Subject: Re: [AUDITORY] Differences between RETSPLs for different transducers From: Raul Sanchez-Lopez <raul.sanchezlopez@xxxxxxxx> Date: Tue, 23 Jul 2024 11:46:55 +0200Hi Ben, This is a very interesting question and very important indeed. Let's go=20 step by step so I can share my understanding. (1) RETSPLs are the result of testing the hearing thresholds of a group=20 of young typical hearing listeners (usually 25 people under 25 years=20 old) with specific transducers. (2) These transducers have been previously calibrated in dB Sound=20 Pressure Level (SPL) using an ear simulator or just a microphone in the=20 case of free field or diffuse field. (3) The specification of the RETSPL is then the averaged hearing=20 thresholds corresponding to the hearing tests. The RETSPL is then the deviation required to play the stimulus in the=20 equivalent dB HL (hearing level). Applying that deviation to the=20 transducers calibrated in dB SPL (i.e. with a flat response using the=20 ear simulator) we assume that we are then playing dB HL. Does anyone know of any analyses that explain the differences in the=20 RETSPLs for different transducers obtained with ear simulators (rather=20 than just standard couplers)? I do not know, but I would say that it is not the standard couplers but=20 it is probably related to the losses (leakage). If you look at the=20 RETSPL of the TDH39 (supra-aural), you can clearly see that at low=20 frequency you need 15 dB more than with the HDA-200 (circumaural). To=20 calibrate these two in transducers you need different couplers, but they=20 are also the couplers used in the first place. On the other side of the coin, we have the Free-field RETSPL which show=20 a characteristic dip at 3-4kHz. That is probably because the calibration=20 of the transducers (2) for the test (1) was done with a free-field=20 microphone, so the hearing threshold results reflect the resonance of=20 the ear canal. I don't know about any study that compares hearing thresholds using=20 different transducers after applying the RETSPL. That would be very=20 interesting to see, especially for people with hearing loss (symmetric=20 and asymmetric). Best Regards Raul Sanchez-Lopez Hearing Researcher | Audio Engineer | Technical Audiologist On Fri, 19 Jul 2024 at 16:20, Ben Lineton=20 <000002db5b7c3f83-dmarc-request@xxxxxxxx> wrote: Dear list, Does anyone know of any analyses that explain the differences in the=20 RETSPLs for different transducers obtained with ear simulators (rather=20 than just standard couplers)? If we assume a person=E2=80=99s hearing th= reshold=20 is determined by the SPL at the eardrum roughly independent of tranducer=20 and further assume that an ear simulator achieves a transfer impedance=20 close to that of the average adult ear, then we would expect these=20 RETSPLs to be very similar (and equal to measurements of the minimum=20 audible pressure under headphones). The attached figure shows the RETSPLs for six transducers measured with=20 three ear simulators: the IEC318-4 (was IEC711) for insert phones, the=20 IEC318-1 for TDH49 supra-aurals, and the IEC318-1 with flat plate for=20 the circumaurals (HDA200,250,300). The figure below show a large=20 differences between RETSPLs (around 20 dB) at 125 Hz and 3 kHz. (Also=20 shown for comparison is the freefield RTSPL measured at the head=20 location plus the head-related transfer function to give the eardrum SPL=20 plus a 2 dB correction for binaural listening; see e.g. Brian Moore,=20 Intro to Psychology of Hearing for explanation). Explanations for these differences that I=E2=80=99m aware of or seem plau= sible=20 are these: 1. Detection thresholds are not just determined by the SPL at the=20 eardrum, but are affected by physiological noise which depends to some=20 extent on the ear canal termination of different headphones (hence=20 violating the first assumption above). 2. The acoustic transfer impedance of the ear simulators differ=20 systematically from that of the notional target population (hence=20 violating the second assumption above). 3. Possibly effect of the point acoustic impedance which may differ=20 between the average real ear and the ear simulator. 4. Sampling errors in determining the RETSPLs. I wondered if there other factors that I haven=E2=80=99t thought of, and = is=20 there any analysis quantifying these different factors (and their=20 frequency dependence)? Thanks Ben Ben Lineton University of Southampton --=20 Raul Sanchez-Lopez Researcher