Psychoacoustics - what is it and why does it matter?

[A.S.]

This new section of the Harbeth Knowlwedge Archive looks at psychacoustics, meaning the way the human ear and brain perceive sound and interpret it.

Our entire auditory system developed not for listening to music or even speech, but to make us aware of our environment and specifically, predators in close proximity. In fact, in evolutionary terrms, music is a complete irrelevance, as it appeared only about 35,000 years ago.

I am not an expert on the hearing system, but even a rudimentary appreciation of how we hear is absolutely essential to be able to understand and intrpret why we like some sounds, and not others. And above all, to caution us about making irrational, emotional decisions when purchasig audio equipment because we've allowed (or encouraged) our ears to be seduced by our hearts and overrule our brain. In my opinion, audio should never be critically evaluated with the eyes open; a blindfold is essential. Failing that, look at the floor in the middle distance, but never at the equipment.

Some links: Overview here

This thread is in the objective, knowledge-tree part of the HUG. I welcome objective posts that further illuminate this fascinating and complex subject and draw on published research or professional experience. This is an important subject, and is Moderated.

[kittykat]

I now remember the name of the documentary about musicians' brains. It's a Canadian TV production for National Geographic and its called "The Musical Brain" looking at the work by Dr. Daniel Levitin of McGill University. He has also written 2 books called "This is your brain on Music" and "The World in Six Songs".

The documentary features amongst others Sting, Michael Buble and Feist, exploring music, creativity and the mind. Very interesting as it also looks at brain activity under an MRI during listening, creative and imaginative phases of Sting’s brain.

[STHLS5]

We used to have a sub category under "Hearing and Perception" where I posted a few link regarding recent progress made in understanding our auditory system. The category isn't visible any longer. Would discussion contained be relevant to this topic?

ST

{Moderator's comment: seems some threads have (temporarily?) vanished during relocation ... }

[Kumar Kane]

I think too that the brain hears what it wants to hear. To wit, better sound after an exotic component is dropped into the system. Ears are just transducers after all, it is the brain that hears at the end of the day. So if I know from reviews that the exotic adds an extra dimension to the sound, I end up hearing it. Particularly if I have also paid fancy money for it.

Given this, the effect of alcohol on the brain must also be the reason why the system sounds a lot better to me after moderate imbibing. Turning the lights down low is another useful audio tweak!

Which brings me to the famous glow of tube amps. I wonder how the music would sound with a powered up tube amp in the component rack, even if it wasn't connect to anything. It wouldn't fool the person that has set it up, since he or she would know the situation. But I am pretty sure it would fool a lot of audiophile visitors into gushing over the liquid warmth of tube sound:-)

[Pharos]

The importance of recognising that the brain predominates in the perception of sound, rather than we just receiving pure sound, is hard to exaggerate; we filter it, process it, habituate to its quality, and learn to focus on aspects of it to the almost complete exclusion of all others.

The ear, as Alan says evolved in a survival context, very much unconcerned with music, with the minor exception that I believe music evolved before speech; one can imagine the use of music as an inter-tribal communication form. (War drums).
Dr Oskar Heil did much work in evaluating the sensitivities of the ear, and came to the conclusion that its main sensitivity was to transient sounds and not to the relative intensity of different frequencies, and he developed a tweeter with a good transient response as a result.

I think that our brains, metaphorically like very powerful computers, apply massive amounts of processing to sound, these dependant on contextual objectives in differing environments, and this process is never free from the psychosocial needs, political pressure, and the aesthetic considerations of the individual.

Personally I find a day at a Hi-Fi show exhausting, and a listening session with a dealer more so because of the perceptions of his work load in changing equipments, and pressure to buy.

To me a very difficult and often ignored factor in listening to music on Hi-Fi, is that evaluation of Hi-Fi requires the scientific objectivist evaluative approach, but the musical medium, which we so often use to do such is one of the most emotional mediums there is.

This is in my experience compounded by the excitement I have experienced, and with which I have also seen others overcome, when a much loved piece of music is played in a way which seems more revealing. It may occasionally be so, but I think that often a change in frequency response can be found to be responsible for emphasis of a particular instrument, and hence it is perceived more clearly and separately. Kumar Kane's comment about the brain hearing what it wants to hear regrettably can, I feel, be extended very widely to much perception; often people perceive only that which they wish to, they being more than happy to reside in fantasy, this being described by psychologists as a survival strategy.

[STHLS5]

Very interesting post. You mentioned a few things that I have observed too.*

I am not so sure about perception when we judge sound. At least it wasn't *the case when I first heard Harbeth singing. It was a Compact7 which looked so old styled and frozen in time. I think I didn't wait for the track to finish to recognize its quality. That's the first time i heard the name Harbeth. It is possible that we can chose not to hear what we don't want to recognize, IMO.*

Could you explain what you mean by :-

To me a very difficult and often ignored factor in listening to music on Hi-Fi, is that evaluation of Hi-Fi requires the scientific objectivist evaluative approach, but the musical medium, which we so often use to do such is one of the most emotional mediums there is.*

Btw, it is probably true that we are preprogrammed to appreciated music long before we learn to speak.

[Pharos]

I think that the technical/scientifc/evaluative, and the receptive/emotive artistic aspects of the pursuit of Hi-Fi as an interest, are entiely different paradigms, and care has to be taken, and discipline applied, to avoid confusing them. We all by now know about the objective/subjective divide, originating I believe in the 80s.

To evaluate Hi-Fi we use certain well established technical criteria which show a correlation between their increase or decrease, and that also of the percieved sound quality, though there are many more to be discovered I suspect.

I have with many friends/colleagues strained over the years to try to screen/filter out the pleasurable/artistic response, which probably occuring at a 'lower' level of the brain, perhaps towards the limbic system, and to perceive purely the tonal changes due to a modification or change in equipment whilst also bearing in mind the established technical criteria. This struggles reflects the fact that music at its best can send one into raptures of enjoyment, but in each of these states of receptivity, I think that the brain is in a very different configuration or mode.

I know it may be a rather crude analogy, but one may be a motor enthusiast, and tune up/modify or tweek one's vehicle, and subsequently take it for a test drive. Whilst sweeping through the beautiful country passing through gloriously green and browning Autumn foliage, one has to deliberately focus on the sound of the engine, or feel of the gear stick in an effort to observe any improvement resulting from one's work. I believe music to be the greatest of the arts, it providing me with enormous pleasure, this making only harder the struggle invoved to be in the 'objectivist' mode.

We all in life have to screen out things and focus on the specifics we are trying to affect, but in this case, when the thing we are trying to screen out is so enamouringly pleasurable and potentially overpowering, it is I think harder. Another analogy has just occured to me.

Imagine being a top breast surgeon at a Harley St. clinic, and being presented with an already very beautiful woman who is requiring an improvement to her breasts. The surgeon has to remain very filtered about his task, ignore his own personal reactions to her, and pay attention to the technical aspects of his work, the resulting aesthetics of it, and how she feels about his intentions.

With regard to your case when you "first heard Harbeth singing", I have also come to the conclusion that the division between two brain modes or functions is not always clear. After a modification I listen concsiously to evaluate the results, but do not always appreciate them fully in so doing. Then, whilst in a less conscious state, not in intense focus, I often hear other improvements not noticed when trying so to do, and also, an improvement can 'slap one in the face' with its substantialness, as though 'caught of guard'.

I have over the last few years come to the conclusion that there are for me two criteria which mark a genuine improvement in sound quality; hearing things never heard before on well know music, and, the removal of general mushioness, clutter and din, this latter often revealing just how flakey and amateur a perfomance is. I recently heard an old 'Stones track, and was amazed at how 'clunky' and amateur it sounded.

[A.S.]

...Imagine being a top breast surgeon at a Harley St. clinic, and being presented with an already very beautiful woman who is requiring an improvement to her breasts. The surgeon has to remain very filtered about his task, ignore his own personal reactions to her, and pay attention to the technical aspects of his work, the resulting aesthetics of it, and how she feels about his intentions....

Two good analogies.

Regarding the surgeon and (female) cosmetic surgery, it is one matter for the woman to be self-motivated to seek out the scalpel, but quite another to feel peer pressuriesed into taking action. What I strongly object to is to the extreme language (which is to my mind so obvioulsy tongue in cheek) we read in the media and on fringe forums celebrating this or that new height in audio when the reality is that of extremely small incremental steps over a generation. This constant bombarding of the vulnerable (female) with ideas of inadequacy are precisely the same tricks used in many industries - including this - and miss the point entirely. A beautiful person has nothing to do with external appearance* and the appreciation of music can be greatly enjoyed on a portable radio.

*Just a few nights ago, in the UK TV series "Beauty and the beast, the ugly face of prejudice" we were introduced to a London narcisist and the terribly disfigured Regie Bibbs from Houston. As my wife and I watched and Regie's quiet personality revealed itself, we ceased to focus on his appearance and became completely beguiled by his normality. Self evidently, under that misfortune, Regie is a truly beautiful man with a deep care for others. The outcome of the programme was most unexpected and refreshing.

The analogy between the body beautiful (or not) and high-end audio was striking indeed. Look under the skin - be sensitive to whatever is motivating you. Reject the shallow.

[Pharos]

I agree entirely about the miniscule, if any, progress in audio reproduction, and have discussed with friends, the use of changing appearances throughout the 80s to the present with; shiny cones, frame finishes obviously aimed at th eye rather than function, excessive use of gold, twee mouldings which add nothing to function, and very excessively elaborate cable structures and finishes etc. Audio jewellery.

The real improvements are small, and I liken the changes to the process of traveling up a helical spring; if we are not careful we go round and round in accordance with the latest fashion, this seeming to me in many walks of life to be based only on the idea that we have found a new truth, but in reality the vertical assent, (real progress), if any, is very small. We have had in my audio interest, numerous swings towards each of the bass loading techniques, illustrated by manufacturers each producing models following this new trend.

The pressure to conform seems ubiquitously used in capitalism, and the pressure to change and aquire a new technology obviously promotes sales, and hence the profits of manufacturers. Group sociology does much to ensure compliance of the individual, as discovered by Soloman Asche in the 40s or 50s, and if a hen with a white head feather is placed in a coop with a hundred others with red ones, they will peck it to death.

I do however think that physical beauty can be part of person's beauty, but it is only one aspect of them and it is given by nature, not aquired by personal endeavour. I too watched the programme, and found the end refreshing, but I wondered how sincere the narcisist was. Yes, reject the facile, the easy, and work towards producing the substantive and ultimately more sutainable and rewarding.

Regarding the apparent emphasis currently on superficial 'beauty', I am reminded of a song by Al Stuart from "Modern Times" entitled "I really don't believe what's going on"; Group"She walks like Greta Garbo, and talks like Yogi Bear".

[A.S.]

...Yes, reject the facile, the easy, and work towards producing the substantive and ultimately more sutainable and rewarding...

Very succinct. That sums up what we strive for at Harbeth.

[Bromo33333]

Human perception is the most difficult thing to norm out.

I have a high school friend of mine that is now a researcher in Cognitive Science - and does research in the "familiar/unfamiliar" processing - indicated last time we chatted about his research he indicated to me that the brain appears to be a pattern processor first and foremost - and in things that are familiar (his work is visual not auditory so take it with the grain of salt) the brain spends less time processing new sensory data - but actually spends most of the energy on memory. ( papers )

If applicable (big "if") to auditory stimulation - this might imply that you don't really listen to music if it is familiar to you - but use the pattern of it to recall a memory that is what you really "hear" - this would cast a shadow over A/B testing in general unless this phenomenon were worked out. Might also mean only an unfamiliar piece of music is workable - or perhaps the experience is so wholly subjective it is impossible to norm it out scientifically? Or we are such pattern beasts we might completely miss over a gross tonality difference with an A/B switch?

I am making no such claims myself - but I am throwing out the idea for discussion that the psychological portion of perception may be important enough that we have to figure out a way to norm that away before we can truly trust our testing due to the way we process sound? It is a mystery to me why in isolation differences can seem plain as day, but in an A/B test it is exceedingly difficult to do so! Can we really have a truly universally applicable "objective" test when so much is "in your head?"

Food for thought, anyway - I will caution everyone that "I have no dog in this fight" - a long time ago I realized that for me that the whole thing is an aesthetic experience - and as such fundamentally subjective and beyond anyone's judgement except mine own. And when I feel transported to a live show or into the studio as much as possible, that's my metric.

[A.S.]

.... that the brain appears to be a pattern processor first and foremost - and in things that are familiar (his work is visual not auditory so take it with the grain of salt) the brain spends less time processing new sensory data - but actually spends most of the energy on memory. ( papers )

If applicable (big "if") to auditory stimulation - this might imply that you don't really listen to music if it is familiar to you - but use the pattern of it to recall a memory that is what you really "hear" - this would cast a shadow over A/B testing in general ...

Again, A-B testing is designed to highlight the sonic difference between A and B at the instant of switch-over and this it does extremely well. Are you saying that if I presented you with a slide show of two pictures of family members that you were familiar with but one picture had a colour cast that you wouldn't be able to recognise it on switch-over? Of course you would: the instantaneous comparison would jolt your brain into appreciating the differences. It's exactly the same with A-B audio testing - shown on a video clip here just recently.

Conversely, if the slide show image A transitioned (faded) progressively into the B image over a period of minutes as you watched, do you think you would suddenly say at any one point 'wow, that family member seems to have green skin'? I don't think so. Your visual/audio memory is not designed to make long-range comparisons; there is no evolutionary benefit for that behaviour. Our senses are designed to produce snap conclusions, side by side comparisons, better/worse, yes/no, pass/fail, go/no-go, danger/safe, attractive/unattractive, loud/soft, bitter/sweet, hot/cold ....

[Simon]

Human perception is the most difficult thing to norm out.
If applicable (big "if") to auditory stimulation - this might imply that you don't really listen to music if it is familiar to you - but use the pattern of it to recall a memory that is what you really "hear" - this would cast a shadow over A/B testing in general unless this phenomenon were worked out. Might also mean only an unfamiliar piece of music is workable - or perhaps the experience is so wholly subjective it is impossible to norm it out scientifically? Or we are such pattern beasts we might completely miss over a gross tonality difference with an A/B switch?

I suspect that this is one of those cases where the devil is in the detail of what is done in the auditory experiment. On the one hand we have an effect whereby what is perceived by the higher faculties of the mind is modulated by memory; on the other hand we have the fact that many of the senses are “tuned” to detect change. (If we look elsewhere in the animal kingdom, it is not hard to find examples of vision that are poor in terms of absolute acuity but extremely good at detecting movement).

One might hypothesize that the first effect (memory) might mask the second effect (change detection) by arguing that what is perceived by the mind is (mostly) the content of memory pointed to by the auditory sensation. But equally one might hypothesize that our ability to detect change in sensory stimuli might actually be enhanced when the auditory input is familiar; it is not inconceivable that evolution might have tuned our senses to detect changes occurring in our most familiar surroundings.

For a pair of hypotheses like these the test is, of course, experiment. Informally, an A-B test (with instant switchover) using passages of music of varying familiarity to the listener would seem to be appropriate. Of course, one would have to quantify the notion of familiarity so that one could use this in a statistical model of the outcome. But, appropriately modelled (and with big enough sample size to do this!) one could determine the functional relationship between familiarity and ability to detect difference.

Since the mind is so amenable to suggestion, it is not hard to think of a sequence of variations on such an experiment. For example:

1) The subject is played a 30 second excerpt of music in which there is a “transition” at a random location in the music. Afterwards the subject is asked whether they detected a change.
2) The subject is told that they are about to listen to a piece of music that includes a “transition” somewhere and are asked afterwards whether they noticed it.
3) The subject is told that they are about to listen to a piece of music that includes a “transition” at the twenty second mark and are asked afterwards whether they noticed it.

Of course, the subject is randomly assigned to a piece of music that either has or does not have such a transition. Each of these variations is asking a subtly different question to do with how the initial state of mind affects the process of perception.

[A.S.]

I suspect that this is one of those cases where the devil is in the detail of what is done in the auditory experiment. On the one hand we have an effect whereby what is perceived by the higher faculties of the mind is modulated by memory; on the other hand we have the fact that many of the senses are “tuned” to detect change. (If we look elsewhere in the animal kingdom, it is not hard to find examples of vision that are poor in terms of absolute acuity but extremely good at detecting movement).

One might hypothesize that the first effect (memory) might mask the second effect (change detection) by arguing that what is perceived by the mind is (mostly) the content of memory pointed to by the auditory sensation. But equally one might hypothesize that our ability to detect change in sensory stimuli might actually be enhanced when the auditory input is familiar; it is not inconceivable that evolution might have tuned our senses to detect changes occurring in our most familiar surroundings....

I think that it's valuable to discuss and refine and explore the academic approach to the listening process as applied to audio equipment evaluation. But we mustn't lose sight of the fact that virtually no one who puts pen to paper - particularly those doubters of the validity of the A-B process - have actually tried it for themselves. That is, put themselves in front of two pieces of equipment (speakers, electronics, accessories - whatever), taken care to equalise the levels so they are certainly playing A and B at the same loudness (by far the most influential variable), and controlling the switch-over mechanism themselves to experience A juxtaposed against B with just a tiny fraction of a second gap as the relays change over.

Actually putting oneself into that hot seat is a possibly a life changing experience. It was for me 30 years ago. A rationalist would surely admit that it is simply not valid to continue along the exclusively subjectivist path if before the A-B switchover the listener held (as so many do) rock solid, unshakeable, 'swear on my grannies life', 'cross my heart...' opinions of marked differences between A and B, but after actually taking the test is neither able to hear any differences at all or is unable to state a preference for one or the other or is even unable to positively identify A or B on more than the 50/50 occasions you'd expect by throwing a dice. If the listener refused to accept what he heard for himself in the A-B comparison and reverted to his former preconception (as the majority would) then that tells much about human nature and nothing about the construction of the test.

The reason the audio industry is in the deep rut it is is because the good products and the bad ones are presented side by side in the market, looking equally credible to the non-technical consumer. This does the consumer no favours. It stifles R&D investment and hence true progress. The audiophile industry is a law court where entirely innocent, decent upright defendants are daily found wrongly guilty and where flamboyant and more media savvy defendants are let of the hook or even lauded. We in the gallery know the truth about A v. B and shout out 'you've got it wrong - you've picked the wrong man yet again' but are powerless bystanders, drowned out by the howling of the public. Who can stand against the Law of Subjectivism prevailing at the audio court?

As the judge might say 'don't trouble me with the facts .... I've already made up my mind'. Simple, $20 A-B relay comparators are exceptionally valuable for getting to the facts of the comparative performance of A v. B.

[kittykat]

Scientifically and mathematically, fortunately or unfortunately depending on which side one is on, it takes more than one person's opinion or event/s to make a choice statistically significant. Who is one Mr. X to say that product A is superior over product B (and on what basis)? Trial and event wise, under no pressure, insignificant perceived differences WILL draw closer to 50/50 likelihoods, some quite obviously even after a very small number of trials.

It takes conviction (and bravery) for one person to say there is less difference than more.

[Pluto]

A rationalist would surely admit that it is simply not valid to continue along the exclusively subjectivist path if before the A-B switchover the listener held (as so many do) rock solid, unshakeable, 'swear on my grannies life', 'cross my heart...' opinions of marked differences between A and B, but after actually taking the test is neither able to hear any differences at all or is unable to state a preference for one or the other or is even unable to positively identify A or B on more than the 50/50 occasions you'd expect by throwing a dice. If the listener refused to accept what he heard for himself in the A-B comparison and reverted to his former preconception (as the majority would) then that tells much about human nature and nothing about the construction of the test.

If the test results demonstrate that there is no discernible difference between state A & B, when 'everybody' knows that there is, then clearly, 'the test is flawed or the rest of the equipment involved isn't of sufficiently high resolution to exhibit the difference of which 'everybody' is aware.

Thus the test process is fatally flawed and, by inference, useless. Because 'everybody' knows better.

[Simon]

I think that it's valuable to discuss and refine and explore the academic approach to the listening process as applied to audio equipment evaluation. But we mustn't lose sight of the fact that virtually no one who puts pen to paper - particularly those doubters of the validity of the A-B process - have actually tried it for themselves. That is, put themselves in front of two pieces of equipment (speakers, electronics, accessories - whatever), taken care to equalise the levels so they are certainly playing A and B at the same loudness (by far the most influential variable), and controlling the switch-over mechanism themselves to experience A juxtaposed against B with just a tiny fraction of a second gap as the relays change over.

Oh, I agree absolutely with what you say. In my own field (of medical statistics) there has been gradual shift over the last fifty years from “expert” opinion based practice towards evidence based practice. In particular, the use of randomised controlled trials (or of well-run observational studies) has become mandatory before the introduction of new drugs or medical procedures. This sort of trial can, of course, be difficult and/or expensive and there have been murmurings from some quarters against what are claimed to be over-bearing requirements.

For example, there’s a famous paper in the British Medical Journal (2003; 327:1459) entitled “Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials”. This paper (which was partly tongue-in-cheek and partly serious) argued that the evidence base in favour of the efficacy of the use of parachutes when jumping out of airplanes is entirely anecdotal; no controlled trials have been done! The (serious) point being that there are circumstances where we are so sure of the underlying science of the situation, we simply don’t need to do experiments when the outcome can be predicted from theory or when the result is blinkin’ obvious.

It strikes me, when reading around the audio forums, that many have convinced themselves that their uncontrolled observations lend weight to the hypothesis that the differences they are “observing” fall into the blinkin’ obvious category. Of course, they have fallen foul of the bane of observation: confounding by uncontrolled factors. You’ve already given the example of the need to control for volume levels; it would not be difficult to come up with a long list of others!

Another more pernicious aspect (and this occurs in the medical field as well and is beginning to be recognised as an issue) is the self-need of “experts” to be seen as “key opinion formers”. There is a very strong thread of “damn the evidence, I’m an expert” running through much of hifi as well as medicine.

[A.S.]

If the test results demonstrate that there is no discernible difference between state A & B, when 'everybody' knows that there is, then clearly, 'the test is flawed or the rest of the equipment involved isn't of sufficiently high resolution to exhibit the difference of which 'everybody' is aware'. Thus the test process is fatally flawed and, by inference, useless. Because 'everybody' knows better.

And that is the usual argument that is trotted-out by the nay-sayers when confronted with the possibility that their core belief system is threatened.

I just don't see get it. I cannot understand the benefit of living a lie. If someone proved to me with my willing participation in such an A-B test that impregnated lace, hand woven in an Amazonian jungle by mysterious ancient methods was, under A-B testing, better sounding* than our RADIAL™ speaker cones, do you think I would spend the rest of my life flogging a dead technical horse to dwindling numbers of die-hards? I'd be mad to. My income would suffer as the competition gathered pace and I'd spend my remaining years bitter and twisted and in shivering poverty. But why do so many humans - including audiophiles - snuggle under a 20 tog duvet of beliefs and perceptions which they mistakenly convince themselves protects them from the reality of life? The actuality is that the world is not static: one needs to be flexible enough to review even ones most deeply held beliefs regularly, to keep as open a mind about as many issues as possible, to be brutally objective and honest, especially concerning technology. To be self-aware that there are no absolutes and that if we want the best from our audio lives, we have to work with our senses and not against their reality. Simple stuff!

Who knows, preaching that dogma even I may eventually come to truly appreciate the life and works of our Great and Glorious former Leader, Anthony Charles Lynton B.. No, forget everything I've said above about keeping an open mind .....

*Better means under A-B comparison seemingly more true to life, however one may define it. As at today, RADIAL™ does seem to be the best solution.

[kittykat]

This sort of trial can, of course, be difficult and/or expensive and there have been murmurings from some quarters against what are claimed to be over-bearing requirements.

How overbearing does it have to be to prevent unplanned outcomes? fluoxetine, hpv vaccine, thalidomide?

We have nothing to learn from the self interest of the drug industry in general. We all have something to learn from math, science and common sense. There is no common sense from maiming people or hiding those facts.

[A.S.]

How overbearing does it have to be to prevent unplanned outcomes?....

Again this is where I scratch my wooden head and wonder just what is wrong with people! (I agree with the full post).

Do we know what an A-B comparison looks, smells and feels like? All it is is a willing listener alone or with others (best alone) in a comfy chair, in familiar warm surroundings, with music of his/her choice, at whatever time of day or nights suits, in the dark or in sunlight at whatever listening level is desired. The only equipment (and this is no more than high school grade technology) is some sort of audio multimeter to check for equivalence of levels between A and B and a relay switch-over box. There is no time pressure; the comparison can take as long as the listener has the patience or curiosity for. He can switch a few times or hundreds of times until his clicking finger is red raw - it's up to him. He can go and make a meal, have a cup of tea, walk around the block, take an ocean cruise or have whatever interruptions in the routine he likes. He can drag it out over hours or days if he prefers, providing that the levels are not tampered with*. There are no chains, whips, electric prods; no dungeons, no enleathered taskmasters, no hallucinatory drugs, no truth serums. An audio A-B comparison is not at all like this. Only if you really want it to be and pay.

So what's the big deal? The listener is in total control of the process. Why wouldn't you want to learn the truth of the relative merits of A v. B? What is the problem folks?

* The levels must not be adjusted, not even a little bit. If the equipment is calibrated for equal loudness at the outset, if any part of the system (other than a common source) is moved, disassembled or touched, the listener must, as a precursor to further listening, measure the loudness. If comparing electronics which has digital or motorised volume controls, if these remember the volume setting on power down/up that's OK: if they reset to some other value, then the levels must be reset and validated by instrument before further listening is undertaken. It goes without saying that mechanical volume controls also cannot be touched. This means the replay volume for the entire music selection must be decided in advance as the volume of amplifiers cannot be adjusted during the session unless a complete re-measurement is undertaken. A work around would be to have a CD source which does have a (remote) volume control. Thus, if comparing two amplifiers, their volume is fix and set, and the common source is allowed to go up and down in volume to suit the music.