Materials Transactions, Vol. 52, No. 10 (2011) pp. 1966 to 1971
#2011 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Improvement in the Frequency Response of Loudspeakers
by Using Diamond-Like Carbon Film Coatings
Chii-Ruey Lin1 , Shin-Hwa Liu2 , Wang-Jeng Liou1; * and Chien-Kuo Chang1
1
Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology,
Taipei 10608, Taiwan, R. O. China
2
Graduate Institute of Vehicle Engineering, National Taipei University of Technology,
Taipei 10608, Taiwan, R. O. China
It is well known that a diamond-like carbon (DLC) film has a high mechanical hardness and Young’s modulus. One of the beneficial
properties of a DLC film is its ability to change the sound velocity in loudspeakers through its application as a hard coating. In the present study,
DLC films were coated onto polyetherimide (PEI) diaphragm substrates at low temperature with radio-frequency (RF) magnetron sputtering.
Amorphous DLC films deposited at an RF power of 150 W and with a deposition time of 3 h have a high ID =IG ratio and a low surface roughness.
The ID =IG ratio and surface roughness were 2.27 and l.21 nm (Ra), respectively. From frequency response analysis of the DLC film on the
diaphragm, we found that the frequency response increased by 0:21:2 dB on average. This confirmed the excellent adhesion of DLC films onto
PEI (or polymer) substrates for future potential applications in acoustic wave devices. [doi:10.2320/matertrans.M2011080]
(Received March 10, 2011; Accepted July 11, 2011; Published September 25, 2011)
Keywords: diamond-like carbon (DLC) thin-film coatings, acoustic, frequency response, diaphragm
1.
Introduction
The mini dynamic loudspeaker was successfully developed by Mr. Eugen Beyer, a German scientist, during the
1930s. We now find its ubiquitous application to mobile
devices, such as mobile phones, MP3 and MP4 players, and
laptops. The more we become accustomed to these technological conveniences, the more we rely upon mini loudspeakers of better quality. The total quality of the loudspeaker depends on the performance indexes of its 4 main
constituent subsystems: the vibration system, magnetic
system, the coupling system between the aforementioned
two systems, and support systems. We must see it from a
holistic viewpoint instead of a compartmentalized viewpoint.
Apart from the vibration system, the other 3 systems have
been well developed. The quality of the diaphragm of the
vibration system will be based on 3 essential requirements,
outlined below.1,2)
(1) The modulus of the diaphragm material will be as high
as possible to provide a wider range of frequency
responses and lower distortion.
(2) The density of the diaphragm material will be as low as
possible in order to increase the fidelity.
(3) The intrinsic damping capacity of the diaphragm
material will be such that loudspeaker partition vibration is attenuated.
The audible frequency of human hearing ranges from
20 Hz to 20 kHz.3) Since the demarcations of low, medium,
and high frequency ranges are not clearly defined, this study
assigns a definition of low frequency (LF) as 100710 Hz,
mid-frequency (MF) as 7505:6 kHz, and high frequency
(HF) as 640 kHz. Each frequency band is further divided
into three discrete narrow bands, LF: 100180 Hz,
190355 Hz, 375710 Hz; MF: 750 Hz1:4 kHz, 1:5
2:8 kHz, 35:6 kHz; and HF: 611:2 kHz, 11:822:4 kHz,
*Corresponding
author, E-mail: s3669014@ntut.edu.tw
Fig. 1
The Impedance curve and fo from the examination.
23:640 kHz. Resonance frequency fo of a loudspeaker is
representative of the loudspeaker at the initial point of the
frequency response. According to this theory, we could find
out the resonance frequency fo by eq. (1). This research used
a CLIO audio test system to conduct sampling and measurement by tracing through the characteristic impedance curve,
as shown in Fig. 1.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
1
fo ¼
ð1Þ
2 mo Co
where
fo : Resonance frequency (Hz)
Co : Compliance
mo : Total mass of vibration system
mo ¼ md þ mc þ mad
md : Mass of diaphragm
mc : Mass of coil
mad : Mass of additional air, equal to 8a d 3 =3
a : Density of air
d: Effective diameter of diaphragm
Improvement in the Frequency Response of Loudspeakers by Using Diamond-Like Carbon Film Coatings
Table 1 Physical properties of different vibration diaphragm materials.
Material
Young’s modulus
E (1011 Pa)
Density
(g/cm3 )
Coil
Suspending
Sound velocity
C (km/s)
Paper
0.002
0.5
2.0
PEI
Stainless
steel
0.03
1.27
0.13
2.0
7.9
5.1
Al.
0.74
2.7
5.2
Ti.
1.1
4.5
5.2
1967
carrier
Dome (Cap)
edge
3.25mm
10.5mm
Fig. 2
Coil
Sectional view of a loudspeaker.
W.C.
7.2
15.6
6.8
Al2 O3
4.3
3.9
10.4
Be.
DLC
2.8
3.3
1.8
1.8
12.3
18.3
Coating pattern
(PEI)
RF Power
(W)
Ar. flow rate
(sccm)
Pressure
(Pa)
Deposited time
(h)
Diamond
11.5
3.5
18.5
C2aC4a
150
25
1.3
3
Since 1887, beryllium has been used innovatively by
Yamaha for loudspeaker diaphragms in treble units. In 1968,
they accomplished automatic completion of cone and forming equipment used in low-frequency diaphragms material
from plant fiber to animal fibers, man-made fibers, and
reinforced fiber polymer. The qualities of the frequency
response and the maximum frequency have therefore
increased significantly. In the late 1970s, many kinds of
polymers and ceramic materials were developed, such as
carbon fiber (1973), beryllium composites (1976), aluminum,
titanium, boron alloy, magnesium alloy, pure beryllium
(1978), carbon-graphite (1979), ceramic graphite (1987),
graphite composite materials, and all-crystalline diamond
material (1989). In 1986, Japan Sumitomo Electric Company
primary produced a DLC/Ti composite cone.
A loudspeaker diaphragm needs to have high rigidity,
low density, high sound propagation velocity, and high
heat conduction, as shown in Table 1.4,5) The mass of the
diaphragm plays an important role in determining the
frequency response directly; that is, a lighter loudspeaker
diaphragm manifests better frequency response. A highquality tweeter diaphragm must be made of highly rigid,
thin, and light material. In recent years, DLC films have been
used in loudspeaker diaphragms, which would improve the
frequency response drastically. DLC films retain most
diamond properties, such as chemical inertness, high mechanical hardness, high Young’s modulus, and high sound
conduction velocity.6) The properties of DLC films are
conducive for their use in loudspeaker diaphragms owing to
the fact that their properties can vary from those of diamond
(sp3 bonding) to those of graphite (sp2 bonding). The
diamond (sp3 bonding) properties can be used to increase
the sound velocity (E=) in the loudspeakers.7,8)
Microelectronics and measuring systems (MEMSs),9) and
electro-optical systems, piezoelectric systems10–12) have been
previously employed in acoustic wave device studies.
Furthermore, materials combined with various kinds of
metal-substrates, such as aluminum13) and titanium,14) have
been developed and investigated to enhance frequency
response performance. Moreover, improvement has been
made through studies15) that used finite element methods
(Abaqus) to simulate the diaphragm coating with DLC in
different areas and to study the frequency response that is
influenced by different coating areas on the diaphragm,16) and
Table 2
Parameters of R.F. sputter deposition.
through studies that investigated issues related to the
sputtering process, such as substrate holder rotation, in order
to achieve uniform deposition.
DLC films can be deposited by various physical vapor
deposition (PVD) methods in combination with radio
frequency (RF) plasma and chemical vapor deposition
(CVD) methods. RF magnetron sputtering was used in this
study because of its advantages such as deposition of large
areas with uniformity and processing at room temperature.17)
In this study, we deposited DLC coatings on polyetherimide
(PEI) diaphragm substrates with varying parameters for
instance deposition area. The corresponding microstructural
properties of DLC films were identified by Raman spectroscopy, atomic force microscopy (AFM), and scanning
electron microscopy (SEM).
2.
Experimental Methods and Procedures
The aim of this research was to deposit DLC onto PEI
diaphragm films to improve the performance of mini loudspeaker frequency response. Because the glass transformation temperature of PEI materials (216 C) is relatively lower
than the CVD working temperature (600 C–800 C), we
could not use the CVD process with such materials. We as an
alternative used RF magnetron sputtering to deposit DLC
thin films to avoid deformation or deterioration of the
diaphragm.
RF magnetron sputtering was employed to deposit a DLC
thin film onto a PEI substrate with a diameter of 17.0 mm and
a thickness of 25 mm. Owing to the use of the vibration
technique, the suspending edge (Fig. 2) areas should not be
coated. Appropriate flexibility of suspending edge should be
maintained. For verification, documentation on this experiment is included in following section. With the aim of obtain
the relevant coating parameters, data on composition and
thickness was acquired using a Si wafer, through Raman
spectroscopy, AFM, and SEM. Experimental parameters are
listed in Table 2.
Next, the PEI substrate was replaced with the Si-wafer, and
its surface was cleaned with methanol in ultrasonic equipment for 3 min so as to remove contaminants from the surface
to guarantee adhesion. The substrates were placed at a
distance of about 9 cm to 12 cm from the target. The PEI
substrates were rotated at a speed of 15 rpm. And two shields
1968
C.-R. Lin, S.-H. Liu, W.-J. Liou and C.-K. Chang
8.5mm
5.25mm
Diaphragm No. C1
Diaphragm No.C2a
Diaphragm No.C3a
Diaphragm No.C4a
Note
1. Diaphragm type P2024 - 25 BPEI, thickness 25 µm
2.
Deposition area
Fig. 5 Frequency response before and after DLC coating. (C1, without
coating, C2aC4a coated).
Fig. 3 4 types DLC coating pattern of diaphragm in this study: (C1)
without coating, (C2a) whole area coating, (C3a) suspending edge coating,
(C4a) dome coating.
Fig. 4 Photos of coated with DLC thin films show (a) Diaphragm, and (b)
Loudspeaker assembly.
model were designed for three different coated areas, as
shown in Fig. 3.
After assembly, the mini loudspeakers were tested at a
distance of 10 cm from microphone in a semi-anechoic
chamber having the SoundCheck and CLIO audio test
system, B&K microphone, and other equipment, for sound
measurement and validation.
3.
Results and Discussion
(1) After several previous failed studies, including tests that
used shielded fixtures, readjusted the sputtering parameters, and fixed the sputtering chamber diaphragm, the
present study succeeded in depositing a 120-nm DLC
film on PEI diaphragms by sputtering. Some finish
diaphragm assemblies are shown in Fig. 4. The color
and uniformity of coatings can be observed clearly.
(2) The frequency responses before and after sputtering are
shown in Fig. 5. Regardless of whether sputtering was
carried out, the frequency bands remained above 75 dB.
In particular, at more than 30 kHz, the reduction trend
of frequency response of the diaphragms after sputtering is lower than that of uncoated one.
(3) Figure 6 shows the cross-sectional SEM images of the
amorphous DLC film deposited for 3 h. The film
thickness was found to be 120 nm. Further, the detailed
bonding structure of the carbon samples was observed
Fig. 6 Shows that the SEM cross-sectional view of DLC film had been
deposited for 3 h, 1.3 Pa, with Ar. 25 sccm sputtering.
clearly by Raman scattering. Raman spectroscopy is a
non-destructive method to characterize the structure of
graphite, diamond, and DLCs. The single Raman peak
located at 1580 cm1 in this study is attributed to highly
crystalline graphite in an as-deposited DLC film
structure. The peak located at 1350 cm1 is attributed
to micro-crystalline graphite with disordered sp3 sites.
The ID =IG ratio of the DLC film was calculated by two
Gaussian-fitted peaks in the Raman spectra. An ID =IG
ratio of 2.27 was obtained at a deposition time of 3 h, as
shown in Fig. 7. For comparison of the surface topography under varying parameters of the DLC films, the
surface roughness values of DLC films were obtained
by AFM analysis. The surface topography was analyzed
by AFM in a scanning area of 10 mm 10 mm of the
DLC film under an RF power of 150 W and deposition
time of 3 h, as shown in Fig. 8. The surface of the DLC
film was very flat, with an average roughness (Ra) of
1.21 nm. The Young’s modulus and density of the
film were around 3:3 1011 Pa and 1.82 g/cm3 , which
the Young’s modulus of DLC film was obtained by
adopting nano indenter, and the density of as-grown
DLC thin film was estimated as corresponded to their
hardness,18) respectively.
Improvement in the Frequency Response of Loudspeakers by Using Diamond-Like Carbon Film Coatings
1969
Fig. 9 Impedance compares curves.
Fig. 7 Raman spectra of the DLC films deposited with deposition time of
3 h. (with Gaussian fitting of D and G peaks).
Table 3 Impedance comparison table.
Coating pattern
Impedance
()
Frequency
(Hz)
Fre. Ratio E
(C a/C1)
C1
C2a
13.32
14.77
714.4
783.0
1
1.09
C3a
14.45
749.9
1.05
C4a
14.08
718.2
1.01
(5) Assume that the mass (mo ) of the diaphragm is
unchanged; then, according to eq. (2), if the Young’s
modulus of the material at the suspending edge is
doubled (E2 =E1 ¼ 2), then the resonance frequency of
the diaphragm increases by 1.4 times (E ¼ 1:4).19)
Assume mo ¼ const.
sffiffiffiffiffiffi sffiffiffiffiffiffi
fo2
C1
E2
ð2Þ
E ¼
¼
¼
fo1
C2
E1
Fig. 8 AFM surface topography images for the DLC films deposited at RF
power of 150 W and with deposition time of 3 h with a scanning area
(10 mm 10 mm), and the average roughness (Ra) value is 1.21 nm.
(4) For this study, the loudspeaker resonance frequency fo
was 714.4 Hz. The result of measured fo was shown in
Fig. 9. Table 3 presents data obtained after the sputtering process and testing. All of the resonance frequencies were found to have increased significantly, which
is supported by the principle in eq. (1).
where
mo : Effective mass of vibration system
fo1 , fo2 : Resonance frequency of raw and coated
vibration systems, respectively.
C1 , C2 : Compliance of raw and coated diaphragm,
respectively.
E1 , E2 : Young’s modulus of raw and coated diaphragms, respectively.
(6) The DLC film thickness of 120 nm, in comparison to
the PEI diaphragm thickness of 25 mm, is about 1/200
of the thickness of the raw materials, and according to
eq. (3), density increments (2.07/1000) of the film are
considered to be negligible. However, according to the
transformed-section method,20) eq. (4), and eq. (5), the
Young’s modulus of the DLC film is evaluated to be
1.496 times that of the raw materials. According to
eq. (2), resonance frequency should be 1.223 times the
original frequency. Instead, the C2a resonance frequency increased only 1.09-fold, C3a increased 1.05fold, and C4a increased 1.01-fold during experiments.
This is because the transformed-section method assumes that the diaphragm section is a flat plate, but the
section is actually an arc. Moreover, for the computations in this study, we assumed that the whole area
was coated, i.e., the C2a coating pattern was followed.
1970
C.-R. Lin, S.-H. Liu, W.-J. Liou and C.-K. Chang
Table 4 Frequency response comparison table in individual central
frequency of type Ca.
No.
fc .(Hz)
Type
C1
C2a
C3a
C4a
Note
(dB)
(dB)
(dB)
(dB)
(Hz)
125
66.2
64.5
62.9
65.5
100180
250
78.4
76.3
74.9
77.5
190355
500
1k
92.5
96.2
91.0
97.4
88.7
97.6
91.2
97.0
375710
7501:4k
1:5k2:8k
2k
93.5
94.1
94.8
94.7
4k
93.8
93.9
87.4
93.0
3k5:6k
8k
94.5
95.3
86.6
88.8
6k11:2k
16k
95.2
95.7
91.5
94.2
11:8k22:4k
31.5k
90.0
90.3
88.8
90.1
23:6k40k
Fig. 10 Frequency response comparison in individual central frequency of
type Ca.
Note: Bold letters are meaning larger than original.
However, C3a and C4a coatings were only partial.
The Young’s modulus for a partial coating will be
inevitably lower than that for a whole-area coating.
Therefore, the change in fo reduced along with the
reduction in coating area. It is reasonable that fo will
increase with a small increase in the Young’s modulus.
Essentially, they are well matched. C4a sputter only a
‘‘dome’’-shaped part. Theoretically, it should not affect
the initial fo .
1 t1 þ 2 t2
c ¼
ð3Þ
t1 þ t2
E2
n¼
ð4Þ
E1
E1 I 1 þ E1 I 2
Ec ¼
ð5Þ
I0
Where
c : Compound density
t1 , t2 : Thickness of raw and coated materials,
respectively
I1 , I2 : Moment of inertia of raw and coated materials, respectively
E1 , E2 : Young’s modulus of raw and coated materials,
respectively
Ec : Compound Young’s modulus
I 0 : Moment of inertia of real shape at its neutral
axis.
(7) The purpose of our research was to improve the
performance of mini loudspeakers, whose low frequency response is well known to be poor. The lower limit
of frequency in the study was 100 Hz. To evaluate
performance across every range of frequency, the study
divided the frequencies into nine bands, in high
frequency, intermediate frequency, and low frequency
ranges, as shown in Table 4. The band of the center
frequency, fc , was defined the same as the Acoustics.
The value of each band frequency response (sound
pressure level) adopts its arithmetic mean so as to avoid
the interference of noise and ensure objectivity, as in
Fig. 10.
A. This type of loudspeaker cannot have acoustic
fidelity at a frequency lower than 714.4 Hz
Fig. 11 Frequency response curve of C3a diaphragm.
( fo ). Below 714.4 Hz, the frequency response
decreased by 0:73:8 dB on average (indicated by the curve shift to the right). This
is because fo increases with an increase in
the Young’s modulus relative to diaphragm
sputtering.
B. Except in the case of the C3a and C4a
resonance frequencies at 4 kHz. The effect of
sputtering was greater at frequencies between
750 Hz and 5.6 kHz, where the increases were
from 0.1 to 1.4 dB.
C. Except for the increase in frequency response
in the case of C2a from 0.3 to 0.8 dB, the highfrequency range was generally ineffective in
improving frequency response. First, for C3a,
a DLC film with a high Young’s modulus was
deposited on the suspending edge, which was
likely to increase the stiffness (K) of the
vibration system, as shown in Fig. 11. The
result was not good as we had expected.
Second, for C4a at 3.5 kHz and between
5 kHz and 9 kHz, the 2nd and 3rd harmonics
caused a large distortion because of the
partition vibration, as shown in Fig. 12.
Above 31.5 kHz, the frequency response of
uncoated diaphragm was decrease faster than
that of coated diaphragms.
Improvement in the Frequency Response of Loudspeakers by Using Diamond-Like Carbon Film Coatings
1971
the properties of DLC films. These results also offer useful
parameters for devising modern applications of acoustic
wave devices.
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Fig. 12 Distortion curve of C4a diaphragm.
4.
Conclusions
In this study, DLC films were successfully deposited on
polymer substrates at a low temperature (around 100 C) by
RF magnetron sputtering. The physical behavior of DLC/PEI
structures could be improved to bring their average frequency
response values up to 0:21:2 dB, their ID =IG ratio up to
2.27, and their roughness to less than 1.21 nm (Ra). At
frequencies over and above 20 kHz, which is beyond human
hearing, a remarkable improvement is expected, because this
frequency is related to auditory compliance of human.
Moreover, the C2a coating pattern was found to be optimum
for sputtering over the whole area of the substrate, simplifying the manufacturing process, and improving yield. On the
basis of the results of this study, we validated that it was
practicable to sputter DLC thin films onto PEI diaphragms for
commercial processing, with the aim of improving highfrequency response. The investigations reported here have
highlighted the importance of RF power deposition for tuning
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