| Electronics |
| Rear Speakers
With Home Cinema
|
In the
home cinema, theatre or complex, rear speakers are what make up the
films surround track. A supplement found on most VHS and all DVD soundtracks,
this surround sound can be a number of systems. You have the most
found system called Dolby Pro Logic, encoded into a stereo channel,
this gives a stereo front channel, a centre channel, and a mono rear
channel. Found in a lot of small cinemas Dolby Digital makes up for
the most widely available surround encoding, Dolby Digital is encoded
digitally and compressed as a 5.1 audio source. DTS while not as extensively
used as Dolby Digital is still a used system, this time being encoded
into the soundtrack and requiring its own decoder. If you are going
to build or already own a modern home cinema, theatre, or complex,
then you will by now have a system compatible with all of these new
technologies. This guide aims to guide those who are unsure about
their rear speakers and the impact they make to the experience. Positioning
them is quite easy, it is knowing where that counts.
Common Rear Speakers
Just like sound systems, speakers fall into numbers of categories,
that describe the way the sound is produced. To date there are about
four rear speaker types. These are;
- Standard
- Di-Pole
- Bi-Pole
- Tri-Pole
Standard rear speakers work just like normal transducer type
speakers, they project sound in a 180 degree range forwards towards
the direction they are pointed. These types of rear speakers are known
for their benefits in Surround Sound Music found on SACD and DVD-Audio.
Di-Pole rear speakers work in a new way to normal speakers.
The idea is to generate the same sound but project it in as multiple
directions as possible. In many cases they are projected of a face
plate which is at a 45 degree angle to each side, the effect is said
to sound similar to what cinemas sound like – as they have multiple
rear speakers – aimed to decrease the accuracy of the source. These
speakers are much smaller than regular standard speakers because of
their unique design. Because the two speakers are pushing out, they
must have a vent to allow air to depress.
Bi-Pole rear speakers may look similar to Di-Pole speakers
except they have a unique design element. Their speaker drivers work
in a push-pull way, in this one speaker would produce sound as it
would normally, the other however would do it with an opposite current.
This works by simply wiring the other speaker so it has a positive
on the negative and a negative on the positive plates. This creates
two sounds that are not in sync, they appear less focussed than Di-Pole
speakers.
Tri-Pole rear speakers are less common than the others, these
are slightly different. They are based around a standard speaker but
use the elements of a Di-Pole too. As an example, an M&K speaker had
a regular speaker line up of a tweeter and woofer on the front, but
on the two sides had two large sized mid-tweeters. These are said
to produce much more 3 dimensional sound to the other types of rear
speaker.
New Sound Systems
Recently their have been extensive new models of receivers which are
compatible with Dolby Digital Surround EX, DTS ES Matrix 6.1, and
DTS ES Discrete 6.1. These two audio miracles have been implemented
only in a few DVD movies. The principle is to implement a 7th channel
of audio, to be used as a centre rear. Receivers have now adapted
to it and as a bonus offer the owner to add two centre rear speakers
to the rear surround channel, this being a mono set. The target for
these two new audio channels is to develop a better sense of depth
to the rear sound track.
Another sound system often found on cinema material, if you use it
is a Sony Dynamic Digital Sound, this gives up to eight channels,
which is useful if you use that many speakers. Unfortunately we have
yet to see it emerge onto DVD as it takes up a lot of data space.
SDDS, you’ll see this at the end of most films.
Where To Put Your Rear Speakers
In this part of the guide we’ll look at where you should place your
rear speakers, depending on what type they are, and how large or sensitive
they are.
Standard Rear Speakers
Standard rear speakers, usually bookshelf speakers, should be placed
to the sides of the room, facing the seating position or in line with
it. You should have the rear speakers slightly back of the main seating
position, perhaps further to achieve a deeper sound. If space is lacking,
you can position them so that they are on the floor but facing the
seating position, in this case you should increase their input on
the receiver, this will give you a discrete rear channel which is
still audible, but less direct.
Positioning the speakers to the rear wall in general is not a good
idea. In the case where the seating position is forward and you can’t
put them in line with the seat, then this case must be adjusted properly.
You will lose the sense of surround sound, to cure this lose of depth
or a “hollow” rear section, you can point them or “toe” them into
the listening spot. However this may make them a little accurate,
this is the best position for surround sound music.
Di/Bi-Pole Rear Speakers
Di/Bi-Pole are less likely to give you accurate sounds, they are designed
to smoother the sound so that it appears to come from multiple directions.
The best position for a Di/Bi-Pole is on the back wall, hung mid-way
down from the ceiling, you should have them close to a corner. The
best advice to use is to estimate a line coming from the speaker side
(one nearest a wall) which reflects to a 45 degree line off the wall,
align this line to the seating area and you have a perfect area for
the rear speaker. Just move it along the wall until you can see a
line that goes from the speakers side to the wall then to the seating
area. The other side of the speaker will give you a direct sound,
the second side will give you the same sound in the same time span
but from a second direction.
You don’t need to have them at ear level, you may find that a higher
position gives deeper atmosphere, and lower ones a less accurate source.
If you must place it on the side walls, you can use a similar method
to the last one, you may find some Di/Bi-Pole speakers are more flat
than others, some quite bulky and some using a 45 degree shape. The
bulkier shaped ones work better on side walls. You should line these
up with the seating position, or better, slightly behind the seating
area. The plan is to get a sound come to the sides of the seating
area, and another from behind, reflected off a back wall.
In rare cases, some find rotating the Di/Bi-Pole 90 degrees, so it
is facing the floor and ceiling, works better in areas where rear
walls and side walls are too far apart. In this case the sound is
sent down to the seating area, and at the same time in front of them
from a reflection in the ceiling area. This works better when at the
rear wall, with the seating position right back against the wall.
The speakers need to be far wide of the seat.
Tri-Pole Rear Speakers
Tri-Pole speakers are much more difficult to adjust and position easily.
You are not only finding that you have three sounds are coming from
it at any one time, but those sounds are going to go somewhere, and
reflect. An option is to simply put them at the back wall, and have
them produce sounds going straight ahead - either side of the seating
area – to the inside – between the seating area, but then what about
the other one? The other sound will be going straight into a side
wall, and reflecting back to the speaker, a waster of sound and will
interfere with the other side firing sound.
The answer is simple. All you need to do is toe them in, so the front
is facing the seating area. What this does is create a sound travelling
straight to the listener. A sound travelling to the rear wall. A sound
travelling to the side wall. Now what you need to do is adjust it
so that the sound travelling to the rear wall, is going to reflect
to the dead centre of the seating area. The sound going to the side
wall should be also adjusted to reflect to the area surrounding the
front sides of the listening area. Adjust the distance from the centre,
until both reflects are at their best place.
Some, but not all, Tri-Pole speakers allow you to switch off the two
side speakers, this enables you to listen to music with the perfect
setup in place already. This process for adjusting the tri-pole speaker
makes a lot of difference to the sound of the film.
Dealing with 6.1 audio?
In the case of 6.1 audio, you have many options available, which are
complex to explain thoroughly. For example, you could mix standard
speakers for the rear main speakers, and adding Di/Bi-Pole speakers
for the seventh channel of audio. Think about the options, how another
channel would effect your current system, or what the drawbacks could
be of adding another channel. For instance a tri-pole setup would
no longer be as impressive by removing a rear wall.
You will find that the effect of reflecting sounds makes a large difference
to the way movies are perceived. Remember that movies are made for
cinemas, not homes, and most cinemas have around a dozen rear speakers.
|
|
