"-rick-" wrote in message
...
Ken Heaton wrote:
Basic physics: take note of the distance sound leaving the rear surface
of
the speaker has to travel through the air to reach the front of the
speaker.
Any sound with a wavelength longer that this distance will cancel itself
out
as the sound from the front is perfectly out of phase with the sound from
the rear. No bass in other words. An enclosure effectively creates an
infinite distance from front to rear as the two out of phase sound waves
can't get to each other. they are blocked form meeting by the walls of
the
enclosure. If the enclosure is too small the speaker ends up using up
most
of its power trying to compress the air inside the enclosure resulting in
little low bass as well. So the enclosure has to be both well sealed and
big enough to be effective. It is possible to vent an enclosure with a
tuned port (often called bass reflex) allowing a smaller enclosure but
that
isn't simple physics any more...

Good explanation. It also helps to know the wavelengths. Sound travels
about 1127 ft/second in air and wavelength = velocity/frequency so...

freq. wavelength
(Hz) (ft)

20 56
40 28
80 14


All good points. If he can't build a perfect enclosure, a good ballpark
figure (at least with my experience) is that most car doors are a "pretty
good size" for decent 5" to 6" speakers, assuming they've got at least 10-15
watts of CLEAN power driving them.

"-rick-" wrote in message
...
Ken Heaton wrote:
Basic physics: take note of the distance sound leaving the rear surface
of
the speaker has to travel through the air to reach the front of the
speaker.
Any sound with a wavelength longer that this distance will cancel itself
out
as the sound from the front is perfectly out of phase with the sound from
the rear. No bass in other words. An enclosure effectively creates an
infinite distance from front to rear as the two out of phase sound waves
can't get to each other. they are blocked form meeting by the walls of
the
enclosure. If the enclosure is too small the speaker ends up using up
most
of its power trying to compress the air inside the enclosure resulting in
little low bass as well. So the enclosure has to be both well sealed and
big enough to be effective. It is possible to vent an enclosure with a
tuned port (often called bass reflex) allowing a smaller enclosure but
that
isn't simple physics any more...

Good explanation. It also helps to know the wavelengths. Sound travels
about 1127 ft/second in air and wavelength = velocity/frequency so...

freq. wavelength
(Hz) (ft)

20 56
40 28
80 14


Holy cow. This ain't rocket science folks.

"JimH" wrote in message
...

"-rick-" wrote in message
...
Ken Heaton wrote:
Basic physics: take note of the distance sound leaving the rear surface
of
the speaker has to travel through the air to reach the front of the
speaker.
Any sound with a wavelength longer that this distance will cancel itself
out
as the sound from the front is perfectly out of phase with the sound
from
the rear. No bass in other words. An enclosure effectively creates an
infinite distance from front to rear as the two out of phase sound waves
can't get to each other. they are blocked form meeting by the walls of
the
enclosure. If the enclosure is too small the speaker ends up using up
most
of its power trying to compress the air inside the enclosure resulting
in
little low bass as well. So the enclosure has to be both well sealed
and
big enough to be effective. It is possible to vent an enclosure with a
tuned port (often called bass reflex) allowing a smaller enclosure but
that
isn't simple physics any more...

Good explanation. It also helps to know the wavelengths. Sound travels
about 1127 ft/second in air and wavelength = velocity/frequency so...

freq. wavelength
(Hz) (ft)

20 56
40 28
80 14


Holy cow. This ain't rocket science folks.


Actually, it's *is* science, although it's not always possible to apply it
accurately in cars & boats. You just do the best you can, using the rocket
science as a guideline. And even if you're building freestanding speaker
cabinets for home or stage use, the science assumes a perfect speaker cone
which doesn't add color of its own, which is pretty much fiction. Even so,
the science works.