Chapter 9. Borderline cases: bone-conduction device versus middle ear implant versus cochlear implant

9.0 Introduction

During a round table discussion on borderline cases (CI2024, in Las Palmas), an overview was presented of Nijmegen studies aiming at borderline areas between conventional hearing aids (behind-the ear device or BTE) and implantable hearing devices: Cochlear implant (CI), middle ear implant (MEI) and implantable bone-conduction device (‘BCD’). 

Four borderline areas are discussed 

–Sensorineural hearing loss:

  • Severehearing loss: when to switch from a BTE to a CI?
  • If the hearing loss is moderate to severe, what is the better option: a BTE or MEI?

–Conductive or mixed hearing loss, 

  • What is the best amplification option? A BTE or ‘BCD’ or MEI?
  • In case of mixed hearing loss with a severe sensorineural component, when should a powerful ‘BCD’/MEI be replaced by a CI?

The focus is on device benefit in terms of speech recognition in quiet. 

Note. In contrast to other chapters, references are listed at the end of this chapter


9.1 Sensorineural hearing loss

9.1.1 When switching from a BTE to a CI in severe/profound sensorineural hearing loss? 


The concept of ‘equivalent hearing loss’ is used, comparing speech recognition score of CI users with speech recognition scores of a large group of BTE users with varying degrees of sensorineural hearing loss (Snik et al., 1997). The speech outcome chosen in this overview (and in all Nijmegen studies) is the (unilateral) phoneme recognition score in quiet obtained with monosyllables (MPS), presented at a normal conversational level (65 dB SPL; Snik, 2007, Verhaegen et al., 2008). 

In order to determine the ‘equivalent hearing loss’, data of (53) BTE users with moderate to profound sensorineural hearing loss were gathered; Appendix 9.1 presents the selection criteria and the BTE-fitting procedure. In Figure 1, individual, unilateral MPS scores are presented as a function of the PTA3 (mean hearing loss at 0.5, 1 and 2 kHz); the MPS of the ear with the best hearing (lowest PTA3) is presented. The blue line connects median MPS scores of the BTE users divided into classes, according to their PTA3.  These data will be used for a comparison with CI data.

Figure 1. The individual MPS values as a function of the PTA3 of the BTE users and the median (p50) and p25 MPS data of the group of CI users (dotted lines). MPS stands for Monosyllables Phoneme Scores

Table 1 presents MPS scores, obtained one-year after cochlear implantation of 139 successive but selected CI users with postlingual deafness. Inclusion criteria and details about the types of CI used are presented in Appendix 9.1. Table 1 presents the median and mean MPS as well as the p25 MPS (the 25e percentile). The p50 (median) and p25 values are indicated in Figure 1 as dotted lines. The figure shows that the median and the p25 MPS values are ‘equivalent’ to those of BTE users with a PTA3 of 75 dB HL and 85 dB HL (expressed in multiples of 5 dB), respectively. In other words, if a CI candidate has a PTA3 > 85 dB HL, then the chance that his/her MPS with CI is higher than with his/her BTE is >75%. If the PTA3 is 75 dB SPL, the chance level is 50%. This suggests that the p25 value (and thus 85 dB HL value) might be the better criterion. 

Table 1. The median, p25 and mean MPS scores and the related word recognition scores (WRS) of the CI users

measure MPS WRS
median (p50) 77% 57%
p25 63% 30%
mean 75% 53%
mean, from a later study* 79% 60%

* more recent data from a Nijmegen study (Huinck et al., 2019), see Appendix 9.1 for details

The last line of the table shows the outcome of a study carried out one decade later, by Huinck et al. (2019). They studied the advantage of relatively early implantation by comparing the MPS of two groups of CI users. Group 1 comprised CI users selected using rather conventional inclusion criteria (speech recognition with BTE less than 30% and/or PTA3 > 85 dB HL). Their data are presented in Table 1, last line and are rather well comparable to those used in the previous analysis.

Huinck et al. used the data of Group1 for a comparison with CI users selected according to less strict criteria (MPS with BTE between 30% and 60% and/or PTA3 between 65 dB HL and 85 dB HL). The one-year post-implant MPS of the second group was significantly higher than that of Group 1; viz. 85% versus 79%. The authors suggested that relatively early implantation is beneficial, which was ascribed to less ‘auditory deprivation’. 

This suggests that the less conservative PTA3 > 75 dB HL criterion might be the better choice. 

9.1.2 Moderate to severe sensorineural hearing loss, better results with a MEI or a BTE?


Nowadays, only the Vibrant Soundbridge MEI is widely used (VSB; Med-El, Innsbruck, Austria). In Nijmegen the VSB has (only) been applied if a BTE is contraindicated (owing to chronic external otitis; Snik and Cremers, 2001). MPS data were gathered and used to answer the question which device is more powerful, a BTE or a VSB. The same straightforward method was performed as in the previous section; Figure 2 presents the MPS as a function of the PTA3 of 30 patients, fitted with a VSB (for details regarding inclusion criteria and devices used, see Appendix 9.1). A best-fit line is presented; the black line. The blue line in the figure is copied from Figure 1, referring to BTE users. Evidently, the VSB scores are below the BTE scores. E.g., at a PTA3 of 65 dB HL, the MPS with VSB is approx. 25% worse than that obtained with BTE, increasing to 45% at 75 dB HL.

The figure suggests that, if the PTA3 exceeds 50-60 dB HL, a BTE should be fitted for better speech recognition, unless a BTE is contraindicated. Compared with the p50 MPS from CI users, the ‘equivalent hearing loss’ is approx. 60 dB HL, so, if a BTE is contraindicated, cochlear implantation might be considered if the PTA3 exceeds 60 dB HL. 

Although the capacity of the VSB is limited, it should be noted that this device is appreciated by patients with moderate sensorineural hearing loss and chronic external otitis. They can use this device all day without pain or itching (Snik and Cremers, 2001). The Med-El Company advocates the present VSB only for patients with sensorineural hearing loss suffering from chronic external otitis.

Figure 2.  The individual MPS of MEI users plotted as a function of the PTA3. For reference purposes, more recent literature data (Maier et al., 2015); mean MPS versus mean PTA3, presented as a red dot (see Appendix 9.1 for details) 


9.2 Conductive and mixed hearing loss


9.2.1 What is the best option: a BTE or a ‘BCD’? 


Most often, the cause of a (chronic) conductive/mixed hearing loss is aural atresia/stenosis or a chronic infected ear. In either case, a BTE is contraindicated. However, in other aetiologies like otosclerosis or tympanosclerosis, a BTE might be used. 

Whether a (powerful) BTE or a ‘BCD’ is more effective, depends on the width of the AB-gap (gap between the air- and bone-conduction thresholds). Assume a powerful BTE is used with a mean maximum output (MPO) of 112 dB HL and a mean gain of 50 dB. If the AB-gap is e.g. 55 dB, then the effective MPO is really limited: viz. 112 minus 55 thus 57 dB HL, and the effective gain is -5 dB. Even the less powerful conventional ‘BCDs’ can provide such a MPO and gain. 

The MPO and gain of a ‘BCD’ are not affected by the width of the AB-gap. The mean MPO of more powerful ‘BCDs’ like the percutaneous ‘BCD’ (pBCD; see section 9.2.2) is significantly higher than that 57 dB HL; a MPO of 85 dB HL with a mean gain of up to 20 dB is achievable.

Mylanus et al. (1998) and de Wolf et al (2010) compared outcomes (MPS) obtained with powerful BTEs and p’BCDs’ in (Nijmegen) patients with conductive/mixed hearing loss. They found that if the AB-gap exceeds 30-35 dB, the pBCD provided higher gain and, therefore, better speech scores than the BTE. Below that AB-gap, the BTE provided the best result.

In other words, if a BTE is not contraindicated while the mean AB-gap exceeds 35 dB, then a powerful (thus percutaneous) ‘BCD’ might be the better option.


9.2.2 If a ’BCD’ is indicated, which type of ‘BCD’ should be used? In other words, can we rank today’s ‘BCDs’?

Before discussing the capacity of ‘BCDs’, an overview of the different types that are  available. 

  1. The non-surgical, conventional ‘BCD’, which is kept in place by an elasticsoftband, steal headband or adhesive coupling. Such devices are referred to as conventional transcutaneous passive ‘BCDs’, e.g., the Baha Softband (Cochlear BAS, Goteborg, Sweden), Ponto Softband (Oticon Medical, Askim, Sweden) and the Adhear (Med-EL, Innsbruck, Austria)
  2. ‘BCD’ with magnetic coupling. This device requires a surgically placed subcutaneous magnet for coupling purposes and is also referred to as a transcutaneous passive device, e.g., the Sophono device (Medtronic, Minneapolis, USA) and the ‘Baha Attract’ (Cochlear BAS; see Figure 3) 
  3. Another transcutaneous approach consists of an actuator (the vibrator) implanted subcutaneously in the mastoid, driven by an externally worn audio-processor. This type of ‘BCD’ is referred to as a transcutaneous active ‘BCD’. Available devices nowadays: the ‘Bonebridge’ (Med-El; see Figure 3), the Osia (Cochlear BAS) and the Sentio (Oticon Medical)

Figure 3. Different types of ‘‘BCDs’’


  1. The percutaneous ‘BCD’ (pBCD) with a skin penetrating implant, connecting the ‘BCD’ to the skull without attenuation caused by the damping tissue. Available devices: the Baha (Cochlear BAS) and the Ponto (Oticon Medical; see Figure 3). It should be noted that (only) for these ‘pBCDs’ more powerful audio-processors are available besides the standard processors


The different types of ‘BCDs’ are not equivalents. In contrast to BTEs, all ‘BCDs’ (and MEIs) have a limited capacity. E.g., the loudest sound that ‘BCDs’ can produce (the MPO or maximal power output) varies between 50 dB HL and 85 dB HL, while humans can tolerate sounds up to 105-115 dB HL.

The MPO of ‘BCDs’ and also that of the VSB have been measured. Table 2 gives a literature overview. The table shows that data from different studies are rather consistent. It should be noted that in contrast to the other ‘BCDs’ and VSB, the MPO of pBCD can easily be derived from the available data sheets, following the procedure introduced by Carlsson and Hakansson (1997). 

The Table doesn’t present the MPO of the passive transcutaneous devices. Their mean MPO lays between 50 dB HL and 60 dB HL (Van Barneveld et al., 2018); included in the Table are only the more powerful options.

What are the consequences of a low MPO? Figure 4 presents the audiogram of a patient with mixed hearing loss. Bone-conduction-thresholds are shown as well as loudness-discomfortable-levels (LDLs). The patient was fitted with a passive tBCD with magnetic coupling as the bone-conduction thresholds laid well within the application range of that device. The measured MPO values are also displayed. The mean bone conduction threshold is 32 dB HL and the mean MPO of the tBCD is 55 dB HL. Consequently the ‘auditory window’ with this ‘BCD’, is just 23 dB (55 minus 32 dB HL). 

Figure 4. Audiogram of a patient with mixed hearing loss. As a ‘BCD’ is applied, only the bone conduction thresholds are relevant. MPO values are also displayed 

Figure 5.  On the right side, the (simplified) audiogram with (mean) MPO levels of different types of ‘BCDs’. Now, on the left side, the level of every-day sound sources, expressed in dB HL. Arrows indicate  ‘projections’ of the (grey) speech area into the patient’s auditory window. tBCD: transcutaneous passive ’BCD’; pBCD: percutaneous ‘BCD’; BCD-SP: pBCD with powerful sound processor.

Figure 5, left side, presents the outside world with all kinds of sounds, expressed in dB HL. The grey area represents the speech area (normal conversational speech at 65 dB SPL). Evidently, projecting relevant sounds properly (not only speech at normal conversational level, but also with raised voice or someone talking at some distance or the patient’s own voice and the other sounds) in the narrow ‘auditory window’ of 23 dB is not easily possible. 

Now, the right figure also shows the MPO of more powerful ‘BCD’ solutions (values taken from Table 2). With a standard pBCD, the auditory window becomes 37 dB and if a more powerful processor is used, like the Baha 5 SP, then the window becomes 55 dB, the maximum achievable window with todays ‘BCDs’. With the VSB, this window is 50 dB. Thus, from an audiological point of view, pBCD is the best option followed by the VSB device. As Figure 5 shows, the speech area is approx. 35 dB wide. This value is used to define application ranges for different types of ’BCD’ and the VSB. We use the following assumption: for a given patient, a specific ‘BCD’ can be used only if the ‘auditory window’ is at least 35 dB. Simply means that the highest acceptable PTA4bc (average bone-conduction threshold at 0.5, 1, 2 and 4 kHz) is the mean MPO value minus 35 dB. Table 3 gives an overview; it should be noted that, at date, the MPO of the relatively new transcutaneous active ‘BCDs’ (Osia en Sentio) is not known yet (not published). 

Evidently, the different types of ‘BCDs’ and the VSB are not equivalents; if the PTA4bc of a transcutaneous passive ‘BCD’ user exceeds 20-25 dB HL, then an update to a percutaneous device might significantly improve the patient’s performance. Another option is to apply a VSB. For Bonebridge users with a PTA4bc exceeding 30 dB HL, a pBCD or VSB are the next options. 

9.2.3 In progressive severe mixed hearing loss, when to replace a ‘BCD’ by  a CI?

Following the same procedures as used before, Figure 6 presents the MPS versus PTA3 (bone-conduction) of 26 patients with mixed hearing loss with moderate to severe sensorineural component using the most powerful ‘BCD’ at that time, the Baha Cordelle (Verhaegen et al., 2009). For selection criteria etc. see Appendix 9.1. The PTA3 for bone conduction varied between 30 dB HL and 75 dB HL. As the figure shows, when using the CI-p50 (77%), the ‘equivalent hearing loss’ is 60 dB HL. 

Figure 6. Individual MPS values as a function of PTA3 if 26 users of the most powerful pBCD


Verhaegen et al. (2009) also presented the data of the first five Nijmegen patients switching from a Baha Cordelle to a CI. Two of them suffered from advanced otosclerosis and external otitis and three from chronic middle ear problems. The MPS of these five patients improved significantly from 10% (range 0 to 30%) with ‘BCD’ to 75% (range 60% to 90%) with the CI. 

Regarding advanced otosclerosis, performance with a CI might be worse than in patients with normal cochlear anatomy (Rotteveel et al., 2010). Poor performance is caused by otosclerosis-related anomalies of the cochlea often leading to partial insertion of the electrode array. This might lead to a partial insertion. Because of poor bone quality, facial nerve stimulation might occur. Then, certain electrodes have to be switched off (Rotteveel et al., 2010). Therefore, the MPS scores of CI users with otosclerosis are worse than those of patients with a normal bony structure; the p50 and p25 MPS values for CI users with otosclerosis are 55% and 40% respectively (Rotteveel et al., 2010), thus more than 20% below those of CI users with anatomically normal cochleae (Figure 1). Using Figure 6 the otosclerosis-specific p50 and p25 (not shown) result in ‘equivalent hearing loss’ values of 70 dB HL and 75 dB HL, respectively.

Cochlear implantation in the other group of patients with severe mixed hearing loss, suffering from chronic middle ear problems, is also challenging. Nevertheless, cochlear implantation in such patients remains of utmost importance, as it is a ‘last resort’ solution. 


9.2.4 Which acoustic implantable device to use for patients with conductive/mixed hearing loss?


Previous sections clearly suggest that the pBCD is the best option from an audiological point of view. That device has some other specific advantages as well, but also a disadvantage. The advantages:

  1. Of course application of a pBCD requires surgery, but minimal invasive surgery, in contrast to all the other implantable options. Implantations have even been performed in outpatient clinics 
  2. pBCD is the only implantable device that can be upgraded when hearing deteriorates. Besides the standard sound-processor, power and superpower processors are available
  3. The distortion of MRI images is limited in contrast to most other implantable ‘BCDs’ and VSB. Explantation of MEIs and active t’BCDs’ because of MRI have been reported 

Disadvantage of pBCDs are infections around the percutaneous implant and implant loss owing to poor fixation in the skull bone (poor osseo-integration).


Regarding point 1: Minimal invasive surgery lowers the burden for the patient and the costs of the intervention. In contrast, placing a MEI in patients with conductive/mixed hearing loss might be challenging, especially in patients with congenital anomalies (e.g., aural atresia) and in patients with chronic otitis media. In the latter case, subtotal petrosectomy is a required first surgical step (Verhaert et al., 2013).

Regarding point 2: Longterm effectiveness is an important issue, especially when treating elderly people, because their hearing loss might significantly deteriorates over time. This is illustrated in Table 4, which shows the effectiveness of several types of BCDs and the VSB in relation to age of patients with mixed hearing loss. Few data were found on the deterioration of the sensorineural component in patients with mixed hearing loss. Therefore, we used age-related hearing thresholds taken from the ISO standard (ISO 7029, 2017), with an additional cochlear loss of 20 dB HL owing to cochlear damage caused by middle ear infections and/or medical treatments in the past (group ISO 7029+; see Appendix 9.1 for details). We used these data for illustrating the effect of becoming older on the effectiveness of several devices.  


Table 4. Effective use of devices in progressive (cochlear) hearing loss in patients with cochlear damage caused by chronic otitis media in the past (ISO 7029+). 


device PTAbc max,


ISO 7029+


Passive ‘tBCD’* 25 up to 50 
Active ‘tBCD’** 30 up to 60
VSB 45 up to 80
‘pBCD’ 50 up to ±85

* with magnetic coupling; ** the ‘Bonebridge’


Table 4 shows that when using a passive tBCD with magnetic coupling, the PTAbc max for applying that device (25 dB HL; taken from Table 3) is passed at age 50 years. Evidently, the pBCD and VSB are the best solutions as they might be used effectively life long.

Disadvantages of percutaneous implantation are skin problems around the implant and implant loss, what might lead to revision surgeries. Through the years, the number of revisions has dropped, owing to more than 35 years of experience with percutaneous implants. New implantation procedures have been introduced as well as optimized implants. Table 5, first line, presents Nijmegen data. Systematically, all minor and major complications have been documented for each patient, starting in 1988. Patients were seen at least 5 times during the first years after implantation and at least twice a year, later on. Apart from planned visits, additional visits took place at the patient’s initiative. Dun et al. (2012) published data on medical complications of 903 successive adult patients, implanted from the start in 1988 up to 2008. Table 5 shows that one revision surgery has been performed per 30 years of follow-up (column 5). Wazen et al. (2008) published a similar study. They started later with providing Baha devices, namely in 1998. That might be the reason why their result is better than that of Dun et al. More recently, prospective multicentre studies have been published using a specific new, improved percutaneous implant and/or implant procedure (Nelissen et al., 2014; Kruyt et al., 2018; see Table 5). Indeed, further progress has been made, as relatively less revision surgeries were needed. 

It should be noted that the follow-up period of the latter two studies is relatively short, 3 years. However, as has been shown by Dun et al., most complications occur in the first year post implantation. 

The last columns of the table presents data related to skin problems around the percutaneous implant (percentage of visits with some skin reaction). Severity of skin reaction was expressed in Holgers grades 1 to 4. Grade 2, red and moist tissue, requires local treatment; grades 3 and 4, more severe conditions, might require revision surgery (Wazen et al., 2011).  Indeed, skin problems remain common; however, the percentage of serious skin problems requiring some type of intervention is decreasing (Holgers grades 2 to 4; last column). 


Table 5.  Number of revisions related to the total follow-up period and the percentage of soft tissue reactions

Author, year n revisions Summed follow-up relatively All skin reactions3 Holgers grade > 2
Dun, 2012 9031 1452 4.267 yrs 1 in 30 yrs 15% 4,3%
Wazen, 2008 218 23 990 yrs 1 in 43 yrs 4
Nelissen, 2014 52 2 156 yrs 1 in 78 yrs 23,1% 1,8%
Kruyt, 2018 39 2 117 yrs 1 in 58 yrs 15,5% 1,5%

1 only the adults; 2 revision surgeries because of skin problems plus lost implants; 3Holgers grades 1 to 4; 4no Holgers grades reported 


Similar systematic studies on complications with the other types of ‘BCDs’ and MEIs are lacking. 

It is suggested, Based on the data of Table 5, we suggested that one revision surgery in 40 years might be considered as the standard for all implantable devices.


9.3. Concluding remarks


Fortunately, nowadays several amplification options are available for deaf and hearing-impaired patients for whom conventional amplification (BTE) is no longer effective or contra-indicated (CI, ‘BCD’, MEI). Thanks to the investigators and the companies that put these devices into the market! 

Regarding the future; a main concern is the costs of these interventions; each update of any implantable device leads to a higher price. Rising cost of medical healthcare is a major issue in many countries; already in 1998, Daniels & Sabin concluded: ’no country in the world can afford all of the medical care that providers can render to populations’ (Daniels and Sabin, 1998). In other words, there is a great need for better affordable hearing devices including implants, see also the ‘World report on hearing’ (WHO, 2021).


Appendix 9.1 


Section 9.1.1. Regarding the 53 BTE users: the inclusion criteria were: age between 18 and 70 years, with flat or mildly sloping audiogram (difference in threshold between 0.5 kHz and 4 kHz less than 40 dB HL) with hearing thresholds < 110 dB HL up to 4000 Hz, using BTEs for at least 3 years. They were all fitted with state-of-the-art digital hearing devices using the NAL-NL fitting rule in the Nijmegen clinic, using one and the same validation protocol (Snik, 2007; Verhaegen et al., 2008).  

Selection criteria for the 139 CI users: postlingual deafness with normal cochlear anatomy. Age: between 18 and 70 years. Devices used were the Cochlear CI (N22, N24, Freedom) and Advanced Bionics CI (C1, 90K), almost equally divided. The patients were implanted between 1990 and 2009. Same evaluation protocol was used as for the BTE users. The data used are relatively old, however, comparable to that for a group of 52 CI users implanted in Nijmegen between 2010 and 2016, using similar conservative inclusion criteria. Their mean MPS was 79% (Huinck et al., 2019), close to MPS of reference CI group (75%).


Section 9.1.2. Regarding the 30 VSB users: inclusion criteria were: age 18-70 years, with flat or mildly sloping audiogram (difference in threshold between 0,5 kHz and 4 kHz less than 40 dB HL) and with hearing threshold < 80 dB HL up to 4 kHz, using their VSB for at least 1 year. The patients were fitted between 1998 and 2008 (unilaterally) with the AP303 or AP404 speech processor. For the device-fitting, the NAL rule was applied. The evaluation protocol was the same as used to test the BTE users (Snik, 2007; Verhaegen et al., 2008). 

Evidently, the included data are relatively old data. Maier et al. (2015) presented data of a more recently implanted group (their group 1, n=34). They were fitted with the Amade speech processor, using the NAL fitting rule. Individual data were not presented: in Figure 2, the average MPS is presented at the average pre-intervention PTA3 (red symbol). This average MPS is in agreement with the Nijmegen data (data-point on the best fit line) suggesting that the MPS values used might still be relevant. Note that Maier et al. reported the mean WRS using monosyllables; that score was expressed as a MPS, using a look-up table. 


Section 9.2.3. The study group comprised 26 patients who were using the most powerful ’BCD’ at that time, the Baha Cordelle (Verhaegen et al., 2008). The inclusion criteria were: age 18-70 years, with flat or mildly sloping bone-conduction audiogram and measurable bone conduction thresholds up to 2 kHz. They were all using a pBCD for more than a year. The mean MPO of the Baha Cordelle (0.5 to 4 kHz) is 82 dB HL, 1 to 4 dB below that of more recent powerful pBCDs, as presented in Table 2. 


Section 9.2.4. Deterioration of hearing loss in aging patients with chronic middle ear problems. 

For subjects with pure sensorineural hearing loss, ISO 7029 describes the deterioration of hearing sensitivity owing to aging, however, only for otologically normal subjects (ISO, 2017). In case of chronic middle ear problems, progression in cochlear hearing loss might follow the ISO 7029 norm, however, with an additional cochlear loss owing to either the infections or medical interventions in the past. We assessed that additional cochlear loss by comparing the bone-conduction thresholds of patients between 50 and 70 years, using a ‘BCD’, with the age-specific ISO 7029 thresholds. It is likely that the cochlear damage caused by the middle ear problems occurred in the past when the patients were still using a BTE, thus before using a ’BCD’ (e.g., Macnamara et al., 1996). 

Three studies with relevant data were identified. Table A9.1 shows that the ‘additional sensorineural hearing loss’ is approx. 20 dB (note: the ISO thresholds used were the average for those of men and women).


Table A9.1. Bone conduction thresholds compared to age-specific ISO 7029 norms of patients with mixed hearing loss using ‘BCDs’; the additional cochlear loss owing to chronic ear infections.

Study; first


n (ears) 0.5 kHz


1 kHz 


2 kHz


4 kHz




Bosman, 2013 18 15  20 25 20  20 
Flynn, 2012 20 15 20  20  10  17 
Pfiffner, 2011*  44 19

*their group C; no frequency specific data available




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