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Old 19th Apr 2018, 10:00 am   #1
GrimJosef
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Default Were there any 'high quality' early headphones ?

In the earliest days of wireless - I'm thinking the early 1920's, before the moving coil loudspeaker was widely available commercially - a good deal of listening was done using headphones. If all you had was a crystal set then the performance figure that mattered most was generally the phones' sensitivity. A 'better' set in this respect would let you get away with less of an aerial and/or would let you hear more distant stations and/or would let you work with more lossy but also more selective tuning circuitry. However the price you paid for greater headphone sensitivity might well be poorer sound quality - in particular a narrower frequency range. In his book Loudspeakers Gilbert Briggs shows the frequency response of a very early speaker based, more or less, on a headphone driver coupled to a horn. It peaks at 700Hz or so, is 10dB down at 300Hz and 1.2kHz and is essentially useless below 100Hz and above 3kHz.

The above response might have been OK for speech but wouldn't really have done justice to music, although music was quite often played. I was wondering whether anyone had come across a brand or a model of headphones which were better than usual at reproducing music, perhaps at the expense of sensitivity ? I see from sites such as this one http://oldheadphones.com/crystal/index.html that there was a huge range of brands and models. Some of the early ads do make grand claims for their products' tone as well as sensitivity, durability, comfort etc. But I suspect they're not entirely impartial .... Do you know of a headphone set which really does sound better than the rest ?

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Old 19th Apr 2018, 1:42 pm   #2
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Default Re: Were there any 'high quality' early headphones ?

Well,

That is a really great question.

I did a bit of work analyzing the frequency responses of radios made in the 1920's. I got interested in this area for a few reasons, one was I had a 1920's radio, a Grebe MU-1 and I became interested in the speaker technology and the audio amplifiers of this era, specifically their frequency response, that was limited by their audio coupling transformers at the low end and the radio's detector time constants at the high end.

Most people in radio work, interested in RF circuitry, understand the notion of the gain-bandwidth product, but actually, back in the 1920's, this notion applied to audio amplifiers and both speakers & headphones. This was because gain came at a premium with the cost of valves and transformers, coupling audio, were expensive parts and for a transformer with a better LF response it had to be bigger and more expensive.

Most speakers of the time (and headphones) had metal diaphragms. The frequency response was not in fact diabolical for speech, somewhere from about 500 Hz to 3.5Khz.

Because at the the time, the gain of radio frequency and audio stages was limited, the detector frequency response in the radio could only just resolve an upper frequency of a few KHz and the transformer coupling in the audio stages set the low frequency response.

So , just as in RF work, the gain was optimized at the expense of the audio frequency bandwidth, but both the speakers and headphones of the time were only up to that audio bandwidth anyway.

In the late 1920's and early '30's, it became obvious that paper cone speakers had a much flatter response, slowly these replaced the metal diaphragms in horn speakers and to allow for this radios had extra gain stages.

So if you are looking for any headphones, with a better frequency response, my guess is they would have to be post war. Certainly by the mid 1960's wide frequency response headphones became available, I think from companies like AKG. Many were used on medical instruments, like Audiometers for hearing testing, where they required a pretty flat response from 100Hz to 10KHz at least.
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Old 19th Apr 2018, 2:01 pm   #3
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Default Re: Were there any 'high quality' early headphones ?

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like Audiometers for hearing testing,
I had one hearing test on the late 60's I can remember it 50 odd years later. I am sure the headphones where the typical bakelite 2000 ohm jobs, perhaps they where calibrated? After all it was only a multiple fixed frequency test.
 
Old 19th Apr 2018, 2:52 pm   #4
Argus25
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Default Re: Were there any 'high quality' early headphones ?

The headphones for audiometers were generally calibrated by coupling them into a capacitance microphone (made by B&K) in a soundproof box, then to a B&K signal level meter to ensure the actual audio output across the full range of audio frequency signals was standardized. The driver circuitry in the audiometer, could be adjusted, to compensate for the actual frequency response of the headphones so that the sound pressure level was correct at all audio frequencies. B&K also made suitable headphones. They must have had a reasonably flat response or else massive drive level adjustments would have been required across the audio frequency range, as I recall that was not the case.
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Old 19th Apr 2018, 2:58 pm   #5
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Default Re: Were there any 'high quality' early headphones ?

Modern moving coil headphones certainly claim an impressive degree of flatness when it comes to their frequency response. My not very expensive Sennheiser HD 202s are specified as only being 3dB down w.r.t. 1kHz at 18Hz and 18kHz https://assets.sennheiser.com/global...584.1524143998. Strictly speaking I suppose there could be enormous resonances at intermediate frequencies, but I suspect there aren't.

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Old 19th Apr 2018, 3:27 pm   #6
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Default Re: Were there any 'high quality' early headphones ?

Those audiometric headphones are funny. The supra-aural design (= old bakelite thing which sits on the ear with no cushion) offers - as I remember - better fit reproducibility for one subject, and between subjects, than circumaural designs with cushions. Open back ones of the latter are better in that regard than close-back, but still. B&K came up with their 'artificial ear' which doesn't really do very much more than couple the 'phone to the mic repeatably - there's not much more physical modelling, and I guess there didn't need to be. The 'correction curve' for calibrating the 'phones and audiometer goes up and down by 10dB-ish in places - again as I remember - and the whole thing might be thought rather lo-fi. But they key thing is that it is linear over a wide dynamic range, and repeatable - and that's all that matters.

(Incidentally - you need to get to a 20dB loss in one band before you can start to wonder if you actually have a hearing loss that would stand up in court. That's quite a deviation!)
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Old 19th Apr 2018, 3:34 pm   #7
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Default Re: Were there any 'high quality' early headphones ?

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Originally Posted by GrimJosef View Post
Modern moving coil headphones certainly claim an impressive degree of flatness when it comes to their frequency response. My not very expensive Sennheiser HD 202s are specified as only being 3dB down w.r.t. 1kHz at 18Hz and 18kHz https://assets.sennheiser.com/global...584.1524143998. Strictly speaking I suppose there could be enormous resonances at intermediate frequencies, but I suspect there aren't.
It's interesting too, to wonder what 'flat' means, or what we would want it to mean. For instance, the head-related-tranfer-function (HRTF) for - what shall we say, random incidence, frontal incidence? - is not flat wrt freq, and we might hope that the headphones recreate a binaural representation to suit.

It's tricky. I guess the material will be stereo (which is strictly 'wrong' over headphones as it includes the cross-talk paths) and recorded to sound 'right' using loudspeakers with 'flat' response in a reasonably well-damped room. So we might want to use a dummy head to see what kind of response that is, then put the headphones on the dummy head and equalise accordingly.

I used to work on DSP binaural 'auraliations' of real, and projected, acoustic environments, where this kind of thing mattered a bit.
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Old 19th Apr 2018, 8:10 pm   #8
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Default Re: Were there any 'high quality' early headphones ?

When I joined the BBC I was recalled to BH for a 'repeat audiometric test'. Likely I hadn't got one of the cans over my ear properly and it made a big difference.

This leads me to think that, like the GPO receiver 2P (the later equalised receiver in the ubiquitous bakelite type 164 hand-microtelephone), one of the resonant cavities for getting the best from a decent pair of headphones necessary to optimise the frequency range is the outer ear cavity, and not part of the receiver itself.
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Old 22nd Apr 2018, 1:04 am   #9
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Default Re: Were there any 'high quality' early headphones ?

That's very true. Headphones and the pinna cavity are a lot like loudspeakers in rooms - a whole load of modal variability. Lumped parameter modelling (no modes) will get you to about 2kHz and a really fancy FEA model will stretch that to 4kHz. It will appear to get to 20kHz, but every time you take the 'phones off and put them back on the response changes so much that 4-20kHz modelling is a bit pointless

If anyone really wants to get into this, there's a good chapter in the 'Loudspeaker and headphone handbook' (ed. Borwick) by Poldy, who might have been at Philips or AKG - I can't remember. The second edition is much better than the third, as all the acoustic networks are written out in full with passive electronic components - you can analyse it in maths or even using 'spice' etc I guess. Also, Elliot Berger of 3M did quite a lot of papers on hearing protectors which are also relevant to understanding the acoustics of headphones.
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Old 22nd Apr 2018, 9:23 am   #10
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Default Re: Were there any 'high quality' early headphones ?

Thanks for all the detail everyone. I guess little of this would have been known to the early headphone builders. They were just faced with the electromechanical problem of making a diaphragm 'floppy' enough to allow the large excursion needed to produce bass while keeping the whole assembly both durable and sensitive. Given the fact, well-known to PA installers, that bass actually tends to obscure the intelligibility of speech (and in the first instance phones might well have been optimised for speech) and also that the bass content of broadcasts might not have been that strong anyway (was it filtered out ?) maybe the phones just needed to be 'good enough' that they weren't the frequency-limiting element in the chain. In that case my quest for an unusually good pair might just come down to sucking-it-and-seeing.

Cheers,

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Old 22nd Apr 2018, 9:28 am   #11
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Default Re: Were there any 'high quality' early headphones ?

Interesting re the audiometric phones.
I recently checked my own hearing using a reasonably decent pair of Sennheisers, and was interested to note that around 18kHz (IIRC) they produced an f/2 artefact, which could easily be mistaken by an inexperienced subject as hearing the fundemental frequency.
I imagine that this sort of thing is commoner in older phones - how was it avoided when testing?
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Old 22nd Apr 2018, 9:57 am   #12
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Default Re: Were there any 'high quality' early headphones ?

The standard-issue BBC monitoring / comms 'phones manufactured by ST&C had notched earpieces. Wonder if this was to flatten the characteristic by altering the resonance of the cavity formed between earpiece and outer ear by making it 'leaky'? That said, some of them have clip-on pads or big rubber ear-cups. I feel an experiment coming on, as I have sets of all three earpiece types!

Or maybe it was simply to drain off sweat?

These headsets use very sensitive rocking armature inserts and have a surprisingly good frequency response.
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Old 22nd Apr 2018, 1:54 pm   #13
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Default Re: Were there any 'high quality' early headphones ?

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Originally Posted by dseymo1 View Post
Interesting re the audiometric phones.
I recently checked my own hearing using a reasonably decent pair of Sennheisers, and was interested to note that around 18kHz (IIRC) they produced an f/2 artefact, which could easily be mistaken by an inexperienced subject as hearing the fundemental frequency.
I imagine that this sort of thing is commoner in older phones - how was it avoided when testing?
I don't think tones at frequencies lower than the stimulus itself can be produced for a single excitation through a non-linear transfer function, but any version of that kind of distortion process will produce 'difference tones' (as they're called in hearing research) - I guess that's what others would call intermodulation distortion - when presented with more than one tone at once. Distortion wasn't really avoided way-back-when in testing, which is why early graphs for 'detection thresholds of one pure tone, in the presence of another' - auditory masking - have whacking great dips in them at the harmonics of the reference tone, which wan't remotely 'pure'! The test-tones created auditory 'beats' with the harmonics of the distorted reference tone, and gave the game away.

Off-topic a bit, it's interesting that cochlear mechanics themselves will generate quite noticeable distortion (those 'difference tones'), and at gigs these days the PA system can be considerably more linear than the ears of those listening. If you're in that environment try putting your fingers in your ears - often the sound cleans up lovely, revealing the lack of distortion in the well-engineered PA system which has been driving your ears into clipping!

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Old 22nd Apr 2018, 2:03 pm   #14
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Default Re: Were there any 'high quality' early headphones ?

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Originally Posted by russell_w_b View Post
The standard-issue BBC monitoring / comms 'phones manufactured by ST&C had notched earpieces. Wonder if this was to flatten the characteristic by altering the resonance of the cavity formed between earpiece and outer ear by making it 'leaky'? That said, some of them have clip-on pads or big rubber ear-cups. I feel an experiment coming on, as I have sets of all three earpiece types!

Or maybe it was simply to drain off sweat?
Yes, designed-in 'leaks' are a big part of creating a know-able sound field at the entrance to the ear canal; know-able between different users, and also for one user with phones refitted on multiple occasions. If you know what it is going to be, you can shape it. Open-back designs are all about this, and only really fall down where excluding background noise (headphones as hearing protector) are part of the design.

Sweat is a big deal, and is one reason why fluid-filled cushions (which fit multiple users' jaw-ear combo really well, and are great at stopping the cup bouncing up and down and transmitting LF energy) have never been very popular.

I was once part of a university project which was apparently going to make a large industrial concern a global leader in hearing conservation products. Well, they lost patience with our worthy but rather incremental progress and bought the existing leading global brand for $$$$$$. They then told us to 'carry on, carry on' without being very interested in what we did, or didn't. Happy days .
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