Unfortunately, a baffle of 3 x 3 meters is sometimes not practical. Other
solutions had to be found.
But this method also has disadvantages:
The closed cabinet also has advantages: an accurate pulse response
and is easy to build
The bass reflex box is constructed and built up like a closed cabinet, however,
an opening (or port) is extended into the interior of the cabinet by means
of a tube. This bass reflex port should ideally be located close to the bass
driver. The air pressure generated within the cabinet doesn't simply exit
into the ambient environment, the air rather starts resonating inside the
tube and acts as an independent sound source radiating with a phase shift.
If drivers, cabinet size and bass reflex tube and their dependencies are
tuned properly, the low frequency sound generated inside the tube is still
radiating while the sound radiating from the front of the driver is already
diminishing. Therefore, the bass driver's excursion doesn't need to be as
long as in closed cabinets.
The long channel acts also as a resonator, closed on the side where the bass
driver is located and open on the opposite side. Therefore, the channel resonates
with 1/4 or 3/4 or 5/4 of the wavelength. Since the higher frequencies ((3/4
and 5/4 wavelength) cause interference, the channel needs to be damped slightly
with light material, somewhat more towards the bass driver, somewhat less
towards the opening.
Transmission line speakers have the disadvantage that they are difficult
to tune - DIY only makes sense having extensive measuring instruments available
and with a lot of know-how.
This is not practical. Partial horns or folded horns are used to get by,
sometimes even the walls of the listening room are used (e.g. Klipsch Horn).
In hi-fi applications horns play a secondary role, although, there will always
be enthusiasts preferring this concept. At rock concerts where extreme volumes
are often required, they are quite common.
This concept is suitable for subwoofers, especially for passive radiators
since no steep filter is necessary. This is a filter prohibiting the subwoofer
of transmitting frequencies around 200 Hz that can be located by the human
If you want to calculate the correct cabinet size yourself, please download
from our server or you may use our online tools (link top right).
In a listening room they collide, a state often called acoustic short circuit.
In places where rear and front sound radiation are equally high, they cancel
out one another. That's the case with the two first dipolar speakers vertical
to the radiation axis; their radiation characteristics resemble a figure of
eight while the others rather resemble a cardioid since the backwards radiating
sound is more restricted.
In the picture to the right, the radiation axis is in light green and the
radiation areas are marked in red. Above they are symmetrical (like a figure
of eight), below - due to smaller opening on the left side of the dipole -
they are generating a smaller area (minor area) and on the right a larger
area (major area), encircling the minor area even further should the radiating
differences become bigger.
More about dipolar speakers you'll find in Hobby HiFi 2/2005, or in our discussions
with Axel Ridtahler, having
patent rights for two types of dipolar speakers that are called RiPoles
by us. Axel Ridtahler also wrote the PDF