Sharp PG-C45S Operation Instrukcja Użytkownika Pobierz pdf (Strona 10)

F E A T U R E D P R O D U C T ST E C H N I C A L L Y S P E A K I N G . . .

10 ExtroNews 15.1 Spring 2004

Fast-forward 30 years. Here I am writing

about that pushbutton. That pushbutton

recently created considerable ﬂurry here at

Extron. Since the inclusion of much more au-

dio support in our products, that loudness

pushbutton ﬁnally crept into a prototype

product. Someone in Product Management

pushed it, but it didn’t do what it was sup-

posed to do: make the system “sound

good.” That launched an investigation into

the real intent of the loudness control func-

tion and a bit of re-evaluation by those indi-

viduals designing audio products.

We learned something. Different people

and different companies have different de-

sign philosophies toward functionality of

the loudness control. So, when you push it,

what should the loudness control do? Let’s

begin answering that question with a look

at the historical basis for the loudness con-

trol and why it should make an audio system

“sound good.”

Equal Loudness for All

Research characterizing the human hear-

ing range generated an areal map of auditory

response as shown in Figure 1. The shape of

the minimum audibility curve tells us some-

thing about the frequency response of the ear

at low sound pressure levels. At the bound-

ary of minimum audibility, perception of low

frequencies and high frequencies requires a

signiﬁcant level boost as compared to the mid-

range frequencies. Conversely, at the upper

boundary the threshold of feeling represents a

much ﬂatter response. The incomplete outline

of the graph indicates regions of extreme vari-

ability where performance data collected on

human subjects is not altogether consistent.

In the 1930s, two Bell Labs researchers,

Harvey Fletcher and Wilden

Munson, organized a re-

search project in which they

asked that each participant

match the perceived level of

two pure tones by adjusting

the level of one tone source

against a 1000 Hz tone at

a pre-determined reference

level, until the two tones

were perceived as equal in

loudness. Their test results

embodied data at many fre-

quencies at various sound

pressure levels across the

by Steve Somers, Vice President of Engineering

The Mysterious Loudness Control

What Does It Do?

he ﬁrst really decent stereo audio system I owned had a pushbutton on the front panel

called “loudness.” I pushed it. I decided the audio sounded better so I left it pushed in;

never changed it. OK, occasionally I would push it to the off position; nope, sounded better

on. Thinking back, I really didn’t care what it was for. I left it pushed ON because it made

the system sound better, so I continued to concentrate on my video career. All the while, my

stereo system “sounded good.”

human hearing range. Since this is a highly

subjective way to conduct research, many

subjects performed the experiment. Results

were averaged to obtain loudness contours

intended to represent a “normal” response.

Their results, called the Fletcher-Munson equal

loudness contours, provide signiﬁcant insight

toward our understanding of how humans

perceive relative sound levels. In 1956, D.W.

Robinson and R.S. Dadson reﬁned this work

in a similar study described as having more

reliable measurement results. The Robinson

and Dadson equal loudness contours shown

in Figure 2 are most widely used today, but

are often referred to as the Fletcher-Munson

curves. Eventually, the International Standards

Organization adopted the Robinson-Dadson

curves as a basis for “Normal Equal-Loudness

Level Contours,” ISO 226:1987, which is

now the current standard. Since these curves

describe the perception of only pure tones

in a free ﬁeld, they do not necessarily apply

to noise band analysis or diffused random

noise.

Additional research continues in an

attempt to further characterize our hearing

perceptions in real audio applications.

If we invert any of these contour curves

at a particular intensity level, we now have

the relative frequency response plot of the

human ear for all tones across the frequency

range on that particular contour. Inversion of

the lower curves illustrates human ear fre-

quency response deﬁciencies at low sound

intensities. Conversely, invert any of the up-

per, higher intensity curves to realize a more

Frequency (Hz)

Sound Level (dB)

Minimum

Audibility Curve

Threshold of Feeling

100

120

140

50 100 200 500 1K 2K 3K 10K 20K

Auditory Response AreaAuditory Response Area

Figure 1: Realm of human auditory response

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 19 20

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