The primary complaint of people with hearing loss and the primary complaint of people wearing hearing aids is speech understanding in noise.

There are two main causes as to why it is difficult to hear in background noise

1) Hearing loss

A common type of hearing loss is a high-frequency sensorineural hearing loss, which is a permanent loss of high pitch sounds. Below is a classic example of a high- frequency hearing loss, as shown in red circles. The dark grey region above the red line is an area where it is inaudible to the person. The lighter region below the line represents the audible area. The consonants, which gives clarity in speech are mainly located in the inaudible, dark grey region. The vowels that convey volume are located in the audible, lighter grey region. The problem with background noise is that it mainly consists of low pitch sounds which mask the speech in the audible area.

Hence a person with this hearing loss is losing both speech volume an clarity in background noise

2. Brain Hearing

The ear is simply a microphone to the brain. The brain is where hearing and understanding of speech takes place.

Two factors that may affect speech understanding at the brain level are

The damage in hearing might cause distortion of the speech signal, so the brain does not receive a clear enough sound to be able to understand.
The brain itself may have trouble separating speech from noise. So if the signal is not clean enough, the brain cannot make sense of the speech.

Recent research by Petersen et al. (2016) explains the second point.

In quiet environments, the speech sound of interest is present in isolation. The acoustic information is clear and well-defined, largely because it has no competition. Therefore, understanding speech is generally effortless.

In noisy environments (restaurants, bars, cocktail parties, etc) the primary speech sound is acoustically mixed with multiple secondary speech sounds (i.e., background noise) at the ears. To solve this dilemma, the brain attempts to organize and prioritize sounds present in the sound scene by focusing on the primary speech sounds while ignoring all others, i.e. secondary sounds. For listeners with more significant hearing loss, there is no significant difference in the encoding of the two speech signals, thus indicating that people with hearing loss, are less able to ignore the disturbing secondary speech (i.e., background noise).

How can Hearing aids help?

Hearing aids cannot change how the cochlea or the brain work, but hearing aids can change the sound that enters the ears. Hearing aids attempt to reduce disturbing background noise to support the brain’s ability to organize complex sounds and thereby better focus on the primary speech sound. Many hearing aids do this with directionality microphones and algorithms that filter out excess background noise. ( Click here to see the latest hearing aid technologies)

Reference

Petersen, E.A., Wöstmann, M., Obleser, J., Lunner, T., (2016) “Neural tracking of attended versus ignored speech is differentially affected by hearing loss”, Journal of Neurophysiology