A speaker produces two sounds one in phase one at 180 deg to that,
the inphase comes out the front, and the 180 deg out the back.
They also have a resonent frequency, this is the about the
lowest frequency you can get from that speaker. There are two
diferent breeds of speaker one with strong suspension, and one with weak
suspension. The weak suspension speakers are ment to
compensate for the build up of pressure in a sealed enclosure,
unless you have this back pressure, you soon ruin the speaker. The
strong suspension can be used in applications where there is
littel or no back pressure.
Car speakers are usaly strong suspension, so they can be mounted
on a singel baffel.
Base reflex (ported) are inbetween.
Seald normal hifi speakers are weak.
The more excursion your speaker gives the louder it goes, so two
speakers one with twice the area, but half the excursion, will
both produce the same volume.
All speaker enclosures are built to stop the two phases canceling.
The problem with a baffel speaker is when the wavelenght of the
sound is grater than the distance around the speaker, them you get
canceling below that. To produce a good sound down to 20 Hz you
end up with a 32 x 64 x 96 foot area. Of cource you also need a
room 32 foot long.
Seald enclosures are called infinit baffel speakers, because they
are no edges to the baffle. Of cource they have a resonat
frequency, depending on the volume of are inside which fixes the
lowest frequency. They tend to have a gental first order slope of
frequency volume.
Base reflex are the same as infinite baffel, except they have a
port that changes the order of slope. The air in the port acts
like two ping-pong balls hitting each other at that frequency and
changes the phase. Below it allows the wrong phase to cancel out,
so you get a second order curve.
Transmition line speakers (like my home ones), this is a speaker
with the back going
to a long tube. The tube is 1/4 the wave lenght of the resonat
frequency of your speaker (approx), for mine this is 8 foot.
These change the phaze over the length of the tube, and yeald less
lumpy frequency responce, and it comes out the other-side in
phase.
They are also by far the most efficient (loudest) speakers
mentioned. For example mine run from one driver 2"x4" designed for a
car, and produce a responce of 50to14,000 Hz from the one
driver, I also have a pizeo electric tweeter so I have no need of
a crossover. The tube I use is 2"4" and folded in a box. With
these speakers you get an extra octave over most "Bass" speakers.
You can also use flexibul fire flu hoze (steel stuff), and run it
about. My plan is to have the hoze come round the side of the
petrol tank in my spitfire, and as long as both sides are in the
compartment, speaker on one end, and are on the other you should
have a speaker system, that dosent look much, but sounds alot
more.
As you can see I read a good book on the subject.
--
James Carpenter
Yellow '79 spit wired by a trained marmot
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