I just bought a Stax SRM Monitor unit to try out Stax's attempt at diffuse field equalisation and thought I'd post a little summary of what I understand of this topic. It is such a rare unit that I couldn't let it go - only ever seen one for sale before in the past 10 years. It seems to be about as infrequently put up for sale as an SRM-T2!!!
Diffuse Field Equalisation vs Free Field Equalisation.
Headphone measurements conducted with a microphone in front of the headphone driver, much as speaker measurements were conducted, observed that headphones with a measured flat frequency response did not sound as flat as they measured. Comparing the sound of a flat measured speaker with a flat measured headphone revealed extreme tonal differences that started off a whole lot of investigation into how a headphone’s frequency response could be altered to make it sound like flat measured speakers.
An experiment was set up as follows:
1. Loudspeaker replay of a frequency sweep was recorded by a high quality, miniature microphone in a room – either an anechoic chamber or an approximation of an ordinary room (see later) – and the frequency response was charted.
2. The same microphone was inserted into a subject’s ear canal and the same speaker replayed frequency sweep was charted again.
3. It turned out there was quite a difference between the charted frequency response of the two recordings.
4. It was postulated that if the frequency response of the recording made by the microphone in the ear (see 2) could be altered by pre-equalization to ultimately match the shape of the frequency response of the recording of the same microphone when not in the ear (see 1), that replay of that in ear microphone recording should sound the same as that of replay of the recording made by the same microphone in a room.
5. This gave rise to a target measured frequency response for a headphone to sound like a flat measured speaker i.e. if the headphone had a measured frequency response that looked like the target response, it should sound flat when reproducing a recording that had been mixed with speakers in front of the mixer, and sound as if one was listening to speakers in front of him/her, rather than via headphones.
6. The concept of pre-equalisation of headphones was thus born.
Pre-equalisation could either be mechanical (i.e the driver frequency response was manufactured to behave that way e.g. the AKG 240DF – not so easy) or electrical (which should be cheaper, easier and field-adjustable), and meant that although the headphones now had a frequency response that had been altered to something that looked decidedly non-flat when measured, it reproduced the sounds coming from a sound source with the same frequency response at the ear canal as if recording and replay over headphones had not been introduced into the chain i.e. the headphone replay should now sound the same as sitting in the room and listening to the speakers.
Two main theories of the correct pre-equalisation curve were forwarded. The first, called free-field equalization, suggested that the above experiment be conducted in an anechoic chamber (like a field, free of reflective, asbsorptive and refractory surfaces). So, to reiterate, a free field equalized headphone is designed to sound like the reproduction of speakers as if a listener is sitting in an anechoic chamber. Although an anechoic chamber is more reproducible as a standard, it was argued that nobody listens in an anechoic chamber (and indeed, most listeners find even speaking in an anechoic chamber uncomfortable) and a reasonable approximation of a standard listening area be used to conduct the above experiments. This was called diffuse field equalization.
There are many things that alter sound between the release from the sound source and arrival at the ear canal. Reflections, diffraction and absorption from objects in the listening environment, reflection, diffraction and absorption by the head, hair and ears all contribute to alteration of sound before it reaches the ear canal. Diffuse field equalization, as mentioned before, is an attempt to make the replay of a recording on headphones sound like you are listening to the same recording through speakers in a non-anechoic room. Experiments were also done so that headphone users were asked to equalise various sharply limited frequency bands’ playback on headphones until they had matched the loudness of the same playback through speakers and with headphones removed. A good correlation was obtained between this method and the probe microphone recording method.
The direction of sound (from the front in a reverberant field) with speakers is far removed from actually injecting the sound directly into the ear canal. Stax, pondering this problem, possibly because they couldn’t successfully mechanically create a diffuse field equalized headphone, and any electrical equalizer would have to be a custom unit, first decided to create a new headphone that coupled its own reproducible miniature room (complete with uneven diffractive, reflective and absorptive surfaces) called the Stax Sigma. It had headphone drivers that fired from anterior to posterior instead of laterally into the ear canals. The sound was bounced off irregular “wool” into the canals, creating a mechanical diffuse field room for each ear as well as having "speakers" that fired sound from the front, rather than straight into the canals. It was partially successful, but listeners either hate it or absolutely love it. Personally I love it, but they were inefficient headphones, they sounded quite rolled off at both ends of the frequency spectrum. They were also huge and very odd looking. Better drivers than the original Sigma drivers (which were the same as the then current Lambda) improve the frequency extremes, allowing the merit of the theory to finally shine through (e.g. the very rare Sigma/404 hybrid).
Stax later decided, instead, to bite the bullet and build custom equalisers to electrically equalize their latest headphone range to provide individual target diffuse field responses for each of its various then current headphones (the ED-5 for the SR5 normal bias headphone, the ED-1 and SRM-Monitor for the Lambda Professional high bias phone and the ED-Signature for the high bias Lambda Signature). The headphones could then be less bulky than the Sigma and more fashionable (see my avatar for what the Sigma looked like – it definitely had a style only a mother could love). Again, reactions to Stax engineers’ diffuse field equalized headphones literally polarized listeners into “hate it” or “love it” camps. I would guess that economically, this proved to be a dead end, and any research into diffuse field equalization has never been publicly mentioned again by Stax.
As mentioned above, the three ED diffuse field equalisers were designed for three different Stax phones (the ED-5 for the SR-5, the ED-1/SRM Monitor for the Lambda Pro and the ED-Signature for the Lambda Signature). The ED-5, ED-1 and ED-Signature were placed between the source and the headphone driver and are connected by way of RCA cables. The ED-1 matched the construction and size of the SRM1 Mk2 and was finished, like those units, in either black or silver. The ED Signature matched the chocolate brown of SRM-T1/S/W. The SRM-Monitor incorporated an ED-1 and an SRM1Mk2 Professional into one large package and was finished in either black or silver, and had both RCA and XLR inputs.
The ED-Signature would most likely also match the 404 and Lambda Nova Signature. The ED-1 equalisation (in my case, provided by a very rare SRM Monitor) sounds rather nice with the Lambda Nova Signature and surprisingly good on the Omega 2 Mk 1, despite being the wrong equalisation for the latter. The upper midrange/lower treble, in particular, sounds quite a bit flatter and the low end remains in good balance with the mid and high. As Bill Sommerweck said in his review of the ED-1 in the April 1989 issue of Stereophile, track 9 on Stax' own “Space Sound” CD changes from objectionable (without the equalizer switched in) to quite listenable with the equalisation switched in. In my opinion, there is no magical out of the head experience, except when listening to the aforementioned CD, or the Ultrasone binaural tracks (i.e. binaural recordings). These are seriously spooky. Try them with someone who is not used to listening to headphones and see what happens when you cue up track 1 or 2 of the former, or the fireworks track of the latter. Sabine whispering in your ear - Oh yes! Shower spraying on your shower cap - OMG!
Now, here is where things start getting weird. I had a listen to the Sigma/404 and the SR-007 phones with the equaliser on and they both sound great - it may just be happenstance, but I've never heard "Kind Of Blue" sound so wonderful and with plenty of lower bass (which even the SRM-717 doesn't seem to match). This is strange, as the above frequency response for the ED-1 is flat in the bass, and should not be suited to the Sigma/404 or SR-007 anyway. Maybe it's a de-emphasised treble spike? I don't know, but whatever, this is really good.
