Note: Descriptions are shown in the official language in which they were submitted.
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Maqnetic Fluid Loudspeaker Assembly with Ported Enclosure
and Method of ~etermininq Parameters thereof.
DESCRIPTION
5 TECHNICAL FIELD:
~ This invention relates to loudspeaker assemblies and
methods of determining parameters thereof and is especially
applicable to loudspeaker assemblies in which a magnetic fluid
is provided between the voice coil and the magnetic poles.
~0 The invention is especially concerned with small loudspeakers,
for example loudspeakers of "hands-free" telephone sets,
loudspeakers of multimedia personal computers, and so on.
BACKGROUND ART:
Magnetic fluids comprise very fine magnetic particles
suspended in a viscous liquid, such as an oil. Such magnetic
fluids have been used in loud~peakers to carry heat away from
the voice coil. This decreases the temperature rise in the
voice coil for a given applied power (and hence the
20 corresponding change in impedance), as well as increasing the
power handling capabilities of the loudspeaker. This
is particulary beneficial for tweeters, where power handling
is more often restricted by voice coil heating. In low
fre~uency drivers, power handling is more often restricted by
25 the suspension and voice coil characteristics required for
large cone excursions, and less likely to be improved by
magnetic fluid.
Loudspeakers using magnetic fluid have been disclosed in
US patent No. 5,335,287 (Athanas) issued August 1994 and US
30 patent No. 4,017,694 (King) issued April 1977, to which the
reader is directed for reference. In a conventional
loudspeaker, the diaphragm is attached to a voice coil former
which carries the voice coil and extends into an annular
cavity within the- usual magnet assembly. The voice coil
35 former is attached to the surrounding frame of the loudspeaker
by a corrugated annular suspension In designing the
loudspeaker disclosed in US 5,335,287, Athanas dispensed with
the corrugated annular suspension and relied upon magnetic
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fluid to support the voice coil former and voice coil. Athanas
focused upon venting arrangements to prevent displacement of
the magnetic fluid.
US patent specification number 4,017,694 issued April 12,
5 1977 discloses a loudspeaker drive unit of conventional
configuration but with a magnetic fluid enveloping the voice
coil. The magnetic fluid is introduced into the annular cavity
which contains the voice coil and is retained there by the
magnetic field. According to US 4,017,694, providing the
10 magnetic fluid has a viscosity between about 1000 centipoise
and 10,000 centipoise, air gap underdamping of the loudspeaker
drive unit is eliminated, leading to improved bass response.
Also, it is claimed that the power rating of the loudspeaker
drive unit can be increased 200% to 300% without introducing
15 gross distortion and avoiding the use of heavy magnets. US
4,017,694 also addresses dust cap venting to prevent hissing
and possible displacement of the magnetic fluid. However, US
4,017,694 does not address the design of an enclosure for such
a loudspeaker drive unit.
When designing an enclosure for a conventional
loudspeaker, one may use computer modelling techniques
operating with an equivalent circuit of the loudspeaker.
Employing such techniques to design an enclosure for a
loudspeaker with a magnetic fluid around the voice coil, the
25 inventor found that the techn; ques did not work properly and
concluded that the magnetic fluid was not behaving as
expected.
One of the problems encountered in designing
loudspeakers for telephone sets, and other applications where
30 size is restricted, is that the small enclosure size results
in poor sound quality. It is generally accepted that, for
optimum frequency response of a particular loudspeaker drive
unit in a sealed enclosure, the volume of the enclosure must
be much larger than the compliance equivalent volume of the
35 loudspeaker drive unit itself, typically by at least a factor
of four. At frequencies which are low compared with the
resonance frequency of the loudspeaker drive unit, the sound
pressure at an external point rises at 12 dB/octave. At high
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frequencies, the pressure is roughly constant (neglecting cone
breakup, standing waves, and other resonances). At the
resonance frequency of the loudspeaker drive unit, the
pressure may rise a little above the high frequency asymptote
5 depending upon the Q factor of the loudspeaker drive unit.
When the volume of the enclosure is reduced, the effective
resonance frequency increases because the back pressure of the
air in the enclosure effectively stiffens the drive unit
suspension. This increased resonance frequency reduces the
10 effectiveness of the drive unit at low frequencies, in view
of the "roll off" at 12 dB per octave. In addition, the Q
factor of the system increases, resulting in a pressure
increase at the resonance frequency. Both effects degrade
performance.
lS Consequently, it is difficult to obtain good sound
quality in telephone set loudspeakers, multimedia computer
loudspeakers, and the like, where enclosure size is limited.
Sound ~uality depends upon many factors, but generally
designers try to obtain a substantially flat frequency
20 response characteristic over a wide range of frequencies.
Although adding magnetic fluid improves the frequency response
of the drive unit, particularly at the resonance frequency,
it does not necessarily follow that the performance will be
the same when the drive unit is mounted in an enclosure. The
25 magnetic fluid comprises small magnetic particles suspended
in a viscous fluid. The magnetic field retains the fluid
within the voice coil cavity. The presence of the viscous
fluid between the voice coil and the magnet poles increases
the damping. When designing an enclosure for such a
30 loudspeaker drive unit with increased damping, a skilled
person would expect to have to reduce the size of the
enclosure in order to obtain a reasonably flat response. The
reduced enclosure size would cause the lower frequency part
of the fre~uency response to "roll off 1l at a higher frequency,
35 decreasing performance at low frequencies.
The inventor has discovered that, by taking the magnetic
fluid characteristics into account when designing the
enclosure, it is possible to design a loudspeaker enclosure
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which, for a given performance, is surprisingly smaller than
expected.
DISCLOSURE OF INVENTION:
According to one aspect of the present invention, a
loudspeaker assembly comprises a loudspeaker drive unit having
a magnet unit defining a magnetic air gap, a voice coil
extending at least partly in the air gap and movable to and
fro relative to the magnet unit, a magnetic fluid within the
10 air gap and occupying interstices between the voice coil and
the magnet unit, and a diaphragm coupled to and driven by the
voice coil, the loudspeaker drive unit being housed in an
enclosure having a volume between about one eighth and about
double a compliance equivalent volume of the loudspeaker drive
15 unit.
Preferably, the enclosure volume is less than, or equal
to, the compliance equivalent volume of the loudspea~er drive
unit.
The enclosure may have a port, in which case the
20 loudspeaker may have a low frequency response extending
significantly lower than the free space resonance frequency
of the loudspeaker drive unit.
In preferred embodiments of the invention, where the
enclosure has a port, the free space resonance frequency of
25 the loudspeaker drive unit is between about 50 per cent and
about 60 per cent, and preferably about one half, of the
resonance frequency of the enclosure dete- ine~ approximately
according to the expression:
f ~ 1 ¦ pc2
1 2~ V~MA
where MA is the acoustic inductance of the port,
30 given approximately by the pl expression:
A 1~a2
p is the density of air (# 1.18 ~g/m3);
a is the radius of the port (m);
1 is the length of the port (m);
-
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VAB is the internal volume of the enclosure ~m3);
c is the speed of sound (~ 344 m/S);
The parameters of the loudspeaker drive unit, magnetic
fluid and enclosure preferably are predetermined such that
rl S ~ VAS + VAB
A2L '\ pc2MA
5 where M~ is the acoustic inductance of the port,
as above;
VA~ i5 the volume of the enclosure (m3)
VAS is the compliance equivalent volume of the
loudspeaker drive unit (m3);
~ is the viscosity of the magnetic fluid (Pa -
S);
S is the voice coil surface area in contact with
the magnetic fluid (m2);
A is the area of the loudspeaker diaphragm (m2);
L is the mean distance between the voice coil
and the magnet poles (m); and
p is the density of air (kg/m3).
According to a second aspect of the invention, a method
of dete, i ni ng the parameters of the loudspeaker assembly
20 comprises the step of deriving an effective impedance ZFF for
the magnetic fluid as follows:
Magnitude: 12~/Re ( F) 2 + Im ~ F) 2 Ns/m 5
Phase: tan~1( Im ( F) )
where:
Re ~F) = ~Sk sinh 2kl + sln2kl
tanhkl _ tankl
Im(F) =-~S~ cos2kl cosh2kl
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A is the surface area of the loudspeaker diaphragm (mZ)
~ is the viscosity of the magnetic liquid (Pa-s)
S is the voice coil surface area in contact with the
magnetic liquid
5 k =
2~
p is the density of the magnetic liquid (kg/m3) and
l is the mean distance between the magnet and the voice
coil (m).
An embodiment of the invention will now be described by
10 way of example only and with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 is a plan view of a loudspeaker assembly
15 embodying the present invention;
Figure 2 is a sectional side view of the loudspeaker
assembly;
Figure 3 is a schematic sectional view of the loudspeaker
drive unit;
Figure 4 is an equivalent circuit of the loudspeaker
assembly used to model its performance;
Figure 5 shows plots of the electrical impedance of the
loudspeaker drive unit;
Figure 6 shows the frequency response of the loudspeaker
25 drive unit without magnetic fluid and on an IEC st~n~rd
baffle;
Figure 7 shows the frequency response of the loudspeaker
drive unit on the IEC standard baffle after the addition of
magnetic fluid;
Figure 8 shows the frequency response of the loudspeaker
drive unit with magnetic fluid and mounted in a ported
enclosure;
Figure 9 is a plan view of a modified loudspeaker
assembly; and
Figure 10 is a side view of the modified loudspeaker
assembly.
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MODE~S) FOR CA3~RYING OUT TE~E INVENTION:
In the drawings, corresponding items in the different
Figures have the same reference numeral.
Referring to Figures 1, 2 and 3, a loudspeaker comprises
5 a loudspeaker drive unit 10 housed in a parallelepiped
enclosure 12. The drive unit 10 is of conventional
construction in that it comprises a conical diaphragm 14
carried by a voice coil unit 16 which extends into an annular
cavity 18 defined by opposed magnetic poles 20 and 22 of a
10 magnet assembly 24. Magnetic fluid 26 is provided in the
cavity 18, in the interstices between the voice coil unit 16
and the magnetic poles 20 and 22. A suitable magnetic fluid
is marketed under the trade name FerrofluidTM by Ferrofluidics
Corporation, Nashua, New Hampshire. The magnetic fluid may
15 ~e inserted into the cavity using a syringe, as described in
US 4,017,694. A dust cap 28 with a small vent (not shown)
spans the inner end of the conical diaphragm 14. A flexible
surround 30, which extends around the outer rim of the conical
diaphragm 14, attaches the diaphragm 14 to the support frame
20 32 of the drive unit 10. The construction of the loudspeaker
drive unit may be as described in US 4,017,694 and so will not
be described in more detail here.
The enclosure 12 comprises an oblong, cast aluminum box
34 closed by a lid 36 which is secured to the box 34 by screws
25 38. The lid 36 is sealed to the rim of box 28 by a gasket
(not shown) and has a central aperture 40. The loudspeaker
drive unit 10 is attached to the inside of lid 36 by screws
42 which extend through aligned holes (not shown) in the lid
36 and flanges 44 and 46 of the support frame 32, the rim of
30 the diaphragm 14 coinciding with the rim of aperture 40. A
hole 48 is provided in one end wall 50 of the box 34. One end
of a cylindrical tube 52 is attached to the end wall 50 and
communicates with the hole 48. The tube 52 extends, with its
cylindrical axis coincident with the longitudinal central axis
35 of box 34, away from the end wall 50 for a distance slightly
greater than the length of the box 52. The tube 52 forms an
acoustic port and may be made of aluminum or a synthetic
plastics material.
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In one practical embodiment, the drive unit lO was a
model TF050-A90822 by NMB Precision Incorporated, with about
1 x 10-7 m3 (100 microliters) of Ferrofluid ~Mwith a viscosity
of 1 Pa-s injected into its voice coil cavity. The box 34 was
5 108 mm. long by about 67 mm. wide and about 43 mm. deep, with
a net internal volume, i.e. not including that occupied by the
drive unit 10, of about 250 cc. The port tube 52 was 115 mm.
long with an internal diameter of 16 mm.
These dimensions of the enclosure and port which
10 optimized the acoustic performance of the loudspeaker were
determined by a series of iterative computations using the
parameters of the loudspeaker drive unit lO, the port 52 and
the magnetic fluid 26 in an equivalent circuit for the
loudspeaker system as shown in Figure 4. In Figure 4, the
15 drive unit 10 is represented by the voltage source VG,
resistance ~AE for losses due to the electrical circuit,
inductance LAS representing the mass of the diaphragm 14,
capacitance CAS representing the compliance of the loudspeaker
drive unit suspension and RAS representing mechanical losses.
20 The magnetic fluid 26 is represented by complex impedance ZFF.
Capacitance CDC represents the compliance of the cavity
beneath the dust cap 28, RDC and LDC represent, resistance and
inductance, respectively, of the vent 29 in the dust cap 28.
LAP and RAP represent inductance LAP and resistance RAP
25 represent the compliance of the port 52. Inductance LAL and
resistance RAL represent leakage. CAB represents the
compliance of the enclosure 12. Losses in the enclosure 12 are
insignificant. The turns ratios of ideal transformers T1 and
T2 are 1:(1 ~ SC/SR) and 1:(1 + SR/SC), respectively, where
30 SC is the cross-sectional area of the volume swept by the dust
cap 29; SR is the area of the diaphragm excluding the dust cap
29.
The optimized dimensions were obtained as follows:
1. The electrical impedance of the loudspeaker drive unit
35 was measured. The results are shown in Figure 5, curve A
showing the variation of impedance with frequency with the
drive unit hanging in free space and curve B showing the
variation of impedance with frequency with the drive unit in
a sealed volume.
2. Available ranges of magnetic fluid parameters were
determined, i.e. viscosity, density, magnetic susceptibility).
3. Commencing with an enclosure volume approximately equal
to the compliance equivalent volume of the loudspeaker drive
unit 10, and using the equivalent circuit shown in Figure 4,
the frequency response was calculated and plotted.
5. The various parameters were adjusted and the calculations
repeated.
6. The above steps were repeated until a predetermined
satisfactory frequency response was obtained.
The values of VG, RAE, RAS, LAS and CAS were derived from
the electrical impedance curves shown in Figure 4. The values
of CDC, LDC, RDC and SC/CR were determined from the geometry
of the loudspeaker drive unit 10. The values of RAP and LAP
were derived from the geometry of the enclosure. The
impedance ZFF for the magnetic fluid was derived from an
analysis of the effects of magnetic liquid in the structure,
as follows:
Magnitude: Image
Phase: Image
where: Image
Image
A is the surface area of the loudspeaker diaphragm (= 1.45
x 10 3 m2)
~ is the viscosity of the magnetic liquid (= 1 Pa-s)
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S is the voice coil surface area in contact with the
magnetic liguid
k =
2~
p is the density of the magnetic liquid (= 1100 Kg/m3)
5 l is the mean distance between the magnet and the voice
coil (0.225 x 10-3 m).
It should be noted that the magnetic fluid could be
represented by an equivalent voltage source (EFF) rather than
the impedance (ZFF). The value of the voltage source would
10 be obtained by multiplying the impedance ZFF by the acoustic
current/volume velocity uO.
At the end of the process, optimized values had been
determined for the frequency response of the final assembly,
the volume of the enclosure, the dimensions of the port
15 (radius and length), and the viscosity, density, volume and
magnetic susceptibility of the magnetic fluid. It will be
appreciated that the calculations were carried out using a
computer. For the loudspeaker illustrated in Figures 1, 2 and
3, the final values were as follows:
20 VG 1.270 x 102 N/m2
RAE 3.313 x 105 Ns/ms
LAS 3.114 x 102 kg/m4
CAS 1.011 x 10-9 m5/N
RAS 1.041 x 105 Ns/ms
25 CAB 1.426 x 10-9 m5/N
RAP 2.389 x 104 Ns/m5
~AP 7.841 x lo2 kg/m4
CDC 7.770 x 10-l2 m5/N
LDC 2.295 x 102 kg/m4
30 RDC 6.811 x 105 Ns/m5
SC/SR 0.167
The magnetic fluid viscosities considered ranged between
0.05 Pa-s and 2.0 Pa-s, the actual value used being 1.0 Pa-s.
The density of 1100 kg/m3 did not change appreciably from one
35 magnetic liquid to another. The susceptibility varied between
100 and 200 Gauss but, since it had a much smaller effect than
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variation of the viscosity, it was neglected in the
calculations.
It should be appreciated that these parameters were
arrived at for a particular drive unit and frequency response.
Figure 6 shows the frequency response of the loudspeaker
drive unit 10 without the magnetic fluid and on an IEC
standard baffle. As shown in Figure 7, addition of the
magnetic li~uid had the effect of "overdamping" the drive
unit, resulting in a reduction in the response to the lower
10 frequencies. It is generally known that a suitable enclosure,
with a port, can restore the response at lower frequencies.
However, in conventional loudspeaker units, the improvement
is at the expense of a reduction in the uniformity of the
frequency response, the effect being more pronounced as the
15 enclosure size is reduced. As shown in Figure 8, with the
loudspeaker drive unit lO mounted in the ported enclosure 12,
the lower frequency response is restored. It is noticeable,
however, that the frequency response curve in Figure 8 does
not show the usual high Q resonances one would expect from
20 such a small enclosure. The reason for such surprisingly good
results attained by embodiments of the present invention is
not known precisely. It is thought, however, that it might
be attributable, at least in part, to the fact that the
magnetic fluid not only increases the damping, thereby
25 reducing the high Q resonances, but also effectively increases
the voice coil mass. Moreover, the change in mass is
frequency dependent.
It should be appreciated that the above-described
enclosure is of prototype construction. In practice, it
30 could, and probably would, be made differently. For example,
the port tube 52 might extend within the box 34.
Figures 9 and 10 illustrate a modified loudspeaker
assembly in which the port tube 52' still is external but
extends alongside the box 34, the prime signifying that the
35 tube 52' is not identical to that of Figure 1. The tube 52'
is cylindrical, as before, and is bonded to the exterior of
one of the longer walls of the box 34. A junction piece 54
~ comprising a short section of cylindrical tube e~ual in
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12
diameter to tube 52 is bonded to one end of the port tube 52.
Its other end is cut obli~uely and bonded to the edges of an
elliptical hole 56 provided in the wall of the box 34. The
other end of tube 52' protrudes slightly beyond the end of the
5 box 34. Hence/ the port tube 52' communicates with the
interior of the box 34 by means of the hole 56.
Figure 9 also illustrates another modification, namely
the repositioning of the drive unit 10 further away from the
hole communicating with port tube 52', which could also be
10 used with the port arrangement of the loudspeaker assembly of
Figure 1. With such an arrangement, the air from the port 54
is "less hindered" by the drive unit lO because it will be
travelling more slowly when it reaches the drive unit 10.
15 INDUSTRIAL APPLICABILITY
The invention is applicable to small loudspeakers, for
example loudspeakers of "hands-free" telephone sets,
loudspeakers of multimedia personal computers, and so on.
