FLOYD TOOLE, 3. UTGAVE
I påvente av Floyd Tooles tredjeutgave av boken "Sound Reproduction" snappet jeg opp et ferskt sitat fra Toole. Jeg tror sitatet er av interesse for flere lesere av denne tråden:
"My entire lifetime's work began with a series of blind listening tests in 1966 at the National Research Council of Canada. It produced surprising results - people could reliably hear differences between loudspeakers, they generally agreed on which ones sounded good, and those that sounded good exhibited the best looking measurements. For me, the aspiring scientist, this seemed eminently logical, but for me, the audiophile, this was contradiction of popular beliefs. For the next fifty years my colleagues and I, and other researchers around the world have pursued the underlying truths, encountering more eye-opening revelations. The scientific process begins with double-blind listening tests - if something sounds good, it is good. If not, we need to discover why and chase down the gremlins. All of our listening tests involved comparisons of three or four loudspeakers, presented in randomized sequences. In the beginning we compared single loudspeakers for simplicity, but later discovered that listeners heard more problems and were more critical when listening in mono than in stereo. Several stereo vs. mono tests since then have shown that winners in the mono tests also win the stereo tests. The latest tests show that we are even less critical in multichannel comparisons. Interesting. The reality is that much of what we hear is mono - the dominating center channel in movies, the hard left and right, and the double-mono phantom center in stereo.
Having multiple loudspeakers in the comparisons helps listeners to separate the timbral contributions of the loudspeakers from the essential timbre of the recordings. A simple A vs.B test is a start, but if the speakers share the same problem it is not likely to be noticed. It is a remarkable feat that two ears and a brain perform in such tests. They are able to substantially separate the sound of the loudspeaker from the sound of the room it is in (except at low frequencies where the room dominates). They separate the timbral contributions of the loudspeakers from the inherent timbre of the recordings - even with studio recordings for which there is no "live" reference. The fact that listeners with normal hearing agree on what is good indicates that they are not only recognizing excellence, but are rejecting unnatural sounds. Listener comments usually describe shortcomings of flawed loudspeakers - sometimes at length and in colorful language - but reward more neutral loudspeakers with simple words of praise. If given the opportunity in an unbiased situation, humans are remarkably good "measuring instruments". In non-blind tests anything is possible.
Even now, I know of almost nobody who follows that degree of scientific rigor within the consumer or professional audio domains. It happens in some university research investigations, but most of the information available to consumers and professionals is seriously inadequate. Most loudspeaker specifications are an insult to intelligence forcing us to choose on the basis of imperfect listening evaluations and/or other persons' opinions. In product reviews we get few or no measurements, and subjective evaluations of the "take it home and listen to it" kind, where the test product is known, adaptation (a.k.a. breaking in) and bias are omnipresent. It doesn't mean they are without merit, but it does mean that opportunities for error are present. When a review begins with a sentence like: "I have always liked ##### loudspeakers" one can anticipate the conclusion, whatever the true merits of the product. Without trustworthy measurements the reader is at a severe disadvantage.
Fortunately a few consumer audio publications show measurements and some of them are reasonably accurate, although not everything one might wish for gets measured. Still, it indicates an admirable respect for the technical side of audio. If measurements indicate a poor performance and the subjective reviewer raves there is something wrong. It happens. Professional publications almost totally ignore measurements raising the question, who is more professional?
It takes time and money to do good product evaluations, and publishers and reviewers have lives to live and expenses to cover. Making meaningful loudspeaker measurements is not easy. Anechoic chambers are rare and very expensive, but the great outdoors or a large room can, with modern measurement methods, provide very useful data. However a loudspeaker cannot be described in a single curve, and the multiple curves that result from comprehensive measurements need to be processed to reveal to the eyes what the ears hear.
Over the years we have learned that human listeners cannot hear everything we can measure, and the possibility exists that there are things we can hear that we cannot measure. The problem with measurements is that they need to be in a form that correlates with audibility - not all are. E.g. we basically do not hear the ringing of resonances but we are very sensitive to the spectral component - the bump in the frequency response. That can be confidently identified in high resolution measurements of the right kind. Surprisingly, by this metric, we are less sensitive to high-Q resonances than lower Q ones. This is an example of the eyes anticipating what we hear, and being wrong.
It turns out that humans are essentially phase deaf, and can tolerate up to 2 ms of group delay. Non-linear distortions are tolerated at frighteningly high measured levels (LPs are a perfect example of more coming off the medium than went in) - simultaneous masking is at work on our behalf. So, right away we see an opportunity for the "we can't measure what we can hear" argument. It is absolutely true for some of the traditional audio measurements.
In the digital domain one cannot deny the existence of digital jitter. The question is "how audible is it?", "what are its effects on what we hear?" What are the technical metrics that correlate with audibility? The existence of a "distortion" does not ensure its audibility, but if it is audible, then let's nail down some meaningful metrics of it and establish thresholds of audibility. "Zero" is always an admirable objective for distortions of any kind, but in the real world it is not necessary.
Imaging is an undefined parameter, yet people routinely talk about being affected by everything in the playback chain, and some things that aren't, up to an including the plug that goes into a wall outlet. It clearly means different things to different people, and different people exhibit different expectations. I did my PhD on sound localization, so I have followed this topic with great interest. See for example, pages 126-138 in my existing book where three pairs of loudspeakers were compared - double blind - using positional substitution in the same room. Listeners responded in detail on sound quality and imaging. The evidence to date indicates that the principal determinant of a soundstage is the recording itself - the microphone choice and placement, and electronic control-room manipulations that went into it. For playback the principal requirement is that the direct sounds from the left and right loudspeakers be identical. Distortions in the signal paths are merged with the signal itself and become part of the soundstage.
Loudspeaker directivity is also a factor, but the effects of reflections turn out to be much less of a problem than might have been believed - instinct is not always right. This is especially true if the loudspeakers have relatively constant directivity with frequency, a parameter that is very rarely measured. When the timbral signature of the reflection is different from that of the direct sound, the reflection becomes more apparent. Significant lateral reflections have been shown to have insignificant effect on placement or precision of stereo images, again counterintuitive. The explanation: it is the direct sound that is responsible for sound localization. However reflected sounds can modify the sense of space which may or may not be desired. They also fill in the massive 2 kHz dip in the phantom center image spectrum (caused by acoustical crosstalk), so some reflected sound actually improves the sound quality of the featured artist in stereo reproduction. That is another reason to have a center channel.
I consider "envelopment" to be an important component of imaging. The impression of being in an acoustical space with the performers is the most important parameter of concert halls. It is deficient in two-channel stereo, as I explained in detail in my book, but multichannel audio can deliver as much of it as a recording engineer decides to incorporate. Upmixing of stereo to multiple channels is often a pleasurable addition, but some of the popular algorithms are - for my taste - overly aggressive in sending signals to the surround loudspeakers. Some experimentation with system setup may be required to find a good balance between leaving the soundstage intact, while adding the right amount of envelopment information.
All of this and much more is discussed in my new nearly 500 page book out in August if all goes well. The last chapter is a display of anechoic measurements on 50 years of loudspeakers. It is obvious that great improvements have been made. It is also obvious that over that entire period most loudspeaker designers started with the same objective: a flat, smooth on-axis frequency response. As the years passed they were able to achieve it more closely, and at the same time improve the off-axis response: the frequency dependent directivity. Sound quality improved. Both professional monitor loudspeakers and consumer loudspeakers benefitted. Now, the best of both domains are essentially identical in sound quality, and neutral is the objective. That is what we need in the audio industry, but sadly folklore and fantasy are still influential in both domains.
One always needs to remember the existence of the "circle of confusion" (the recordings we play back are influenced by the loudspeakers used in the recording process). We know how to make very "neutral" loudspeakers; transparent "windows" through which to view the art. But even the most perfect loudspeakers cannot always sound perfect - recordings are now the weak link. The recording industry is gradually adopting neutral loudspeakers as the new norm, but there are holdouts, mostly at the mixing end of the process. I think I hear their personal biases in some recordings. However smart mastering engineers are aware of the problem and can fix it before it gets into the final product.
I had a recording engineer at my home a week or so ago. He is successful, has two studios and has strong opinions. We ended up listening to TIDAL streaming and music Blu-rays until 2 AM. He was very impressed and wants to come back for more. My home system is better than he has in his control rooms or his home. My room looks more like an art gallery than a high performance listening room. Good sound is possible mainly because the origin of that sound is properly designed loudspeakers. The sound field managed multiple subwoofers were a special bonus - smooth, deep non-resonant bass. Bass management is not only for home-theater-in-a-box systems.
It is not a mystery how to get superb sound quality. Price does not correlate at all well, except that small inexpensive loudspeakers cannot play as loud as larger, more substantial ones. The information explaining the process is in the public domain. There are no excuses for less than excellent sound. I love science!
And then we have the whole topic of auto room EQ. Lots to talk about there, but a topic for another time and post…
As William Gibson said: "the future is already here, it is just not evenly distributed yet"
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Den nye boken kommer i august. Her er en lenke til en annonse med 30 prosents rabattkupong:
http://www.avsforum.com/forum/attachment.php?attachmentid=2192505&d=1497720714