Claro Adaptive digital AudioZoom TM

TM

Active, adaptive separation of speech from noise

Introduction One of the main challenges facing those with sensorineural hearing impairment is the understanding of speech in the presence of competing noise. In addition, the ability to understand speech in noise decreases with increasing hearing loss (Killion, 1997), Figure 1. In noisy surroundings the use of amplification often does not ameliorate the problem. It is enhancement of Signal-to-Noise Ratio (SNR) which improves intelligibility in noise (Agnew, 1997; Wolf et al., 1999). The challenge for hearing instrument manufacturers is to apply technologies that can increase the level of the signal of interest, while attenuating the background noise. In the past, many approaches have been used (e.g. low frequency suppression, automatic changes to the frequency response and digital signal processing). The most successful method to date for improving the SNR is the use of directional microphones and multi-microphone technology (Valente, 1998).

Claro Adaptive digital AudioZoom

Development of directionality in hearing instruments

Hearing loss in dB (PTA)

Directional microphones Hearing instruments incorporating directional microphones have been available since the early 1970s. While the first products did not gain popularity, many studies have shown them to be effective (Nielsen and Ludvigsen, 1978; Madison and Hawkins, 1983; Mueller et al., 1983; Hawkins and Yacullo, 1984). The polar response from a classical directional microphone tends to have a cardioid pattern (maximal suppression to the rear at around 180°), and when well designed, classical directional microphones generally offer a 3-4 dB enhancement in SNR (Nielsen and Ludvigsen, 1978; Hawkins and Yacullo, 1984). Despite the SNR advantages associated with these early directional microphones some practical disadvantages concerned with their use in everyday listening environments were apparent: most importantly they did not offer the option to choose an omnidirectional mode if required.

90 80 70 60 50 40 30 20 0

5

10

15

Signal-to-Noise Ratio in dB (ref.: normal hearing) Figure 1 SNR required for 50% word recognition score as a function of hearing loss, compared to people with normal hearing. (Killion, 1997)

2

20

At around the same time In-the-Ear (ITE) hearing instruments were introduced and their popularity grew rapidly. Although these hearing aids could not out-perform directional microphones for improvement of SNR, user preference for the smaller instruments meant that ITE technology became very popular. Furthermore, many manufacturers offered directional technology as an extra on their instruments, which implied that the use of directional microphones was just for specific situations rather than for everyday use. At the time technical and cosmetic considerations in ITE manufacture precluded the use of directional microphones, and a choice between ITE and directional technology became necessary. Multi-microphone technology An important advance in directional microphone technology took place with the introduction of multi-microphones. Phonak pioneered this technology with the introduction of AudioZoom in Behind-the-Ear (BTE) instruments in1993, and MicroZoom ITEs in 1997. This technology employs two separate, well matched, omnidirectional microphones designed to allow users to electronically switch between omni and directional modes. In addition, use of this technology has allowed increased noise reduction (average 7-8 dB SNR for those with mild-to-severe losses). Recent studies with high-power AudioZoom instruments have shown an average 13.7 dB SNR improvement for those with severe-toprofound hearing loss (Kühnel et al., 1999). The effectiveness of AudioZoom technology has been documented by a large number of studies. These have shown significant improvements in SNR and high levels of consumer satisfaction (Valente et al., 1995; Kochkin, 1996; Agnew and Block, 1997; Voss, 1997; Gravel et al., 1999; Kühnel et al., 1999; Pumford et al., 1999; Schuchman et al., 1999; Valente et al., 1999).

While multi-microphone technology has helped overcome many of the disadvantages of classical directional microphones, these microphone arrays still have a fixed direction of maximal noise suppression. In real-life use, however, noise interference does not always come from the same direction. The digital challenge Digital technology in hearing instruments offers the potential for further improvement of multi-microphone technology. Despite high expectations from digital signal processing, it has been shown that digital signal processing alone is not sufficient to improve the understanding of speech in noise (May et al., 1998a; May et al., 1998b; Bille et al., 1999). The integration of directional or multi-microphone technology into digital hearing instruments, has shown no significant improvement over multi-microphones within analog instruments (Ricketts and Dhar, 1999). Recognizing the limitations attached to fixed polar responses, some digital instruments with multi-microphone technology offer manual switching between various fixed polar patterns. However, this requires not only that the user decides on the appropriate program setting but also recognizes the direction of the noise and manually switches the microphone response accordingly. This is not an easy task as it requires a knowledge of the acoustic environment and good manual dexterity on the part of the user. Multi-microphone technology has maintained steady research interest over the years, largely due to the proven user advantages. However, despite the current growth of interest in digital processing, no improvements in directional hearing instruments have been achieved. Therefore the question still remains: How can the power of digital processing be harnessed to improve on current AudioZoom technology?

QuietAdapt Auto Select

NoiseAdapt

Figure 2

dAZ Adaptive digital AudioZoom

The Claro Solution

dAZ stands at the front of a comprehensive noise control system within Claro. The programs for quiet and noisy enviroments are automatically selected by AutoSelect. dAZ works in line with the Fine-scale Noise Canceler and the NoiseAdapt DPP algorithm for effective noise control.

Adaptive digital AudioZoom (dAZ) Digital technology within Claro uses advanced processing power to move multi-microphone technology into the next generation. dAZ stands at the front of a comprehensive noise control system, designed to optimize speech discrimination in noisy situations (Figure 2). Claro uses sophisticated digital signal processing algorithms to target and destroy noise for the hearing impaired client. With Claro we apply adaptive, digital multi-microphone technology to hearing instruments and take full advantage of digital processing power. This unique system incorporates all the proven benefits of AudioZoom but with very important additional advantages. To overcome the problem of manual switching and fixed polar patterns dAZ will adapt to the environment automatically and attenuate noise in the rear plane in whichever direction is necessary. The noise source is tracked as it moves and the directional performance is continually optimized. In Claro, noise control is an adaptive process which responds to changes.

3

DPP QuietAdapt

FNC

DPP

Fine-scale Noise Canceler

NoiseAdapt

This has obvious advantages for the hearing instrument user. In everyday situations when there is head movement, or the direction of the noise source changes, the Adaptive digital AudioZoom characteristics will automatically compensate to ensure that there is maximum noise suppression at all times. Improved microphone technology In order to maintain optimum directional performance it is important that the microphones in a multi-microphone array are carefully matched. As a vulnerable component in the hearing instrument, microphones are susceptible not only to wind noise but to damage from moisture and debris. To address the issues of wind noise and microphone damage a patented protective

Figure 4

microphone cover has been designed, for the Claro BTE models (Figure 3), which is resistant to moisture and debris while remaining totally transparent to sound. Thanks to the careful integration of the cover into the hearing instrument housing it also serves to substantially reduce wind-noise. How does dAZ work ? The diagram in Figure 4 demonstrates the principle of the dAZ system. The basic operation of a dual microphone system is to combine a real-time output from one microphone with a delayed signal from a second microphone to form a characteristic directional response. In Claro´s dAZ system, each microphone output feeds into a separate Analog to Digital (A/D) converter, to allow any necessary delay to the signal to be accomplished in digital mode. The spatial processor adjusts the polar response to ensure a minimum total output power. This is performed with the restriction that useful signals come from the front and results in an optimal attenuation of noise from the side or behind the user depending on the situation. This continual minimizing of the output specifies the adaptive nature of the array. In effect Claro searches for the point at which the noise must be attenuated to the greatest extent and places the area of minimal sensitivity in that direction so that optimal noise reduction is achieved.

Figure 3 The specially designed microphone cover in the Claro BTE range. Protects the microphones from moisture and debris together with the annoying effects of wind noise. The dual microphones in the Claro ITE model are ideally positioned and discretely protected.

Front microphone

The basic principle of Adaptive

AD-Converter

digital AudioZoom. The two microphone outputs are fed

Spatial Processor

directly into separate Analog to Digital converters. The adaptive algorithm is applied in the spatial processor.

Back microphone

AD-Converter

4

The diagram in Figure 5 illustrates four possible polar patterns in the dAZ system. Due to the adaptive nature of the array, however, any configuration between bi-directional or cardioid could be applied in order to best attenuate noise. This means that the noise source can be pinpointed automatically, regardless of it’s direction and without the restrictions imposed by fixed polar patterns. Bidirectional

HyperCardioid

SuperCardioid

Cardioid

Front to Back Ratio

0 dB

6.0 dB

11.4 dB

infinite

Directivity Index

4.8 dB

6.0 dB

5.7 dB

4.8 dB

The angle of maximal suppression, θmax

90°

110°

125°

180°

Summary

Table1 provides an overview of the progression of multi-microphone technology and the challenges met in applying digital technology to enhance AudioZoom further through the development of dAZ. The dAZ system uses adaptive algorithms to optimize the directivity pattern and reduce noise levels regardless of the rear plane angle of incidence. The digitally controlled microphone array continually adapts to changes in the environment to preserve the directional characteristics and optimize SNR. An additional unique feature of Adaptive digital AudioZoom is automatic activation: the AutoSelect feature in Claro continually classifies the acoustic environment and appropriately switches programs. The NoiseAdapt program is selected in specific noisy environments, activating all the Claro solutions for hearing in noise, including dAZ. Adaptive digital AudioZoom is the next generation in effective noise control and multi-microphone technology: • automatically activated • automatically adapting to maintain optimum noise suppression in dynamic environments

Figure 5 An example of four of the many possible polar configurations for dAZ. The Directivity Index is defined as the ratio of the output power due to the target signal from the front and the average output power of the noise originating from all directions. Table 1

The development of directional technology.

Frequency response

Classical Directional Microphone

Multi-Microphone Technology (Analog or digital)

The Claro Adaptive digital AudioZoom

Low frequency attenuation.

Possible to compensate for low

Digital Perception Processing

frequency "roll-off". Useful for

(DPP) optimizes sound

users with severe-to-profound

perception adaptively.

hearing impairment.

Polar pattern

Fixed area of attenuation.

One or more fixed or manually

Adaptive attenuation in the

selectable polar responses. Client

direction of the most dominant

intervention required.

noise source. No need for client intervention.

Microphone

Problems with wind noise, may

Problems with wind noise, may

Unique, patented microphone

be susceptible to moisture and

be susceptible to moisture and

cover to protect from moisture

debris.

debris.

and debris. Reduction in annoyance from wind noise.

Claro Adaptive digital AudioZoom

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