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 Reduction
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 Reduction
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.
Hearing aids with this feature can remember your volume and program preferences in specific listening environments. You can train the hearing aids with a push button or remote control. For example, if the volume of the hearing aids is reduced every morning by using the volume control or remote control, eventually, the hearing aids will automatically turn on at a lower volume setting.
Many hearing aids internally record the number of hours the hearing aids are worn, which programs are being used, how often and how much the volume is increased or decreased, and, in some cases, the nature of the sound environments. This tool can often be very helpful in fine-tuning the hearing aids and helps the audiologist identify and resolve certain types of difficulties you might experience.
Understanding speech on the telephone can be difficult for some individuals with hearing impairment and hearing aids can help in various ways. Signals from both cell phones and landline telephones can be heard through the hearing aid, either by simply placing the receiver near the microphones of the hearing aid or by utilizing the electromagnetic induction coil (telecoil) contained in many hearing aids. Specific hearing aid compatible telephones work best with the telecoil.
An automatic telephone sensor is available in some instruments, and it automatically perceives the presence of the electromagnetic signal from a hearing aid compatible phone and switches the hearing aid to either an acoustic telephone or telecoil program. Premium hearing aids are also able to present the telephone signal to both ears when the telephone is placed over one ear.
Wireless and Bluetooth Connectivity
This technology uses a Bluetooth or wireless streaming device (either worn around the neck or kept in the pocket) to receive sound from Bluetooth transmitters and send the sound to the hearing aids. For example, the streaming device will pick up the Bluetooth signal from a cell phone and directly transmit the signal to your hearing aids. Wireless devices are also available to stream other audio signals directly to the hearing aids, like the audio signal from a television or MP3 player. Recently, wireless devices can also stream the signal from a lapel microphone that a speaker might wear.
The transmission from these streaming devices is then heard in stereo with little interference from competing noises in the environment. The signal can be received from up to 30 ft away.
Many hearing aids can be operated using a remote control. For some individuals and certain hearing aids, remote control can be very helpful, allowing the changing of programs and/or volume without touching the hearing aids. The increased automatic functioning of modern hearing aids has somewhat reduced the need for remote controls, however, many hearing aid users still find them beneficial. Some remote controls can also act as a Bluetooth streaming device.
Some hearing aids have a feature described as frequency shifting or frequency lowering. When the hearing loss in the high pitch region is severe to profound, it can be difficult to provide adequate amplification to those pitches. With frequency shifting or frequency lowering, high pitch sounds are shifted down to lower frequencies where hearing is typically better. Consonant information in speech is typically present in the high pitches and by shifting these sounds down to an area of better hearing, speech understanding may be improved. This feature may require a period of adjustment to learn to use these different speech cues.
Sound Generators or Tinnitus Maskers
Several hearing aids have the capability of internally generating sounds that are not present in the environment. The sound generators are used to produce various sounds that can help reduce the perceived loudness of tinnitus (ear or head noise). This feature is relatively new and more detailed information is discussed in the Tinnitus Section of this website.
With all of the features now available in hearing aids it can be somewhat confusing to decide which feature(s) are needed. The audiologist you are working with will help analyze your listening needs and help you evaluate which features are appropriate for your listening environments and needs.