ELECTRONICS
APPLICATION
NOTE AN-6
FerrofluidTM Damped ED Receivers
Introduction
A new damping technique has been
developed that provides adjustable damping
without a screen. Ferrofluid injected into
the magnetic gap of the receiver’s motor
acts as a viscous damper to reduce the
motion of the armature at resonance (Figure
1). The fluid is a suspension of microscopic
magnetic particles (Fe3O4) in viscous oil that
is retained by the magnetic field in the
receiver’s motor.
Fluid-damped Response
appropriate fluid volume. Figure 2 on the
next page shows that the range of damping
options extends from as little as 2dB up to
critically damped (i.e., delta peak ± 0.5dB
relative to 1 kHz). The peaks in the
response are reduced without affecting the
nominal sensitivity at 500Hz. Figure 3 (next
page) shows the same receivers under
constant current drive conditions.
“Damping” refers to the change in the
mechanical resonance, which is the first
peak in the frequency response with shorter
ITE tubing (i.e., the peak between 2kHz and
3kHz voltage drive for the ED receiver).
The amount of damping can be adjusted
over a wide range by selecting the
Figure 1: Cross-section of the Knowles FED receiver (i.e., ferrofluid-damped ED receiver)
application note
Ferrofluid-Damped ED Receiver
120
Constant voltage drive with ITE tubing
(10mm x 1mm) into 2cc Coupler
115
Sensitivity (dB SPL)
110
105
100
95
90
delta peak = 8dB (undamped)
delta peak = 6dB (2dB damping)
delta peak = 4dB (4dB damping)
85
delta peak = 3dB (5dB damping)
delta peak = -0.5dB (8.5dB damping)
80
100
10000
1000
Frequency (Hz)
Figure 2: Frequency response curves under constant voltage drive conditions
for fluid-damped FED receivers and a response curve for an undamped ED
receiver. Labels in legend are the delta peak values.
Ferrofluid-Damped ED Receiver
120
Constant current drive with ITE tubing
(10mm x 1mm) into 2cc Coupler
115
Sensitivity (dB SPL)
110
105
100
95
90
delta peak = 13dB (undamped)
delta peak = 10dB (3dB damping)
delta peak = 8dB (5dB damping)
85
delta peak = 6dB (7dB damping)
delta peak = 3dB (10dB damping)
80
1000
1000
10000
Frequency (Hz)
Figure 3: Frequency response curves under constant current drive conditions
for fluid-damped FED receivers and a response curve for an undamped ED
receiver. Labels in legend are the delta peak values.
Damping is calculated by subtracting the
damped delta peak from the undamped
delta peak and is specified as “dB damping
relative to X”, where X = 8dB constant
voltage drive for the ED receivers (see
Equations 1 and 2).
Delta Peak = Peak Sensitivity (dB SPL) - Sensitivity (dB SPL) @ 1kHz
(1)
dB Damping = Delta Peakundamped - Delta Peakdamped
(2)
Knowles specifies fluid damping under test
conditions that include ITE or BTE tubing
connected to a 2cm³ coupler and constant
voltage drive. The amount of damping will
2
be similar under different acoustic loads
(i.e., various tubing lengths and 2cm³ or
Zwislocki-type couplers). Ferrofluid can
dramatically reduce the mechanical
resonance and the tubing resonances
above the mechanical peak (i.e., > 2kHz).
The amount of peak damping is shown in
Figure 4 below for the FED receiver
connected to ITE tubing. The second peak
(tubing resonance) falls off much faster than
the mechanical peak. However, when using
longer tubing (i.e., BTE tubing), the fluid is
not as effective in damping the tubing
resonance below the mechanical peak (i.e.,
peak at ~1.3kHz).
barometric relief), and Type III (modified
barometric relief). For ITE and CIC
applications, eliminating a screen in the port
tube is desired because of potential
problems due to wax build-up. Ferrofluid
can be substituted for screen damping.
Figure 5, on the next page, shows a
magnified view of the peaks for several
types of damping when ITE tubing is used.
For BTE applications, the multiple resonant
peaks created by the longer tubing are
affected to different degrees by a screen
(Type I), a diaphragm pierce (Type III), and
ferrofluid. Ferrofluid alone is not as
effective as Type II in reducing the first
(acoustic) peak in the BTE response curve
(i.e., peak near 1.3kHz). A combination of
fluid damping and Type III damping is
recommended to decrease all of the peaks
when long tubing is used (Figure 6, next
page).
Damping Options
The choice of damping method depends on
the application. Until recently there were
three damping options available: Type I
(screen), Type II (screen + modified
Peak Damping
0
Constant voltage drive with ITE tubing
(10mm x 1mm) into 2cc Coupler
2
Damping (dB)
4
6
8
10
Mechanical resonance (2-3kHz)
First tubing resonance (4-5kHz)
12
0dB
undamped
1dB
2dB
3dB
4dB
5dB
6dB
Damping (dB)
Figure 4: Damping for the first and second peaks in the frequency response
of the FED receiver connected to ITE tubing and a 2cm3 coupler under
constant voltage drive conditions.
3
7dB
8dB
9dB
critically
damped
ITE Frequency Response
120
Constant voltage drive with ITE tubing
(10mm x 1mm) into 2cc Coupler
Sensitivity (dB SPL)
115
110
105
100
95
undamped
Type I
ferrofluid
Type II
90
1000
10000
Frequency (Hz)
Figure 5: Frequency response of damped and undamped ED receivers
connected to ITE tubing and a 2cm3 coupler cavity under constant
voltage drive conditions (magnified view).
BTE Frequency Response
125
Constant voltage drive with BTE tubing (8mm x 1mm + 28mm x
1..5mm + 25mm x 2mm + 18mm x 3mm) into 2cc coupler
Sensitivity (dB SPL)
120
115
110
105
100
undamped
ferrofluid + Type III
ferrofluid
Type II
95
1000
10000
Frequency (Hz)
Figure 6: Frequency response of damped and undamped ED receivers
connected to BTE tubing and a 2cm3 coupler cavity under constant
voltage drive conditions (magnified view).
4
Hot / Cold Response Data
Improved Shock Resistance
The viscosity of the ferrofluid changes with
temperature and will affect the frequency
response. At higher temperatures there will
be slightly less damping. At lower
temperatures, the sensitivity of the receiver
will be reduced. However, the receiver
recovers as soon as it is warmed up again.
For example, Figure 7 shows the data for
an FED receiver at the limits of the
operating range specifications (0°C and
63°C), room temperature (25°C), and body
temperature (37°C). There is about a 1dB
increase in the delta peak at body
temperature. At lower temperatures, the
stiffening of the fluid decreases the output of
the receiver by 2dB.
The ferrofluid-damped receivers have
improved shock protection compared to
standard receivers. Furthermore, there is
no detectable fluid loss even under the most
extreme conditions of shock testing. The
data in Figure 8 at the top of page 6
illustrates the improved shock resistance of
the fluid-damped ED receivers (FED
receivers). Samples were oriented cover up
and cover down and were dropped at
progressively increasing heights starting at
12.7cm (5 inches) up to 196cm (77 inches).
This is equivalent to decelerations of 5,000g
to 20,000g.
Temperature Variation
120
Constant current drive with ITE tubing
(10mm x 1mm) into 2cc Coupler
115
Sensitivity (dB SPL)
110
105
100
95
90
85
25°C
37°C
63°C
0°C
80
100
1000
Frequency (Hz)
Figure 7: Frequency response of an FED receiver at
temperatures: 0°C, 25°C, 37°C, and 63°C.
5
10000
Shock Resistance
24 Undamped & 48 Ferrofluid-Damped Receivers
100
90
80
% Survival
70
60
50
40
30
20
ferrofluid-damped
10
undamped
0
0
5000
10000
15000
20000
Deceleration (g)
Figure 8: Percent of surviving ferrofluid-damped and undamped
ED receivers for increasing drop height.
voltage drive conditions. Distortion is
typically highest at 1/2 and 1/3 resonance
where the effects of the second harmonic
and third harmonic distortion are dominant.
Therefore, these trends represent the “worst
case”. The distortion at other frequencies
will be less.
Harmonic Distortion
The total harmonic distortion will rise with
increased damping (Figure 9). Figure 10 on
page 7 shows the average percent total
harmonic distortion (% THD) at 1/2 and 1/3
resonance versus damping under constant
Percent Total Harmonic Distortion vs. Frequency
100
6dB damping relative to 8dB peak
5dB damping relative to 8dB peak
% Total Harmonic Distortion
undamped response
10
1
0.1
100
1000
10000
Frequency (Hz)
Figure 9: Percent of the total harmonic distortion versus frequency for
FED receivers under constant current drive conditions.
6
Percent Total Harmonic Distortion vs. Damping
100
% THD @ 1/2 resonance
% Total Harmonic Distortion
% THD @ 1/3 resonance
10
1
0.1
0dB
1dB
2dB
3dB
4dB
5dB
6dB
7dB
8dB
9dB
Damping (dB)
Figure 10: Percent of total harmonic distortion at 1/2 and 1/3 resonance
versus damping under constant voltage drive conditions (FED receivers).
Impedance
7000
ED receiver connected to ITE tubing
(10mm x 1mm) into 2cc Coupler
8dB (undamped)
6dB (2dB damping)
6000
4dB (4dB damping)
Impedance (Ohms)
3dB (5dB damping)
-0.5dB (8.5dB damping)
5000
4000
3000
2000
1000
0
100
1000
10000
Frequency (Hz)
Figure 11: Impedance curves for fluid-damped and undamped ED receivers connected
to ITE tubing and a 2cm3 coupler. Labels in legend are the delta peak values under
constant voltage drive conditions.
APPLICATION NOTE
7
AN-6
Impedance
Fluid damping also reduces the peaks in the
impedance curve. Figure 11 on the
previous page shows the impedance of FED
receivers attached to ITE tubing and a 2cm³
coupler.
Conclusion
Ferrofluid provides a wide range of damping
values and eliminates the need for a
damping screen in the port tube. Fluid
damping has the additional benefit of
improved shock resistance. There is no
fluid loss even under the extreme conditions
of shock testing. The fluid viscosity will vary
with temperature causing the delta peak to
increase about 1dB at body temperature.
There is a slight rise in the nominal
harmonic distortion as damping is
increased.
Knowles Electronics LLC
1151 Maplewood Drive
Itasca, Illinois 60143
Phone: (630) 250-5100
Fax:
(630) 250-0575
www.knowles.com
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York Road, Burgess Hill
West Sussex, RH15 9TT, England
Phone: (44) 1444 235432
Fax:
(44) 1444 248724
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Fax:
(81) 3 3439 8822
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P.O. Box 1238
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2500, Australia
Phone: (61) 2 4226 6777
Fax:
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53 Pao Hsing Road
Hsin Tien City, Taipei, Taiwan
Republic of China
Phone: (886) 2 2911 4931
Fax:
(886) 2 2918 6868
NOTE: Specifications are subject to change without notice. The information on this Application Note reflects typical applications. Specific test specifications
defining each model are available by requesting Outline Drawing Sheets 1.1 and Performance Specifications Sheets 2.1 of that model number. Knowles’
responsibility is limited to compliance with the Outline Drawing and the Performance Specification application to the subject model at time of manufacture.
Knowles Electronics LLC
8
Issue 01 - 0403