Yamaha P2075 Operating Manual page 13

UNDERSTANDING DYNAMIC RANGE AND
HEADROOM
Every sound system has an inherent noise floor, which is the
residual electronic noise in the system equipment (and/or the
acoustic noise in the local environment), The DYNAMIC
RANGE of a system is equal to the difference between the
peak output level of the system'and the noise floor.
A concert with sound levels ranging from 30 dB SPL (near
silence) to 120 dB SPL (threshold of pain) has a 90 dB
dynamic range. The electrical signa! level in the sound system
(given in dBu) is proportional to the original sound pressure
level (in dB SPL) at the microphone. Thus, when the
program sound levels reach 120 dB SPL, the maximum line
levels (at the console's output) may reach *24 dBu (12.3
volts), and maximum power output levels from the amplifier
may peak at 250 watts. Similarly, when the sound level falis
to 30 dB SPL, the minimum line level falls to —66 dBu
(0.388 millivolts) and power amplifier output level falls to
250 nanowatts (250 billionths of a watt).
The program, now converted to electrical rather than
acoustic signals, still has a dynamic range of 90 dB: +24 dBu
— (—66 dBu) = 90 dB. This dB SPL to dBu or dBm corres-
pondence is maintained throughout the sound system, from
the original source at the microphone, through the electrical
portion of the sound system, to the speaker system output.
A similar relationship exists for any type of sound reinforce-
ment, recording studio, disco or broadcast system.
Тһе average line level in the typical commercial sound system
just described is +4 dBu (1.23 volts), corresponding to an
average sound level of 100 dB SPL. This average level is
usually called the "nominal" program level. The difference
between the nominal and the highest (peak) levels in a pro-
gram is the HEADROOM. In the above example, the head-
room is 20 dB. Why is this so? Subtract the nominal from the
maximum and see: 120 dB SPL — 100 dB SPL = 20 dB. The
headroom is always expressed in just plain "dB" since it
merely describes a ratio, not an absolute level; "20 dB" is the
headroom, not "20 dB SPL". Similarly, the electrica! head-
room is 20 dB, as calculated here: +24 dBu — (+4 dBu) = 20
dB. Again, "20 dB" is the headroom, not "20 dBu".
Provided the amplifier is operated just below its clipping level
at maximum peaks of 250 watts, and at nominal levels of 2.5
watts, then it also operates with 20 dB of headroom (20 dB
above nominal = 100 times the power).
If another sound system were equipped with a noisier circuit
somewhere along the line, and a less capable line amplifier
than the previous example, it might have an electronic noise
floor of —56 dBu (1.23 millivolts), and a peak output level
of +18 dBu (6.16 volts). The dynamic range of this system
would only be 74 dB. Assuming the original program still has
an acoustic dynamic range of 90 dB, it is apparent that 16 dB
of the program will be "lost" in the sound system. How is it
lost? There may be extreme clipping of program peaks,
where the output does not rise higher in response to higher
input levels. Quiet passages, corresponding to the lowest
signal levels, may be buried in the noise. Typically, portions
of that 16 dB difference in dynamic range between the sound
system capability and the sound field at the microphone will
be lost in both ways. A system with +24 dBu output
capability and a —66 dBu or better noise floor, or +18 dBu
output capability and —82 dBu noise floor, would be able to
handle the full 90 dB dynamic range. Thus, for high quality:
sound reinforcement or music reproduction, it is necessary
that the sound system be capable of low noise levels and high
output capability.
SOUNO LEVEL TYPICAL GAIN STRUCTURE IN ELECTRONICS
AT MICROPHONE MAXIMUM SOUND LEVEL
{SOUND {THRESHOLD OF PAIN! AUDIO CLIPPING
PRESSURE) 120 dB SPL SIGNAL POINT +24 dBu балада
12048 SPL — — +25 dBu POINT
115 dB SPL — — +20dBu +18 dBu
11048 SPL —
105 dB SPL —
10048 SPL —
95 dB SPL —
— ^15dBu 44dBu 2048
— новь NOMINAL HEADROOM | 1448
LEVEL HEADROOM
2038
HEADROOM
— +59Ви
100 dB SPL —. OdBu
AVERAGE
90dBSPL — SOUND — -SaBu
85 dB SPL ~ LEVEL — -10dBu
8008 SPL — — -16 ави
75 dB SPL — — -20d8u
7038 РЕ — 7048 30d8sPL — -25dBu
esdBspL. . S/N AMBIENT -80 dBu
RATIO Nols: =
60 98 SPL — chal
55 48 SPL — — -40dBu
50dBSPL . — -45dBu
45 dB SPL — -- —50dBu
40 dB SPL —— — -55 dBu
35 dB SPL — —— -60dBu
30dBSPL —
25 48 SPL — AMBIENT
20 dB SPL — {№013
15dBSPL — ЕЕ
10dBSPL —
5 dB SPL
048 SPL
Fig. 9 DYNAMIC RANGE, HEADROOM & S/N RATIO.
as the program is retrieved from the tape. Compact (laser)
discs, and digital audio tape recording, and the FM recording
used in modern
stereo VCR
soundtracks are
additional
methods of recording wide dynamic range programs which,
in turn, demand playback systems with wide dynamic range.
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