Have you ever wondered why some home theatre receivers sound rather poor with stereo music? I mean, not just short of audiophile quality, but actually thin in sound as though something’s going wrong. Brands like Denon and Marantz and Yamaha can build perfectly competent stereo amplifiers, so why should their multi-thousand-dollar home theatre receivers sound so dreadful with stereo music?
Well, truth is, they don’t sound unusually poor. Unless the user has made the wrong setting during the setup process. It’s a setting that’s all too easy to get wrong. And, indeed, there was a while when for several brands it was the default setting, almost guaranteeing lousy stereo sound out of a home theatre receiver.
What is this setting?
This setting goes under various names depending on the brand, such as Audyssey Dynamic EQ, Dolby Volume, YPAO Volume and so on. What they do is what the “Loudness” control used to do on stereo amplifiers back in the day: adjust the frequency balance of the sound to deal with the “problem” that our ability to hear frequency extremes diminishes as the volume level is reduced.
Of course, these new processes do the intended job far better than the “Loudness” control ever could and overcome some of the inevitable issues it had. But that kind of misses the point: all of these things, and the original “Loudness” button as well, were and are fundamentally misconceived.
That’s because they are based on a misunderstanding of how our hearing works. Our hearing, note, not our ears.
What is the “problem” to be fixed?
It all started in 1933 when the Fletcher-Munson curves were published. This came out of research that indicated that human hearing was not linear, but the sense of loudness varied by frequency. Over the decades further experimental work has been conducted, so there are significant variations the original Fletcher–Munson curves and the equal loudness contours specified in ISO 226.2003, but the details don’t matter too much to us here.
What does matter is what those curves mean. They show that if you play a 1kHz tone at 60dB SPL, a 30 hertz tone will have to be more than 80dBSPL to seem like it’s the same loudness. Taking things to their end points, extreme treble and bass can evade detection by the human ear at much higher levels than mid frequencies. A healthy young person can hear a 3kHz or 4kHz at as low a level as -8dB SPL. That is, at an even quieter level than that originally specified as the threshold of hearing. But to hear 100 hertz it has to be at least 35dB SPL, and to hear 30 hertz it has to be at least 60dB SPL.
Likewise, at 15kHz a healthy young person’s hearing is 15dB to 20dB less sensitive than it is at 3kHz.
Home theatre receiver makers seeing those things, and noting that they had plenty of DSP power in their equipment, thought: ah, we can shape the sound to get address that “problem”. We can boost the bass and treble when the volume control is low so that balance is “restored”. (And yes, those are all “scare” quotes.) That’s what a “Loudness” control does. It boosts bass and treble. The better ones would boost them more at lower settings of the volume knob, and not at all at higher settings.
There was at least one obvious problem with the original Loudness control. The amplifier didn’t know how big your room was, how far away from the speakers that you were sitting, how efficient your speakers were. And it turns out, the varying sensitivity of the ear depends not only on frequency but on volume level. At 100dB SPL (on the newer equal loudness curves) 30 hertz at 100dBSPL sounds close as dammit in level to 1kHz at 100dBSPL. So not only were “Loudness” controls fundamentally misconceived, as I’m arguing here, they wouldn’t even work to properly address the “problem” they for which they were designed, unless your system happened to sound exactly as loud as their ‘corrections’ were designed for.
Modern DSP-based “correction” processes (at least in theory) deal with this, because they are set up using data gathered by the receiver in the automatic room calibration process. So, the processor knows how loud the sound is where you are sitting and can apply the inverse of the appropriate equal loudness contour.
Signal processors
But here’s the thing, you and I and everyone else does not hear low level sound as deficient in those frequencies to which our ears are less sensitive. Our ears are only part of our hearing mechanism. The signals from them are fed into an enormously sophisticated organic signal processor, the human brain. (Not just humans: other animals would have similar mechanisms).
This OSP does astonishing things. By comparing subtle phase differences between the left and right ear signals it can determine the direction from which the sound is coming (which is why bass isn’t directional – the wavelength is too large for the distance between your ears to allow the accurate determination of the gap between the crests of the wave).
And, it turns out, our hearing mechanisms have a powerful EQ processor. Here I make a bold claim: the world around you will continue to sound subjectively the same as you age. The upper reaches of your hearing sensitivity roll off as the years accumulate, undeniably and measurably so. As a result, in crowded and noisy environments you will find it harder to distinguish what someone is saying to you because those high frequencies are used, if available, by the brain to better localise sound and to allow focus on a particular source. But all you will know is that you find it harder to understand what someone is saying. It will still seem like it sounds the same as it always did.
This works a bit like the auto-white-balance in your eyes. Back when we used to use film cameras, taking a photo under incandescent lighting would result in an orange hue, which was invisible to your eyes when you were taking it. Because your visual mechanism takes the video feed from your eyeballs and processes it to look “right”, based to a degree on averaging over time. So, the orange glow of the lights was invisible to you while you were under them. But when you looked at the photo, you’d see that orange shift right there on the print.
And so it is with your ears. Test it yourself. Take a little cotton wool, loosely ball it up and put it in your ear canal. Gently, at the very surface, in just far enough so it won’t easily fall out, but so that you can readily take it out. Leave it there for a couple of days -- actually, replace it a couple of times along the way. Note the dullness. But after a while sounds will no longer seem dull. They’ll sound kind of normal. But you’ll have greater trouble understanding what’s being said to you, of picking out one voice among many while conversing in a crowd.
Then take out the cotton wool, and suddenly your world will be full of the tinkle of glassware, birds tweeting outside the window, the oddly bright voices of those around you. Not like it was before, but noticeably brighter. For a little while. But soon enough everything will sound pretty much the same as normal as your hearing recalibrates to the newly bright signal.
That is, I suspect, why some hearing-impaired people detest their hearing aids. Everything sounds harsh and bright on first use, and only by leaving it in for a reasonable period without interruption can the hearing mechanism adjust to the tonal balance of the new signal and make it sound right.
The Fundamental Misconception
So, the fundamental misconception of all those processors, starting with the “Loudness” control, is that the undeniable differences in hearing sensitivity at different frequencies and volumes requires some kind of redress.
It doesn’t, because our hearing already provides this. For sure, the treble component of the music is reduced in level relative to the midrange when the music is being played at background levels. But our hearing expects it to sound lower in level. If doesn’t, the music is going to sound harsh. The same is the case with respect to bass, although in this case it tends to make the deeper bass sound disconnected from the rest of the sound.
Your hearing expects the world to have a certain frequency balance at any given volume level. Alter that and it’s going to sound wrong.
Do note, though, that this kind of volume-level-dependent correction is quite separate from overall system EQ. Regular static Audyssey EQ, for example, is pretty good at room correction (and, quietly, loudspeaker correction, since there is no system that can tell the room apart from the loudspeakers!)
Gradual improvements
There was a period a few years ago when many home theatre brands not only had “Dynamic” “Volume” processors, but switched them on by default. That was a dark period indeed for sound quality and I wonder how many people found their equipment less satisfying because of this issue.
Indeed, do you recall reading any reviews in home theatre magazines from back in those days complaining about the sound quality? Or noting that the sound was poor unless those processors were switched off? If you do recall that, then they were probably reviews written be me. It did make me wonder how in an industry in which reviews routinely comment on the differences in sound produced by two different $10,000 amplifiers, reviewers didn’t seem to notice the distinct and obvious degradation of sound caused by, for example, Audyssey Dynamic EQ.
These days home theatre receivers mostly still have the processors, but switch them off by default. Thank goodness, although better yet would be to eliminate them altogether.
And if, perchance, you are using a home theatre receiver, make sure that any such processing is disabled. Look under the Audio or Audyssey setup menu or consult the receiver’s manual.
And remember, it takes time for your hearing to adjust. When you first switch it off, if you’re used to it, your system may initially sound a little dull. Just persist for a while until the true character of music has a chance to assert itself.
