Even though you could get away with just one, how many
subwoofers do you actually need to achieve natural sound at a reasonable cost?
By: Ringo Bones
Believe it or not, subwoofers were already sold by hi-fi
retail stores around the mid 1970s. Sadly, those early subwoofers were very
notorious for sounding as if an electric bass guitar connected to a loud bass
guitar amp is playing along with the music in your listening room – even when
playing small-scale chamber music. After the early design excesses were weaned
out, subwoofer in the home has undergone a hiatus that made it desirable again
in a few years time.
The subwoofer gained popularity again in the 1980s due to a
great number of affordable small loudspeaker systems designed with very small
woofers / main drivers – i.e. 5 to 7 inches or so and, sadly, most
pain-in-the-ass interior decorators find them attractive. Sadder still, the
growing impracticality for many hi-fi enthusiasts to own loudspeaker systems with
12-inch or 15-inch or larger main drivers for reasons of cost, space or room
aesthetics (thanks to your pain-in-the-ass interior decorator) finally made
subwoofers a viable option when it comes to system building.
During the early 1990s, a partial, but interesting solution
to the problem was the introduction of the three-piece system. This generally
consists of two quite small loudspeakers that cover the frequency range down to
100-Hz or so. Such a loudspeaker system with a 5-inch or 6-inch main driver are
fairly easy an inexpensive to design and manufacture. But a loudspeaker system
with the bottom two octaves missing is not very satisfying to the seasoned
audiophile, so a third component is added – the subwoofer.
The rational of using a subwoofer is that the main front
left and front right loudspeakers can be designed to give appreciable power
output and have optimal performance at very low frequencies because the two
main left and right front loudspeakers no longer has to reproduce the very low
frequencies. And while the subwoofer no longer has to reproduce the higher
frequencies assigned to the front left and right loudspeakers, that one
subwoofer box can now – in theory – be placed in a more or less out-of-the-way
location in the room since the low frequencies – i.e. below 100-Hz – have very
little directionality. It was generally thought – since the mid 1960s – that a
single monaural channel for frequencies below 100-Hz was needed and also
because frequencies below 80-Hz were, allegedly, mixed to monaural anyway. This
latter factor was a result of the limitations – and still is - of the
mechanical reproduction process used in making vinyl records and back then,
open-reel tapes are a way, way more expensive alternative to vinyl playback. But
when properly connected, two subwoofers operating in stereo absolutely sounds
more gorgeous that a system with just one, even in casual listening.
The assumptions that justified using a single subwoofer has
some basis in fact because three piece systems have been, and are still popular
when space and room styling (thanks pain-in-the-ass interior decorator) take
precedence over other acoustic considerations. Audiophiles who preferably
listen to rock music usually place the subwoofer to the left because the bass
guitarist is usually mixed to the left of the soundstage, while those who
listen to large scale Classical music place the subwoofer to the right because
in a majority of orchestral recordings, the low frequency instruments tend to
sound as if they are located on the right of the soundstage. However, over the
years, I have some concerns and reservations about the technical justifications
for the three-piece system.
It can also be quite a shocker to know that not all, or even
most, loudspeaker setup absolutely needs a subwoofer or two to make them
complete in the low bass. To my ears, it might be the wisest decision to get a
truly full-range loudspeaker in the first place, since it will have been
designed as a unit and have a smoothly balanced response over its full range.
However, in the case of smaller bookshelf-type loudspeakers, considerable advantage
can be had by removing the lower two octaves of the sound from the two main
left and right loudspeakers and diverting them into a good subwoofer. The
advantages are that the subwoofer now handles the hard-to reproduce very low
frequencies due to much higher power requirements freeing the smaller
loudspeakers from the greater power and displacement demands of reproducing the
very low frequencies. This can result in lower distortion in the small
loudspeakers and considerably improved sound quality from them. It also allows
planning for system growth – that is if you first invest in a good, small pair
of loudspeakers, you can augment them later by increasing bass capacity with a
subwoofer.
But the question now is whether one, two or perhaps more,
subwoofers should be added. If only one subwoofer is added, the result would be
a three-piece system with monaural bass response below 80 or 100-Hz. If two
subwoofers are added, the result is the two bookshelf-sized loudspeakers
virtually becoming two full range loudspeaker system with a full stereo low
bass capability. But does it matter if we have stereo or monaural low bass? To
my ears and my over 30 years experience in being an audiophile, I believe that
it does for at least two reasons. The first is that we have a much better
program material today from the Redbook Compact Disc, various high resolution
digital audio formats like DSD and 24-bit 192-KHZ sampled PCM audio – and even
low rate data online streaming in MP3 and the like compared with entry level
vinyl in the low bass frequencies.
The second reason is not quite so obvious, but very
important if you want your system to accurately reproduce low bass frequencies
in a timbrally accurate manner, whether it is from a Fender Mustang electric bass
played through an Ampeg GS-12R equipped with 7591A output tubes, an upright
acoustic bass, or even a full-sized church organ with 64-foot pipes. Believe it
or not, there is considerable interaction between the room and the low frequency
loudspeaker units. We all know that placing the bass loudspeakers in near a
wall or in a corner of the room changes the amount and distribution of the bass
frequencies in the room. At low frequencies, strong and widely spaced room
modes are occurring. These modes manifest itself by the fact that as we move
about the listening room, some locations have a lot of bass while other
locations have a lack of bass. Also, the locations where more or less bass
frequencies are heard also moves around the room when you shift the position of
the low frequency loudspeaker.
The problem with a three=piece system is that we only have
one loudspeaker producing the low frequencies, thus the room is only excited
from one point and we have a set of modes – i.e. those parts in your room where
the low frequencies are either too strong or too weak – that are quite audible.
When two bass loudspeakers – or two subwoofers – are placed in a room and are
excited by a full stereo bass signal, each will have its own set of modes. But the
benefit this time is that it is not as likely that the lack or excess of bass
from one loudspeaker will fall exactly the same point as for the other
loudspeaker. This would result in a considerably better uniformity of bass
response is obtained when the room is excited from two low frequency sources –
in comparison when excited from just one low frequency source. But if using two
subwoofers is better than using just one, would using more than two provide even
far better results?
A fellow audiophile of mine has a set-up that uses eight REL
Acoustics Q100E subwoofers in the frontal 60-degree arc of his hi-fi rig. Four were
connected to the left channel and the other four were connected to the right. According
to him, this set-up was a quantum leap in terms of performance from his
previous set-up when he just used two REL Acoustics Stadium II subwoofers –
about four times larger in terms of volume than the REL Q100E. But how does it
sound? To my ears, it is probably the most timbrally accurate set-up when it
comes to producing bass below 100-Hz. Not only that, on some minimalist chamber
music recordings, it transformed his listening room into a medieval era chapel
where the music was recorded!!! Probably due to the room modes are now
vanishing low.
