Tech Topic: CROS and BiCROS Systems | Hearing Review June 2014
By Francis Kuk, PhD; Petri Korhonen, MSc; BrYan Crose, BSc; and Chi-Chuen Lau, PhD
An asymmetric hearing loss may refer to any one or more of the following conditions:
1) Differences in audiometric thresholds between ears of greater than 30 dB HL;
2) Significant differences in speech identification scores between ears when tested at the most comfortable listening (MCL) level, in quiet or in noise; and
3) Differences in the tolerance for sounds (either from loudness or from distortion) between ears.1
The prevalence of this hearing loss configuration in adults is difficult to estimate partly from its varied etiologies and partly from the differences in the criteria used to estimate this condition. The management for this hearing loss configuration can also be controversial.
In this paper, we briefly summarize the current audiological thinking in the treatment of this patient group. The rationale and evidence to support the use of the Widex CROS system to manage this patient group is also provided.
Management Options for Asymmetric Hearing Losses
It is generally accepted that bilateral fitting is the norm in managing someone with a symmetrical hearing loss. The goal is to preserve as much of the binaural benefits as possible.2 To a large extent, the same philosophy is applicable for people with an asymmetric hearing loss unless evidence suggests otherwise.
Managing an asymmetric hearing loss may be most straightforward if we can ask three questions about the hearing loss:
1) Is the hearing in each ear normal or impaired?
2) Is the impaired ear aidable or unaidable?
3) Is the unaidable ear due to a technology limitation (such as insufficient gain for a profound hearing loss) or distortion at the ear/brain?
One normal-hearing ear, one impaired ear. An asymmetric hearing loss may include one normal-hearing ear (thresholds <20 dB HL across frequencies) and one impaired ear. The impaired ear may have any degree of hearing loss (from mild to profound) or any degree of distortion/tolerance such that it may be aidable or unaidable with amplification.
Although a mild-to-moderate degree of hearing loss is typically aidable, the audiogram is not a good predictor of aidability. Knowing if the speech understanding score is commensurate with the degree of hearing loss may be a better predictor of aidability. Amplifying an aidable ear with conventional amplification devices typically leads to improved audibility and speech understanding.
The impaired ear can be unaidable for two reasons: either the degree of hearing loss is so severe that conventional amplification is insufficient to bring about adequate audibility, or hearing is so distorted the wearer cannot accept the sound quality provided by the hearing aid on the impaired ear.
In the case of a profound loss, use of a Contralateral Routing of Signal (CROS) hearing aid to route sounds to the normal ear provides a good solution. Alternatively, the use of a unilateral, super-power hearing aid fitting with a deep, tightly occluding earmold may provide some benefits. In that case, amplification provided at the impaired ear creates a vibration so the cochlea of the opposite (normal) ear is stimulated through bone conduction. This is also called a Transcranial CROS (TC) fitting.1
The impaired ear is also unaidable when sounds (amplified or unamplified) result in significant distortion. Amplification only aggravates the condition, making its use unacceptable to the wearer.
An unaidable ear may express itself in any degree of hearing loss. Typically, it is accompanied by a speech identification score that is significantly poorer than what the audiogram suggests. A CROS hearing aid is typically the best solution.
Both ears impaired, with one better ear and one poorer ear. Both ears may be impaired in an asymmetric hearing loss, except that the degree of hearing loss, the speech understanding ability, and/or the tolerance for amplification between ears is significantly different.
When the degree of hearing loss in the two ears is up to a moderately severe/profound range with commensurate speech understanding abilities, both ears are likely aidable despite any asymmetry between ears. In such a case, the use of bilateral hearing aids has proven to be the most helpful in over 70-80% of cases.3
The choice is less certain when one of the ears is aidable and the other one is unaidable because of the severity of the loss. Obviously, the aidable ear should be fitted with the appropriate hearing aid. The unaidable ear may be fit with a BiCROS, where the signal from the profound ear is routed to the aidable ear for processing. An alternative is a trans-cranial CROS for the ear with the profound loss, making this essentially a bilateral fitting.
When the reason for the unaidable ear is due to perceived distortion in the unaidable ear, a BiCROS solution is the most appropriate.
[Click on figure to enlarge.] Figure 1. Six asymmetric hearing loss conditions and their management options.
CROS/BiCROS in the Management of Asymmetric Hearing Loss
The previous discussion proposes that a CROS/BiCROS may be the only solution if one of the ears in an asymmetric hearing loss is unaidable because of distortion.
CROS systems. A CROS system is used for someone with one normal-hearing ear and an unaidable ear. In a CROS system, the hearing aid on the unaidable side picks up the signal through the microphone on that side and routes it (through either a wire or wireless connection) to the receiving hearing aid on the normal ear. There is not a microphone on the hearing aid for the normal ear; thus, the routed signals from the unaidable ear are processed alone and transduced through the receiver. A CROS system has one microphone and one receiver separated into two hearing aid housings.
BiCROS systems. A BiCROS is used for someone with an aidable ear and an unaidable ear. It has a microphone on the hearing aid for the unaidable ear (transmitter side) and another one on the receiving hearing aid for the aidable ear. Thus, sounds are picked up on both sides (from transmitter and receiver). They will be summed, processed, amplified, and transduced through the receiver on the aidable ear. Thus, a BiCROS has two microphones but only one receiver.
Considerations for CROS/BiCROS
Ideal listening conditions. Because a CROS/BiCROS hearing aid transmits sounds from the side of an unaidable ear to the aidable (or normal) ear, the ideal situation where the system outperforms the “hearing aid on the better ear alone” condition is when a speech (or other desired) signal is presented to the unaidable (or poorer) ear. This is because speech presented to the unaidable ear is not audible to that ear. The same sound could reach the aidable ear, but it would be attenuated by the amount of the head shadow effect.4 By routing sounds directly from the side of the unaidable ear to the aidable ear, a CROS system or microphone eliminates the head shadow effect.
More challenging conditions. On the other hand, if noise (or an undesirable sound) is presented to the unaidable ear, the noise that was once inaudible would become audible to the better ear in a CROS arrangement. It may interfere with the speech sounds that are received on the side of the aidable (or normal) ear and reduce the signal-to-noise ratio (SNR). Thus, noise (or undesirable signals) presented to the side of the unaidable ear would not be an optimal condition for a CROS/BiCROS system.
This information is important to understand for at least two reasons. From the product perspective, it requires device manufacturers to have features within the CROS system to circumvent the potential degradation introduced by the CROS system in non-ideal listening situations. From the users’ perspective, the listener needs to understand the conditions where the use of the CROS may be counterproductive so they may develop coping strategies.
For signals that are presented directly in front, a CROS/BiCROS system should have minimal impact on the audibility of the sounds. However, the two microphone inputs in a BiCROS system would result in a louder input (3-6 dB) than a hearing-aid-alone condition.
Desirable Features in a CROS Hearing Aid
Because a CROS system transmits sounds from the unaidable ear to the aidable ear, the goodness of the transmitter, the technology used in the transmission, and the quality of the receiver are important determinants of sound quality, efficacy, and usability of the CROS system.
Transmitter. The transmitter in a CROS system picks up the input signals and transmits them to the receiving hearing aid. Thus, any components that affect the pickup and transmission affect the overall sound quality.
- Input stage saturation. Microphones used in analog hearing aids (including CROS) typically saturate below an input level of 115 dB SPL. In a digital hearing aid (and CROS system), the use of a 16-bit analog-to-digital converter (ADC) at the input stage means that many digital hearing aids saturate or start compressing at an input level lower than 115 dB SPL—sometimes as low as 96 dB SPL. When the CROS transmitter uses the same ADC, it will have the same problems as hearing aids with a low input stage. This could affect the sound quality (and intelligibility) of the transmitted sounds. Thus, a transmitter with a high input range is desirable for good sound quality. Kuk et al5 reported on the impact of a high input range on speech intelligibility in noise.
- An on/off switch and volume control (VC) on the transmitter. It was indicated earlier that a CROS system is optimal when speech is presented to the side of the unaidable ear. On the other hand, when speech is presented to the aidable (or normal-hearing) ear and noise to the unaidable ear, the once inaudible noise may become audible and the SNR decreases.
A convenient on/off switch on the CROS transmitter allows the wearer to deactivate the CROS/BiCROS system so no input from the side of the unaidable ear is transmitted to the aidable ear when the wearer judges the situation to be unfavorable for a CROS system. Similarly, a VC on the side of the transmitter allows the wearer to adjust how much sound is transmitted to the hearing aid receiver for a better balance of transmitted and direct sounds, and thus a better SNR. These two critical features allow the wearers better control of their listening environments.
- Directional microphone. In cases where noise is presented to the side of the unaidable ear, or when speech and noise are presented to the side of the unaidable ear (but spatially separated), a directional microphone on that side may further improve the overall SNR for the aidable ear.
Nature of transmission. The first CROS devices used a wired connection between the transmitter and the receiver to ensure the consistency of the transmitted signals. Unfortunately, many wearers objected to the arrangement because of cosmetics and flexibility/freedom. Newer forms of CROS devices use wireless transmission. This frees the wearers from the need to physically connect two hearing aids.
Despite this advantage, the quality of the wireless transmission affects the integrity of the transmitted signals. The sampling frequency that is used could affect the bandwidth of the transmitted signals, and the nature of the coding could also affect the stability of the transmission. In addition, how the signal is coded and transmitted will affect the current drain on the transmission. Kuk et al6 provided a summary of the considerations affecting the quality of wireless transmission, and they are as relevant for hearing aids as they are for wireless CROS devices.
Receiver. The CROS receiver picks up the transmitted signals and amplifies them accordingly. In essence, it serves as a traditional hearing aid. All the factors that affect the performance and quality of a hearing aid would affect the performance of the CROS receiver. This includes sophisticated signal processing features to process the transmitted sound, and a broad receiver to output the sounds (see Kuk et al7 for a summary about factors related to sound quality).
A Unique CROS System
Figure 2. Widex CROS transmitter (left) and the DREAM hearing aid styles that can be paired to form a CROS/BiCROS system.
The Widex CROS is part of the Widex wireless assistive device series (DEX system); it is a stand-alone transmitter device that can be paired with any wireless DREAM hearing aids to form the CROS/BiCROS hearing aid system. This includes all DREAM hearing aid styles at all price points (440, 330, 220, and 110, as shown in Figure 2).
The Widex CROS takes the appearance of the DREAM Fashion miniature BTE. When worn, it is coupled to a thin tube and an open-ear insert to maintain wearer comfort. Widex CROS integrates the WidexLink wireless platform6 and the DREAM True Input Technology platform. WidexLink uses bit-true transmission with a sampling frequency of 25 KHz so that input frequencies up to 12 KHz can be sampled and transmitted accurately. In addition, many features to maintain the security and stability of the transmission are implemented in the Widex CROS for stable and consistent sound transmission. Widex CROS uses near-field magnetic induction (NFMI) with a carrier frequency of 10.6 MHz for safe and stable wireless connection. PowerSave technology also is utilized in the Widex CROS such that the typical current drain of the transmitter is only 0.89 mA.
The Widex CROS can be used in the CROS and BiCROS modes. It is equipped with user-friendly features that improve the usability and satisfaction with the device. On the bottom of the BTE is an On/Off switch that allows wearers to engage or disengage the use of the CROS transmitter. In addition, a volume control that has a range of over 35 dB allows wearers to control sounds that are transmitted to the receiving ear. These two features are critical in situations where undesirable sounds are presented to the unaidable ear. Properly adjusting the level of transmitted sounds can improve listening comfort and enhance the SNR of the listening environments.
The Widex CROS also uses True Input Technology to ensure a clean input signal. Like all DREAM hearing aids, the input stage of the Widex CROS only saturates above an input of 113 dB SPL to ensure an almost distortion-free transmitted signal. The option of a fully adaptive broadband directional microphone on the CROS transmitter adds to improve the SNR of the listening environment.
Because the receiver of the Widex CROS can be any model and style of the DREAM hearing aids, all the features available in the series are available to the wearer, including multichannel processing, slow-acting compression, noise reduction and speech enhancer, fully adaptive multichannel directional microphone, active feedback cancellation algorithm, frequency transposition, and Zen (for tinnitus management). Thus the wearer does not have to sacrifice any features to receive the benefits of a CROS system.
Clinical Study
A study at Widex ORCA-USA was conducted in order to document the improvement in speech understanding and SNRs offered by the Widex CROS device. A secondary objective was to estimate the additional SNR improvement offered by the fully adaptive directional microphone on the Widex CROS transmitter.
Methods
Subjects. A total of six hearing-impaired subjects ranging from age 63 to 78 years participated. All but one subject had the left ear as the poorer or unaidable ear. All were BiCROS candidates with a mild-to-moderately severe degree of hearing loss in the aidable ear and a severe-to-profound hearing loss in the unaidable ear. An exception was one subject who had severe distortion and poor speech recognition in the unaidable ear, albeit a moderate hearing loss. All but one subject had been wearing premium digital hearing aids on the aidable ear. One subject was wearing a wireless BiCROS from another manufacturer.
Test hearing aids. The DREAM Fashion hearing aid was fit to the subjects’ aidable ear using the default Widex gain target. A thin-tube with appropriate instant ear inserts (open and tulip inserts) were used to couple the aids to the subject’s ears. The Widex CROS was then paired with the hearing aid in the BiCROS mode.
Test conditions and setup. All speech testing was conducted with the speech materials presented to the unaidable or poorer ear. For speech-in-quiet testing, the ORCA-NST speech test8 was presented at 50 dB SPL to the poorer ear. Subjects were tested in the unaided, own aids, DREAM Fashion only (HA on aidable ear), and BiCROS with DREAM Fashion (BiCROS).
For speech-in-noise testing, the Hearing in Noise Test (HINT) was used. The speech sentence materials were presented to the side of the unaidable ear, while the continuous noise was presented to the front (0°), back (180°), and the side of the aidable (or better) ear (either 90° or 270°) at an overall level of 68 dB SPL. The level of the speech was adaptively varied to estimate the speech level for 50% correct identification of the test sentences.
Subjects were tested in the same conditions as listed previously. In addition, for the hearing aid (HA) and BiCROS conditions, the microphones on the hearing aid and CROS transmitter were systematically changed between omnidirectional and directional modes in order to examine the contribution of a directional microphone in a BiCROS system.
Results
Figure 3. Speech identification scores in quiet measured across four hearing aid conditions.
Speech in quiet testing. Figure 3 compares subjects’ performance across the 4 hearing aid conditions, separating the consonant and vowel scores. It is clearly seen that performance with subjects’ own aids was not much better than the unaided condition. An average of 3% improvement (23% – 20%) was seen in the consonant scores (although almost 20% improvement was seen in the vowel scores). This may be related to the limited high frequency output provided by subjects’ own hearing aids.
On the other hand, the DREAM Fashion hearing aid in the “aided better ear alone” condition yielded a consonant score that is 30% higher (53% – 23%) than the subjects’ own hearing aids. This may be due to the unique prescriptive target used in all Widex hearing aid fittings. The use of the BiCROS further improved the speech score by 10% over the DREAM Fashion HA alone condition. This shows that the use of the BiCROS can improve consonant identification over the unaided and own aid conditions by over 40% in an ideal or intended condition!
Figure 4. Reception thresholds for speech on the HINT for four hearing aid (unaided, own aid, aided, BiCROS) and two microphone (omnidirectional, directional or locator) conditions.
Speech in noise testing. Figure 4 summarizes the SNR required for 50% correct identification on the HINT at a 68 dB SPL noise level across the various hearing aid conditions. A higher number would suggest poorer performance in noise. Comparing with the unaided condition (11 dB), subjects’ own hearing aid improved the SNR by about 2 dB (9 dB). The DREAM Fashion (in the HA in better ear only) improved the SNR by 2.5 dB in the omni mode (8.5 dB) and 3 dB in the directional (or locator) mode (8 dB) over the unaided condition.
The use of the Widex CROS further improved the SNR even with the use of the omnidirectional microphone in both the DREAM Fashion and the CROS transmitter. In the BiCROS mode, a SNR of 6.5 dB was obtained when both the aid and the CROS transmitter were in the omnidirectional mode. A SNR of 6.2 dB was obtained when the aid was in the omnidirectional mode and the CROS transmitter was in the directional mode. A SNR of 5.2 dB was obtained when the aid was in the directional mode and the CROS transmitter was in the omnidirectional mode. A SNR of 3.2 dB was obtained when both the aid and the CROS transmitter were in the directional mode.
These observations have several important implications. First, the use of a hearing aid with or without a directional microphone on the aidable ear can improve the SNR of the listening environment by 2-3 dB over the unaided condition when speech is presented to the unaidable ear. Greater improvement can be expected when the hearing aid has a directional microphone than an omnidirectional microphone.
Second, the use of a BiCROS system can further improve the SNR of the aforementioned condition over the “hearing aid on better ear only” condition, even when both the microphones on the BiCROS system are omnidirectional and the “hearing aid in the better ear” has a directional microphone. A minimum of 1.5 dB improvement (8.0 dB – 6.5 dB) can be expected.
Third, when both microphones on the transmitter and HA receiver are directional, a 4.8 dB improvement in SNR (8 dB – 3.2 dB) can be expected between the BiCROS system and the “hearing aid on better ear only” condition, even when the HA is in a directional mode. These findings are similar to what Dillon reported.1
Fourth, the Widex CROS (when both transmitter and HA are in directional mode) improved the SNR by almost 8 dB over the unaided condition and 6 dB over the subjects’ own aid condition.
Lastly, a directional microphone on the CROS transmitter improves the SNR by 2 dB (5.2 dB – 3.2 dB) when the receiving hearing aid is also in a directional mode. However, the benefit of the CROS directional microphone is significantly reduced (0.3 dB) when the receiving hearing aid is in an omnidirectional mode (6.5 dB – 6.2 dB).
Conclusions
People with an asymmetric hearing loss (including those with a unilateral hearing loss) are not restricted to unilateral hearing aid fitting. The use of CROS/BiCROS can provide benefits above and beyond what is possible with unilateral hearing aid fitting on the better (or aidable) ear only.
The current study demonstrated that, when speech is presented to the unaidable ear of a hearing-impaired person, the Widex CROS hearing aid system improved speech understanding in quiet by >10% and SNR by almost 5 dB over the DREAM hearing-aid-alone condition. However, the benefit can be as much as 40% improvement in speech understanding in quiet and 8 dB improvement in SNR over the unaided condition! Considering that many people with an asymmetric hearing loss do not wear or only wear one hearing aid on the aidable ear, the Widex CROS, which can be configured as a CROS or a BiCROS hearing aid, could provide substantial improvement in the wearers’ speech understanding ability under some listening conditions.
In selecting a CROS system, it is important to understand how the CROS system is designed to ensure the highest sound quality and SNR for as many situations as possible. In that regard, the details of the wireless platform that is used for the transmission, ergonomic factors such as user controls, cosmesis, current drain, cost, and the quality of the receiving hearing aid are all important factors to evaluate.
References
1. Dillon H. Hearing Aids. 2nd ed. New York: Thieme Publishers; 2012.
2. Palmer C. Fitting strategies for patients with symmetrical hearing loss. In: Valente M, ed. Strategies for Selecting and Verifying Hearing Aid Fittings. 2nd ed. New York: Thieme Publishers; 2002:202-220.
3. Sandlin B, Bongiovanni R. Fitting binaural amplification asymmetrical hearing loss. In: Valente M, ed. Strategies for Selecting and Verifying Hearing Aid Fittings. 2nd ed. New York: Thieme Publishers; 2002:221-254.
4. Shaw E. Transformation of sound pressure level from the free field to the eardrum in the horizontal plane. J Acoust Soc Am. 1974;56:1848-1861.
5. Kuk F, Chi-Chuen L, Korhonen P, Crose B. Evaluating hearing aid processing at high and very high input levels. Hearing Review. 2014;21(3):32-35. Available at: https://hearingreview.com/2014/03/evaluating-hearing-aid-processing-high-high-input-levels
6. Kuk F, Korhonen P, Crose B, Kyhn T, Mörkebjerg M, Rank M, Kidmose P, Jensen M, Larsen S, Ungstrup M. Digital wireless hearing aids: Part II: Considerations in developing a new wireless platform. Hearing Review. 2011;18(6):46-53. Available at: https://hearingreview.com/2011/06/digital-wireless-hearing-aids-part-2-considerations-in-developing-a-new-wireless-platform
7. Kuk F, Jessen A, Baekgaard L. Ensuring high fidelity in hearing aid sound processing. Hearing Review. 2009;16(3):34-43. Available at: https://hearingreview.com/2009/03/ensuring-high-fidelity-in-hearing-aid-sound-processing
8. Kuk F, Lau C, Korhonen P, Crose B, Peeters H, Keenan D. Development of the ORCA Nonsense Syllable test. Ear Hear. 2010;31(6):779-795.
Original citation for this article: Kuk F, Korhonen P, Crose B, Lau C. CROS your heart: Renewed hope for people with asymmetric hearing losses. Hearing Review. 2014;21(6):24-29.
