How Directional Microphones in Hearing Aids Work

Sound does not arrive from one place. In a room, a voice, a fan, a television and a clatter of plates all reach the ear together. Directional microphones in hearing aids are one of the ways hearing technology deals with that mixture. This article is a plain-English explainer of how they work, and of how they relate to the feature people often call noise cancelling. It describes the engineering only and makes no promise about what any person will experience.

The Starting Point: An Omnidirectional Microphone

A microphone that is not directional is called omnidirectional. A 2021 review in the journal Seminars in Hearing, by Jespersen, Kirkwood and Groth (all employed by the hearing aid manufacturer GN Hearing), describes an omnidirectional microphone as equally sensitive to sounds coming from all directions. It picks up everything around it in much the same way.

A directional microphone is built to respond to directions differently. The National Acoustic Laboratories (NAL) guide to hearing aid features describes a directional microphone as the part of a hearing aid that picks up sound from a given direction, often from in front of you. The same review describes the fixed version as optimised to be most sensitive to sound coming from directly in front.

How Two Microphones Create a Direction

A single tiny microphone cannot tell where a sound comes from, so hearing aids use more than one sound opening. A 2007 Hearing Review article by Mills and Martin described the earlier approach as a single microphone with two sound ports, with a time delay network applied to the rear port to create a front-to-back effect.

Modern digital hearing aids usually do this electronically. The Seminars in Hearing review explains that processing in dual-microphone designs applies a delay to the input from the rear microphone. Sound from the front reaches the front microphone first. Sound from behind arrives at the rear microphone first, and after the delay, the two signals can cancel completely or partly, which reduces sound from those directions. In plain terms, the aid compares the two microphone signals and handles sound that arrives from the sides and back differently from sound arriving from the front.

Adaptive and Automatic Directionality

Because the work is done digitally, it can change from moment to moment. The review describes adaptive systems in which the angle at which the microphone is least sensitive, called a null, can be moved. This lets the pattern follow a noise source rather than staying fixed.

It also describes automatic switching, in which the aid moves between directional and omnidirectional patterns by itself. A steering algorithm may use environmental classification to estimate the speech to noise ratio, and it will select a directional response when that ratio is low. The NAL guide describes a related idea under the heading environmental adaptation: the aid adjusts its settings using the sounds around you, your location or other information.

Wireless Links Between a Pair of Aids

When a person wears two hearing aids, the pair can work together. The review describes a bilateral beamformer, where wireless communication between the hearing aids enables all four microphones to be used to form a more directional pattern. The NAL guide describes this wider category as ear-to-ear communication, where information can be shared between the aids.

Where "Noise Cancelling" Fits In

People searching for noise cancelling hearing aids are usually asking about the second half of this picture. In hearing aids, the term is used as a general label. The NAL guide uses the heading noise reduction, and describes it as features that reduce noise, aimed at listening comfort. The 2007 Hearing Review article by Mills and Martin describes digital signal processing as being about managing the comfort level of noise, and says that on its own it may not improve speech understanding. It describes the combination of a directional microphone system and digital signal processing as an attempt to achieve two goals at once.

The difference between the two is the way they work. Directionality depends on where a sound is coming from. Noise reduction depends on the character of the sound itself. They can be present in the same device.

Conditions That Apply to Directional Processing

Jespersen and colleagues describe directional processing as working under certain conditions. They say the sound of interest should be within about two metres of the wearer, depending on the amount of reverberation, and that a directional response has the potential to reduce awareness of the surroundings. The pattern favours sound arriving from the front. The details of any device are set out by its manufacturer.

For listening situations where the talker is further away, a separate accessory exists. Our article on remote microphones for hearing aids explains how that works, and our article on AI in modern hearing aids covers how some devices use computing to select settings. The wider range is outlined on the hearing aid solutions hub.

A Quick Summary

• An omnidirectional microphone is equally sensitive in all directions. A directional microphone is built to favour a direction, usually the front.
• Most modern aids create direction by comparing two microphones and applying a delay to one of them.
• Adaptive and automatic systems can move their pattern or switch between patterns.
• Noise reduction is a separate feature that deals with the character of sound. Both can sit in the same device.
• Directional processing works under conditions, and the details of any device are set out by its manufacturer.

If you'd like to chat about your hearing, the friendly team at CQ Audiology in Rockhampton is here to help. You can read about hearing aids in Rockhampton, call (07) 4848 6528 or book online.

This article is general information, not personal medical advice. For advice about your own hearing, speak with your GP or a hearing care provider.

Sources: Jespersen, Kirkwood and Groth, Seminars in Hearing, 2021 (Increasing the Effectiveness of Hearing Aid Directional Microphones); Mills and Martin, The Hearing Review, 2007 (Why Don't Directional Microphones Work Better?); NAL, NALguide: Hearing aid features explained (the Australian Government Department of Health Guide to hearing technology states it was developed with the assistance of the National Acoustic Laboratories). The authors of the Seminars in Hearing review are employees of the hearing aid manufacturer GN Hearing.