Features of Hearing Aid Technology
Analog versus Digital Hearing Aids
Analog hearing aids have been available for many years. They have a microphone to collect sound and convert the sound into electrical energy, an amplifier to increase the strength of the electrical energy, and a receiver or speaker to convert the electrical energy to an acoustic sound. Analog hearing aids may help amplify soft sounds without over-amplifying loud sounds through a process called automatic gain control (AGC). However, analog hearing aids do not typically have other advanced features.
Digital hearing aids are more complex. With digital hearing aids, a microphone picks up sound, which is then converted into digital signals. The digital signals are then processed by the small computer chip in the hearing aid. Once the digital signal is analyzed and processed using Digital Signal Processing (DSP), it is converted into acoustic sound. DSP allows for changes in volume, but can also provide noise reduction and other features to help improve communication in difficult listening environments.
Currently, very few analog hearing aids are available, and most hearing aids contain DSP. Within digital hearing aids, however, many different features are available and are discussed further in this section.
Gain (volume) Processing
For many years, hearing aids have been able to automatically increase or decrease volume depending on the input sounds. This feature minimizes the need to physically adjust the volume control. However, for most patients with hearing loss, simply increasing or decreasing the volume does not improve the clarity of speech. You may be decreasing sounds that are too loud, but at the same time decreasing sounds that need to be increased to improve speech understanding. You may often notice this with the volume control settings of your television or radio.
More recently, hearing aids are able to separate sound into different frequency (tonal) regions, called channels. The volume of each channel can be adjusted independently, allowing for certain sounds to be amplified more than others, similar to an equalizer on a stereo. The amount of amplification in each channel can generally be adjusted by your audiologist using the hearing aid programming software.
Manual volume controls to change the overall volume are available on many hearing aids. Volume controls may be accessed by a button or volume control wheel on the hearing aid or, in some cases, with remote control.
Number of Frequency Channels
The number of channels available for programming differs among hearing aids. With more channels, hearing aids can be programmed to more accurately fit your hearing loss. Also, with more channels, the analysis of the sound environment is more accurate, which can enhance the function of other hearing aid features. However, more is not always better. More than 15 to 20 channels can cause sounds to become ‘muddy’. With some hearing losses, hearing aids with many channels may not be a significant improvement over hearing aids with fewer channels.
One of the most difficult listening situations for individuals with hearing loss is understanding conversations in noisy environments. The most effective way of minimizing the negative effects of surrounding noise is to have two microphones on each hearing aid – one for the area in front and one for the area behind. Each microphone provides information to the hearing aid processor, which analyzes the sound in the environment. When the analysis shows a high level of noise, the sensitivity of the back microphone is reduced, to decrease the noise from the back.
In less expensive hearing aids this is done by pushing a button on the hearing aid or remote control to reduce the noise. In moderate or more premium hearing aids, the processor may be powerful enough to automatically reduce the sensitivity of the rear microphone when the environment gets noisy. It will then also increase the sensitivity of the rear microphone to normal when the environment quiets down, so you do not miss the soft sounds behind you.
One thing to remember is that directional microphones help reduce noise, but do not eliminate noise.
Digital Noise Reductio
In addition to directional microphones as a tool to help in situations with competing for noise, hearing aids can reduce amplification in certain channels. Typically, amplification is reduced in the channels that provide little benefit to overall speech understanding. This can be helpful in reducing the noise that is arriving from the front and the overall noise in the room.
More premium hearing aids may also work to enhance the speech arriving from the front by increasing the amplification in the channels that carry important speech information so that the speech is more pronounced than the noise. In some very noisy environments, however, understanding speech can still be very difficult even with the most sophisticated processing.
Digital Feedback Reductio
Acoustic feedback in hearing aids is the high-pitched whistling sound that you may have heard from some older hearing aids. It is a result of the amplified sound leaking out the ear canal and being picked up by the microphone of the hearing aid. Fortunately, feedback is now much less common because most digital hearing aids have a feedback manager that reduces feedback. Manufacturers differ in the way that feedback is controlled, but generally, the premium instruments are more effective. The probability of feedback also depends on the configuration and severity of the hearing loss. Therefore, not every patient needs the most sophisticated feedback management system. Feedback also depends on the fit of the instrument. Feedback can be reduced if hearing aids are fit properly.
Multiple Programs or Memories
Multiple programs or memories can be stored in hearing aids and accessed using a push button or through a remote control. These programs optimize hearing aids for different listening environments. Multiple programs can also be available for special uses, such as for listening on the telephone or to television. More advanced hearing aids analyze the sound environment and adjust automatically for specific environments. For example, premium hearing aids can correctly identify that you are in a noisy restaurant and activate the directional microphones and noise reduction. In less expensive hearing aids, the program may have to be manually changed by pressing the buttons on the hearing aids or using the remote control in order for the directional microphones and noise reduction to be activated.