Sharp PG-C45S Operation Manual do Utilizador descarregar pdf (Página 11)

T E C H N I C A L L Y S P E A K I N G . . .

ﬂattened frequency response. Fletcher and

Munson found human hearing response con-

sistently deﬁcient at low sound intensities, for

both low frequencies and higher frequencies

compared to the 1000 Hz reference point

used to establish these curves. But, the ear is

particularly sensitive in the range of about 300

to 6000 Hz. This happens to be the frequency

range that includes the sound of most human

speech patterns and, curiously, the pitch of a

crying baby. For average background sound

pressure levels, the rolled-off response at low-

er sound pressures allows us to more easily

listen and understand human speech in the

presence of low or high frequency noise.

Meet the Phons

Each contour is identiﬁed as a level in

“phons.” A phon is a subjective unit for loud-

ness equal to the sound pressure level in

decibels when compared to an equally loud

standard note. The standard note is a 1000

Hz pure tone or narrowband noise centered

at 1000 Hz.

Note that the level in phons

matches the sound pressure level in decibels

only at the 1000 Hz standard reference point

on the graph. Therefore, the 40 phon con-

tour represents a 40dB SPL at 1000 Hz, but

a different SPL at most other frequencies.

Essentially, each phon contour represents a

10dB step that we perceive as about twice

as loud as the previous level. This may be a

bit confusing, since we know that a measured

3dB increase represents a doubling of sound

power, but only a perceptible

increase in volume to the ear.

The red dashed line at the bot-

tom of Figure 2 describes the

minimum audible level for hear-

ing sensitivity in a free ﬁeld.

The effect of applying these

curves suggests that some form

of ﬁltering is required within

measurement equipment if we

wish to synthesize the normal

hearing performance of the

ear when calibrating systems

or making value judgments as to sound qual-

ity. Most often, the sound pressure level (SPL)

meter is used to setup listening levels for an

audio system. The SPL meter includes select-

able ﬁlters that modify its calibration so it

approximates the ear’s response at a given

range of sound pressure levels. The most of-

ten-used ﬁlter settings are the A-weighted

and C-weighted. What are these and how do

they relate to our hearing response?

The concept of weighting refers to the rel-

ative shaping of the ﬁlter’s response so as to

mimic the ear at a given loudness level. Four

weighted ﬁlter functions, A, B, C, and D, are

used to simplify and apply regions of the loud-

ness contours that are most meaningful for

describing the frequency response of the hu-

man ear toward real world applications. Refer

to Figure 3 for the following discussion. A-

weighting deﬁnes the shape

of the ﬁlter (and the human

ear response) at low sound

pressure levels, namely the

40 phon loudness contour

curve. Sound level measure-

ments in decibels relating

to A-weighting are denoted

with the units – dB(A).

Shaping for this curve means

that low frequencies are at-

tenuated and the speech

frequencies are ampliﬁed

within the measuring equip-

ment. B-weighting describes

an intermediate level approx-

imating the 70 phon curve. Notice how the

ear’s response begins to ﬂatten. C-weighting

utilizes the 100 phon curve, which describes

the nearly ﬂat response of the ear for high

levels. The C-weighting response is most use-

ful for typical home theater listening levels

and for evaluating system performance for

ﬂat response characteristics. The D-weighting

Curve is a special case developed for aircraft

ﬂy-over noise testing, which penalizes high

frequencies.

Likewise, sound level measure-

ments in decibels relative to these weighting

curves are recorded as dB(B), dB(C), and

dB(D), respectively. The A and C weightings

are most often used since the former relates

to normal everyday sound pressure levels and

the latter relates to higher listening levels

where the ear’s response is nearly ﬂat.

Sounding Good

We’ve covered some signiﬁcant back-

ground, but how does all of that relate to the

loudness control feature on an audio system?

Understanding how the ear perceives sound

intensity versus frequency leads us directly to

that loudness feature. The loudness control

is simply intended to signiﬁcantly boost low

and high frequencies when listening at low

levels so that the ear perceives an overall ﬂat-

ter sound pressure level. In other words, if the

loudness contouring control is not enabled at

low volume levels, bass and treble appear to

be lacking. This effect corresponds to the re-

cently described A-weighted condition where

Relative

Response, dB

B & C

Frequency, Hz

-50

20 50 100 200 500 1000 2000 5000 10,000 20,000

-45

-40

-35

-30

-25

-20

-15

-10

-5

+10

+15

Frequency

Sound-pressure

Level, dB Re 2 x 10

-5

20 Hz 30 40 60 80 100 200 300 400 600 8001000 2 kHz 3 4 6 8 10 15

100

110

120

130

140

Normal Binaural

Minimum Audible Field (MAF)

Phon

100

110

120

130

Figure 2: Equal Loudness Contours

Figure 3: Weighting ﬁlter response curves used in sound level meters

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