Installation & Operation Manual

Wilat_Op_Man_23Oct08.doc CONFIDENTIAL
Aearo Technologies Page 16 of 21 18/07/2007
5. Background Noise Reduction
A major benefit of the E-A-RTONE Insert Earphone in combination with the E-A-RLINK
disposable foam eartips is the reduction of background noise that may adversely affect threshold
determination. When E-A-RLINK eartips are appropriately selected and fitted to the test subject,
the expected noise attenuation will typically exceed 30 dB in the frequency region 125-8000 Hz.
This additional attenuation will improve test reliability in environments where the ambient noise
levels exceed those listed in Table 5.1 below for the supra-aural earphone. Typically 0 dB HL
threshold responses can be obtained in environments where the ambient noise level does not
exceed 45 dB(A).
Table 5.1 (from: ANSI S3.1-1999) - Ears covered: octave band MPANLs using a supra-aural or insert earphone for
three test frequency ranges; in dB re: 20Pa to nearest 0.5 dB.
Supra-aural Earphone
Insert Earphone
Octave
Band
Intervals
125 to
8,000 Hz
250 to
8,000 Hz
500 to
8,000 Hz
125 to
8,000 Hz
250 to
8,000 Hz
500 to
8,000 Hz
125
35.0 39.0 49.0
59.0 67.0 78.0
250
25.0 25.0 35.0
53.0 53.0 64.0
500
21.0 21.0 21.0
50.0 50.0 50.0
1000
26.0 26.0 26.0
47.0 47.0 47.0
2000
34.0 34.0 34.0
49.0 49.0 49.0
4000
37.0 37.0 37.0
50.0 50.0 50.0
8000
37.0 37.0 37.0
56.0 56.0 56.0
The ears covered MPANLs shown in the above table were derived using mean earphone
attenuation values. To decrease the chance that individual listeners for whom less than average
attenuation is obtained will experience threshold shift, the MPANLs can be lowered by reducing
the amount of earphone attenuation. An appropriate reduction would be equal to the mean
attenuation minus one standard deviation. The net effect would be that the MPANLs would be
lower (more stringent) for ears-covered testing. This reduction would cover about 84% of all
subjects. The standard deviations for Insert Earphones for 125 to 8000 Hz are between three (3)
and six (6) decibels. Refer to ANSI S3.1 – 1999, Table A.2 for specific standard deviation values
by frequency.