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Description  |
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FIELD OF THE INVENTION
This invention relates to eleotrodynamic loudspeakers. In particular, it is
an improved phasing plug for those types of loudspeakers known as
compression drivers.
BACKGROUND OF THE INVENTION
A compression driver comprises a pole piece made of ferromagnetic material
which has a bore therein, the front end or opening of which is adaptable
for coupling to the throat of a horn. A diaphragm, usually circular with a
central dome-shaped portion, is mounted adjacent the rear opening of the
bore so as to be freely vibratable. Attached to the edge of the
diaphragm's dome is a cylindrical coil of wire, the voice coil, oriented
so that the cylindrical axis of the coil is perpendicular to the diaphragm
and coincident with the axis of the pole piece bore. A static magnetic
field, usually produced by a permanent magnet, is applied so that an
alternating signal current flowing through the voice coil causes it to
vibrate along its cylindrical axis. This in turn causes the diaphragm to
vibrate along the axis of the bore and generate sound waves corresponding
to the signal current. The sound waves are directed through the bore
toward its front opening. The front opening of the bore is usually coupled
to the throat of a horn which then radiates the sound waves into the air.
In the description that follows, the term "throat" is used to mean either
the front or downstream end of the pole piece bore or the actual throat of
a horn. Interposed between the diaphragm and the pole piece bore is a
perforated structure known as a phasing plug for impedance matching the
output of the diaphragm to the horn. Within the phasing plug are one or
more air passages or channels for transmission of the sound waves. The
surface of the phasing plug opposite the diaphragm is of corresponding
sphericity and positioned fairly close to the diaphragm while still
leaving an air gap, or compression region, in which the diaphragm can
vibrate freely.
The phasing plug effects two basic functions. First, because the
cross-sectional area of the air channel inlets are smaller than the area
of the diaphragm, the air between the diaphragm and the phasing plug
(i.e., the compression region) can be compressed to relatively high
pressures by motion of the diaphragm. This is what allows a compression
driver to output sound at greater pressure levels than can conventional
loudspeakers where the diaphragm radiates directly into the air. The
efficiency of the loudspeaker is thus increased by virtue of the phasing
plug being placed in close opposition to the diaphragm to minimize the
volume of air between the diaphragm and the phasing plug Secondly, as the
name "phasing plug" implies, the path lengths of the air channels within
the phasing plug may be equalized so as to bring all portions of the
transmitted sound wave into phase coherence when they reach the throat.
Without such path length equalization, sound waves emanating from
different air channels would constructively or destructively interfere
with one another at certain frequencies so as to distort the overall
frequency response.
Phasing plugs have been made with many designs. Perhaps the most frequently
used type is one having annular cross-sections that usually increase in
area as the principal radius of each annulus decreases in moving toward
the throat of a speaker. This is shown, for example, in U.S. Pat. No.
2,037,187, entitled "Sound Translating Device," issued to Wente in 1936
and hereby incorporated by reference. Another type is the saltshaker
design, so called because holes at the spherical outer surface of the plug
that extend through to the throat of the speaker resemble the holes of a
saltshaker. Another design that has been used, shown in U.S. Pat. No.
4,050,541, entitled "Acousticla Transformer for Horn-type Loudspeaker" and
hereby incorporated by reference, couples the diaphragm region to the
throat by radial slots extending from the axis of cylindrical symmetry of
the speaker.
In order to provide a low reluctance magnetic pathway for the applied
static magnetic field, the permanent magnet and the voice coil are
disposed within a surrounding environment of ferromagnetic material. As
both the magnet and voice coil are commonly located on the side of the
diaphragm facing the pole piece, the magnetic pathway includes both the
phasing plug and the surrounding pole piece. In order for the voice coil
to be free to vibrate, however, it must be disposed within an annular air
gap which will be referred to herein as the coil space. Ideally, the coil
space should be made as small as possible since air in the magnetic
pathway adds reluctance to the magnetic circuit which lessens the field
strength at the voice coil. Nevertheless there is a considerable volume of
air in the coil space surrounding the voice coil as well as in the spaces
along the inner edge of the surround and outer edge of the diaphragm which
are continuous with the coil space. This region, comprising the coil space
and the space along the surround and outer edge of the diaphragm, is thus
an uncoupled region since it is so far from the inlets of the phasing plug
air passages that variations of air pressure in that region are coupled
little or not at all to the phasing plug and thence to the throat. Such an
unused volume is shown in the Wente patent referred to above. These
pressure variations thus result in energy losses which lead to heating of
the loudspeaker but do not result in the generation of useful sound
output. The uncoupled region also causes cavity resonance effects which
distort the overall sound output of the speaker due to anomalies in its
frequency response. Such resonances, known as parasitic resonances,
present a significant design problem for the speaker designer. (See, e.g.,
"The Influence of Parasitic Resonances on Compression Driver Loudspeaker
Performance" by Kinoshita, et al. presented at the 61st Convention of the
Audio Engineering Society in 1978 and available as preprint no. 1422
(M-2).)
It would be useful to couple the pressure variations in the uncoupled
region around the voice coil to the throat of the horn, in addition to the
pressure variations produced by the diaphragm, to improve the efficiency
and sound quality of the loudspeaker. Use of the additional pressure
variations could be expected to reduce heating in the region around the
voice coil as a result of repeated compression and rarefaction of the same
air in that region, to produce an increase in the efficiency of the
loudspeaker, and to reduce parasitic resonances.
SUMMARY OF THE INVENTION
The present invention is a compression driver with an annular auxiliary air
passage for providing an acoustic pathway between the uncoupled region
outside of the voice coil and the throat. Sound waves generated by the
vibration of the voice coil and surround are then output from the
loudspeaker which thereby reduces heating, increases the efficiency of the
loudspeaker, and reduces cavity resonance effects.
Further advantage may be obtained in accordance with the present invention
if the auxiliary air passage is made thin so that the added magnetic
reluctance is minimized. In order to achieve a compromise between
minimizing added reluctance and providing an optimum air passage for
soundwaves, part of the auxiliary air passage may be filled with
ferromagnetic material.
It is an object of the present invention to provide a compression driver
with increased efficiency, and which reduces cavity resonance effects. It
is a further object of the present invention to provide a means for
accomplishing the above objective in a manner that minimizes any added
magnetic reluctance.
Other objects, features, and advantages of the invention will become
evident in light of the following detailed description considered in
conjunction with the referenced drawings of a preferred exemplary
embodiment according to the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional side view of a compression driver in accordance with
the present invention, taken along an axis of cylindrical symmetry.
FIG. 2 is a cutaway rear view of the phasing plug of the compression driver
of FIG. 1, taken along section lines 2--2 of FIG. 1.
FIG. 3 is a sectional side view of an alternate embodiment of a compression
driver in accordance with the present invention.
FIG. 4 is a cutaway rear view of a compression driver as shown in FIG. 1,
taken along section lines 2--2 of FIG. 1, but with a combination of
annular and salt-shaker type air passages in the phasing plug.
FIG. 5 is a cutaway rear view of a compression driver as shown in FIG. 1,
taken along section lines 2--2 of FIG. 1, but with a radial slot type of
phasing plug.
FIG. 6 is a cutaway rear view of a compression driver where the auxiliary
air passage has been partially filled with ferromagnetic material so as to
leave a plurality of salt-shaker type passages.
DETAILED DESCRIPTION OF THE INVENTION
Shown in FIG. 1 is an exemplary embodiment of a compression driver
according to the present invention. All of the components in FIG. 1 which
are to be described have cylindrical symmetry about a longitudinal axis. A
diaphragm 34 is suspended from a mounting plate 74 attached to the back of
annular pole piece 52 by means of a resilient surround 72 so that the
diaphragm 34 is freely vibratable along the longitudinal axis. A cover
housing 82 fits over the pole piece 52 so as to cover the diaphragm and
extends over the pole piece's sides to its front surface. Mounted at the
front of the pole piece 52 is a horn 80. The pole piece 52 has within it a
bore through which sounds waves generated by the diaphragm at the bore's
rear opening are transmitted to the horn. The pole piece bore's front
opening is continuous with the throat of the horn and both are designated
66 in the figure. Within the bore of the pole piece 52 is phasing plug 30.
FIG. 2 is a rear sectional view of the driver where the surround 72 has
been partially cut away and the diaphragm 34 removed. As can be seen from
FIGS. 1 and 2, coursing through the phasing plug 32 are annular air
passages 60, 62, and 64 which are referred to herein at main air passages.
Each of the main air passages 60, 62, and 64 serves as an acoustic pathway
through the bore of the pole piece 52, as does a surrounding annular
auxiliary air passage 70 to be described more fully below. As shown in
FIG. 2, each of the air passages 60, 62, 64, and 70 are segmented rings
being separated by longitudinal ribs 71 which connect concentric portions
of the phasing plug 30 a well as connect the phasing plug 30 to the pole
piece 52. The ribs 71 of air passage 70 do not extend completely to the
rear face of the phasing plug so as to leave an annular recess 42 in which
the voice coil is free to vibrate.
The diaphragm 34 is mounted adjacent the rear surface of the phasing plug
30 being separated by a thin space or compression region 32 in which the
diaphragm is free to vibrate in a direction along the longitudinal axis.
The diaphragm 34 is shown as having a central dome-shaped portion with the
rear surface of phasing plug 30 being of corresponding sphericity.
Attached to the diaphragm 34 around the circumference of its central
dome-shaped portion, is a cylindrical voice coil 36 to which the signal
voltage is applied. The coil 36 is wrapped perpendicular to the
longitudinal axis usually around a longitudinally extending rim or form
(not shown) of the diaphragm 34. In this embodiment, and in most
compression drivers, the diaphragm 34 is mounted with its concave surface
adjacent the phasing plug 30 in order for the mean path length through the
annular air channels of the phasing plug from any point on the diaphragm
to the throat 66 to be substantially uniform.
The voice coil 36 must be subjected to a static magnetic field in order to
experience oscillation forces corresponding to the oscillatory signal
current flowing through it. This is accomplished in all electrodynamic
loudspeakers by disposing the voice coil within an air gap which is part
of a magnetic circuit, the coil being free to vibrate with in the air gap.
The magnetic circuit usually comprises a permanent magnet embedded within
ferromagnetic material with the air gap being within the ferromagnetic
material. The air gap, which will be referred to herein as the coil space,
is made as short as possible in order to maximize the magnetic field
intensity impinging on the coil for a given size magnet. For reasons of
design simplicity and efficient use of material, it is desirable to place
the magnetic circuit on the concave side of the diaphragm (i.e, the
compression side) and construct the phasing plug and surrounding pole
piece from ferromagnetic material. (Actually, only the outer portion of
the phasing plug need be made of ferromagnetic material since no magnetic
field lines which impinge on the voice coil pass through the inner
portion.) This means that the voice coil and coil space must necessarily
also be located on the concave side of the diaphragm. (It is possible,
however, to design otherwise so that the voice coil is mounted on the
convex side of the diaphragm. See, for example, U.S. Pat. No. 2,832,844,
issued to Matsuoka. The present invention is not applicable to those
designs where the phasing plug and voice coil are located on opposite
sides of the diaphragm).
The embodiment in FIG. 1 thus shows the voice coil 36 being disposed within
an annular coil space 42 in which it is free to vibrate in a direction
along the longitudinal axis and cause corresponding vibration of diaphragm
34. An annular permanent magnet 14 is embedded within the outer concentric
portion of the phasing plug 30 so as to produce a magnetic field having
field lines such as that designated 46 in FIG. 1. In accordance with the
present invention, the coil space 42 is continuous with annular auxiliary
air passage 70 which serves as an acoustic pathway for soundwaves
generated by the vibrating voice coil 36 (as well as vibrations of the
surround 72 and outer edge of the diaphragm) to reach the throat 66.
Without the auxiliary air passage, the sound energy generated by the voice
coil 36, surround 72, and outer edge of the diaphragm, in addition to
causing cavity resonance effects, would be wasted. Thus the present
invention increases the efficiency of the loudspeaker, serves as a means
for heat dissipation, and reduces parasitic resonances.
It should be noted, however, that the sound output from the vibrating voice
coil 36 and surround 72 only adds to that from the vibrating dome of the
diaphragm when the entire structure vibrates in phase in the diaphragm's
fundamental mode. When the driving frequency (i.e., the frequency of the
signal voltage) equals the second resonance frequency of the surround of
the diaphragm, the dome and surround 72 vibrate in opposite phase causing
their sound outputs to subtract from one another. Thus, only below the
second resonance frequency does the auxiliary air passage 70 actually
increase the efficiency of the loudspeaker. The reduction in cavity
resonance effects is accomplished, however, at all driving frequencies.
Also in accordance with the present invention, the auxiliary air passage 70
may be designed so that its cross-sectional area increases in going from
the coil space 42 to the throat 66. Adding an auxiliary air passage in the
proximity of the magnet necessarily attenuates the magnetic field
impinging on the voice coil because the air passage adds reluctance to the
magnetic circuit. To minimize this added reluctance, the auxiliary air
passage should take up no more volume than necessary. In order to
compromise between this objective and providing an optimum path for
soundwaves, the auxiliary air passage may be constructed so that its
cross-sectional area is small in the proximity of the coil space and
increases toward the throat 66. Additionally, the auxiliary air passage
may be partially filled with ferromagnetic material so as to leave a
plurality of narrow air passages (e.g., of the salt-shaker type) for
transmitting sound from the coil space to the throat.
FIG. 3 shows another embodiment of the present invention in which the
magnet 14 is located within the pole piece 52 instead of the phasing plug
30. The operation of this embodiment is exactly as described above with
reference to the first embodiment. Also, the main air passages of the
phasing plug 30 do not have to be annular but can be either of the
salt-shaker or radial slot design as shown in the rear sectional views of
FIGS. 4 and 5, respectively.
FIG. 6 shows a rear cutaway view of another embodiment of the present
invention in which the auxiliary air passage 70 is partially filled with
ferromagnetic material so as to reduce the reluctance added to the
magnetic circuit. In this embodiment, the ribs 71 (made of ferromagnetic
material) form a segmented annulus separated by round air passages 70
which are shown to be essentially of the salt-shaker type. The round air
passages 70 extend all the way to the throat 66 and the ribs 71 may also
so extend. As in the previous embodiments, however, the annular ribs 71 do
not extend all the way to the rear of the phasing plug so as to leave an
annular recess 42 (i.e., coil space) in which the coil is free to vibrate.
Although the invention has been described in conjunction with the foregoing
specific embodiment, many alternatives, variations, and modifications will
be apparent to those of ordinary skill in the art. Those alternatives,
variations, and modifications are intended to fall within the scope of the
following appended claims.
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