“A Bone of Contention” 
Concerning the Curious Case of Bone-conduction Headphones

Prelude 
At the time of writing this (August 2009), it has been all of 18 months since last I indulged in the eminently enjoyable pursuit of CD reviewing. By anyone’s reckoning, that’s an unconscionably long period of abstinence, particularly from something purportedly pleasurable. If you harbour any curiosity about the reasons for my apparent perversity, please persevere - all will be explained anon, because, as it happens, it does have some considerable bearing on the matter in hand.

I’d set out to write a straightforward audio equipment review, but the unusual, even bizarre nature of the bit of kit in question sucked in wider issues - including elements of topical overview, relevant personal experience, some investigative reporting (of a sort!), and related audiological health matters.

The outcome is this essay, looking at certain aspects of the apprehension that’s a prerequisite for the appreciation of music. Being primarily concerned with the degeneration or failure of the apprehensive apparatus, this is, I must admit, not the happiest of subjects. However, it’s something that will - sooner or later, in some degree, and for one reason or another - affect almost anyone who currently derives pleasure from listening. Hence, I rather hope it won’t (if you’ll pardon the expression) fall on deaf ears.

It’s rather long, but that’s mainly because it covers a lot of ground. I did toy with the idea of trading content and readability for relative pithiness. In the end, I thought, “Blow it - if it’s interesting enough they’ll read it, if not they won’t!” Instead, for the convenience of the “butterfly browser”, I’ve provided lots of section headings.

Acknowledgements 
I would like to express my gratitude to:

Direct Imports (NZ) Ltd., Hastings, and in particular Grae Gillespie, for bending over backwards to help me. Their sympathetic customer service, when faced with a fairly unusual requirement, sets an impeccable example that many others would do well to follow.

Jackie Clemmer B.A., M.A., A.S.H.A., F.A.A.A., C.C.C., M.N.Z.A.S., of Northland Hearing Clinic, for giving me the invaluable benefit of her expertise, and indeed for “marking my work” to make sure that I’d got my audiological facts right.

A Bit of Background 
A couple of months ago, I’d never even heard of bone-conduction headphones (BCHs). I rather suspect that I wouldn’t have been alone in my recently-relieved ignorance. Not that it matters. My awareness having been awakened, curiosity demanded I did a bit of nosing around the Net. I was fairly taken aback to discover that BCHs have actually been around for something like ten years! That’s a long time. Why, I wondered, hadn’t I got wind of them ages ago?

The reason is - at least in part - that they’ve been mis-marketed, or so it seems to me. If you come up with a bright idea, what do you do with it? A reasonable response would be to explore its potential, then exploit its strengths. There were two angles to the bone-conduction idea. One - surely the main one - was that, although it probably would never outclass existing technology, bone conduction could still serve a purpose both serious and valuable. The other was that BCHs would make a “fantastic” quirky accessory for sporty types, fashion-conscious air-heads, and anyone greedy for gimmicky gizmos.

We live in a world where, increasingly, we find appearance (“cool!”) taking precedence over substance (“boring!”). So, not surprisingly, the great god Profit Potential dictated that the former angle be sidelined in favour of the latter. This may be the reason why the BCH idea has generally been neglected by established (i.e. “boring”) audio manufacturers. Admittedly, I’ve seen mention of a Sennheiser BCH, but as I can find no confirmation of any such on Sennheiser’s own web-site, I can only conclude that either it was just a wicked rumour, or Sennheiser had second thoughts, perhaps dissatisfied with the quality of performance, or otherwise preferring to preserve its “boring” image.

Thus far, the only recognised audio manufacturer to offer BCHs is Teac, whose Filltune Hi-fi BCHs may have acquired that auspicious brand-name through a timely and successful takeover bid. Otherwise, all the BCHs come from relatively obscure oriental companies - such as Goldendance (Audiobone), PHICOM (Alljoy), Pamsh, and Vonia.

Not surprisingly, BCHs soon became - if you’ll pardon the expression - a bone of contention. Some swallowed the chic - or cheeky - image promoted by the publicity blurb, whilst others gypped on it, and a right old ding-dong ensued. I had to applaud the chap who wryly observed that folk seen sporting a pair of BCHs “looked like they’d missed their ears when they’d put them on.” All good, clean fun, of course, but regrettably all this fuss tended to focus on the “fluff”, further submerging the already floundering serious purpose.

Similarly, many of the published BCH reviews and other commentaries positively glisten with compliments, whilst lots find just as much to complain about. What those that I’ve come across typically have in common is yet further cause for concern: with a few notable exceptions, they are at best only partially comprehending, generally ill-informed, and at worst so misleading as to be downright dangerous. It saddens me to say that much of the mish-mash of misunderstanding originates from manufacturers’ own product pamphlets, whose several trumpetings sound suspiciously similar. As we go along it’d be as well to try to prick some of these, what for obvious reasons I’m going to call “fantasy-bubbles” (FBs).

How They Work 
Air Conduction vs. Bone Conduction 
Firstly, then, just what are BCHs? Conventional headphones are small loudspeakers, supported in more or less intimate proximity to your ears. You hear the sound they generate in the manner that Nature intended - incoming sound passes down the outer ear canal to the ear-drum, and thence, via the auditory ossicles of the middle ear, to the inner ear’s cochlea. This last contains an array of tiny hairs which, when excited by the vibrations, send appropriate nerve impulses to the brain. For convenience this, the normal hearing path, is termed “air conduction”, although it actually comprises a succession of four media - air (ear canal), skin (eardrum), bone (ossicles) and water (aqueous medium in the cochlea).

BCHs simply use an alternative, we might say unnatural path. Experiencing this in operation for yourself is a simple matter - just scratch your head (which is something I do a lot!). Instead of a loudspeaker diaphragm generating air-borne sound waves, a vibrating driver, pressed firmly against your skin, transmits sound through your skull. According to one FB, this is down to the BCHs’ remarkable ability to “transform sound into vibrations” - about as remarkable, I reckon, as transforming common salt into sodium chloride.

The cochlea, being firmly coupled to the inside of your skull, finally feels these “good vibrations” in the time-honoured fashion. Yet, another FB, which you’ll find in the list of advantages of BCHs, solemnly states that “the sense organ is vibrated directly”, insidiously implying that this is somehow superior to the natural, “indirect” route. Well, try totting up the succession of media. The only one in the natural path that’s absent from this path is “air”. This falls a long way short of “direct”, although it is “more direct”, even if only just. However, the insidious implication hinges on that general assumption that “simpler is better”. Usually it is, all else being equal. But here, all else is not equal. Far from it - eardrums and ossicles are custom designed and built for conducting sound, whereas hair, skin and the temporal bone are not.

Bone-conduction Drivers 
Of course, even in normal hearing some sound is bound to be transmitted through the bone, but the mass of bone is so high, and its conductivity is so low that, in normal circumstances, it isn’t a viable auditory medium (otherwise, let’s face it, we’d have evolved without ears!). For this reason, you get little or nothing from placing a conventional headphone driver against your skull. A BCH driver has to be capable of the considerably tougher job of “punching” sufficient energy into the massive bone to penetrate sensibly to the cochlea.

Broadly speaking, thus far BCHs have employed one of three driver technologies: electromagnetic (similar to the voice-coil principle of conventional speakers), piezoelectric (strictly, reverse piezoelectric, deformation of a material by an electric field), and magnetostrictive (deformation of a ferromagnetic material by a magnetic field). In all cases, the objective is to cause a solid plate, a sort of “piston” rather than a diaphragm, to vibrate in response to an electrical audio signal. For optimum efficiency, this piston has to be closely coupled to the bone, by pressing it in some place where the layer of skin is thin.

We need to bear in mind constantly that the hair, skin and bone involved in bone conduction have been pressed into service - Mother Nature never “intended” them for effective sound transmission. Just because it’s possible does not automatically make it even viable, never mind better at the job. I know this seems blatantly obvious, but it nevertheless needs underlining - out there you’ll find a whole clutch of FBs that would have us believe otherwise. The emptor needs to keep a very tight grip on his caveats.

Dire Warnings and a Proper Purpose 
Audibility of Ambient Sounds 
Now, what about that “purpose both serious and valuable”? First we need to prick a couple more of those FBs. One FB claims, with a certain degree of pride, that BCHs provide effectively “ears-free” listening, being oft-touted as permitting “listening to music whilst simultaneously being able to hear [external] sounds” - sounds, I guess, such as that of the horn of a truck coming up fast behind you, as you drift, oblivious to everything but your iPod, across the road. The Filltune instruction book’s version is typical: “These headphones do not isolate you from the surrounding sound.” However, two paragraphs further on it says, “With its advantage of blocking surrounding noise, . . .” - there’s nothing like having it both ways, is there? 

Notice, though, the insinuation that only BCHs can do this - whereas you know as well as I do that even well-lagged, circumaural headphones never have been all that brilliant at blocking out ambient noises, whilst the far more common open-backed and/or supra-aural models are, by design, almost completely transparent to external sounds. Why else would “noise-cancelling” headphones recently have been gaining in both capability and popularity?

The rock-bottom truth of the matter is that, in any listening situation, with or without whatever sort of headphones, the louder the foreground sound gets, the more our hearing faculties ignore - or fail to discern - the background. This is an inherent feature of the ear/brain system - if you fill your head to bursting with pulsating “heavy metal”, you will have trouble hearing the telephone ring, and might even miss the crack of Doom itself. 

Risks of Hearing Damage 
I mentioned that the range of FBs extended to the downright dangerous. Regretfully, I must confirm that I wasn’t saying this just for dramatic emphasis. Please consider carefully these widely-disseminated, often authoritatively-pronounced claims: “BCHs are a safer means of sound delivery”; “BCHs prevent damage to the ear-drums”; and especially, “The use of BCHs eliminates the risk of hearing damage known to result from extended use of conventional headphones at high volume levels”.

Would any folk actually be daft enough to be taken in by such claims? Well, yes, they would, and they wouldn’t need to be “daft enough” - even perfectly sane and sensible people can be taken in. That’s because we, in our consumer-protected society, not unreasonably expect to be responsibly and expertly informed. If we are given authoritative-sounding claims, we expect them to be correct. What we do not expect is to be led up a gum-tree. Yet, that is exactly where these claims are leading us.

You don’t have to take my word for it. Go and ask any professional audiologist or ENT specialist. You will be told, in no uncertain terms, that a normal, healthy ear-drum is as tough as an old boot, and that normal, healthy ossicles, whilst not quite that rugged, are still made of pretty stern stuff. Both can happily cope, long-term, with sound levels that would broil your brain. However, these same experts will also tell you, equally emphatically, that your cochlea would fry long before your brain broiled.

That’s because the auditory chain’s weakest link by far is the cochlea’s array of delicate sensory hairs, which can, all too easily, be gradually done to death by excessive vibration. And, it matters not one whit how the vibrations arrive there. Whether the sound comes in through the normal route, or via the skull-bone, or by some marvellous manifestation of telekinesis, the louder it sounds to your brain, the greater the risk of irreversible hearing damage. Hearing aid users should rest assured that, by the self-same token, pumping up the volume specifically to compensate for conductive hearing loss poses no additional risk of damage to hearing. 

The Real Potential Beneficiary 
If you’ve been adding up the pointers along the way, by now you’ll have come to the conclusion that BCHs offer no advantages whatsoever over conventional headphones. Provided, that is, you have normal, healthy outer and middle ears. It’s a very different kettle of fish, however, if obstructions, physical damage or disease inhibit the passage of sound through to a normally-functioning cochlea. Conductive hearing loss is the sensible outcome of various conditions ranging from the trivial - such as the temporary and reversible losses imposed by excess wax or ear-plugs - to the serious, such as severe infections and tumours. Whilst not the most common hearing impairment, it nevertheless affects a vast number of people.

For sufferers of conductive hearing loss, especially if they are also music-lovers, BCHs are potentially an absolute God-send. In fact, I’ve heard it said that the first to make practical use of the bone-conduction principle was none other than Beethoven, who for a time managed to offset the effects of his encroaching deafness by clenching one end of a wooden rod in his teeth and wedging the other against the sounding-board of his piano.

Whilst conductive hearing loss is more prevalent amongst older people, it is by no means confined to that corner of the population, which brings us yet another FB: BCHs are of benefit to “middle-aged or elderly people suffering mild conductive hearing loss” or to “those with presbycusis”. Yes - and no! Presbycusis, more commonly known as age-related hearing loss, is effectively due to wear and tear, fair or otherwise, of the cochlea. In this case BCHs, far from being “of benefit”, will simply be banging their heads against a brick wall.

Neither is the benefit limited to those whose conductive hearing loss is “mild” - a BCH’s effectiveness is utterly independent of the severity of the conductive hearing loss, simply because the problem area is bypassed altogether. In the extreme case, someone who is conductively as deaf as a door-post is effectively imprisoned behind soundproof walls. For such prisoners, BCHs are like an escape tunnel in reverse - a tunnel to let the sound in. In terms of proper purpose, that’s got to beat “quirky fashion accessory” into the proverbial cocked hat, hasn’t it? What’s more, for this proper purpose BCHs have a potential market that may be rather bigger than most manufacturers seem to imagine.

Case Study - A Personal Interest 
This is the appropriate point to clear up the mystery of my CD reviewing “drought”. Just three months after emigrating from the UK to New Zealand (in late August 2007) I started suffering from spasmodic pains in my left ear. Inevitably, it turned out to be an infection, which should have been no big deal. However, the infection was in what turned out to be a cholesteatoma. Suffice it to say here that this charming manifestation takes pleasure in gradually eliminating the contents of the middle ear - before moving on to greater things.

Through six months of waiting for the surgery that would hopefully arrest its progress, I felt rather like HAL, the computer in 2001, A Space Odyssey - I could feel, not my mind, but my hearing steadily going down the pan. For over 45 years of my life, music had been my daily bread. Now, with shocking suddenness, I found myself suffering from musical malnutrition. Some folk, I believe kindly, pointed out that there were many worse off than I was. I replied, I hope not unkindly, that although this was true enough, knowing that someone else has toothache and an abscess never made anyone’s plain, ordinary toothache any the less painful.

Typically, hearing losses affect both ears more or less equally, generally comprising a tailing off at extreme frequencies due to age or aural abuse, and a lowering of overall “volume”. In cases like mine, however, you can’t really avoid ending up with ears having drastically disparate frequency responses. My L/R spectral difference curve wiggled wildly up and down, so that both the location and spread of a musical instrument in the stereo image depended markedly on what note it was playing. Each instrument became increasingly a peripatetic, pulsing blur. Multiply that by 100 and - well, let’s say that it created much more mayhem in Mahler than ever the composer intended.

The surgery arrested the decline, and subsequently some small recovery of hearing modestly moderated the musical collywobbles. Nevertheless my left-side hearing ended up depleted by between 10 dB. to over 40 dB., depending on the frequency. For the loss of hearing there is no cure, only a workaround. On the plus side, modern digital hearing aids incorporate “graphic equalisation”, making it feasible to straighten out the frequency response. On the minus side, as yet audiology practices serving the general public (as distinct from audiological research departments) have no proprietary instruments for measuring the “post fitting” response differences needed to steer the adjustments. Whilst the standard audiogram provides a useful first estimate, it can’t be done with the hearing aid(s) in place, and in any case lacks the necessary resolution.

Several frustratingly fruitless passes round the “guess and adjust” loop soon convinced me that a means of measuring “spectral differences” was not an optional extra. So, I figured out a technique that, although very crude and cumbersome in operation (Heath Robinson himself would have been proud of me!), at least enabled me to make these measurements myself, to an accuracy of about ± 1.5 dB. This should have made short work of equalising the aid. It didn’t. Instead there followed further months of frustration, until this self-same technique finally confirmed what was otherwise merely a subjective suspicion: my hearing was fluctuating, frequency-dependently, from day to day and even from one moment to the next. In our cycle of measurement and adjustment, we had been effectively shooting blindfold at an elusively moving target.

More than once it’s been suggested to me that I could still listen with just my right ear, that good quality mono sound is better than bad stereo. Funny, but that’s exactly what I’d always imagined. What I’d failed to appreciate, I discovered very early on (before the hearing had deteriorated too much), is that even when listening to mono sound, two ears are still much better than one. To get the idea, just try listening to a mono recording with one ear plugged, and then again without the plug. I’m sure you’ll find that the difference is nothing short of startling.

There were, and still are, further configuration complications - for example, there’s the need to preserve a residual dynamic range comparable to that of a live symphony orchestra or a CD - but until this equalisation problem is resolved CD reviewing is never going to be easy. Also, because hearing aids are largely incompatible with headphones, I have little or no prospect of resuming numerous other activities for which headphones are essential, activities like making and editing recordings, audio restoration, remastering of my old LP collection, or, for that matter, partaking of the simple pleasure of listening to music whilst sitting or walking in the sunshine. In summer, this last is no joking matter - my office-cum-listening room can sometimes pass muster as a sauna. 

One perhaps less obvious peculiarity of conductive hearing loss is that internal sounds, being completely unaffected, become predominant; this not only makes it much harder to hear conversation whilst chewing and swallowing, but also acts as a constant reminder that, as I found myself complaining, “I can hear well enough, it’s just that the sound can’t get through.” Eventually, my audiologist totted up two and two, and came up with the “four” of this suggestion, “I’ve no idea how good they might be, but have you thought of trying bone conduction headphones?”

After a few minutes of intense, increasingly eager discussion, I was wondering whether, if not out of the blue then at least out of the current problems and considerations, an answer to the proverbial maiden’s prayer had dropped into my lap. BCHs would simply bypass all the imbalances and fluctuations, feeding an external sound down the same route as chewing noises. Provided the sound quality was anywhere near good enough, equalising the hearing aid would become a secondary issue, and my life could, as they say, be as back on track as it’s ever likely to get. 
After a few weeks of frantic scrabbling around, a pair of Teac Filltune BCHs was on its way through the post. Other than my treasured, but sadly now redundant Sony MDR-CD1700s, these headphones are the most expensive I’ve ever had in my sticky little mitts - so I was more than grateful for the supplier’s promise of a “no quibble” money-back guarantee. Needless to say, and regardless of the discomfort involved, I’d crossed everything of which I possessed at least two! - which brings us very nicely to a consideration of some actual BCHs.

Teac Filltune HP-F100 Hi-Fi Bone-conduction Headphones 
Quality Expectations 


Once upon a time there was a maxim, often quoted in audiophile circles. This piece of pre-packed profundity went something like this: “For higher quality you have to pay more, but paying more doesn’t necessarily get you higher quality.” Considering that Teac’s Filltune BCHs are, as far as I am aware, the most pricey on the market, you may well be wondering which half of this maxim applies to them. That I can’t say, as I have no basis for comparison. So, what can I say?

Teac’s product pamphlet makes various claims of superior qualities, expressly labelling these BCHs as “hi-end” and “hi-fi”. According to the table in the product pamphlet, magnetostriction, the Filltune’s driver technology, out-performs the alternatives on all fronts bar one. That sole exception is low-frequency reproduction, where both magnetostrictive and piezoelectric drivers are classed as “good”.

The most remarkable performance parameter Teac quotes is frequency response, claimed to be 25 Hz. to 25 kHz! Possibly significantly, no roll-off points are given - not “-3 dB. points” or even “-6 dB. points”. They could be “minus anything”. Does it matter? I’d suppose not, unless, that is, either the “anything” is an embarrassingly large number or you’ve got cochleae that would make even a baby weep with envy.

So, what I can say is that Teac’s quality claims, allied to the high price, compel me to regard - and judge - these headphones as being in the crème de la crème bracket.

Features  
Some BCHs look truly bizarre, like something out of a Pokemon cartoon or, even more unnervingly, accessories for Dyson vacuum cleaners. By comparison the Filltunes look - no doubt to the immense relief of most potential purchasers - really quite respectable (see picture). The headphones connect, via a small, flat multi-pin plug that fitted rather less securely than I’d have liked, to a fairly small (about 30 x 25 x 135 mm.) control unit. This incorporates an on/off switch, an amplifier, a volume control knob, two stereo mini-jack inputs (one for line level signals, the other for a stereo electret microphone), and a compartment to house three “AAA” batteries - but no belt-clip, so if you’re going to use the Filltunes on the move, you’ll need a handy pocket.

Ergonomics 
The driver heads are spring-loaded to apply the appropriate operational pressure, and when not in use the springs fold these right up against the inside of the headband. The required pressure is described as “low” compared with other driver types, but it is still high enough to be considered uncomfortable by some, and to make putting them on something of a wrestling match - at least until you’ve mastered the secret technique. Once on, though, they have a tendency to come back off, especially if you’re on the move, or even if, in an unguarded moment, you glance down at the CD booklet nestling in your lap.

This predisposition to self-eject, seemingly spontaneously, is due to a combination of the firm pressure, smooth driver contact plates, and the fact that, unlike conventional headphones, BCHs lack the inestimable advantage of a convenient pair of outer ears to hold on to. Regrettably, it is also due to what I’d regard as an ill thought-out headband design - if, indeed, any thought at all has been expended in that direction. If you wear them in the recommended position, with the driver plates pressing on your cheekbones just in front of your ears, the slim, slippery headband sits precariously on the slippery slope of the front of your head, from where it suffers a natural tendency to, well, slip.

There’s another, perhaps even more important reason why the headband is inadequate. Unlike conventional headphones, whose positioning is universal and predestined (i.e. over, on, or in your ears, depending on the particular design), BCHs will work almost anywhere on your skull-bone. I tried holding the driver plates in many positions. Generally, they work best when pressing on the temporal bone, which means close to, and somewhere in front of, above or behind the outer ears. However, because - as I discovered - loudness, left/right balance, and tonal response are all sensitive to placement, it really does matter exactly where you put them and, having got them there, that they stay put.

Reviewers have commented variously and contradictorily on there being, for example, “too much” or “not enough” treble. I have found positions - admittedly on my particular skull, which might not be typical! - which could account for both of these complaints. Wearing them with the headband adjusted too short, say, brings the driver plates up near the temples, where the sound is definitely on the papery side. The longer you set the headband, the “darker” the sound becomes. The all-round best position, for me at least, was actually behind my ears, on the upper foothills of the dome-shaped bumps of the mastoid bone (the backmost part of the temporal bone).

The thing is, improving the headband wouldn’t take rocket science, although the application of rocket science might well furnish a more elegant and effective improvement. A twin headband, such as already adorns some conventional headphones, would provide two points of support. With one in front of and one behind the very top of the head, the headphones would immediately be more secure and, given sufficient scope for adjustment, would permit a relatively wide range of positioning options, including both before and abaft of the ears. Then, for the benefit of those who otherwise would baulk at headphones masquerading as a “head-clamp”, and to lend a little extra security against slippage, why not provide a nice bit of comfy circumferential cushioning?

Sound Quality 
Of course, the most important thing is how they sound. Notwithstanding the peculiar path the sound must take, do they fulfil Teac’s impressive-sounding array of “hi-fi” promises? Will users really be able to “re-capture the pleasures of virtually all audio, including high-quality music” and to “feel rich sound”? Will they experience “clear, distinct sound quality”? Will “those who simply enjoy listening to loud Hi-Fi music” find their hearts’ desire? Do they indeed offer “the original sound including harmonics”? Most particularly, are they really “ideal for anyone (especially middle-aged and elderly) with hearing difficulty or impairment”? All that - need I say? - is a lot to live up to. If the Filltunes, in actuality, come anywhere near this ball-park, my own “maiden’s prayer” would be answered with knobs on.

Right, so there I was, with the Filltunes sitting, albeit precariously, in the position I’d found most favourable. Taking my cue from the Immortal Bard, I let “the sounds of sweet music creep in [my] ears.” (At this point, I must at all costs resist the temptation to add, “Soft, what sound in yonder ear-hole breaks?” You’ll see why before long.) And indeed, quiet music sounds very good. In fact, even to a cynical old sod like me, it sounds astonishingly good. Subjective sensations included silky strings, tingling tremolandi, pizzicati pricking like pins, solo woodwind of almost saintly purity, and light touches of percussion sparkling like champagne under jewellers’ lighting.

On a more prosaic level, I had to agree with Teac’s modest claim, of low frequency reproduction that is merely “good” - bass notes were clear but a bit on the “pale” side, lacking that truly visceral quality. Other than this, I’m tempted to go so far as to say that, if anything, their sound has greater clarity and resolution than even my trusty Sonys.

One prejudicial reservation I had been harbouring was that, with all that bone for the sound to bounce around on its way to one cochlea or the other, would cross-talk significantly mess up the sound-image? That one was quickly knocked on the head - the full-width stereophonic stage played host to precisely-located instrumental images of crystalline clarity. I also noted that the stereo image width also varies with driver placement, though not by very much.

So far, so good; but what happens as the music gets louder, or if you turn up the volume? Well, quite a lot - but regrettably none of it good. Firstly, moving though the broad realm of mezzo-piano, the sound starts to fray around the edges, what my wife was later to describe as “a bit fizzy”. Thereafter, distortion increases rapidly, the sound quickly degenerating into ever-more indistinct mush, and the more complex the texture, the more mushily indistinct it is. By the time the level has gone up to anything even approaching a robust double-forte, the noise is almost unbearably harsh and confused, and the less said about Shostakovich firing on all cylinders, the better. 

Matters are made worse by transient break-up, which in particular affects tympani or bass drum beats. This effect was very well illustrated on Mercury’s original monaural LP of Tchaikovsky’s 1812 Overture. In the sound sample of the cannon firing directly towards the microphone, you hear, not a mighty “boom”, but an emaciated, splintered “crack”, the result of severe sound-pressure overload.

This is, by almost anybody’s standards, seriously poor sound quality, which makes me wonder what sort of music was used by the writers of glowing reviews. It is similar to, but not quite as good as, an old, ill-tuned medium-wave “tinny tranny” whose battery is on its last legs. To put it in some sort of perspective, I fed the same music through the “in-ear” earphones that came with my portable MD recorder. These are the sort of cheap and cheerful things that typically cost a few dollars or pounds, or less than a fiftieth of the Filltune’s price, and yet their sound (heard mainly through my good ear) was far, far superior, even at levels bordering on the uncomfortably high. My reaction? One of utter disbelief, not to mention equally utter dismay. I had two options. One was to dismiss these BCHs out of hand. I took the other.

Troubleshooting the Distortion 
I dutifully turned first to the “troubleshooting” section of the owner’s manual. Under the heading, “The Sound Distorted” [sic] were listed three possible causes. These were of little or no help. The first said, “If an audio device or TV set is connected to the Filltune amplifier, do not connect to the MIC jack.” This was a trifle ambiguous, but I already had both bases covered, because I was using only the LINE jack, and I never connected a microphone at all. The second suggested, “When the batteries have run out, change all three batteries with [sic] new ones.” Ignoring the fact that, if the condition as stated was true, there would be no sound at all, and hence no distortion, and in any case jumping the gun, I installed a trio of brand new, good-quality, heavy-duty batteries. The distortion, albeit only marginally, became more clearly audible.

The third brought on a spasm of head-scratching: “Turn down the volume of the connected device.” The outcome of the scratching spasm was this: I concluded that this is verily a prince among solutions. It is completely infallible. For, sooner or later, it’s bound to work - when the volume gets to zero then, just as in the case of the dead batteries, there’s no sound, hence no distortion. However, this solution also implied the possibility of overloading the Filltune amplifier’s input stage. Disregarding the fact that the distortion became painful long before the volume itself did, I tried a wide range of settings of my amplifier’s headphones output level, for each one adjusting the Filltune volume control to maintain the same subjective listening level. The distortion pattern persisted, blissfully unaware of my machinations.

That left the possibility that my audio amplifier’s headphones outlet was ill-matched to the BCHs’ LINE input, so I tried feeding it instead from a standard RCA-type “line out”. For good measure, I also tried plugging the Filltunes into my portable minidisc recorder. In both cases, blissful unawareness continued to be the order of the day. Since I had never before experienced appreciable distortion from any of these sources via any other headphones, I was forced to suspect the fault lay somewhere after the input stage of the Filltunes.

A Second Bite at the Cherry 
At this point, I had one of those paradigm-shift thingies - my concept of “something being at fault” slid slightly sideways, to “something being faulty”. Surely, I reasoned, expensive equipment of ostensibly hi-fi quality from a respectable manufacturer, even where the said manufacturer might be gilding the lily a wee bit, must be at least tolerably good? Well, there was only one way to check that. I popped my “maiden’s prayer” on hold, while I waited for the suspect Filltunes to be replaced, courtesy of my unfailingly helpful supplier.

I re-ran the entire battery of tests on the replacement set. The most, if not only remarkable thing about the results was their disconcertingly close kinship to the first lot. I tried - and failed - to calculate the probability of getting two identically faulty sets of the same kit on the trot. The only reasonable alternative was that this “distortion” must be something other than what it seemed. And, although it does indeed sound like overload distortion, in truth it is altogether too progressive. It creeps in almost imperceptibly at quite a low sound level, and steadily gets worse as the volume rises - whereas the onset of overload distortion, by its very nature, is abrupt.

The Hunt for an Alternative Cause . . . 
Obviously, other than working my way through the World’s entire stock of Filltunes, there was no way of directly “proving” the truth of the coincidence. So, instead I tried some systematic elimination, working from the outside inwards. Was there interference from the vibration of the driver housing? Damping as much as I could of the relevant surface with my fingers made no perceptible difference. Perhaps the vigorously vibrating drivers were “bouncing” against my skull? Pressing them harder against my head didn’t do any good at all, nor was the distortion influenced by different placements. Was there “rattling” due to the unstable intervention of my hair, such as still remains of it? Holding the drivers against naked skin soon laid that hypothesis to rest.

An ENT consultant came up with a couple of suggestions, which he stressed were no more than possibilities, regarding possible side-effects of “flooding” the middle and inner ear with bone-conducted sound. Firstly, a normal middle ear cavity resonates at several frequencies in the 1 to 3 KHz. band. As a result of the surgery, my cavity had changed in respect of size, shape and reflectivity of affected surfaces. This will have modified those resonances, which could cause the impression of distortion. Secondly, there are two distinct “banks” of cochlear hairs. One lot senses the frequency spectrum of the incoming sound, the other moderates the sound level, cranking up the gain when you strain to hear something very quiet or pulling it back if things get uncomfortably loud. It is conceivable that the surgery could have adversely affected the operation of this latter bank.

I could think of any number of logical counter-arguments, but I felt disinclined to argue the toss with a professional expert. Fortunately, I didn’t need to - because straightforward practical observations settled the matter. My left ear may be half-wrecked, but the right one, normal age-related deterioration apart, works well enough. The two ears should therefore hear different qualities and/or quantities of distortion. Yet, the distortion sounds just the same in both.

That left the possibility that my skull-bones were somehow mangling the sound. So, I enlisted the aid of a completely different skull. I got my wife to listen to a musical extract, which started quietly and then became loud, firstly via loudspeakers (as a benchmark), then through the Filltunes. Her impression was marginally worse than mine. She described the same excruciating racket that I had heard, but also found it “a little bit fizzy” even before the loud part began.

. . . and Is This It? 
Unless I’d missed something, that exhausted the possibilities. I was left with the conclusion that the “fault” boils down to a dreadfully deficient dynamic range. You see, although you can find a gain setting where the loudest music sounds undistorted, at such a setting even very loud music seems to be emanating from a remote planet. Moreover, feeding the Filltunes with a line-level signal - or a headphones output adjusted to yield the same perceived volume - the distortion sets in with a vengeance by the time you’ve advanced the Filltunes amplifier volume control scarcely beyond half-way.

Is it purely a coincidence that, while the Filltune specifications quote an impressive-sounding “output force level” of better than 102 dB. for the 1 KHz - 20 KHz band, they omit any mention of distortion figures? In actuality, the usable “output force level” is a lot less than that quoted. I have to wonder: does this mean that all those less expensive makes of BCHs actually sound worse than the Filltunes? What really lies behind the apparently ecstatic expressions of the impeccably handsome youngsters whose images adorn all those websites and brochures?

A New Model - but Is It “Improved”? 
Teac have brought out an “F200” model, with somewhat swisher design lines - but no evident improvement to the headband - and a slightly narrower frequency response (the upper limit being a “mere” 20 KHz., as I recall). I suspect that the former would probably be of interest only to the dedicated follower of fashion, whilst the latter might imply that something has been done to reduce the distortion or, what amounts to the same thing, improve the usable dynamic range.

Interestingly, Teac are marketing the F200 model through their Tascam subsidiary, which specialises in professional and top-end equipment (paradoxically, though, the F200 model is actually less expensive than its predecessor). I’m forced to wonder what motivated this move. Unfortunately, lacking access to any F200s, “wonder” is all I can do.

Overall Conclusions 
I shouldn’t make too much of the ergonomic drawbacks. Of course these are inconvenient, and the designers could have paid a lot more attention to them. Nevertheless, a user can learn to get round, or otherwise put up with them. However, the same cannot generally be said regarding the problem of the sound quality. To put a “hi-fi” label on your product is not, or shouldn’t be, a mere marketing ploy. It is, or should be, a declaration of compliance with a certain audio quality standard. Yet, look how my experience compares with the half-dozen claims quoted at the beginning of the foregoing “Sound Quality” section: 

If you keep the volume low, and I mean really low, then four of the claims are completely justified. But feeble volume will hardly satisfy those who “enjoy listening to loud Hi-Fi music”, and will make the headphones far from “ideal for anyone . . . with hearing difficulty or impairment”. Turn up the volume, though, and the rampant distortion chucks all six right out of the window.

Nevertheless, I cannot shake off that nagging doubt. Why, oh why is the performance of BCHs such a bone of contention, with the experiences of various reviewers and other commentators so widely at variance? I have described my own experiences as best I can, and with ruthless candour. One thing that I am not mistaken about is that apparent distortion - it is not the subtle sort that “may cause aural fatigue with extended periods of listening”, but the vicious variety that causes instant and all-too-evident aural pain, and it is manifest throughout the four-fold combinations of two separate sets of Filltunes shaking the bones of two entirely separate skulls. If this noise is inherent in the headphones themselves, there really shouldn’t be any glowing reviews at all, should there?

This led me to believe - I needed to believe - that I must be missing something. I racked my brains until they ached, but could find nothing that would alleviate, or otherwise satisfactorily explain the distortion. Hence, I must set down my pen (or whatever is the keyboard equivalent), and wait to see whether anyone else can cast any light on the matter. Teac, in particular, are cordially invited to give it some consideration - I would be more than interested in what they have to say.

Recommendations and Rounding Off 
We are left with one last question. Assuming my findings are valid, just who, if anyone, could actually benefit from these Filltunes? As I’ve already said, people who have no sensible hearing problems simply don’t need them. They have literally hundreds of far better-performing - and much less expensive - conventional headphones and earphones at their beck and call. Sadly, neither can I recommend them to folk like myself, with partial conductive hearing loss. The commentaries - at least, the complimentary ones! - and brochures brimmed with the glowing promise of a panacea, inevitably exciting many hopes and dreams. However, the actuality has proved a bitter blow such as I wouldn’t wish on even my proverbial worst enemy.

So, regretfully, I had to take up the supplier’s “no quibble” guarantee. Dearly as I would have liked to, I cannot in all conscience recommend these Filltunes for most potential users. Nevertheless there are beneficiaries, albeit relatively few and far between - those folk I mentioned earlier, who are conductively as deaf as door-posts. My heart went out to one chap, commenting on the Web, who summed it up in these simple words, “Well, as I can only hear by bone conduction, they are the business.” And there you have it, the legendary “bottom line”: what matters distortion, if the only alternative is utter and unrelieved silence? That seems irrefutable, doesn’t it? Maybe it is - but then again, in my book of fairy tales, praying maidens would far rather be saved by handsome princes than by ugly sisters.

Paul Serotsky 

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