hd414 "classic" headphones (Bob Carlyon )

Subject: hd414 "classic" headphones
From:    Bob Carlyon  <bob.carlyon(at)MRC-APU.CAM.AC.UK>
Date:    Thu, 25 Jan 1996 12:12:21 +0000

Just before christmas I circulated the results of some calibrations which I'd performed on the "new" HD414 headphones recently re-issued by Sennheiser, together with the calibrations performed on an "old" pair which I had been using in the lab. I received some very interesting correspondence which has prompted me to make some new measurements which I present below, together with a description of some of the complicating factors involved in making these measurements. For those of you with insufficient time and/or inclination to read through the entire discussion, the bottom line is that the distortion produced by the new 414s is not as bad as I made out, but that their frequency response leaves a lot to be desired. Rob Hukin wrote to tell me that he'd read in "Studio Sound" (August 1995) that the only similarity between the new and old 414s was in the mouldings, and that the drivers on the new version were one of Sennheiser's latest 'off-the-shelf' numbers. To quote the article, the new phones use "selected versions of Sennheiser's latest drive units employing lightweight Copper Clad Alloy Wire driver coils, Neodymium magnets and OFC (oxygen-free copper) leads". This could explain a lot. Roel Smits pointed out that the level at which we calibrated the phones was rather high, which I agree with. We used an input level of 0 dBV, which corresponds to an output of about 106 dB SPL on the new phones, and about 98 dB SPL on the old ones. Further investigation revealed that the distortion was highly level-dependent, which, given the different sensitivity of the phones, complicates matters. I should have made the comparison at equal OUTPUT levels, and have now done so. The measurements which follow were obtained at output levels of 90 and 100 dB SPL for one "new" and one "old" earpiece. A further complicating matter concerns the age of the cushions. As they wear out they produce less attenuation; I've seen effects as large as 10 dB at 1 kHz, which makes it advisable to re-calibrate your phones every year or so. The effect of cushion wear is relevant to the measurement of distortion, whose level-dependence is, presumably, determined by the level at the output of the driver (before passing through the cushion). This means that if you use a worn-out cushion, you need a lower "driver output" for a given SPL at the output of the cushion, which will reduce the distortion compared to that measured at the same SPL using a new cushion. The distortion measurements described below used new cushions on both the "new" and "old" 414 tested. The numbers in the table represent the attenuation of the 2nd and 3rd harmonics relative to the primary. An asterisk indicates that the distortion was too low to be measured on my HP3561A spectrum analyser. "NEW" FREQ 100 dB,H2 100 dB, H3 90 dB, H2 90 dB, H3 125 43 51 53 61 250 51 51 61 67 500 59 62 * 68 1000 * 67 * 74 2000 * * * * 4000 72 * * * 6000 * * * * 8000 69 * * * "OLD" FREQ 100 dB,H2 100 dB, H3 90 dB, H2 90 dB, H3 125 37 71 47 74 250 58 73 68 * 500 62 67 74 77 1000 65 75 76 * 2000 * * * * 4000 73 * * * 6000 71 * * * 8000 72 77 * * So, the good news is that the distortion levels on the new 414s are quite reasonable except at very high SPLs. Rather worse news is that the frequency response of the new 414s shows some quite sharp peaks and dips, which vary across units For example, the "new" 414 described above has a response which rises gradually to a peak at 2775 Hz, drops by 4.7 dB as frequency increases to 3175 Hz, rises by 7.8 dB to produce a sharp peak at 3700 Hz, dips again by 6.8 dB at 4375 Hz, remains fairly flat up to about 6800 Hz, before dropping steeply by 7.9 dB at 7300 Hz. The dip at 4300 Hz is fairly typical across units. If anyone has a use for 6 sets of headphones which look like the original HD414s but aren't, I'm open to offers. bob ------------------------------------------------------------------------------ Bob Carlyon MRC Apllied Psychology Unit 15 Chaucer Rd. CAMBRIDGE CB2 2EF England Phone: (44) 1223 355294 ext 720 FAX: (44) 1223 359062 email: bob.carlyon(at)mrc-apu.cam.ac.uk ---------------------------------------------------------------------------- --

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