In the past several years, hearing aids having a directional microphone option as one or more of the settings of the hearing aid have had a resurgence, especially with those hearing aids that fit over the ear in either a RIC (receiver-in-canal) or open coupled device using a thin tube, but with the receiver in the hearing aid rather than in the ear.
Directional microphones were first used almost 70 years ago to reduce acoustical feedback oscillation in public address systems to suppress noise at sporting events, and to improve better signal-to-noise ratios than omnidirectional mics.
Directional microphones were introduced into hearing aids in the early 1970s. However, they lost favor in the late 1970s, primarily because custom-molded hearing aids became the rage, especially the in-the-canal aids that did not have the physical space for the directional mic size, and also because the wearer did not have the option to listen in both the directional and omnidirectional modes.
Many current hearing aids offer various forms of directional performance, from a classic front/back sound option, to adaptive directional, and to adaptive directional that focuses on the primary voice signal, regardless of direction (front, back, side, or variations in between).
And while there is substantial research to validate the improved S/N of directional microphone hearing aids, historically there are issues that can present downsides to directional mic aids. These can be identified as: (1) increased awareness of circuit noise, especially when equalization is used to drive more low-frequency amplification, (2) less, or even negative directional advantage in reverberant environments, (3) greater wind noise awareness, (4) questionable results when using open fittings, and (5) although programmed directional, are they really producing results that are directional as intended.
In this, and subsequent blogs, these issues related to directional microphone hearing aids will be addressed. Of course, the primary question to be answered is issue #5. However, to get to that point, you will have to endure a series of discussions related to the first 4 issues. The issue to be addressed in this blog relates to the increased circuit noise awareness and how programming the directional performance can add to or reduce this circuit noise.
Increased Awareness of Circuit Noise With Directional Mic Hearing Aids
Figure 1 illustrates low-frequency reduction in the frequency response as a result of switching from an omnidirectional mic to a directional microphone (red). Equalization is the process of bringing the directional mic performance up to the broader response of the omnidirectional mic (arrow). However, there is a consequence to such action. Circuit noise increases the closer one sets the directional mic performance to the omnidirectional mic performance (in other words, when more low frequencies are added).
Figure 2 illustrates the tradeoff of directional mic equalization and hearing aid circuit noise. As more equalization is added (providing for more low-frequency gain to compensate for the loss of low frequencies created by a directional mic function), the greater is the amount of noise experienced by the hearing aid wearer. This is especially true if listening in quiet, or if the patient has good low-frequency hearing.
Directional Mic Noise Related to Distance of Mic Openings to Each Other
Today’s directional mics mostly use two omnidirectional microphones and software algorithms to generate directionality. Microphone sensitivity falls with additional mics and also as the microphone ports are closer together. This is due to a subtraction process that occurs in the difference mic. The second mic increases the noise level at least 3 dB, and it is even greater if the mic ports are closer together. And, low-frequency amplification makes the situation worse because it amplifies the internal noise.
So, what are the consequences of equalization and mic opening distances to the user? Adding more lows to the frequency response of a directional microphone hearing aid (called equalization – to get the response to look more like the omnidirectional mic response), adds noise to the hearing aid. The closer to the omnidirectional mic performance, the greater the noise. Additionally, the distance between the microphone openings influences the hearing aid noise floor. The closer they are to each other, the greater is the noise floor.
The next blog will discuss environmental acoustics and how it often limits the effectiveness of directional mic hearing aids.