How to choose the test environment for real ear test?

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Where is it going?

First of all, there are two options for audiologists to determine where to conduct a true ear test:

Out & in

In the acoustic field, the reflection time is short, the reflection surface is small, and the background noise is low. However, the space in the sound field is very important. When the real ear test equipment is in the sound field, it artificially increases the reflecting surface. Therefore, it is a helpless choice to set the test outside the sound field. But the question is, can the measurement outside the sound field ensure the same reliability and effectiveness as the measurement inside the sound field? How to arrange the measurement site so that the prepared true ear test results can be obtained outside the sound field?

If the following conditions can be met, the test in the room outside the sound field is also acceptable:

1. The distance between speakers and subjects is appropriate;

2. The sound pressure level of background noise shall not exceed that of loudspeaker;

3. There is no reflecting surface near the test point.

How to control the noise?

After selecting the test site, it is also necessary to control the noise generated during the real ear test to prevent noise interference from affecting the test results. Here we need to consider three main types of noise:

1. Background noise of the test site;

Second, the noise inside the real ear test system;

3. Crosstalk caused by excessive sound intensity.

1. Background noise

In order to ensure the accuracy of the test, it is necessary to eliminate the interference of background noise at the test site. We have a series of anti-interference measures.

Method 1

First of all, the real ear test equipment has its own anti background noise technology. In its test chain, a filter is used to distinguish the background noise in the test room. It tracks and filters the sweep pure tone and broadband stimulus tone, which can eliminate a considerable part of the noise.

Method 2

Of course, the self-contained background noise anti-interference technology can not completely avoid the noise interference. At this time, there is another strategy: choose sweep pure tone or internal tone, which is more anti noise than broadband noise. Because the signal energy of the former is concentrated in a relatively narrow frequency range, rather than scattered throughout the frequency range.

Method 3

However, no matter where the equipment is, the lowest testable strength must be determined first, which is particularly important for hearing aids of nonlinear amplification lines. The quieter the environment, the lower the measurable minimum intensity.

65dB SPL is the lowest test intensity that almost all hearing aids can accept. However, for some non-linear amplification line hearing aids, it is necessary to test with a low sound intensity as low as 40dB SPL. In order to avoid the influence of background noise, it is recommended to replace it with an internal sound measurement in a quieter environment.

2. Internal noise

In order to get a flat frequency response, the detector needs at least 20dB high-frequency amplification, but at the same time, it will also amplify the internal noise of the high-frequency part of the detector microphone. Due to the amplification of the internal noise, the accurate measurement range of the probe microphone is largely limited, especially the intensity of the light sound test.

ANSI regulations

ANSI s3.46 (1997) specifies that the internal noise of the probe microphone must be at least 10 dB lower than the minimum measured sound pressure level.

For example, if the sound pressure level of 2000Hz 60dB is measured, the equivalent internal noise of the probe microphone (i.e. the sound pressure level generated by the microphone measured when there is no external signal) shall not exceed 50dB SPL at 2000Hz, and the error of the test result shall be controlled within 1dB.

3. Crosstalk

In addition to the above two kinds of noise, crosstalk is also a noteworthy problem. Crosstalk refers to the phenomenon that the sound exceeds the normal and “leaks” from one signal path to another, which will lead to inaccurate test results.

Cause

In the real ear test system, crosstalk can occur between the sound source and the microphone of the detector tube, or between two different microphone signal channels.

If the sound source intensity is large enough and the microphone body of the probe tube is not sealed tightly, some sounds may bypass the sound inlet of the probe tube and enter the measurement path directly from the microphone body or the broken probe tube wall, resulting in deviation of the results.

ANSI regulations

In order to solve this problem, ANSI s3.46 (1997) requires that the microphone readings at the entrance of the detector tube in the sound field should be compared under the two states of blocking and opening respectively. The readings at the time of blocking should be at least 10 dB smaller than those at the time of opening, so as to ensure that the error of the test results is controlled within 1 dB.