Micro Receiver Analysis

Micro Receiver Analysis
The following 15mm example specifications are given by the customer:
Diaphragm Material
Properties:
Young’s modulus : 620 kg/mm2
Poisson Ratio
: 0.24
Density
: 1.36 g/cm3
Thickness
: 25mm
Diaphragm mass : 0.0058 g
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FINE simulations
The following is a short presentation of the results of the small 15mm micro speaker
(receiver) analysis in FINECone, FINEMotor and FINEBox.
An acoustical simulation is very helpful to the design engineer. The acoustic FINECone
simulation can help to decide the cone thickness, cone profile and material for obtaining
the best frequency response and resonance. Further the Voice Coil and magnet system
can be designed and optimized for sensitivity and linearity using FINEMotor. Finally
FINEBox can be used to find the best rear and front volumes plus power rating and
temperatures.
FINECone Simulation
The fastest and best ways is to first measure a similar speaker and use that response as a
guide for the FINECone simulation. Then it is very easy and fast to change for example
the cone thickness and see which thickness gives the best response.
The measured response is normally required to obtain correct values for impedance,
damping, stiffness of glue joints and special features like pattern in the cone. Even the
stiffness (Young’s modulus) may be changed from table- or measured values to increase
the accuracy. Methods how to do this is described in the FINECone Manual.
The FINECone simulation is changed to the standard 894mV input (100mW/8ohms) at
10cm distance. This brings up the SPL to about 85 dB as measured by the customer.
First the geometry is entered into FINECone using a DXF file, which can be created in any
CAD program. The file is automatically checked for errors in the input editor.
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Then we define the material specs for the segments. Below is shown the centre dome
spec., where we have entered the specs already known. Note that the damping is set to a
low value in order to see break-up resonances in the response.
In case the material parameters are not known, we can open the built-in database and
select a suitable material containing all data:
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The user can also add new or modify materials to the database.
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Above is shown the simulated response from FINECone, which is close to the measured
response from the customer. The top at ~1000 Hz is the main resonance Fs, which
corresponds with the simulated impedance curve below.
The peaks and dips in the frequency response from 8-20 kHz are caused by break-up in
the diaphragm (cone). Below is shown how the centre dome is breaking up at 10522 Hz.
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The next picture illustrates how to simulate a pattern in the diaphragm. But there is little
effect on this 15mm because most of the break-up is in the center dome part.
Now we have a model that is known and from here we can change the parameters to see
what we need to change to get the best result. Instead of having to build a prototype
receiver and experiment with that, we can make changes in seconds and see if we move
in the right direction.
Next paragraph is an analysis of an existing 15mm micro receiver motor. Following an
improved motor version is developed using the FINE programs.
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38mm Headphone Example
A 38mm headphone has been simulated in FINECone. The first break-up is just above 3
kHz, which is seen on the frequency response on the next page. The mode shape at this
frequency is shown below. There is considerable break-up in the middle of the large
surround.
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Now a number of small holes with damping have been added to the rear side of the
surround. The effect is added moving mass, which lowers the SPL and resonance Fs, but
also reduces the break-up.
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Finally we can simulate the addition of a pattern on the outer (surround) part of the
diaphragm:
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FINEMotor Simulation.
Further the 15mm Neodymium motor system has been modeled in FINEMotor. The
magnet material is Neodymium N42. The DCR resistance is 7.0 ohms (Z nominal imp= 8
ohms). The winding width in FINEMotor was very close to the actual 1.05mm when a
stretch of 5% was set. (The winding is normally stretched some % during winding)
Note that the magnet system has a very thin top plate, which is saturated, indicated by Bt
=2.39T. (Bt must be <2T)
And the Voice Coil is not placed symmetrically in the air gap, but offset 0.25mm up. The
BL(x) curve shows how the force factor BL varies when moving up through the air gap.
The ideal BL(x) curve is flat and symmetrical, but in this case it is NOT symmetrical. The
usable range for getting low distortion is when BL is higher that 82% of the maximum
value. FINEMotor shows that as the green range of BL(x). Here we get Xmax (up)=
0.63mm, but only Xmax (down)= 0.16mm, which is quite bad. In other words the usable
Xmax is only 0.16mm, where we would have expected 0.37mm!
First we move the Voice coil by dragging with the left mouse button. This makes the BL(x)
curve quite symmetrical. Now Xmax=0.35 both up and down. Next we need to increase
the thickness of the top plate to avoid saturation. In the simulation below the top plate
has been increased from 0.3 to 0.4mm. The sensitivity has increased about 1dB and Bt is
still a little too high (2.17T).
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In such cases it may be a good idea to do a separate magnetic simulation in another FEM
program. On the next page is shown a FEMM simulation with the 0.4mm top plate. The
top plate is just saturated, and the back plate must be thicker to avoid saturation here.
Now the flux profile (along the red line) is imported into FINEMotor, see below. Many
import formats are supported and LOUDSOFT recommends the FEMM program
(Freeware).
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The imported flux profile is shown in the lower right corner below. The profile has more
flux outside the gap than normally, because of the dimensions and the saturated top
plate. Therefore the Xmax is actually 0.40mm, slightly larger than the 0.35mm predicted
before.
There is another way to obtain more SPL from the micro speaker: By placing a second
neodymium magnet (magnetized opposite) on top of the top plate, some of the leakage
flux is forced back into the air gap.
However this solution requires some available space below the center of the diaphragm,
which is not possible in this case.
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Finally we may improve the 15mm micro receiver by using Ferrofluid in the air gap. There
are 3 significant advantages by doing so:
1. The power handling is increased considerably due to the cooling with the Ferrofluid
in the air gap.
2. The Ferrofluid can be selected to add much damping at resonance (Fs), which
prevents distortion and increases power handling.
3. The air gap can be made smaller, which will increase SPL and improve cooling.
The next picture shows the exact volume of Ferrofluid to be dispensed in the air gap, the
saturation strength, and viscosity.
More info about Ferrofluid can be found at www.ferrotec.com
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The Ferrofluid is specified (in red). First is the exact volume as given by FerroTec, and
then the necessary magnetic saturation strength and finally the chosen viscosity.
The sensitivity was increased due to the narrow gap which is possible with Ferrofluid, and
the mechanical damping was increased due to the damping from the Ferrofluid.
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Micro Loudspeaker / Receiver Box Design
Micro loudspeakers and receivers can be designed in FINEMotor2008 and imported into FINEBox,
where the acoustic loading / box volume and tuning can be simulated.
Figure 4: 15mm micro speaker in closed___/Band pass___/ damped InterPort___
Now we will start a 15mm box design by importing a FINEMotor file (with T/S parameters and
thermal data) directly into FINEBox by pressing the “Read Unit” button.
(Fig. 5) defines the additional information, first is the distance from winding to diaphragm, which
here is 0, since the VC is glued directly to the diaphragm. The second number is the thermal
conductivity, which here is the lower number 0.45 Wm/K for isolating materials. The linear
excursion Xmax=0.276mm is also imported.
Figure 5: Thermal Info Input
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Fig. 6 shows the complete driver data, imported from FINEMotor. The thermal time constants of
the VC and motor are automatically simulated.
Figure 6: Complete 15mm micro speaker data imported from FINEMotor
First the 15mm micro speaker/receiver unit is put it a closed box volume of 0.1 L (100ccm) by
selecting the upper left button “Closed Box” and adjusting the (Front-) volume to 0.100 L by rolling
the mouse wheel. This is shown as the blue curve in Fig.4 (Button #2).
Figure 7: FINEBox Acoustic Loadings
The blue curve (#2) has an impedance peak close to 400 Hz, which is the resonance Fs. The input
voltage was adjusted to give an Xmax excursion of 0.28mm, (= Xmlin: max excursion with Voice
Coil still in the gap). This gives a max SPL of 81dB at 0.1m defined by the frequency range
indicated by the green line. See also later Fig.10.
In contrast the red curve #3 is a Bandpass design, with a small hole (port) in front of the speaker.
This port is tuned to 5000 Hz, after which the response drops at higher frequencies. Again the
input voltage was adjusted to give a max excursion of 0.28mm, giving a max SPL of ~83.7 dB at
0.1m. However there is a very large peak at 5000 Hz.
Choosing the InterPort option (Figs.4 & 7) and adjusting the InterPort Q to 0.9, (Fig. 8) brings down
the peak and gives a quite flat Bandpass response. The high damping (lower Q) is made by
covering the (Inter-) port with a cloth or felt, which will pass air but add damping. Actually the front
port can be damped in the same way. Max SPL (2-5 kHz) is dB.
The VC and magnet temperature is in the upper right picture. The VC is at 28.9C which is no
problem. See the next two sections regarding high power simulations.
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Figure 8: Setting of Port Q and damping
The ports can be changed by modifying the port diameters (Fig. 9), and the length will
automatically be simulated according to the chosen tuning frequency. A flange (trumpet) can
reduce port noise/whistling.
Figure 9: Change of Port diameters and simulated lengths
Figure 10: Excursion of 15mm closed/Band pass/InterPort from Fig. 4
Fig. 10 shows the VC excursion of the 3 designs, where the input was set to produce 0.28mm
(Xmlin) at the resonance frequency (Fs) in the box. Because the excursion is increased at low
frequencies, the design with the higher box resonance (green #4) can produce a higher SPL in the
pass band. In order to prevent problems, it is advisable to insert a High Pass filter to limit the low
frequencies below Fs.
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