LOUDNESS CONTROLS
Why they’re wrong… just plain wrong… even though the circuit implementations have improved dramatically over the years.
When I was a youngster and an avid reader of Australian Hi-Fi, nearly every amplifier review contained, it seemed, a mini rant on the presence of a particular button fitted to the amp. Now, many years later, I find myself writing mini rants on what is essentially the same subject in most of my reviews of home theatre receivers.
That control was the ‘Loudness’ control. These days that control is a great deal more sophisticated, thanks to digital signal processing technology, and goes under various names depending on the brand, such as Audyssey Dynamic EQ, Dolby Volume, YPAO Volume and so on. These do the intended job better than the ‘Loudness’ control and overcome some of the inevitable issues it had. But that kind of misses the point: all of them—and the original ‘Loudness’ button as well—were and are fundamentally misconceived because they are based on a misunderstanding of how our hearing works. Our hearing, note, not our ears.
It all started in 1933 when the Fletcher-Munson curves were published. The curves came out of research that indicated that human hearing was not linear, but that our sense of loudness varied by frequency. Over the decades further experimental work has been conducted, so there are significant variations from 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 30Hz tone will have to be more than 80dBSPL to seem like it is equally loud. 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 tone at as low a level as –8dBSPL. That is, at an even quieter level than that originally specified as the threshold of hearing. But to hear 100Hz it has to be at least 35dBSPL, and to hear 30Hz it has to be at least 60dBSPL.
Likewise, a healthy young person’s hearing is 15dB to 20dB less sensitive at 15kHz than it is at 3kHz.
Our equipment makers seeing those things think: ‘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 that. The amplifier didn’t know how big your room was, how far away from the speakers you were sitting, or 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 100dBSPL (on the newer equal loudness curves) 30Hz at 100dBSPL sounds close as dammit in level to 1kHz at 100dBSPL. So not only were ‘Loudness’ controls fundamentally misconceived, as I am arguing here, they wouldn’t even work to properly address the ‘problem’ 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.
Misconceptions
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 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 also 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 you to 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 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 seeing mechanism takes the video feed from your eyeballs and processes it to look ‘right’, based to a degree on averaging over time.
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, or 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. 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. It’s because everything sounds harsh and bright on first use. It’s only by leaving them in for a reasonable period of time without interruption that the hearing mechanism can adjust to the tonal balance of the new signals and make them sound right.
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 midranges 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.
Gradual Improvements
There was a period a few years ago when many 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.
Now, AV receivers mostly still have the processors, but switch them off by default. Thank goodness, although better yet would be to eliminate them altogether.
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.
When you first switch it off, if you’re used to it, your system may initially sound a little dull, but just persist for while until the true character of music has had a chance to assert itself.