Frequently Asked Questions

On January 30, 2002, an international reader from the UK wrote,

Hello

I like the audiomtric pages, very good. Do you know, have access to information,specification/suppliers on the various couplers especially those used in the States. I am familiar with the various IEC couplers (60318 parts 1,2 & 3, 60126, 60711 & 60373) but have little information on the 9A coupler and nothing on some of the other devices that you mention (HA1, HA2 etc)

Regards

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Michael Der wrote,

Thanks for your question.  The 9A and HA1 HA2 couplers that you mentioned are used specifically for the calibration of audiometric equipment.  The NBS 9A (there is a diagram on the topic titled Audiometric Earphone Calibration Using and Artificial Ear/Reference Coupler in the Audiometer section) is commonly used for calibrating the TDH style earphones for use with a clinical audiometer.  Theoretically it is supposed to mimic an average (artificial) ear's volume with the earphone cup placed over it.  The HA1 and HA2 couplers are 2cc couplers commonly used to calibrate the ER3A type insert earphones.  I will try to find you additional information on these couplers and their suppliers and I will post it on my website and email it to you. 

Regards,

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Reader responded, 

I thought that the 9A coupler was similar (slightly different volume) to the IEC reference coupler, can you confirm that ANSI allows calibration of TDH49 etc on this device. Under ISO/IEC standards ISO 389-1 states that the reference coupler should only be used with Beyer DT48 earphones with flat cushions and with Telephonics TDH-39 earphones fitted either with MX 41/AR cushions or with the newer style one-piece Model 51 (P/N 510C017-1) cushions, when the RETSPLs given in Table 1 should be used. Hence the artificial ear and RETSPLs given in ISO 389-1Table 2 should be used for any other combination. This implied statement of equivalence between MX 41/AR and Model 51 cushions was included for the first time in this recent issue of ISO 389-1. Until then it had been implied that TDH-39 earphones fitted with Model 51 cushions should use the artificial ear as the calibration device, together with the RETSPLs for that device. This posed many calibration problems and ISO decided to clarify any ambiguity by defining the cushions as equivalent. Metal and plastic cased telephonics earphones are also considered similar (but I disagree but thats another story)

I see you reference the grid on/grid off work I did more years ago than I care to mention, does the 9A coupler specify LS1P microphone or can it be used with grid on, is it used with grid on.

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Michael Der's replied:

Regarding your question on whether ANSI allows calibration on an IEC 318 coupler.   I can confirm that ANSI allows that TDH39/49/50 earphones to be calibrated using an IEC 318 coupler (refer to Annex A:  Correction figrues for free-field equivalent output for certain types of commonly used earphones, ANSI S3.6 -1996).  Correction values are however different for both the IEC 318 and ANSI 9A coupler, so although ANSI does allow for calibration using the IEC 318, a different set of correction values need to be applied to the measured data of SPL. 

Regarding your comment on the equivalence of the MX 41/AR and the model 51 cushion.  I believe that ANSI has considered them to be equivalent for some time.  Have you found that the calibration on the same earphone using a different cushion has lead to some measurement variability? 

Hope this was helpful.  If I can offer further assistance, please don't hesitate to ask.

Regards

Michael Der

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