Chapter 4 Main Points
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In audio production, no one item in the sound chain is more important than the
reference loudspeakers and no one process in evaluating the product is more
important than monitoring.
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The sound chain begins with the microphone, whose signal is sent to a console or
mixer for routing and then processed, recorded, and heard through a loudspeaker.
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Loudspeakers are transducers that convert electric energy into sound energy.
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Loudspeakers that are powered externally are called passive loudspeakers. Those
that are powered internally are called active loudspeakers.
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A single, midsized speaker cannot reproduce high and low frequencies very well; it
is essentially a midrange instrument.
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For improved response, loudspeakers have drivers large enough to handle the bass
frequencies and drivers small enough to handle the treble frequencies. These
drivers are called, informally, woofers and tweeters, respectively.
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A crossover network separates the bass and the treble frequencies at the crossover
point, or crossover frequency, and directs them to their particular drivers.
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Two-way system loudspeakers have one crossover network, three-way system
loudspeakers have two crossovers, and four-way system loudspeakers have three
crossovers.
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In a passive crossover network, the power amplifier is external to the speakers and
precedes the crossover. In an active crossover network, the crossover precedes the
power amps.
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In evaluating a monitor loudspeaker, frequency response, linearity, amplifier power,
distortion, dynamic range, sensitivity, polar response, arrival time, and polarity
should also be considered.
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Frequency response ideally should be as wide as possible, from at least 40 to
20,000 Hz.
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Studios use two or three sets of different loudspeakers to try to get a handle on what
produced audio will sound like on the variety of receivers out there. Although too
many of those receivers produce poor sound quality, it is inadvisable to use poorquality studio loudspeakers for referencing.
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Linearity means that frequencies being fed to a loudspeaker at a particular loudness
are reproduced at the same loudness.
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Amplifier power must be sufficient to drive the loudspeaker system, or distortion
(among other things) will result.
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Distortion is the appearance of a signal in the reproducedsound that was not in the
original sound.
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Various forms of distortion include intermodulation, harmonic, transient, and
loudness.
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Intermodulation (IM) distortion results when two or more frequencies occur at the
same time and interact to create combination tones and dissonances that are
unrelated to the original sounds.
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Harmonic distortion occurs when the audio system introduces harmonics into a
recording that were not present originally.
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Transient distortion relates to the inability of an audio component to respond quickly
to a rapidly changing signal, such as that produced by percussive sounds.
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Loudness distortion, or overload distortion, results when a signal is recorded or
played back at an amplitude greater than the sound system can handle.
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To meet most any sonic demand, the main studio monitors should have an outputlevel capability of 120 dB-SPL and a dynamic range of up to 80 dB.
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Loudspeakers in professional studios have a dynamic range of up to 80 dB (40 to
120 dB-SPL). The dynamic range in consumer loudspeakers is considerably less.
Even the better ones may be only 55 dB (50 to 105 dB-SPL). The limited dynamic
range of mobile devices, such as smartphones, in which dynamic range is usually
compressed, and of computers (laptops in particular) considerably reduces the
quality of audio reproduction.
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Sensitivity is the on-axis sound-pressure level (SPL) a loudspeaker produces at a
given distance when driven at a certain power. A monitor’s sensitivity rating provides
a good overall indication of its efficiency.
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Polar response indicates how a loudspeaker focuses sound at the monitoring
position(s).
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The coverage angle is the off-axis angle or point at which loudspeaker level is down
6 dB compared with the on-axis output level.
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A sound’s arrival time at the monitoring positions should be no more than 1 ms;
otherwise, aural perception is impaired.
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Polarity problems can occur between woofer and tweeter in the same loudspeaker
enclosure or between two separate loudspeakers.
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Where a loudspeaker is positioned affects sound dispersion and loudness. A
loudspeaker in the middle of a room generates the least-concentrated sound; a
loudspeaker at the intersection of a ceiling or floor generates the most.
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For professional purposes it is preferable to flush-mount loudspeakers in a wall or
soffit to make low-frequency response more efficient and to reduce or eliminate
back-wall reflections, cancellation, and cabinet-edge diffraction.
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Stereo sound is two-dimensional; it has depth and breadth. In placing loudspeakers
for monitoring stereo, it is critical that they be positioned symmetrically within a room
to reproduce an accurate and balanced front-to-back and side-to-side sonic image.
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Loudspeakers used for far-field monitoring are usually large and can deliver very
wide frequency response at moderate to quite loud levels with relative accuracy.
They are built into the mixing-room wall above, and at a distance of several feet
from, the listening position.
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Near-field monitoring enables the sound engineer to reduce the audibility of control
room acoustics, particularly the early reflections, by placing loudspeakers close to
the monitoring position.
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Surround sound differs from stereo by expanding the depth dimension, thereby
placing the listener more in the center of the aural image than in front of it.
Therefore, using the 5.1 surround-sound format, monitors are positioned front-left,
center, and front-right, and the surround loudspeakers are placed left and right
behind, or to the rear sides of, the console operator. A subwoofer can be positioned
in front of the listening position, between the center and the left or right speaker; in a
front corner; or to the side of the listening position. Sometimes in the 5.1 surround
setup, two subwoofers may be positioned to either side of the listening position.
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7.1 surround sound, which is growing in popularity, feeds the two added channels to
left- and right-side speakers. The additional speakers improve sound localization.
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A monitor controller is, in essence, a control room monitor source selector. It
facilitates switching from one loudspeaker to another, between stereo and a given
surround-sound format, and between surround-sound formats; it enables selective
monitoring of any channel or group of channels and can dedicate the headphone
output for monitoring.
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As important as personal taste is in monitor selection and evaluation, calibration of a
monitor system is critical to the objective measurement of the correlation between
monitor sound and room sound.
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In adjusting and evaluating monitor sound, objective and subjective measures are
called for.
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Devices such as a spectrum analyzer measure the relationship of monitor sound to
room sound. Although part of testing a monitor loudspeaker involves subjectivity,
there are guidelines for determining performance.
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When evaluating the sound of a monitor loudspeaker, it is helpful to, among other
things, use material with which you are intimately familiar and to test various
loudspeaker responses with different types of speech and music.
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Headphones are an important part of monitoring, particularly on-location. The
following considerations are basic when using headphones for professional
purposes: the frequency response should be wide, flat, and uncolored; you must be
thoroughly familiar with their sonic characteristics; they should be circumaural
(around-the ear), as airtight as possible against the head for acoustical isolation, and
comfortable; the fit should stay snug even when you are moving; and, although it
may seem obvious, stereo headphones should be used for stereo monitoring, and
headphones capable of multichannel reproduction should be used for monitoring
surround sound.
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Headphones can be detrimental to hearing because they are mounted directly over
the ears or, with earbuds, placed inside the ears. Outdoors the problem is
exacerbated by the tendency to boost the level so that the program material can be
heard above the noise floor.
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To help reduce the potential of hearing loss, the active noise-canceling headphone
detects ambient noise before it reaches the ears and nullifies it by synthesizing the
sound waves.
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Passive noise-canceling can be employed using high quality conventional
circumaural headphones that fit snugly over the ears.
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The in-ear monitor (IEM) allows a performer to hear a mix of on-stage microphones
or instruments, or both, and provides a high level of noise reduction for ear
protection. Since their development IEMs are used for other monitoring purposes. A
sound-level analyzer for the specific purpose of measuring in-the-ear monitoring
levels to guard against excessive loudness is available.