Description |
Definition of Artificial Ear in AS 1591.5
“A device for the measurement of earphones which presents to the earphone an acoustic impedance equivalent to the impedance presented by the average human ear”
Placement of test Earphone
The Right or Left Earphone is removed from the head band, and placed face down on the conical section. The retainer is adjusted to hold the earphone in place, the tension is adjusted to 500g and clamped by tightening the knob. Aim to achieve a seal between the earphone cushion, and the top of the adapter cone to keep the presented impedance within specification.
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 B&K 2203 SLM & Artificial Ear - Mounting an earphone on the artificial ear |
Construction of B&K artificial ear
• Coupling cavity - 6cc or 2cc for earphone or insert earphones respectively
• Clamp for holding earphone at a known force- (in-built spring gauge. Set to 200g when using 2cc coupler, & 500g for the 6cc ANSI)
• Microphone mount for the Sound Level Meter
• Vibration absorbing foam rubber feet
Artificial Ear Calibration
If an external calibrator such as the pistonphone is used to set
up the Sound Level Meter without the artificial ear, the sensitivity
of the included microphone is compensated for in the single calibration
adjustment. Connect the Artificial Ear to the SLM in place of the
microphone and gently insert the microphone with its cover into
it. The artificial ear has no insertion loss, so calibration is
unchanged.
When the TDH 39 earphone is correctly sealed against the artificial
ear, its frequency response is flattened in a consistent manner
by the impedance thus presented and is similar to its response on
a human ear. This allows the listing of “Reference Equivalent
Threshold Sound Pressure Levels (RETSPLs) Specified In Terms Of
Measurements on The Reference Coupler”. (AS 1591.2 table 2)
All we need to do to find our working levels at each frequency is to add the Audiometer level setting to these figures. The audiometer calibration level is usually performed at about 60 to 75db, to keep above the noise floor, and in the usual working range. To make it easier we keep ours at one setting; 60dB.
Calculation of Audiometer Earphone Calibration levels (ISO 389, & AS 1591.2 table 2)
Telephonics TDH-39 Headphone |
Audiometer Setting |
Required Calibration Level |
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| Frequency Hz |
RETSPL dB ref 20µPa
|
dBµV or dB20µPa |
|||
| 125Hz |
45.0dB |
60dB |
105.0 dB |
||
| 250Hz |
25.5dB |
60dB |
85.5 dB |
||
| 500Hz |
11.5dB |
60dB |
71.5 dB |
||
| 750Hz |
8.5dB |
60dB |
68.5 dB |
||
| 1000Hz |
7.0dB |
Plus |
60dB |
Gives |
67.0 dB |
| 1500Hz |
6.5dB |
60dB |
66.5 dB |
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| 2000Hz |
9.0dB |
60dB |
69.0 dB |
||
| 3000Hz |
10.0dB |
60dB |
70.0 dB |
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| 4000Hz |
9.5dB |
60dB |
69.5 dB |
||
| 6000Hz |
15.5dB |
60dB |
75.5 dB |
||
| 8000Hz |
13.0dB |
60dB |
73.0 dB |
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Required Audiometer frequency and level accuracy: Ref AS 2586,
Sects 8, 9: Frequency: 3%, Level: +/-3dB, and +/-5dB @ >4000Hz
Ref. AS 1591.2-1987, “Reference Zero For The Calibration Of
Pure Tone Audiometers. Table 1: Reference Equivalent Threshold Sound
Pressure Levels In The Wide Band Artificial Ear”, and Table
2 “RETSPLs in the reference coupler” ref to “Telephonics
TDH-39 & MX41/AR cushion” - listed above.