ANSI S3.1 - 1999:  Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms

ANSI specification for maximum permissible ambient noise levels (MPANLs) for audiometric test rooms are used to ensure that thresholds obtained down to 0-dB HL will not be elevated do to masking by ambient noise.  ANSI S3.1 has been revised several times (1960, 1977, 1991 and now 1999).  In this section, I will highlight the current standard and some of the recent changes to ANSI S3.1 – 1991.

In the standard, ANSI refers to two conditions, ears covered and ears not covered.  The ears covered condition refers to the condition when testing was done with both ears simultaneously covered by supra-aural earphones, as in pure-tone air conduction audiometry.  The ears not covered condition refers to testing done with either one or both ears uncovered as in pure-tone bone conduction or sound field audiometry.   

The main difference between ANSI S3.1 1991 and 1999 is that the 1999 version includes a new way to calculate the MPANLs.  In 1991 the MPANLs were obtained using an NBS 9-A coupler-based RETSPLs which can be found in ANSI S3.6 – 1989 Specification for Audiometers.  This meant that for the ears covered condition, the MPANLs only applied when testing was done using supra-aural earphones.  ANSI S3.6 – 1989 has since been updated to ANSI S3.6-1996 and includes RETSPLs for both supra-aural and insert earphones.  Since the output of an insert earphone is calibrated with an occluded ear simulator or hearing aid coupler (ANSI S3.6 – 1996), a new way to calculate MPANLs was needed that did not use the coupler-based RETSPLs.  This was the primary motivation for the 1999 revision.  The main benefit with the 1999 revision is that ears covered MPANLs can now be specified for any earphone meeting the specification for a supra-aural or insert earphone (specifications in ANSI S3.6 – 1996).

Table 1 indicates octave and one third octave band maximum permissible ambient noise levels for three test frequency ranges specified in ANSI S3.1 – 1999 for ears not covered.

click image to enlarge

Table 2 indicates octave band ears covered maximum permissible ambient noise levels for three test frequency ranges as specified in ANSI S3.1 – 1999 when ears covered testing is done using a supra-aural or insert earphone. 

click image to enlarge

Table 3 indicates one-third octave band ears covered maximum permissible ambient noise levels for three test frequency ranges as specified in ANSI S3.1 – 1999 when ears covered testing is done using a supra-aural or insert earphone. 

click image to enlarge

Compared with the 1991 MPANLs, the 1999 values are about 2 dB higher at 125, 250, 500, and 4000 Hz, 6 dB higher at 2000 Hz, and 6.5 dB lower at 8000 Hz.  What this ultimately means for the Audiologists is that the 1999 standard allows for a little more low frequency ambient nose and a less very high frequency noise than the 1991 standard. 

According to ANSI S3.1 – 1999, ambient noise SPL should be below those values stated in the above three tables whenever possible.  There is a 2 dB variability around values provided in tables 1-3.  That is, if a threshold value was measured in a sound booth equal to those in the table, then you can expect a level of uncertainty within 2 dB of that value.  Because insert earphones proved more attenuation than supra-aural earphones, higher ears covered MPANLs are allowed when testing is done with an insert earphone as shown in Table 2 and 3.    

Ultimately, the Audiologist probably will only be interested in knowing if the current test booth meets the current ANSI standard.  For this to occur, the measured ambient noise SPL from 125-8000 Hz are compared with the MPANLs shown in tables 1, 2, and 3 in reference to the type of measurement (octave or one third octave band) and the test condition and test frequency range to be used in the test room.  If they do not exceed the values stated in these tables, then the sound room passes ANSI 1991 standards and is acceptable for testing.  If this requirement is not met, even at one octave, or at one one-third octave band, then the room fails and is not considered to be acceptable for testing down to 0 dB HL.        

Note:  This site is best viewed using IE 4.X or higher under 800x600 resolution