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2.6
Surround Loudspeakers - Dipoles vs. Direct Radiators
When Dolby Surround and Dolby Pro Logic were the sole surround formats, the idea of using dipole
radiators in the rear was to create a diffuse sound field in the listening environment for the ambient
information. Today, however, there are three good reasons NOT to use this type of loudspeaker design
for the rear channels:
•
Dipole loudspeakers have a limited low frequency bandwidth, which is fine for use in matrix
surround sound formats, such as Dolby Surround and Dolby Pro Logic where the surround
channel bandwidth is also limited (100 Hz to 7 kHz). However, in discrete surround sound
formats, such as Dolby Digital and DTS, the rear channels have an extended bandwidth from
20 Hz to 20 kHz. In addition, in the production stages, direct radiating loudspeakers are
used that typically extend down to 40 or 50 Hz. Most dipoles used for surround applications
do not extend much below 100 Hz so real impact in the bass is absent, however, there is
envelopment in the mid bass and higher frequencies.
•
Often the listening environment is absorbent in the mid range so the effectiveness of dipole
radiation is lost. In fact, the effect of dipole loudspeakers is entirely dependent on the
acoustics of the room that often vary considerably from one room to the next.
•
The advent of multi-channel music formats, such as SACD and DVD-Audio, depends on
greater usage of direct radiators as sides and rears in order to get the intended
performance. These music mixes are monitored in studios with direct radiating
loudspeakers, so playback through them in the home will create the result the engineer,
producer and artists intended.
A Genelec direct radiator, whose off-axis response is smooth and natural sounding, will provide a good
solution for the new extended bandwidth surround sound formats (SACD and DVD-Audio) and still work
in any sonic environment. In addition, 7.1 and other such 'formats' (for example, Logic 7 is actually a post
processing technique added to a 5.1 signal) have changed room design techniques and so dipoles are
no longer as suitable as they once were. The use of direct radiators will yield better front to back effects
panning and more detailed playback of soundtrack events.
See Section 2.10 Selecting the Right Model for a table to select surround models for a given room volume
and listening distance.
2.7
Subwoofers - Loud and Low
The primary function of a subwoofer is to move lots of air. Ideally, it should have a low cut-off frequency
(<20 Hz for movie material, <35 Hz for music material), no distortion and play extremely loud. To play loud
is easy - use big amplifiers. To play low is easy - use big drivers and large enclosure. To play loud AND low
is hard, especially when low distortion is also desired. Do not be fooled by statements such as, "It has a
1200 W amplifier so it must be loud!" as this is only part of the story. Poor efficiency in the acoustics can
make such a system quieter than a 12 watts system.
Number of
Increase in
subwoofers
SPL, dB
1
2
3
4
Table 1. - Adding Subwoofers Increases the Maximum Peak SPL Output
See Section 2.10 Selecting the Right Model for a table to select subwoofer models for a given room
volume and listening distance.
SPL for 5050A
and multiples
0
104
6
110
9.5
113.5
12
116
SPL for HTS3B
SPL for HTS4B
and multiples
and multiples
113
117
119
123
122.5
126.5
125
129
SPL for HTS6
and multiples
129
135
138.5
141
9
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