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A Practical Guide to CROS/BiCROS Fittings

A Practical Guide to CROS/BiCROS Fittings
Donald Hayes, PhD
July 24, 2006
This article is sponsored by Unitron.
Intended Audience

There are Audiology groups in most large urban centers affiliated with teaching hospitals or large scale otolaryngology services. The clinical populations of such groups are often skewed toward a larger than average subset of individuals who have "unaidable" unilateral hearing loss (Sandlin, 1994). In general, the term "unaidable" is used here to mean an ear for which no form of amplification, air conducted or bone conducted, will provide any meaningful benefit. Clinicians who work in such facilities often have considerable expertise in fitting and assessing Contralateral Routing of Signal (CROS) and Bilateral CROS (BiCROS) hearing aid fittings. They may practice these fittings on a weekly or monthly basis and consequently they have developed their own system for testing them.

Consequently, most clinicians may see as few as two to four people in an entire year who have one truly unaidable ear. These types of cases are seen so infrequently that clinicians may not be at ease when fitting or evaluating them. If this describes your caseload, this paper has been written for you. It begins with a statement of the goals for each type of fitting followed by a brief set of evaluation instructions and concludes with a selection of tips, tricks and traps. It has been written in outline form as a clinical aid to help the reader quickly locate sections that are relevant for a specific fitting. As mentioned above, this guide is most applicable for clinicians who do not fit CROS/BiCROS systems on a frequent basis.

Fitting Goals

CROS devices can be placed into one of three categories: wired, wireless and transcranial (Ericson, Svard, & Hogset, 1988; Hayes, 2001; Hayes & Chen, 1998; Hayes, Pumford, & Dorscher, 2005; Pumford, 2005). For the most part, the challenges of fitting wired and wireless CROS aids are about the same. They will be discussed as one type of fitting with the distinctions between them being mentioned where appropriate. BiCROS devices are limited to wired and wireless air conduction hearing aids as transcranial fittings do not readily lend themselves to BiCROS applications. Appropriate candidacy and the expected benefits of each type of fitting have been described elsewhere (Hayes, Pumford, & Dorscher, 2005; Sandlin, 1994). However, here is a short outline to use as a quick reference.


  • Motivation - These systems all include some element of inconvenience beyond that of a traditional hearing aid. For this reason, regardless of the hearing loss configuration, any CROS/BiCROS fitting is doomed to fail unless the wearer is motivated to make it succeed. That motivation typically stems from the recognition of one or more listening situations where performance is compromised by their inability to attend to signals from the side of the unaidable ear. In other words, it does not matter how perfect a textbook example of the appropriate hearing loss I may have. If I do not regularly find myself in situations where my performance is compromised by my unaidable ear, I will not have the proper motivation to succeed with a CROS or BiCROS instrument.

  • CROS - used exclusively to provide awareness and audibility of sounds from the side of the unaidable ear (Ericson, Svard, & Hogset, 1988). The signal is routed to a normal hearing ear and therefore amplification is undesirable. The Power CROS is the only exception to this rule. See Tip 1

    • Wired & Wireless - One unaidable ear and one ear that is either normal or has at most a mild high frequency hearing loss.
    • Transcranial - One completely dead ear and one ear that is either normal or has at most a mild high frequency hearing loss.
    • Power CROS - a special case of the wired or wireless CROS. It is presumed that moving the hearing aid microphone to the other side of the head from the aided ear will allow greater headroom in the fitting before the onset of acoustic feedback. The purpose is exclusively to provide amplification with less feedback and not specifically awareness of sounds from the unaidable ear.
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  • BiCROS - provides appropriate amplification for the aidable ear while also improving awareness and audibility for signals from the unaidable side of the head.

    • The aidable ear must have a degree and configuration of hearing loss that is within the accepted fitting range of the hearing instrument. This means at least a mild loss in the low to mid frequencies, but may be as much as a severe hearing loss at any frequency.
    • An appropriate BiCROS instrument will function as a well fitted hearing aid for the aidable ear even without the input from the microphone on the unaidable side of the head.


The one common goal of all these fittings is to provide audibility and awareness of signals from the unaidable side of the head. However, realistic performance expectations may vary by device.

Assessment Techniques

The efficacy of a given fitting can be determined in a number of ways. They may include, in order from the objective to the subjective:

  • Acoustically using probe microphone measures
  • Functionally using speech and/or speech in noise
  • Experientially using questionnaires or careful follow-up
Instructions for each of these three approaches will now be provided.

Probe Microphone Measures

Acoustic measurements of CROS and BiCROS instruments using probe microphones have been described a number of times (Dillon, 2001; Mueller & Hawkins, 1992; Pumford, 2005; Tecca, 1994). The reader is referred to those publications for an exhaustive explanation of procedures. What follows is a quick overview of one approach they describe.

Speech Assessment

Probe microphone measurements are often the preferred method for assessing CROS/BiCROS systems. They are relatively quick and offer precision for frequency response shaping and gain adjustments. However, not everyone has access to a probe microphone system. Furthermore, they are only an indirect means of determining improvements for speech in quiet and in noise from the unaidable side of the head. (see Trick 4) Fortunately, there are other ways to obtain that information directly. Aided and unaided speech reception thresholds can demonstrate improvements in quiet or changes to SNR scores in noise. Once again, the steps are slightly different for CROS and BiCROS instruments.

  • CROS or BiCROS sound field

    • At least two loud speakers are required as well as a two-channel audiometer to control the relative levels of speech and noise from each speaker.
    • The loudspeakers should be placed from 90 degrees to 180 degrees azimuth apart relative to the listener.
  • CROS - Not only should signals from the unaidable side of the head sound the same as signals from the aidable side (See CROS objectives for probe microphone), but speech perception from the unaidable side should be measurably improved with the CROS aid in place.

    • Unlike probe microphone assessment, speech assessment can be readily applied to transcranial CROS fittings of either type (bone anchored or CIC) and it is undertaken the same way for all types of CROS aids.

    • In the simplest case, the head shadow is thought to account for approximately a 6 dB difference in speech reception threshold when presented from the unaidable side of the head. Functional gain measurement for speech in quiet will demonstrate efficacy for offside hearing.

    • Another way to demonstrate hearing aid benefit is by speech perception testing.

      • Obtain a speech perception score without the CROS aid while presenting the speech from a loudspeaker positioned at 45 degrees azimuth to 90 degrees azimuth relative to the unaidable ear.
      • Repeat the measurement with the CROS aid on.
      • Any difference in speech discrimination is due to the CROS instrument.
      • It is best if this test is done using soft speech for two reasons:

        • Soft speech from the unaidable side adequately replicates the most common complaint of typical CROS wearers.
        • If the CROS wearer has normal hearing on the better side, louder speech will be too audible even without the aid, thereby, creating a ceiling effect.

    • It is not very realistic to expect substantial benefit with a CROS aid in most types of background noise. The resources required for perceiving speech in a diffuse noise background (localization & figure/ground separation) cannot be provided by a device that only aids one ear. Those skills require binaural input. However, there are special (and common) circumstances where spatial separation of the noise and speech are inherent in the environment. A perfect case is in the car. If the wearer's better ear is facing outward toward the car window and the unaidable ear faces inward toward the radio and passengers, the situation becomes untenable at highway speeds. A simple speech in noise test may be used to assess the potential for benefit in such circumstances.


  • Transcranial CIC CROS - This type of fitting represents a very special case in that the assessment can often be done under headphones.

    • A detailed description for fitting transcranial CROS CIC's can be found on Audiology Online (Hayes, 2001). Briefly, the following can be done.

      • Place TDH headphones on the wearer in the unaided condition.
      • Present desired stimulus: pure tones, speech, etc. to the unaidable ear.
      • Adjust the input levels through the headphone to find thresholds at which the desired signal is audible in the better ear.
      • Repeat the procedure with the transcranial CIC in place.
      • The difference in scores demonstrate benefit or the lack thereof in the same way that the sound field measurements do.
    • Much greater spatial separation can be achieved under headphones than in the sound field, thus, eliminating participation of signals entering the aidable ear acoustically rather than via the transcranial route.
  • BiCROS - As stated previously, a BiCROS fitting is only as good as the hearing aid being fit and the approximation to target for an accepted hearing aid fitting formula (NAL-NL1 or DSL[i/o] for example).

      The first step in speech assessment is the assessment of the hearing aid itself without the additional microphone engaged.
    • Therefore, all of the measurement techniques described for a CROS fitting can be applied to a BiCROS fitting with one additional step.
    • Between the assessment of the unaided condition and the assessment for speech from the side of the unaidable ear is the assessment of the hearing aid on its own without the additional microphone. Without this step, it is not possible to determine how much benefit is provided by the additional microphone versus simply providing good amplification to an aidable ear. In other words, the clinician must determine:

    • Remember when you test the additional microphone in a BiCROS fitting, it is a test for signals from the unaidable ear. Spatial separation of the loudspeakers used for the aided versus BiCROS conditions is essential.

Experiential Assessment

The most subjective approach to CROS/BiCROS assessment is the use of diaries, questionnaires or a good thorough follow-up. It is also often the most meaningful. Probe microphones and speech tests are great tools, but they can be somewhat removed from the listener's reality when working with CROS or BiCROS fittings. To be precise, they are the best tools to fit and troubleshoot these instruments, as they give your fitting a fighting chance for success. But more than any other type of fitting, it is the wearer's needs and motivation in difficult listening situations and ultimately the device's ability to provide success and comfort in those situations that will carry the day. A good prefitting and post fitting assessment will provide more meaningful information than all of the objective measurements you can devise. Here are a couple of examples:

  • COSI - Client Oriented Scale of Improvement (Dillon, James, & Ginis, 1997)

    • Hearing aid wearers provide a hierarchical listing of listening situations that they find important and in which they are having difficulty.
    • The amount of difficulty for each situation is recorded as the amount of time that the person has difficulty without the aid.
    • The same situation is scored again with reference to their ability to perform with the aid.
    • Since the COSI does not contain a set of standard listening situations it is hard to compare aided performance across listeners.
    • However, the listening situations of most interest to the wearer of the aid are the ones that are being assessed. This is particularly important for people with unilateral hearing loss as their needs frequently differ somewhat from those queried in standardized questionnaires.
  • APHAB - Abbreviated Profile of Hearing Aid Benefit (Cox & Alexander, 1995)

    • This standardized questionnaire evaluates performance in four categories:

      • EC - Ease of Communication in quiet
      • BN - Background Noise
      • RV - Reverberation
      • AV - Aversion to loud sounds
    • The results can be evaluated by comparing aid and unaided scores or comparing aided scores to published norms. Unfortunately, I know of no norms for individuals with a unilateral unaidable ear.
    • This test is worth doing on a number of potential CROS candidates if only to demonstrate how truly heterogeneous a group they are. For an example, see (Hayes, 2001). They have a very wide range of unaided performance scores for a group of people with nearly identical hearing losses.
    • In general it is probably most acceptable to compare aided and unaided scores to determine the percentage of benefit provided by the CROS/BiCROS instrument.
  • Careful follow-up - Needless to say, following the results of the fitting is absolutely required, especially if this is only one of two or three CROS fittings you may do annually.



This paper is written as a guide to those who get little or no regular fitting experience with CROS/BiCROS systems. It is intended as a set of instructions to the fitter and it is laid out to allow quick access to information about each type of fitting at each step in the process. The purpose is more to provide a set of clinical cue cards than a rigorous scientific exploration of the topic. For more detailed information about any step in these instructions please see the relevant references. I hope you find it useful.

Tips, Tricks & Traps


Tip 1: To determine if a given hearing instrument is CROS or BiCROS, remember a CROS system will have only one active microphone and one active gain control. However, a BiCROS system will have two active microphones and two active gain controls. - back to article.

Tip 2: Unless signals from both ears are present, there can be no binaural summation. Therefore, a reduction in the audibility of soft speech from any azimuth may be a consequence of a large asymmetry, even when one ear has normal threshold sensitivity. - back to article.

Tip 3: In a multimemory BiCROS fitting, it is often desirable to set one memory in CROS mode. This may provide a surprising benefit for the wearer in circumstances where speech is coming predominately from the side of the unaidable ear while background noise is directed toward the better ear. Turning off the microphone on the side of the better ear may provide a 6 dB SNR advantage over the BiCROS or unaided conditions due to the head shadow effect. - back to article.

Tip 4: Just as an improvement in functional gain can demonstrate the efficacy of a CROS fitting, the absence of improvement may help diagnose complaints. For example, wireless CROS transmission strength is affected by a number of variables including the distance between the wearer's ears. When the gain of the microphone transmitter is maximized, there should be a demonstrable improvement in speech reception threshold for speech from the side of the unaidable ear. The absence of improvement confirms that the signal from the transmitter is not being provided to the aidable ear. A test box can not be used for this test. It cannot incorporate the actual transmission distance into the measurement. That can only be done on the wearer's head. - back to article.

Tip 5: You may use any type of speech and noise with which you are comfortable and for which you can get a clinically relevant result. However, proper speech in noise testing requires precise control of a lot of variables and it is best done using well designed and properly calibrated commercially available speech tests. Below is a list of a few recommended titles by way of reference to be used for this purpose:


Trick 1: It is appropriate and correct to help the wearer focus attention on those specific situations creating problems. But it is equally as important for the clinician not to try to overextend the technology. Even successful CROS users wear their aids exclusively in situations where they will provide benefit. Successful BiCROS users only prefer to wear their additional microphone when it is needed, while using just the hearing aid the rest of the time. It is right to encourage full time use. Also, it is wise not to push too hard. - back to article.

Trick 2: When purchasing a probe microphone system, make sure you buy one that allows you to enable a reference microphone on either side of the head relative to the probe microphone. Otherwise, you will have to do your measurements using the substitution method. This can be quite awkward in a busy clinic setting. - back to article.

Trick 3: Overwhelmingly, audiologists fit hearing aids to cochlear hearing losses. Most of those cochlear losses result in large part from outer hair cell damage. Hence, we tend to fit WDRC signal processing by default to replace the absent or damaged cochlear amplifier. However, the aided ear in a CROS fitting has an intact cochlear amplifier. Therefore, the preferred processing strategy should be linear, the gain applied should minimal and the MPO should be low. - back to article.

Trick 4: It is true that speech perception should also be improved regardless of the measurement technique, probe microphone or speech testing. However, it was only implied as an objective under probe microphone testing, while being explicitly stated under speech testing because it is directly measurable only when a speech stimulus is used. - back to article.

Trick 5: As clinicians, we like to assume that hearing aid wearers turn their heads in the direction of the speech they want to hear. If that were always true, no one would ever need a CROS or BiCROS hearing aid in quiet. They would just turn their better ear toward the desirable signal and all would be well. Obviously this is not true. People frequently lack that convenient control over their listening environment. Whenever someone complains about offside hearing, they are sending you a subtle hint that you are wasting your time testing their BiCROS aid using speech from 0 degrees azimuth. If they have the luxury of facing every speaker they meet or every alerting signal they hear, just fit them with a hearing aid and be done with it. But if they have a problem hearing sounds or understanding speech from the side of an unaidable ear, talk to them about BiCROS. Then test them with that in mind. Put the speakers from which the speech is emanating off to the sides and then see how they do. - back to article.


Trap 1: Compared to traditional hearing aid fittings, CROS devices have a low success rate. A number of factors converge to complicate these fittings:

  • CROS candidates do not perceive problems in most listening situations.
  • Most clinicians rarely fit CROS aids and therefore lack the confidence and experience they have with traditional aids.
  • Until recently, CROS fittings primarily used outdated technologies such as wires or analog signal processing.
Consequently, it is not unreasonable for a clinician with an 80% - 90% success rate using traditional amplification to obtain good results as little as 50% - 60% of the time with CROS. -
back to article.

Trap 2: Always remember that amplified sound is being applied to a normal or near normal hearing ear in a CROS fitting. The loss of one ear can severely degrade performance in background noise. If this is the primary listening situation for which the CROS aid is purchased, be prepared for a failure. CROS aids enable the listener to hear signals in their remaining ear that emanate from the unaidable side of the head, but they do not improve localization or figure/ground perception. Therefore, they do not add useful information in background noise. They just make it louder. This may lead to rejection of the device. - back to article.

Trap 3: The BiCROS microphone measurement described above will show you if the microphone on the unaidable ear is working. However, it will not always demonstrate the relative contribution of that microphone versus the one in the hearing shell. This is important because many new BiCROS wearers have difficulty balancing the inputs from the two microphones. Remember they are getting both signals in the same ear. They have no easy basis for direct independent comparison of the two inputs. In older analog applications the contribution of the additional BiCROS microphone was added to the existing signal from the hearing aid. But newer digital instruments mix the inputs from the two microphones before they are processed. Therefore, it is difficult to ascertain the relative contribution of each microphone when both of them are turned on. - back to article.

Trap 4: Speech in noise testing is one way to determine if the CROS aid (or BiCROS) will provide adequate benefit in background noises where the desired and undesired signals are already spatially separated. But they are not a good indicator of speech in noise performance over all unless a realistic and diffuse background noise is presented. Furthermore, speech testing in general may help to validate a fitting that is already mostly done. But it is very time consuming (especially when using noise) and not particularly useful for fine tuning adjustments. Therefore, overall speech testing should be used judiciously especially when assessing performance in noise with either a CROS or BiCROS fitting. - back to article.

Trap 5: A BiCROS instrument has two microphones and two user-adjustable gain controls. However, all of that sound is going into only one ear. New wearers of BiCROS systems often have difficulty balancing the inputs from both microphones. It is desirable for them to set their two gain controls to evenly match the signals from both sides of their heads. This provides the optimal audibility for signals from both sides. When a new wearer returns from a trial period it is best to physically check where their controls are set. This is particularly important if they are not finding the additional microphone helpful or even worse if they find it confusing. But never assume they have the controls set properly or that they can consistently set them properly unless you test them. Remove the aids, adjust the controls to the "Off" position and have them reset them in front of you. If they can not do it easily and consistently, some counseling will be required. - back to article.


Bentler, R. (2000). List equivalency and test-retest reliability of the Speech in Noise test. American Journal of Audiology, 9(Dec), 84-100.

Cox, R. M. & Alexander, G. C. (1995). The abbreviated profile of hearing aid benefit. Ear and Hearing, 16(2), 176-186.

Dillon, H. (2001). CROS, bone-conduction and implanted hearing aids. In H. Dillon (Ed.), Hearing Aids (pp. 434-450). New York: Thieme.

Dillon, H., James, A. & Ginis, J. (1997). Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefit and satisfaction provided by hearing aids. Journal of the American Academy of Audiology, 8(1), 27-43.

Ericson, H., Svard, I. & Hogset, O. (1988). Contralateral routing of signals in unilateral hearing impairment. A better method of fittings. Scandinavian Audiology, 17(2), 111-116.

Hayes, D. (2001, January 17). You Want to Put That CIC Where?? A Primer on CROS Fittings Using CICs. Audiology Online, Article 1572. Retrived July 11, 2006, from the Article Archives on or direct access via

Hayes, D. & Chen, J. (1998). Bone-conduction amplification with completely-in-the-canal hearing aids. Journal of American Academy of Audiology, 9(1), 59-66.

Hayes, D. Pumford, J., & Dorscher, M. (2005). Advantages of DSP instruments for wireless CROS fittings. Hearing Journal, 58(3), 44-46.

Mueller, H. & Hawkins, D. (1992). Assessment of fitting arrangements, special circuitry, and features. In H. H. Mueller, DB. Northern, JL. (Ed.), Probe Microphone Measurements: Hearing Aid Selection and Assessment (pp. 201-225). San Diego: Singular.

Nilsson, M., Soli, S. & Sullivan, J. (1994). Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise. Journal of the Acoustical Society of America, 95(Feb), 1085-1099.

Pumford, J. (2005). Benefits of probe-mic measures with CROS/BiCROS fittings. Hearing journal, 58(10), 34-40.

Sandlin, R. (1994). Fitting binaural amplification to asymmetrical hearing loss. In M. Valente (Ed.), Strategies for Selecting and Verifying Hearing Aid Fittings (pp. 207-227). New York: Thieme.

Tecca, J. (1994). Use of real-ear measurements to verify hearing aid fittings In M. Valente (Ed.), Strategies for Selecting and Verifying Hearing Aid Fittings (pp. 88-107). New York: Thieme.
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donald hayes

Donald Hayes, PhD

Director of Audiology at Unitron Hearing Ltd. in Kitchener, Ontario

Donald Hayes, Ph.D. has been an audiologist for 18 years. He is the Director of Audiology at Unitron Hearing Ltd. in Kitchener, Ontario. Don Hayes is the Director of Audiology at Unitron Hearing Ltd. in Kitchener, Ontario.

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