Okoniewski and Stuchly [2] examined the interaction ofwireless phone fields with hearing aids using a finite-differencetime-domain (FDTD) model in conjunction with a currenttransformer and magnetic resonance imaging (MRI)-derivedhigh-resolution model of the human head. While providing aninteresting model, the authors concluded that experimentationmust be performed to determine the value of their results inelectromagnetic interference (EMI) testing of hearing aids.

Skopec [3] conducted hearing aid measurements infree-space, but mentioned that the effects of the head mayattenuate the influence. In conjunction with Stuchly, Bassen[4] (with the FDA Center for Device and Radiological Health)is currently conducting experiments in this area and expects to

present results in the near future. Although modeling the RFattenuation effects of the head is important, adequate data arenot currently available. The model presented here representsworst-case conditions. Substantial agreement between clinicaldata and acoustic measurements leads us to conclude that theattenuation effects of the head are not sufficient to reduce theinterference to levels that are acceptable to hearing aid wearers.As results from other researchers become available, they canbe incorporated into the model presented here.

A given aid may demonstrate greater susceptibility to eitherthe -field or the -field. Unfortunately, determination of thedominant mode is difficult to make since an increase in phonetransmission power will typically be accompanied by a compa-rable increase in both the-field and -field levels. Many aidsmay also be susceptible in a hybrid fashion; that is, differentcomponents or regions of the aid may be more susceptible tothe -field, while other areas may be more susceptible to the

-field. This distinction becomes important when attemptingto model the responses of a mix of products (both phones andaids). One does not know whether the phone’s-field distri-bution or -field distribution is the determining factor for pre-dicting the interference produced in a particular aid which mayhave hybrid susceptibility. In the absence of such information, astrict -field model may be developed that will provide a goodfirst approximation of the interference prediction.

Although the relative orientation of the hearing aid and theRF antenna has a demonstrated effect on the level of the inter-ference, this effect is also somewhat unpredictable. Since theaid may be susceptible to either-field or -field interference(or both), rotation of the aid within the EM field may align cer-tain components that result in greater-field interference whilesimultaneously reducing -field interference. The complex na-ture of the fields prevents isolation of the two effects and the in-teractive nature of the combined effect has not been determined.The most that one may accomplish is to align the aid in such afashion as to produce maximum interference and use this mea-sured interference as the target of the prediction equation.

The field strength gradient is also of importance in the nearfield as it relates to the size of the receptor component in thehearing aid. If the gradient is extremely large in comparison with

SCHLEGEL AND GRANT: MODELING EM RESPONSE OF HEARING AIDS 349

Fig. 2. Sample plots of wireless phoneE-field andH-field emissions across 5 cm� 5 cm grid encompassing wireless phone speaker.

the receptor, then it becomes more difficult to determine the truefield to which the aid is exposed.

III. EM FIELD CHARACTERISTICS OFWIRELESSPHONES

Since field strength patterns in the vicinity of phone an-tennas vary greatly by design (dipole, monopole, foldedpatch, quadrifilar helix) or, by chance, it is important to haveaccurate measurements of-fields and -fields in the regionin which the hearing aid approaches the phone. These emissionscan measurements are typically obtained with a roboticmeasurement system employing relatively small-field and

-field probes. Fig. 2 presents a sample collection of-fieldand -field scans showing radiation patterns that are typicalof several different phones from a variety of manufacturers.The scans, conducted at Motorola’s Illinois facility using anET3DV4 -field probe and H2DV3 -field probe, cover a 5cm 5 cm grid surrounding the speaker area of the phone ata separation distance of 1 cm. Readings represent root meansquare (rms) values recorded at 0.5-cm increments in the–planes. Each phone was set to transmit at its maximum power(which may differ as a function of the specific technology andits corresponding standard).

A. Digital Pulse versus Continuous Wave

Because of the averaging nature of the probes used, the rawdata provided by the scan system represent an average across

the on and off periods of the digital phone pulse. However,the audio power of the interference is a function of the carrierpeak pulse power. There are three basic digital wireless phonesignal waveforms: TDMA (IS-136) with a 50-Hz pulse repeti-tion rate, TDMA (GSM and J-STD-007) with a 217-Hz pulserepetition rate (often called PCS-1900), and CDMA (IS-95 andJ-STD-008) with a varying pulse structure that changes as afunction of the puncture rate (eighth, quarter, half, and full).For TDMA-50 Hz phones, which have a 1/3 duty cycle, the rawscan data in V/m or A/m must be multiplied by (1.73) orincreased by 4.77 dB. For TDMA-217 Hz phones, which use a1/8 duty cycle, the scan data in basic units must be multipliedby (2.83) or increased by 9.03 dB. CDMA phones may bescanned in a “full rate” configuration that provides continuoustransmission and avoids the need to make any CW conversion todetermine the field strength during pulsing at any other CDMArate.

B. -Field Scans

Across the various phones that were scanned, maximum-fields ranged from 35 to 45 dB re 1 V/m RMS. As seen in

the scans, the -field can vary by as much as 14 dB acrossthe entire 5-cm-square scan area (although typical values wereapproximately 6 dB). Maximum field strength gradients rangedfrom 1.6 to 4.5 dB V/m per cm.


C. -Field Scans

Maximum -fields ranged from 23 to 8 dB re 1 A/mRMS. The -field varied by as much as 13 dB across the 25cm scan area, with typical variation ranging from 5 to 10 dB.Maximum field strength gradients ranged from 1.0 to 2.0 dBA/m per cm.

IV. RESPONSECHARACTERISTICS OFHEARING AIDS

A. Square-Law Relationship (SPL versus RF)

The audible interference in hearing aids is caused bysquare-law detection or rectification of the amplitude variationsin the RF signals appearing at the input of an amplifier [5], [6].The nonlinear characteristic of the input transistor produces thesquare-law detection, with the resulting amplitude variations(whether sinusoidal or pulsed) being amplified along with thedesired audio.

Square-law detection means that a 1 dB increase in RF power(or field strength) should result in a 2 dB increase in audio power(or sound pressure level in decibels SPL), although it is impor-tant to point out that this relationship holds only within the linearresponse range of the amplifier. In particular, this response willnot be observed for low-level signals, which produce interfer-ence levels masked by the ambient environment or the internalnoise of the hearing aid. Likewise, the relationship changes asthe RF level places the amplifier in the saturation region or acti-vates automatic gain control (AGC) circuitry. Data will be pre-sented later in this article to demonstrate the ambient noise re-gion, the square-law relationship in the linear response regionand the saturation region for a sample of hearings aids.

B. Acoustic Output

The normal acoustic output of a hearing aid is a function ofthe acoustic input at the microphone of the aid and the frequencyresponse of the aid. This gain response depends on the designand adjustment of the aid and modifies the output magnitude andspectrum of desired sounds and any interference (RF or other-wise) that is present. ANSI S3.22 defines the acoustic gain ofa hearing aid as “the difference between the output SPL in anearphone coupler and the input SPL” and specifies the measure-ment procedures for determining the frequency response curve[7]. The frequency response curve is the output of the hearingaid across a range of input frequencies for sound presented at60-dB SPL with the volume control set to the reference test gain(RTG) position for a linear aid and set to full-on gain (FOG) foraids with AGC and other signal processing.

C. Input Referenced Interference

In measuring and modeling the RF interference produced in anumber of hearing aids, it is helpful to quantify the interferencein units that are invariant to the frequency response (gain andfilter settings) of the aid. One approach is to convert the acousticoutput of the aid to an equivalentinput referencedsound pres-sure. This can be accomplished by subtracting the gain of the aidat each frequency for which a measurement is made (i.e., singlefrequency, octave, one-third octave, or one-twelfth octave bandmeasurements).

For a single frequency (e.g., 1-kHz sinusoidal 80% ampli-tude-modulated RF test signal), subtracting the gain of the aidat 1 kHz yields the input referenced interference level (IRIL indecibels SPL) as described in [5] and [6]. This equivalent mi-crophone acoustic input usually includes only the fundamentalresponse frequency (no harmonics) by using a one-third octavefilter at 1 kHz.

We introduce the input referenced interference spectrum(IRIS) as an extension of the IRIL. The IRIS represents theequivalent acoustic input for the entire spectrum of interferencefor a pulsed RF waveform. For a typical hearing aid with lowimmunity, Fig. 3 illustrates: a) the hearing aid response curve;b) along with the output spectra and IRIS graphs for CDMA;c) TDMA-50 Hz; and d) TDMA-217 Hz phone signals. Fur-thermore, the sound energy in this IRIS may be accumulated toprovide an overall input referenced interference level (OIRIL indecibels SPL), an unweighted (linear) summation (in pressureunits) that represents the interference from a specific wirelessphone. The OIRIL may be accurately estimated by includingonly three or four frequency components (fundamental and firstfew harmonics) [6]. For a given phone signal format, similarIRIS patterns were demonstrated across a variety of aids [8].However, the patterns differed as a function of the specificpulsed waveform for each of the various phone technologies asseen in Fig. 3.

The IRIL/IRIS approach to measuring interference relies onthe simplifying assumption that the RF reception and detectiontakes place in the first amplifier stage of the hearing aid. Al-though not exclusively the case, this assumption holds in thevast majority of hearing aids and can be confirmed by noting theeffect on the interference of changing the gain (volume) con-trol. Since the gain control follows the first amplifier stage, ifchanging the gain setting has the same effect on the interferenceas it does on an acoustic signal presented at the microphone, theassumption is valid. If this is not the case, multiple-stage inter-ference exists, and the modeling is further complicated by theinability to determine the contribution to interference made ateach stage.

V. EQUATING MEASUREMENTSUSING VARIOUS TESTSIGNALS

Although the interference of concern is produced by thepulsed nature of the digital phone signal, similar interferencecan be generated by an amplitude modulated carrier of the samefrequency. Thus, measurements of hearing aid immunity canbe made using actual wireless phones or using a dipole with asingle standardized test signal. This fact serves as the basis forthe test procedure adopted by the ANSI ASC C63.19 workinggroup developing aStandard for Methods of Measurement ofCompatibility between Wireless Communication Devices andHearing Aids[9]. For this approach to succeed, it is criticalthat the interference generated by the standardized test signal(IRIL) and the interference generated by a pulsed signal from avariety of formats (OIRIL) be related in a mathematically exactfashion and confirmed empirically.

Several groups have selected a 1-kHz sinusoidal 80% am-plitude-modulated RF carrier at 800–950 MHz (low band) or1800–1900 MHz (high band) as the standard test signal ([5], [6],


Fig. 3. Typical response for a low immunity hearing aid. (a) Gain curve.(b) Output and IRIS for CDMA. (c) Output and IRIS for TDMA-50 Hz.(d) Output and IRIS for TDMA-217 Hz.

[9]). This approach possesses the advantage of a simplified testprotocol resulting in a single immunity value at each frequencyband.

A. IRIL to OIRIL Conversion Factors

Using different terminology, [5] and [6] presented the theo-retical relationship between interference produced by a single-frequency, amplitude modulated carrier and a pulse modulatedcarrier with a 1/8 duty cycle (GSM or PCS-1900 phone tech-nology). A similar relationship was developed for CDMA phonetechnology at various puncture rates [10]. We have verified theequations, reconciled differences in terminology and extendedthe results to TDMA-50 Hz phones that use a 1/3 duty cycle.The output of the theoretical development is a set of conversionfactors that permit the prediction of OIRIL values for specificphone technologies based on the IRIL value obtained from thestandard 1-kHz test signal.

Assume that the unmodulated RF carrier in all cases is repre-sented by

(1)

where represents the rms carrier voltage (i.e., the carrierpeak is ).

Sinusoidal Amplitude Modulation:The modulating signal isrepresented by

(2)

If the modulation index , then the modulated wave-form is given by

(3)

Due to the square-law relationship between the input and outputof the amplifier , the output of the amplifier at anypoint in time will be proportional to the square of (3). Termsin the squared output equation can be grouped into dc terms,low-frequency terms (audio), and high-frequency terms (RF).The dc terms become

(4)

If (80% amplitude modulation), (4) becomesand the associated dc power becomes . The RF termsare effectively filtered out by low-pass filters in the hearing aidcircuitry, leaving the low-frequency fundamental and harmonicsas follows:

higher order terms (5)

The rms value of the first term (fundamental) becomes .For , this value becomes , with an associatedpower of or . The rms value of the secondharmonic is or . The power associatedwith the second harmonic is or . Ignoringthe higher order terms that make a negligible contribution tothe total power, it can be determined that the relative power


TABLE ICOMPARISON OFIRIL AND OIRIL FORTDMA-50, TDMA-217, AND CDMA

contributions of the dc term, the fundamental, and the secondharmonic are 56.7%, 41.6%, and 1.7%, respectively. Theeffective power in the audible frequency band (i.e., ignoring thedc component) represents 43.3% of the total power.

Pulsed Modulation:Similar results can be developed forpulsed modulation with a duty cycle of (proportion of timethe pulse is on). During each pulse, the RF signal is the sameas the unmodulated carrier in the previous example

(6)

The output of the amplifier during the pulse will be propor-tional to the square of (6). After filtering the high-frequencyterms (RF), the rectified pulse has a peak-to-peak amplitudeof and produces an average dc shift of across the fullon-off duty cycle. The dc component of the power is ,while the total power is .

By integrating the output waveform across the on and off por-tions of the duty cycle and using Parseval’s equation, the rmsvalue of the low-frequency (audible) term is obtained as

(7)

The rms audio power becomes . Alternatively, thiscould be determined by subtracting the dc power fromthe total power . The relative power contribution in theaudible frequency band depends on the duty cycle and is 87.5%for TDMA-217 with a 1/8 duty cycle and 66.7% for TDMA-50with a 1/3 duty cycle. CDMA at eighth rate has effectively a 1/8duty cycle and is thus similar to TDMA-217. A comparison ofaudio power for other CDMA puncture rates, including a typicalvariable rate, is provided in [10].

Table I compares the IRIL obtained from the standard 1-kHz80% amplitude-modulated test signal with the correspondingOIRIL obtained for digital phones with various duty cycles. TheIRIL is typically obtained using a one-third octave filter cen-tered at 1 kHz and, therefore, only the response at the funda-mental frequency is included. As an example, a 1-V/m unmodu-

lated carrier amplitude will produce an IRIL, which is 10.68-dB(SPL) higher than the OIRIL produced by a 1-V/m pulse witha 1/8 duty cycle (TDMA-217 and CDMA). Likewise, for a 1/3duty cycle (TDMA-50), the IRIL will be 7.60 dB higher thanthe OIRIL. Alternatively, a 1-V/m unmodulated carrier ampli-tude will produce an IRIL with the same numeric value as theOIRIL produced by a 1/8 duty cycle pulse with an amplitude of1.85 V/m (5.34 dB higher).

VI. M ODEL DEVELOPMENT

The model to predict OIRIL from an IRIL immunity mea-surement has three components: 1) determination of the fieldstrength needed to produce a specific IRIL using a standard1-kHz 80% amplitude-modulated carrier at a representative car-rier frequency; 2) determination of the field strength around theear piece of the wireless phone and a corresponding predictionof the IRIL that would be produced by that field strength; and3) a computational adjustment to compensate for the differencebetween IRIL and OIRIL for the specific phone technology inuse.

Due to the square-law effect mentioned earlier, there is a 2 : 1multiplication factor when converting from RF field strength indecibel units to sound pressure level in decibels SPL (whetherIRIL or OIRIL). Thus, within the linear region of hearing aidacoustic response to changes in field strength, the difference be-tween the phone emission scan field strength and the hearingaid immunity field strength at some reference IRIL (say 55 dBSPL) may be multiplied by two and added to the reference IRILto obtain the predicted IRIL. One must then subtract the appro-priate modulation signal compensation factor to convert fromIRIL to OIRIL, the overall level of interfering sound producedby the pulsed signal of the specific phone technology in ques-tion. As determined above, this compensation factor is 7.60 dBfor TDMA-50 Hz and 10.68 dB for TDMA-217 Hz and CDMA.

In reporting the immunity of a hearing aid, it is convenientto use the term ILM(IRIL), which represents the field strength( -field magnitude in V/m or dB V/m) that produces a specifiedinterference level in microphone mode. For example, ILM(55) isthe -field needed to generate an IRIL of 55 dB SPL. Using thisterminology, the IRIL for any given field strength () becomes

IRIL IRIL in dB SPL (8)

where-field strength (dB V/m) which generates an

IRIL of IRIL (should be in the linear regionof the interference response curve);IRIL (dB SPL) generated by (should bein the linear region of the interference responsecurve).

Alternatively,

ILM(IRIL) IRIL IRIL in dB V/m

(9)

After determining the predicted IRIL, the correspondingOIRIL is computed by applying the appropriate modulationsignal compensation factor obtained from Table I. If the


TABLE IIPREDICTEDIRIL AND OIRIL AS A FUNCTION OFHEARING AID IMMUNITY AND

WIRELESSPHONE EMISSIONS FORCDMA, TDMA-50, AND TDMA-217

phone -field is lower than the reference-field, the IRIL iscorrespondingly reduced from the reference IRIL. An exampleusing (8) follows. Table II presents predicted IRIL and OIRILvalues for various phone technologies as a function of hearingaid immunity (ILM 40 or ILM 55) and -field emissions.

Example: Based on an immunity measurement using thestandard test signal, let

IRIL dB SPL

V/m dB V/m

for V/m 46 dB V/m

IRIL in dB SPL

dB SPL

OIRIL(TDMA-50 Hz) in dB SPL

dB SPL

OIRIL(TDMA-217 Hz) in dB SPL

dB SPL

OIRIL(CDMA) in dB SPL

dB SPL

In determining correspondence between predicted OIRIL andmeasured OIRIL, keep in mind that there is an imposed lower

bound to the predicted value; that is, the measured ambientsound level. If the measured ambient is 40 dB SPL, then a pre-dicted level of 35 dB is not valid for comparison purposes. Itmay be valid for predicting the expected level if the ambientcould be further reduced, but there will still be a practical limitdue to the internal noise of the aid. Likewise, there is a potentialupper limit due to saturation of the amplifier or activation of theAGC. These limits must be carefully examined when attemptingto validate the prediction model. Fig. 4 presents the data fromTable II with the additional restriction of upper and lower limitsdue to the ambient noise floor and amplifier saturation, respec-tively.

VII. M ODEL VALIDATION

The validity of the prediction relies on a number of issues.First, it is assumed that the structures of the RF fields producedby the test dipole and the phone antenna are somewhat compa-rable in terms of the field distribution and field strength gradi-ents. It must also be assumed that the relative orientation andseparation distance between the hearing aid and the RF emitterare the same in all measurements. Hearing aid response can bequite sensitive to relatively minor changes in orientation, dueto the small size of the RF pickup elements in the hearing aidcircuitry. Likewise, at the typical 1-cm test distance, the fieldstrength gradient is high and slight changes in separation dis-tance can produce relatively large response changes. In the val-idation process, the separation distances of the field strengthprobes for the phone emission measurements, for the dipole inthe hearing aid immunity measurements, and between the phoneantenna and the hearing aid must all be carefully and uniformlymaintained at the same 1-cm distance.

There are two primary elements of model validation. The firstelement is confirmation of the square-law relationship betweenexposure field strength and measured acoustic output. Thesecond element is confirming the accuracy of the modulationsignal compensation factors and correspondence between thepredicted and measured OIRIL values. This paper presents val-idation of the first element with IRIL measurements obtainedwith a waveguide and OIRIL measurements using wirelessphones.

A. Waveguide Measurements

Fig. 5 illustrates the electronic apparatus used to obtain thewaveguide measurements. In addition, the following test itemswere used:

1) hearing aid A: in-the-ear (ITE) aid with linear gain andelectrostatic shielding;

2) hearing aid B: ITE aid with linear gain and EM shielding;3) hearing aid C: ITE aid with linear gain; self-shielding; and

a filtered microphone;4) FONIX Audio Test System (Frye Electronics Model

6000C).While applying a sufficiently large RF input signal to the

waveguide, each hearing aid was iteratively rotated through360 along three orthogonal axes to determine the alignmentthat maximized the interference response. Once the aid was


Fig. 4. Interference as a function ofE-field emissions and hearing aid immunity: (a) IRIL; (b) OIRIL for CDMA; (c) OIRIL for TDMA-50 Hz; and (d) OIRILfor TDMA-217 Hz.

Fig. 5. Equipment used to obtain waveguide measurements of interference using standard test signal.


Fig. 6. IRIL for (a) low, (b) medium, and (c) high immunity hearing aids measured using the NAL waveguide at RF field strength levels from 10 to 40 dB V/m.

positioned for maximum response, the field strength wasincreased from 3 V/m to 100 V/m and the acoustic interferenceat 1 kHz was measured at each of 12 RF levels. The interferencevalues were converted to IRIL values by subtracting the hearingaid gain at 1 kHz.

Fig. 6 clearly illustrates the square-law relationship betweenRF field strength in decibels re 1 V/m and IRIL in decibels SPLfor three in-the-ear (ITE) hearing aids, all with a nominal gainof 36 dB at 1 kHz. Across the linear region, the empiricallydetermined ratios of change in IRIL to change in field strengthare 2.1, 2.1, and 1.9, respectively for aids A, B, and C. It isalso easy to identify the ambient noise floor, linear responseregion, and saturation region and to see that these differ amongthe three aids as a function of each aid’s immunity and saturationlimit. A comparison of ILM45 values shows the clear distinctionbetween low (A: ILM45 10.7 dB V/m), medium (B: ILM4530.6 dB V/m), and high (C: ILM45 40.2 dB V/m) immunityaids.

B. Phone Measurements

By replacing the RF generator and waveguide with anactual phone, the effect of RF phone power on interferencecan be measured using the actual RF signal structure andantenna system. Hearing aid B was tested with a CDMA phoneoperating at eighth rate. Fig. 7 shows the change in (a) IRISand (b) OIRIL as a function of power output from 0 dBm to30 dBm. The correspondence between the increased spectrumlevels and the OIRIL is easily observed. Note the ambient noisefloor and linear (square-law) response regions as seen in thewaveguide IRIL measurements. Note also that at the maximumpower tested of 30 dBm [continuous wave (CW) power in thepulse], the aid has still not entered the saturation region.

For the phone used in this testing, a phone power of 23.5 dBmwould typically generate an-field strength of 150 V/m or 43.5dB V/m at 1 cm. Using the equations developed in this paper,the predicted IRIL for an -field of 43.5 dB V/m and an ILM45immunity of 30.6 dB V/m would be

IRIL in dB SPL

dB SPL

and

OIRIL(CDMA) in dB SPL

dB SPL

This value is consistent with the OIRIL in Fig. 7(b) for a phonepower of 23.5 dBm.

VIII. SUMMARY

In this paper, we have presented a fundamental model of theRF interference generated in hearing aids by digital wirelessphones. In particular, we have provided the mathematical re-lationship between the input referenced interference level ob-tained using a standardized hearing aid immunity test and theoverall input referenced interference level produced by a wire-less phone with a specific RF modulation envelope. In addition,sample -field and -field emission plots and hearing aid im-munity values were presented as the basis for predicting acousticinterference.

The square-law relationship between RF field strengthand acoustic interference was confirmed, but only within thelimits established by the ambient noise floor and hearing aidamplifier saturation. In addition, data were presented that showthe correspondence between RF waveguide measurements of


Fig. 7. Effects of a CDMA phone at output power levels from 0 to 30 dBm: (a) IRIS pattern for a medium immunity hearing aid (b); (b) OIRIL for low A andmedium B and C—high-immunity hearing aids.

IRIL and measurements of OIRIL using a CDMA 850 MHzphone. The square-law relationship has also been demonstratedusing TDMA-50 Hz and TDMA-217 Hz phones and a differentsample of hearing aids for which waveguide data were notobtained.

Although the model presented in this paper provided an ac-curate prediction of OIRIL for the example presented, furthervalidation must be conducted with a greater variety of hearingaids and wireless phones. Incorporating the impact of AGC andDSP circuitry in hearing aids and-field emissions data in the

prediction model will broaden the applicability of the modeland, thereby, increase its utility in the development of the ANSIC63.19 standard and the design and manufacture of compatiblehearing aid—wireless phone systems.

REFERENCES

[1] F. K. Kuk and K. H. Nielsen, “Factors affecting interference from digitalcellular telephones,”Hearing J., vol. 50, pp. 32–34, Sept. 1997.

[2] M. Okoniewski and M. A. Stuchly, “Modeling of interaction of elec-tromagnetic fields from a cellular telephone with hearing aids,”IEEETrans. Microwave Theory Tech., vol. 46, Nov. 1998.


[3] M. Skopec, “Hearing aid electromagnetic interference from digital wire-less telephones,”IEEE Trans. Rehab. Eng., vol. 6, June 1998.

[4] H. Bassen, “Personal communication with authors regarding FDAresearch activities in characterizing head attenuation effects,” unpub-lished, Nov. 1999.

[5] J. R. Le Strange, D. Byrne, K. H. Joyner, and G. L. Symons, “Inter-ference to hearing aids by the digital mobile telephone system, globalsystem for mobile communications (GSM),” Nat. Acoust. Lab., Aus-tralian Hearing Services, Chatswood, New South Wales, Australia, NALRep. 131, 1995.

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Robert E. Schlegelreceived the Ph.D. degree in industrial engineering withan emphasis in human factors engineering from the University of Oklahoma,Norman, in 1980.

As Professor of industrial engineering at the University of Oklahoma, heteaches and conducts research in human factors and ergonomics, with a focuson human cognitive performance and human-computer interaction. In his roleas Associate Director for Research at the Oklahoma Wireless EMC Center,Norman, he supervised research on the electromagnetic compatibility of wire-less phones and medical devices such as cardiac pacemakers, implantable car-dioverter defibrillators, and hearing aids.

Dr. Schlegel is a two-time recipient of SAE’s Ralph R. Teetor EducationalAward and was named Outstanding Engineer in Oklahoma in 1999.

F. Hank Grant received the Ph.D. degree from Purdue Univerisity, WestLafayette, IN, in 1980.

He joined the faculty at the University of Oklahoma, Norman, in December1993 as Director of the School of Industrial Engineering and Southwestern BellProfessor. He currently serves as Director of the Center for the Study of WirelessElectromagnetic Compatibility at that university. Prior to joining the Universityof Oklahoma, he was with the National Science Foundation, Washington, DC,where he directed programs in production systems, engineering design, and op-erations research. Previously, he had served as Director of the Measurementand Manufacturing Systems Laboratory, Hewlett-Packard (HP), Palo Alto, CA.Before joining HP, he was involved in the startup and development of two in-dustrial engineering software businesses—Pritsker Corporation and FACTROL,West Lafayette, IN.