助听器介绍及其设计要点(中英对照)

助听器介绍及其设计要点概述电子助听器是置于耳内或耳附近用以提高听觉障碍患者听力的小型设备。

助听器的基本单元包括麦克风、信号调理电路、接收器(也称为扬声器)、以及电池。

麦克风将声信号转换成电信号，信号调理单元则可简可繁，简单的仅将音频信号按固定比例放大，复杂的则需利用数字信号处理器进行均衡。

扬声器将电信号转换成声信号，而电池则为电子元器件提供电源。

类型目前市场上主要有4种类型的助听器，体积从大到小依次是包括耳背式(BTE)、耳内式(ITE)、耳道式(ITC)和完全耳道式(CIC)。

BTE位于耳后，用一个软管连接耳内的耳模发声。

由BTE还发展出来一种开放式(OTE)助听器，耳模被一个小耳塞代替，给人耳一种更开放的感受。

其它的变型还包括用导线替代软管，并将扬声器从耳后移到耳内。

ITE将助听器放入外耳，和耳模成为一体，这种助听器几乎将外耳填满，看起来是一大块。

ITC将助听器填入耳道内，减小了占用外耳的空间，但还是容易被看见。

CIC是各类型中最小的，助听器被完全置入耳道内，从外面几乎看不见。

耳背式(BTE)、耳内式(ITE)、耳道式(ITC)和完全耳道式(CIC)助听器，Starkey Laboratories, Inc.授权照片。

技术演进助听器在技术上基本分为两类，模拟助听器和数字助听器。

首先诞生的是模拟助听器，仅在模拟域处理电声信号，而最近才诞生的数字助听器则在数字域处理电声信号。

最早的模拟助听器既放大语音也放大噪音，而且需要先测试患者对特定频率的敏感程度后专门定制。

后来的一些模拟助听器可以在试戴过程中编程，另一些助听器佩戴者可利用一个按键自己选择预设的几种不同频响。

数字助听器也可在试戴过程中编程，并具有多种佩戴者可选的频响。

将声音数字化的技术使降噪、滤波、声反馈(自激)控制等成为可能。

由于数字助听器相对模拟助听器性能和灵活性的大幅提升，目前销售的助听器大部分都是数字式的。

数字助听器功能框图，欲了解Maxim助听器的推荐设计方案，请访问/hearing。

特性目前的助听器有许多新特性，包括音量调整、遥控、拾音线圈、直接音频输入、FM接收、Bluetooth®、指向性麦克、压缩、钳位、移频、风噪管理、数据记录、自学习、防潮、耳模通风。

有一些特性没法在助听器内部狭小空间内实现，需要占用外部空间，而另外一些则可以在助听器内部实现。

音量调整可通过助听器上的按键或滚轮实现，而使用遥控器则可避免在狭小的助听器上安装按键或滚轮，但仍能控制助听器的所有功能。

拾音线圈可以用来替代麦克风，拾音线圈在以前使用电磁线圈扬声器的老式电话机中通过拾取磁信号来帮助打电话的人提高通话质量。

今天的电话或其它收听设备都内置拾音线圈这一功能并专门指出和助听器兼容。

直接音频输入和FM接收是另外两个助听器输入信号的方式，第一个要用有线方式连接，另一个实际就是FM收音机。

另外一个新趋势是集成Bluetooth功能，这样就可以从手机或音乐播放器中接收信号。

Bluetooth既可以集成到助听器也可以作为附件通过拾音线圈或FM接入助听器。

带有指向性麦克风的助听器有两个或以上的麦克风面向不同的方向接收信号，这样做可以提高在嘈杂环境中收听的信噪比(SNR)，而使用数字信号处理技术可进一步提高话音质量。

压缩和钳位在音量过大时降低音量以使人耳感觉更舒适，但有些时候会限制音量。

移频技术通过数字信号处理将语音移到低频，对那些患有高频听觉障碍的人很有帮助。

风噪管理可以检测风声并消除其反馈，以防助听器佩戴者听到啸叫。

数据记录功能记录收听环境以及助听器如何被使用，听觉专家可以根据这些信息来微调助听器提升性能。

自学习功能可利用记录的数据自动微调助听器提升性能。

防潮功能可以降低因潮湿环境导致的返修率。

耳模通风则可提高人耳佩戴耳模助听器时的舒适程度。

一般需求助听器的关键元器件位于音频处理通道。

主要包括一个或多个麦克风、扬声器以及前置放大器(如果需要的话)和扬声器放大器。

D类功放和AB类功放相比功耗低、失真小、尺寸小，因而多用于现今的助听器。

另外，无论音频带宽是20kHz还是被限制于8kHz，音频编解码器都应该具有高SNR以保证高回放音质。

数字助听器的核心是数字信号处理器(DSP)，这是实现数字助听器所有优势的关键。

各厂商采用不同的DSP技术，但通常包括分频段压缩/放大、正反馈消减、降噪、语音增强等，DSP还可以处理方向信息，并可调整助听器特性使其适合不同的佩戴者。

电源和电池管理已经有一些助听器开始使用可充电的单节锂离子(Li+)电池，但多数还是使用一次性锌空气电池，根据助听器的类型和尺寸、电路功耗以及对使用时间的要求，目前主要使用5种尺寸的锌空气电池。

表1对比了这五种最常用锌空气电池的容量和尺寸，以及它们的颜色代码以及通常被哪种助听器使用。

锌空气电池起始电压为1.4V，需要更换电池时的终止电压为1.0V或更低。

如果按每天使用16小时计算，电池寿命在几天到几星期之间，取决于电池容量和助听器的设计。

电池效率最高的设计从单节电池直接供电，然后使用开关调节器升压电路将电压提升来驱动1.8V或3.0V的助听器电路。

使用锌空气电池的理想功耗在1mW至10mW之间。

使用可充电Li+电池的助听器也许需要一个线性或开关稳压器来将4.2V的单节Li+电池满充电压降至电路可以接受的工作电压。

也可以将电池充电电压限制在3.3V，这取决于电路的需要。

为了防止听觉障碍患者在需要助听器帮助时电池电量耗尽，需要一个精确电量计提前做出低电量预警。

Introduction to Hearing Aids and Important Design ConsiderationsOverviewAn electronic hearing aid is a small device placed in or around the ear to improve the hearing of those with hearing loss. The basic components of a hearing aid are a microphone, signal conditioning, a receiver also known as a speaker, and a battery. The microphone converts the sound into an electric signal. The signal then undergoes conditioning that can be as simple asamplifying all of the sound equally, to more advanced equalization involving a digital signal processor. The receiver converts the electronic signal back to sound, and the battery powers the electronics.StylesThere are four main styles of hearing aids on the market today. From largest to smallest, they are behind the ear (BTE), in the ear (ITE), in the canal (ITC), and completely in the canal (CIC). The BTE style sits behind the ear with a clear tube going to an earmold in the ear to deliver the sound. A variation on this style is called an open-fit-behind-the-ear (OTE) where the earmold is replaced by a small tip, resulting in a more open feeling. Other variations include replacing the tube with wires and moving the receiver from the behind the ear to inside the ear. The ITE style moves the hearing aid into the outer ear, where it becomes a single unit with the earmold. This style fills up most of the outer ear and appears as a solid mass. The ITC style moves some of the hearing aid into the ear canal and reduces the space taken up in the outer ear, but is still plainly visible. The CIC style is the smallest of them all, as it fits completely inside the ear canal, thus nearly disappearing from view.Behind the ear (BTE), in the ear (ITE), in the canal (ITC), and completely in the canal (CIC). Photos courtesy of Starkey Laboratories, Inc.Technology TypesThe two basic types of technology for hearing aids are analog and digital. The first to exist, analog hearing aids process electrical sound in the analog domain; the more recent digital hearing aids process electrical sound in the digital domain. The earliest analog hearing aids simply amplified both speech and noise, and were ordered after testing to determine the particular frequency response needed by the patient. Newer analog hearing aids can be programmed during the fitting process, and some have multiple listening profiles that the patient can select with a button on the hearing aid. Digital hearing aids are also programmable during the fitting process and have multiple listening profiles that are selectable by the patient. The digitization of sound allows more advanced signal processing such as noise reduction, filtering, and acoustic feedback (ringing) control. The vast majority of hearing aids sold today are digital because of their increased performance and flexibility over the analog versions.FeaturesThere are many features available for today's hearing aids, including volume control, remote control, telecoil, direct audio input, FM reception, Bluetooth® capabilities, directional microphone, compression, clipping, frequency shifting, wind-noise management, data logging, self-learning, moisture resistance, and earmold venting. Some of these features require external area to implement and become more difficult to include as the size of hearing aids shrinks, while other features can be implemented in all hearing aids.Volume control is performed manually with buttons or a rotary dial on the hearing aid. A remote control eliminates the need for buttons and dials on the hearing aid and can be used to control all the features of the hearing aid. A telecoil is an alternate input other than the microphone, and it originally picked up the magnetic signal generated by older telephones with speakers driven by magnetic coils so that listeners could hear better when talking on the telephone. Today's telephones and other alternate listening devices build-in this capability in order to work with a telecoil and specifically indicate that they are hearing aid compatible. Direct audio input and FM reception are other ways to input sound or speech into the hearing aid, the first using a wired connector as an input, and the other an FM radio receiver. An emerging trend is to include Bluetooth capability to receive sound from a cell phone or music player. The Bluetooth device can either be integral to the hearing aid or an add-on device through the telecoil or FM input.A hearing aid with directional microphones uses two or more microphones to receive sound from multiple directions. This improves the signal-to-noise ratio (SNR) of speech when heard in a noisy environment, and enhances the quality of speech further when used with digital signal processing. Compression and clipping both increase listening comfort by reducing portions of the sound that are too loud but, in some cases, just clip or limit the sound. Frequency shifting uses digital signal processing to shift speech to a lower frequency, which is helpful for people with high-frequency hearing loss. Wind-noise management detects wind and eliminates the feedback that would otherwise cause ringing sounds to be heard by the hearingaid wearer.Data logging records the listening environment and how the hearing aid is used. A hearing professional can use this information to fine-tune hearing aid performance. Self-learning uses the data logs and fine-tunes the performance on its own over time. Moisture resistance helps reduce repairs due to exposure to moisture, and earmold vents provide additional comfort by reducing the closed-in sensation felt when wearing an earmold type of hearing aid.General RequirementsThe critical components of a hearing aid design are in the audio-processing path. The one or more microphones and the receiver are chosen in conjunction with the preamplifiers (if required) and the speaker amplifiers. Class D amplifiers are used in modern hearing aids due to their low-power operation, low distortion, and small size as compared to Class A and B amplifier. Whether the audio bandwidth is 20kHz or limited to 8kHz, the audio codec should have a high SNR to preserve and reproduce sounds accurately.The heart of the system is the digital signal processor (DSP), which is where all of the benefits of a digital hearing aid are implemented. The DSP implementation is manufacturer dependent. In general, it performs compression/expansion by band, positive feedback reduction, noise reduction, and speech enhancement. It also processes directional information and can generate its own signals to help improve fitting a hearing aid to a patient.Power and Battery ManagementSome hearing aids are beginning to use rechargeable single-cell lithium-ion (Li+) batteries, but most hearing aids are still powered by primary zinc-air batteries. There are five main sizes of zinc-air batteries used, depending on the hearing aid style or size, the power consumption of the circuitry, and the battery-life requirements. Table 1 compares the capacity and size of the five most common zinc-air batteries, and includes their color codes for easy selection and the styles of hearing aid in which they are usually used.Zinc-air batteries start at 1.4V and are used down to about 1.0V or lower before requiring replacement. When used for 16 hours per day, battery life ranges from a couple of days to a few weeks, depending on the battery capacity and hearing aid design. The most power-efficient design runs directly off of a single battery, but a switching regulator can be used to boost the voltage to fit design needs, whether 1.8V or 3.0V. The power dissipation is targeted to be 1mW to 10mW when running off of zinc-air batteries. Hearing aids that use rechargeable Li+ batteries may require a linear or switching regulator to step the battery voltage down if the circuitry cannot run directly from the typical 4.2V, single-cell Li+ battery's fully charged voltage. Alternatively, the battery charger can limit the charging to a lower end voltage such as 3.3V, depending on the circuitry requirements. An accurate fuel gauge is critical to provide warning before the battery is depleted so that the patient is not left with a nonfunctioning hearing aid.。