Australian HiFi — May-June 2017

16 Australian

FEATURE

W

hen I was a youngster and an

avid reader of Australian Hi-Fi,

nearly every amplifi er review

contained, it seemed, a mini

rant on the presence of a particular button fi tted

to the amp. Now, many years later, I fi nd 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 process-

ing 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 ‘Loud-

ness’ button as well—were and are fundamen-

tally 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-Mun-

son 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 signifi cant variations from the original

Fletcher-Munson curves and the equal loudness

contours specifi ed 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 specifi ed

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 ad-

dress 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 amplifi er didn’t know how big your

room was, how far away from the speakers you

were sitting, or how effi cient 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 fundamen-

tally 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 defi cient

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 com-

paring subtle phase differences between the left

and right ear signals it can determine the direc-

tion 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.

Avoid The Loudness

Equal loudness curves, such as the one above,

have been modifi ed over the years.