Environmental Acoustics and Directional Microphone Hearing Aids
This is a continuation of the previous blog and identifies additional issues that impact the performance and use of directional microphone hearing aids. The previous blog discussed how directional microphone equalization adjustments and distance between microphone port openings impacts circuit noise that can be heard by the hearing aid wearer. Understanding these issues makes it possible for a hearing aid user to obtain the best benefit from their hearing aids. These effects can be used in counseling sessions by the dispenser with the consumer, or can be understood and used as well by the consumer.
This blog will relate to the following issues:
- Environmental Acoustics
- Wind noise
- Open fittings and directional microphone hearing aids.
Directional microphone performance is affected by the listening environment that the hearing aid is used in. Primary concerns relate to (1) how reverberant the listening environment is, and (2) the distance of the primary signal and the competing noise from the listener.
In a reverberant environment of 1.2 sec or greater, the directional microphone becomes ineffective. It is even less effective as reverberation time increases above this 1.2 second time. In a very reverberant environment (for example, a room with hard walls, tile floors, hard ceiling), the directional microphone may decrease intelligibility to the point that not wearing the hearing aids might be advised.
There is considerable research to suggest that a critical distance for directional microphone advantage is about 6 feet in an open environment (outside). The advantage is less in a closed environment (inside). The best use of directional microphone hearing aids is when the signal source is in front and relatively near the listener, and when the noise is spatially separated from the signal source, and fairly close to the rear of the listener.
Directional benefit cannot be expected in all noisy environments. The advantage decreases at very easy (+6 dB and in very difficult (-18 dB) Signal-to-Noise Ratios (SNRs). See Figure 1.
Figure 1. Directional microphone benefit is related to about a 24 dB SNR range (+6 to -18 dB). When the signal is about 6 dB or greater than the noise, there is no need for directional mic performance because the person hears speech much greater than the noise. When the noise becomes 18 dB or greater than the speech (a -18 dB SNR), the directional mic loses its effectiveness because the noise is so great that it interferes with speech understanding, regardless of what is done.
Wind Noise and Hearing Aids
Wind noise in hearing aids is generated by movement of air very close to the ear, usually within 10 cm, or just under 4 inches. The turbulence created in the moving air as it passes by the ear causes a wind-like sound. In hearing aids that have a microphone(s) on the housing of the aid, as in those that fit over the ear, the microphone is very close to the noise source – closer than the eardrum in an open ear that does not have hearing aids. Additionally, directional microphone patterns are relatively more sensitive to sounds from a near field than from sounds further distant. It is for this reason that wind noise is perceived louder in directional microphone hearing aids than in omnidirectional microphone units.
Open Fittings and Directional Microphone Hearing Aids
Many directional microphone hearing aids today are fitted in an open coupling arrangement. As a result, the conditions that allow for directional microphone performance are varied in ways where the outcome is less predictable because an open fit has a direct affect on phase of the signal impinging on the microphone ports.
For example, an open fitting allows sounds from the environment to enter the ear canal directly as well as having the amplified processed sound being directed into the ear canal at the same time (a two-channel system). Sounds at the eardrum may be dominated by different sources (direct or processed) depending on their levels and phase interactions (Figure 2). With hearing aids, cancellation and reinforcement of sounds may occur depending on these phase relationship between the direct and processed sounds. This may result in unexpected dips and/or peaks in the measured real-ear aided response, and as a result, sound quality may be affected. Strategies and algorithms designed to preserve the phase relationships may not function as advertised in a normal listening environment.
- Figure 2. Illustration of direct sound (left) and combined processed direct and incident sound (right) and the effect it has on the signal at the eardrum, and on reflected sound. Direct sound to the eardrum from the hearing aid is not influenced by the phase and amplitude of incident sound passing from the external environment to the eardrum. Phase modifications to what is being processed at the eardrum comes from the interaction of direct and incident sound.
In a noisy situation, the level of the direct sound can be higher than the level of the processed sounds treated by the directional microphone. Dillon (2001) reported that the directivity index (DI) of a directional microphone hearing aid decreases by 4-6 dB in the lower frequencies when a vent (3 mm diameter) is used. Fortunately, the higher frequencies (about 1000 Hz) are minimally affected. And while this is the case for a 3 mm diameter vent, the decrease in the DI is expected to be even greater for an open fit hearing aid.
Most current directional microphone hearing aids use two omnidirectional microphones and the circuity uses amplitude and time delay before the output is summed and passed to the pre-amp or elsewhere in the circuitry. To provide maximum benefit, they must be matched in phase and amplitude. Their advantage diminishes, if not. Venting, or maximum venting as in an open fitting, introduces a time of arrival at the eardrum interruption, which is the same as a phase interruption. As a result, it is possible that the dual omnidirectional microphones may not remain matched in phase and amplitude, especially in a real world, rapidly-changing listening environment.
In real life the jury may still be out as to whether the open fit and directional mic hearing aid interact positively.
The next blog will look at actual measurements of directional microphone performance to determine if directional microphone hearing aids are really directional.
Dillon, H. Hearing Aids. Sydney, Australia: Boomerang Press; 2001.