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Radio News for November, 1928
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The socket- operated electrodynamic speaker
uses a dry -electrolytic rectifier R to convert
the stepped -dawn A.C. current into 6 -volt direct
current for the field winding LI. This arrangement
may he used by the constructor
with a speaker of the D.C. type, or a trickle
charger may be employed.
Radio Apparatus," which was published on
page 1348 of RADIO NEWS for June, 1928.
However, in a great majority of cases it
will be found that the current passing in
the circuit will satisfy the requirements of
the speaker.
Fig. 2 shows the method of connecting
the speaker to a "B" power unit used in
connection with a large set employing a
push -pull amplifier. In this case the total
current consumed by the set is so heavy that
it would be apt to burn out the winding
of the speaker, and, therefore, the plate
current for power tubes is tapped before
it reaches the speaker's field winding, which
is connected as part of the voltage divider.
In this circuit all the wiring and parts on
the left side of the "13 +JIax." wire is standard,
but on the right side of the line several
changes have been made. Part of the voltage
divider must be removed from the circuit
to compensate for the drop in potential
which takes place across the windings
of the loud speaker, and probably the remainder
of the voltage divider will have to
he redesigned to permit a flow of current
sufficient to operate the field winding of the
speaker. For data on redesigning the voltage
divider the reader is again referred to
the article in the June number of RADIO
NEWS. The circuit also shows the method
of connecting the output circuit of the push-.
pull amplifier with the moving coil of the
speaker. L5 is a standard center -tapped
output impedance unit, and T3 is the output
transformer of the speaker. 'l'he condensers
C9 and C10 are not absolutely
necessary, but, if used, -should have a capacity
of 2 to 4 mf. -
With the low -potential electrodynamic
speakers, the field coil is operated usually
by a storage battery- connected to the winding,
but it is also possible to employ A.C.
operation, as shown in Fig. 3. ln this circuit
T1 and R are a step -down transformer
and a full -wave dry -electrolytic rectifier, respectively,
which deliver direct current at
6 volts and / ampere. A standard trickle
charger may be used to supply this current.
WHAT IS A BAFFLE?
The baffle is the next subject for consideration
in connection with the operation
of electrodynamic speakers. The baffle is
a board on which the speaker is mounted;
itit is absolutely essential in order to obtain
good reproduction from electrodynamic
units of the free -edge cone type. In speakers
of this type two distinct sets of sound
waves are set up, one from the front and
one from the rear of the cone. These sound
waves will alternately neutralize and rein-
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force each other if the proper precautions
are not taken, and it is the baffle which
prevents such interference. Also, the size
of the baffle determines the lowest frequency
which the speaker will reproduce with full
Baffles are of two general types: the flat
baffle and the box baffle. 'l'he two are
equally effective, but the box -shaped baffle
may be much smaller for equivalent results.
The rule which applies in this case is that
the lowest note which the speaker will reproduce
at full volume is - the one whose
quarter wavelength is less than the distance
from the front to the back of the cone
around the edge of the baffle. Therefore,
the baffle should be as large as conveniently
possible, but considerable latitude is permissible.
To determine the wavelength of a note
it is necessary to divide the speed of sound
in air by the note's frequency. To find
the wavelength of a 100 -cycle note, for instance,
it is necessary to divide 1120 feet
(the speed of sound in air) by 100 cycles,
and the result is 11.2 feet. As it is the
quarter wave which is the important figure,
it is next necessary to divide this figure by
4, and this gives us 2.8 feet, or 33.6 inches
for the quarter wavelength of a 100 -cycle
note.
Provided a baffle is flat, there is no limit
to its desirable size. Speakers have been
inserted in the wall of a room, which is
practically the equivalent of a baffle of infinite
extent, with excellent results. However,
under average conditions a 100 -cycle
baffle is entirely satisfactory, although better
results could be obtained if it were still
larger. A baffle of the flat type having a
100 -cycle cut -off frequency will consist of
a board approximately 35 inches square
with a hole cut in the exact center for the
cone.
BUILDING INTO A CABINET
In general, the box -type baffle is used
more frequently than the flat type because
it is more conservative in space require-
ments. As the effectiveness of a baffle is
determined by the shortest air -wave distance
between the front and back of the
cone, it may be seen that a box baffle is
the approximate equivalent of a flat baffle
of much larger size. Therefore, the lower
compartments of radio console cabinets
often serve as very efficient baffles for electrodynamic
speakers, as indicated in Fig. i.
One objection to the box baffle, however, is
its tendency to resonate or "boom."
the box is shallow from front to back,
though high and wide, this effect is not
noticeable. If the box is deep from front
to back, while small in its other dimensions,
the effect will probably be objectionable.
To overcome this trouble, holes should be
bored in the sides, top or bottom; or the
sides should be lined with felt, or other
sound- absorbing material. In general, with
a box less than 18 inches square and deeper
than one foot from front to back, some
precautions should be taken to prevent
resonance. Any box, the back of which
is not almost entirely open or which lacks
some other outlet of equivalent size for
the sound from the back surface of the cone,
will resonate badly unless a great deal of
felt is used. A grille, covered with a light
cloth, is usually the equivalent of an opening
for the purpose.
The remarks made above regarding the
tendency of box baffles to resonate should
not discourage the set builder from employing
this type of construction. From a
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practical viewpoint, the box baffle is much
more satisfactory and, if it is properly designed,
no trouble should be experienced.
The size of the battery compartment of
most radio cabinets is such that satisfactory
results are assured. However, if it is desired
to construct a 100 -cycle box -type
baffle, this should be approximately 16
inches square and 10 inches deep.
In constructing a baffle there are several
things which should always be remembered.
First, it should be made of heavy wood, so
that it will not vibrate excessively on its
own frequency. Secondly, the frame of the
speaker should make an air -tight joint with
the baffle at the opening which is cut for
the cone. Thirdly, no holes should be cut
in the front of the baffle other than the
opening made for the cone. Lastly, the entire
construction should be as rigid as possible.
When the speaker is located in the sanie
cabinet as the receiving tubes, great care
must be taken to prevent its vibrations
from causing the microphonic sounds which
so often ruin reproduction. For this reason,
both the speaker and the receiver should
be mounted on a sound -insulating material,
such as soft rubber. Special brackets are
sold for mounting speakers on a soft rubber
cushion, and rubber pads may be placed
under each corner of the sub -panel of the
set to insulate it mechanically from the
speaker. As an additional precaution, the
sockets used in the set should be of the
cushion type, and the tubes should be fitted
with heavy ballast caps.
Very often, after unsuccessful attempts
to eliminate microphonic noises, it is found
that poorly- constructed apparatus used in
the receiver is responsible. If it is suspected
that the tubes are not the cause
of the trouble the next parts to examine are
the variable condensers. If condensers with
unduly thin plates are used the vibrations
set up in them may sound like tube noises
in the speaker.
Fig. 7
.4 convenient method of mounting an electrodynamic
speaker in a console cabinet is shown
here. The lower part of the cabinet is thus
converted into a baffle box, and the speaker
should be so mounted that it comes in the
exact center of its compartment. It should be
provided with rubber feet to avoid vibration of
the shelf on tchich the set rests.