Challenges and recent developments in hearing aids. Part I. Speech understanding in noise, microphone technologies and noise reduction algorithms

Abstract

This review discusses the challenges in hearing aid design and fitting and the recent developments in advanced signal processing technologies to meet these challenges. The first part of the review discusses the basic concepts and the building blocks of digital signal processing algorithms, namely, the signal detection and analysis unit, the decision rules, and the time constants involved in the execution of the decision. In addition, mechanisms and the differences in the implementation of various strategies used to reduce the negative effects of noise are discussed. These technologies include the microphone technologies that take advantage of the spatial differences between speech and noise and the noise reduction algorithms that take advantage of the spectral difference and temporal separation between speech and noise. The specific technologies discussed in this paper include first-order directional microphones, adaptive directional microphones, second-order directional microphones, microphone matching algorithms, array microphones, multichannel adaptive noise reduction algorithms, and synchrony detection noise reduction algorithms. Verification data for these technologies, if available, are also summarized.

Figure 1.

(A) The relationship between the ratio of the internal and external delays and the polar patterns. (Reprinted with permission from Powers and Hamacher, Hear J, 55[10], 2002). (B) The directional sensitivity patterns of directional microphones exhibiting cardioid, hypercardioid, and supercardioid patterns. A commercial stand directional microphone is placed at the center of the directional sensitivity pattern.

Figure 2.

(A) The relationship between the port spacing (p) and the amount of low-frequency cut-off for directional microphones that use the delay-and-subtract processing. As port spacing decreases, the cut-off frequency for low-frequency gain reduction increases. This is because sound pressures picked up by the two microphone ports/two omni-directional microphones are subtracted at two adjacent points. As frequency decreases, the wavelength increases, the differences between the two points decreases, and the resultant microphone output becomes smaller after the subtraction. Thus, the cut-off frequency for low-frequency roll-off increases as the microphone port spacing decreases. (B) The relationship between the port spacing (p) and the amount of high-frequency directivity index (DI). As port spacing decreases, the high-frequency directivity index increases. This occurs because as the wavelength of the incoming signal approaches the port spacing, directionality breaks down. The smaller the port spacing, the higher the frequency at which the directionality breaks down (AI-DI = articulation index weighted directivity index). (Courtesy of Oticon, reprinted with permission).

Figure 3.

A figurative example of how the first-order dual-microphone directional microphone is implemented. The polar pattern of the directional microphone depends on the ratio of the internal and external delays. (Reprinted (modified) with permission from Thompson SC, Hear J 56[11], 2003).

Figure 4.

Directional microphones (solid line) have higher outputs for wind noise than omni-directional microphones (dotted line). (Original data from Dillon et al., 1999. Reprinted with permission from Kuk et al., Hear Rev 7[9], 2000).

Figure 5.

The modulation detector is composed of a maxima (thick line) and a minima follower (thin line). The maxima follower estimates the level of speech and the minima follower estimates the level of noise. The difference between the two allows the estimation of signal-to-noise ratio in the frequency channel.

Figure 6.

The implementation of a commercially available hybrid second-order directional microphone. The outputs of the front and back microphones are processed to form a first-order directional microphone (1 ord. dir. mic.), the output of which is low-pass filtered. The outputs of all three microphones are processed to form a second-order directional microphone (2 ord. dir. mic.), the output of which is then high-pass filtered. The low-and high-pass filtered signals are subsequently summed and processed by other signal processing algorithms in the hearing aid. (Reprinted with permission from Powers and Hamacher, Hear J 55[10], 2002).

Figure 7.

(A) The effect of sensitivity mismatch at 1000 Hz between the two omni-directional microphones that form a directional microphone. (Reprinted with permission from Edwards et al., Hear J 51[8]), 1998). (B) The effect of sensitivity mismatch at 250 Hz. (C). The effects of phase mismatch at 250 Hz. (Figures B and C reprinted with permission from Kuk et al., Hear Rev 7[(9], 2000).

Figure 8.

(A) Link.It transmits the processed signal to an in-the-ear hearing aid via telecoil. (Courtesy of Etymotic Research, reprinted with permission). (B) The hand-held unit of Lexis transits the processed signal to an frequency modulated (FM) receiver attached to a behind-the-ear hearing aid. (Courtesy of Oticon, reprinted with permission).

Figure 9.

The relationship between the estimated signal-to-noise ratio (SNR) and the amount of gain reduction applied by noise reduction algorithms of two commercial digital hearing aids.

Figure 10.

The interaction between wide dynamic range compression and the noise reduction algorithm when the two systems are implemented in series. (A) The amplitude envelope of two sentences and speech spectrum noise presented in sound field at a signal-to-noise ratio (SNR) of +3 dB. (B). The amplitude envelope of the same sentences and noise after being processed by a directional microphone and a noise reduction algorithm with the compression system set in the linear mode. (C). The envelope of the same sentences and noise after being processed by a directional microphone and a noise reduction algorithm with the wide dynamic range compression system set at 3:1 compression. The frequency responses of the hearing aid in the linear and compression mode were matched at the presentation level.

Figure 11.

The spectrogram of the sentence “The boy fell from the window.” The vertical striations indicate that speech is co-modulated during the vowel or voiced constants production. They represent the synchronous energy emitted during the opening and closing of the vocal folds.

Figure 12.

The decision rules of Oticon Syncro. The synchrony detector, the modulation detector and the noise level detector determine the presence or the absence of speech and the relative level of speech and noise in the incoming signal. No gain reduction is applied to the frequency channels if speech-in-quiet is detected. If speech-in-noise is detected, the gain is reduced depending on the level of the noise, the modulation depth, and the articulation index weighting of the frequency channel. Maximum gain reduction is applied if noise-only is detected. (Reprinted with permission from Oticon The Syncro Concept, 2004).