I changed the subject on this because my first one was
rejected by Spamcannible.
Just to clarify, the coating does NOT lower the
resonance of the speaker. What it does is to damp out any
vibration of the panels of the enclosure. If undamped thin
metal panels can resonate quite audibly. Even wood can be
resonant but is much less likely to be. While no one
expects hi-fi performance from these speakers miscellaneous
resonances can mask both voice and CW especially if there is
noise present.
An open box enclosure acts like a tube with the speaker
in one end. There will be a strong resonance at the
frequency where the length of the tube is 1/4 wavelength and
at other harmonically related frequencies. The only way to
eliminate this is to close the box but that will raise the
resonant frequency of the speaker because the air trapped in
the box acts like a spring. Adding an absorbent, like
fiberglas, to the box, will reduce the effect of this
"stiffness" and so reduce the amount of rise in resonance,
but not a lot.
Many years ago Edgar Vilchur began to use this idea to
make loudspeaker systems capable of radiating low
frequencies despite being small. The principle was to use a
speaker which relied on the internal stiffness of a small
enclosure to act as part of the restoring force on the cone.
As a result he was able to make speakers with low
fundamental resonance in a small box. The compromise for
this was a loss of efficiency. The efficiency of a speaker,
like that of an antenna, depends on matching the radiation
impedance of the air. Air has an inherent acoustical
impedance. This varies with air density but is fairly
constant. Where the cone or radiating area of the speaker is
too small it behaves in the same way as a short antenna,
that is the radiation resistance falls and it becomes
reactive. For a direct radiator (as opposed to a horn,
etc.) the minimum size for decent match is where the
circumference of the speaker is about one wavelength. Below
this the radiation falls off rather rapidly. Add to this
that there is interference from the radiation from the back
of the speaker and you get very little low frequency output
from a speaker just in air. The purpose of the enclosure is
mostly to eliminate the back wave. The response is then a
function of the size of the cone and its fundamental
resonance. The resonance of the cone causes a boost in the
low frequency response over a narrow band and is used to
compensate for the fall off due to the cone being small. The
relative size of this resonant peak in relation to the
radiation level of the cone at higher frequencies, where it
becomes more efficient, is related to the speaker
efficiency. The more efficient it is the less boost can be
gotten from the cone resonance at a given frequency. As a
result very efficient speakers tend to be deficient in bass
when in plain baffles or completely enclosed boxes.
Speakers like the AR of Vilchur were made deliberately
inefficient at mid-range frequencies in order to obtain a
flatter response down to lower frequencies. Where high
efficiency is desired at low frequencies one must multiply
the cone area in some way. One way is to use a multiplicity
of small speakers mounted closely enough to be coupled
acoustically and act as a single radiator. Another way is
to use a horn. The horn is the exact acoustical analogue of
an electrical transformer. Horns have inherently wide
bandwidth and can extend efficient operation down to fairly
low frequencies. But, here again, size matters and the mouth
of the horn must approximate the size of an efficient direct
radiator to work well. Where the acceptable size of a horn
is limited various schemes have been devised to use parts of
a room, such as the corners, as an extension. The famous
Klipsch Horn is an example. Since amplifier power is now
much more economical than it was some sixty or seventy years
ago the use of horn speakers has become much less popular
since good low frequency response can be gotten from arrays
of smaller cones. The direct radiator has some advantages
since horns often tend to have spurious resonances of their
own.
Another method of extending the low frequency response
of a small speaker is to use an acoustical impedance
matching network on one side, usually the back. This is most
often in the form of a "bass reflex" enclosure. The
bass-reflex is the acoustical equivalent of a L or Pi
network impedance matching network. One can also use a 1/4
wave transmission line in back of the speaker to obtain an
impedance match over a narrow range. Enclosures of this type
are called "acoustical labyrinth" or sometimes just
transmission line boxes. They are the exact equivalent of
the bass-reflex but using distributed constants instead of
lumped constants. Both act in exactly the same way as an
electrical transmission line or impedance matching network
does.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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