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Bilateral BCHIs have been shown to produce improvements in speech reception thresholds (SRTs) in quiet of 4–5.4 dB when compared to unilateral BCHI fitting [21–23], with similar improvements also identified for tone reception thresholds in quiet [21, 24]. This benefit appears to derive from the increased bone vibration generated by 2 devices working together. Evidence of benefit when listening to speech in noise is clear, but situation dependent. Bilateral BCHIs are advantageous when noise is diffuse, with a 2.8 dB speech-to-noise ratio (SNR) improvement when compared to unilateral BCHI fitting [21]. For a single localized noise source, the benefits of head shadow sometimes occur for the unilateral BCHI control group as well as the bilateral group. So, when noise is presented to the aided side (in the unilateral group), the reported advantages of bilateral fitting are 2.5 [23] and 3.1 [21] dB SNR, but, if noise comes from the unaided side, bilateral benefits are reduced to 0.8 dB [23] and -1 dB [21] SNR.

Binaural masking level difference (BMLD) results (which are a reflection of the ability to perform binaural processing of interaural time differences) are mixed. Air conduction BMLD results are in the range of 11 ± 2 dB in normal hearing subjects [25], but Priwin et al. [21] found no benefit and Bosman et al. [23] found only a 6–6.6 dB BMLD at 125–500 Hz with no benefit at higher frequencies in bilateral BAHA users. Despite limited BMLD results, improvements in sound localization with bilateral BCHIs over unilateral fitting are consistent. Sound localization when using unilateral BCHIs has been found to be extremely poor with significant improvements following bilateral fitting [21, 23, 24]. Snik et al. [26] identified a 53% improvement in sound localization over the unilateral condition.