Moulton Lectures
On
Electro-Acoustics
|
Lecture
29 Central Armature Transducer 1 |
Presented
by:
Dave
L Moulton
Assisted
by
David Poulten and James Burt
Location: Racal Acoustics Limited (UK)
Date: 05-May-2004
Background
We now move on to another type of transducer which
makes use of the variable Reluctance flux gap as part of its motion generator.
This type is often called the Central Armature Transducer, and is also known as
the Moving Iron Transducer. The mechanics behind the Central Armature is
significantly different to that of the Rocking Armature discussed in lecture
28. As with Lecture 28, I want to focus this lecture on the mechanics and
magnetics of the device, rather than the acoustics, the basics of which will be
covered in another lecture.
Racal Acoustics still manufactures central armature
transducers and have made many process improvements over the years.
Assistance with this lecture
Once again I have called upon the valued experience of my two work colleagues David Poulten and James Burt, to assist with this lecture by showing examples of the central armature build process, and to give an insight into the various test and setting methods.
Content
This lecture will be dedicated to the
electro-magneto-mechanical construction of a Central Armature transducer. A
separate lecture will be held to explain some of the electro-acoustic
properties of the transducer. I intend this lecture to be very descriptive, and
not contain any mathematics. Thus in this lecture I will cover the following.
·
Outline the electro-magneto-mechanical construction.
·
Explain the mechanical force balance.
·
Explain the criteria for setting the acoustic
performance
·
Show a typical frequency response
On With The Lecture
Firstly I would like to build up a simplified construction showing the fundamental elements of a central armature transducer.
29.1 The Magnet , Pole piece and Coil Assembly
Consider a ring Magnet, polarized as shown in figure 29.1.
Figure 29.1
Now consider a pole piece assembly (Also known as a Yoke) made from a high permeability material such as radiometal.
Figure 29.2
Consider a coil of thin insulated wire wound onto the pole piece.
Figure 29.3
Depending on the intended impedance of the transducer, many hundreds, even thousands of turns can be wound onto the centre of the pole piece.
We now bring the the two assemblies together, such that the magnet is attached to the pole piece assembly, we now have what is commonly known as the motor unit, see figure 29.4 below:
Figure 29.4
Looking at this arrangement as a cross section:
Figure 29.5
At this point I think it is a good idea to illustrate the fundamental difference in the magnetic assemblies for both the Central Armature and Rocking Armature transducers. Figure 29.6 illustrates both assemblies side by side:
Figure 29.6
Normally the Pole Piece and Magnet assembly are lapped and polished to ensure that the upper surfaces are perfectly flat and free from contaminants such as dust and grease.
29.2 Fixing
the motor unit into an assembly ring.
The motor unit is soldered into a Brass assembly ring:
Figure 29.7
The combined assembly looks like that shown in figure 29.8 below:
Figure 29.8
29.3 Connection of a terminal block to the underside
of the motor unit.
Figure 29.9
29.4 Cross
sectional view of the assembly so far.
When all of the parts are brought together, we end up with the cross section assembly viewed as shown in figure 29.10.
Figure 29.10
29.5 Adding the Diaphragm and Armature
We are now at the stage where the Central Armature gets its name. The diaphragm is made from a stiff material, usually formed by being punched out of a sheet of spring steel. The diaphragm usually has a small bleed hole pierced, or laser drilled through it. To ensure the correct transducer frequency response it is very important that the diaphragm is completely flat and undistorted.
A circular armature, usually made from a high permeability material like Permendur is precision welded onto the diaphragm.
Figure 29.11
Our cross sectional view of the transducer can now be improved to that shown in figure 29.12, below:
The whole assembly is designed so that the diaphragm sits very precisely into the Assembly ring. The circumference edge of the diaphragm rests on a small inner ridge. This acts as a kind of circular pivot point for the diaphragm.
Figure 29.12
29.6 The
opposing forces of the magnetic air gap Stiffness and the diaphragm mechanical Stiffness
Unlike in the Rocking Armature transducer, where the stiffness of the diaphragm plays no part, in the Central Armature the diaphragm stiffness is critical to the whole operation of the device. Normally, the magnet assembly is fluxed to saturation, thus providing the maximum air gap force. This air gap force is usually much greater than the mechanical restoring force in the diaphragm, and so the diaphragm and armature collapse onto the pole piece. At this stage the magnetic forces are quite considerable and it can sometimes be almost impossible to separate the diaphragm from the pole piece by hand. However, for the moment this is expected and will be dealt with during the impedance setting procedure.
Figure 29.13
Magnetic Forces >> Diaphragm Restoring Force
29.7 Adding
A Spacer and Rear Case Assembly.
Before the force balance problem is address, we can go ahead and complete the build of the transducer. Firstly we must add in a rear case assembly and a solid spacer ring. See figure 29.14 below:
Figure 29.14
The cross sectional view now looks like:
Figure 29.15
29.8 Adding
the Front Cap, Spacer and Membrane
The construction is now very nearly complete. All that is required is the addition of a front cap, a sealing ring and a membrane. A lot of the acoustic performance depends on the acoustic characteristic of the cavity behind the cap and the holes in the cap. The membrane is usually added for environmental protection, however it also has an affect on the frequency response profile of the transducer.
Figure 29.17
Figure 29.18
29.9 Setting The Gap
Now that we have the basic construction of a central armature transducer, we need to reduce the magnetic field strength of the permanent ring magnet, so as to set an appropriate air gap between the diaphragm and the pole piece.
Further reduction in the magnetic field strength will weaken the force across the gap. The result will be that the gap will grow as the magnetic flux density across the gap reduces. For a very small range of gap widths, the magnetic, mechanical forces will remain balanced.
Remember the force across the gap is non-linear with B-field, so the magnetic restoring force per unit displacement (magnetic stiffness) is also non- linear. As the diaphragm is able to relax back towards its original equilibrium position (rest state with no forced flexing), its restoring force will also change, the linearity of the diaphragm restoring force depends a lot on the type of material used.
If the magnetic field strength is continually reduced, eventually the gap will grow such that the mechanical restoring force will be greater than the magnetic force. When this occurs the diaphragm will relax right back to it equilibrium state, and there will be no force holding the two together, thus the diaphragm will no longer be influenced by the magnet and pole piece.
Clearly we do not want to de-flux the magnet too much, but just enough to open up an air gap big enough to allow for adequate linear movement of the diaphragm, without it coming into contact with the pole piece (Pole over).
There is an optimum gap setting where the balance of forces is at its most stable and linear diaphragm motion is at its peak amplitude. When this occurs the mechanical system (including the effect of acoustic loading) and the magnetic systems are perfectly tuned.
We can set the gap for a perfectly tuned system by applying an external demagnetising field to the transducer and at the same time monitoring the electrical impedance across the terminals.
For a 1kHz sinusoidal signal applied to the transducer terminals, the impedance will reach a maximum when the magnet has been demagnetised to the optimum field strength. This is normally achieved in practise by monitoring the impedance level, while at the same time repeating a series of demagnetisation sweeps.
Figure 29.19
The impedance is also affected by the acoustic loading on the diaphragm, this includes the affects of the acoustic impedance of the front and back cavities.
A Typical gap setting at optimum impedance could be in the order of 90 microns, which is thinner than the hair on your head.
Figure 29.19
Normally the transducer impedance is set to be just below the peak by applying a little more demagnetisation beyond the optimum, this helps reduce the likelihood of pole over when the transducer is driven with high input power (10mW to 100mW), or suffers physical impacts
The range for diaphragm linearity is only restricted to a few microns of movement, thus at input powers beyond 10mW the Total Harmonic Distortion (THD) starts to rise significantly.
29.10 Transducer Frequency Response Profile and
sensitivity
Like the Rocking armature transducer the sensitivity as an earphone is normally measured for a constant input power, usually 1mW, or a constant voltage, usually 1Vrms. The graph below shows a typical frequency response profile of a Racal Acoustics 19575 (300 Ohm) impedance transducer. The transducer is measured on a B&K artificial ear with an appropriate coupler.
Typical Sensitivity at 1kHz is 124dBSPL/mW
A comparison between a 300 Ohm Central Armature (Racal 19575/1/BA) and a 300 Ohm Rocking Armature (Racal 27160) is shown in figure 29.21 below:
The radiation impedance of the Central Armature tends to be higher than that of the Rocking Armature. Thus it tends to get used a lot in closed cavity earshells as the main earphone driver, where it is more efficient than the Rocking armature.
Figure 29.20
Figure 29.21
End of Lecture