- Clarasonic : Distributor of ROHACELL

COAXIAL AUDIO TRANSDUCR REVIEW AND CONCEPT DEVELOPMENT
BY STEVE MOWRY
The following discussion concerns the integration of a new electromagnetic induction
drive tweeter and a midrange into a new coaxial audio transducer concept assembly.
Four coaxial transducer implementations from high end loudspeakers will be reviewed
before the new integrated coaxial transducer concept is presented. The primary design
objectives were to minimize geometric diffraction potential and to achieve a high degree
of manufacturability by utilizing modular assembly procedures. The target for rate of
rejection for the final coaxial transducer assembly is no greater than 5% on the first
manufacturing pass, while the rejected transducer assemblies can be subsequently
reworked. Modular manufacturing streamlines incoming Quality Control and limits the
supply chain while adding value at the part and subassembly source that is typically
located in Asia. The induction drive tweeter diaphragm assembly was designed to be
simple but robust with a two plane suspension system. The midrange was also designed
to be simple yet robust and mechanically stable. The integration of the under-hung
moving coil midrange and the EMI tweeter facilitate a motor assembly with a low
reluctance AC flux path except for the midrange’s magnetic gap.
Four 3-way Systems with Coaxial Midrange and Tweeter
1. TAD REFERNCE ONE: Retail price US$80,000 / pair (passive loudspeaker) with Be
tweeter dome and Be midrange cone
http://data.manualslib.com/pdf3/56/5548/554761pioneer/tadr1.pdf?48bb13604da8282d45ac88272a8e54c2
FIGURE 1. EXPLODED AND SECTIONAL ILLUSTRATIONS OF THE
TAD CST COAXIAL TRANSDUCER
Dual NdFeB magnet integrated motor assembly with serial magnetic gaps would seem to
facilitate typical bottom-up gauged midrange assembly with quasi-butterfly tweeter
diaphragm self-alignment. We will see later, that assembly can be simplified by utilizing
modular assembly; however, this must be designed-in to the transducer assembly.
FIGURE X. THE TAD R1 CST
INTEGRATED COAXIAL TWEETER AND
MIDRANGE WITH ISOLATION POT
FIGURE X. THE TAD R1
LOUDSPEAKER
FIGURE X. TAD R1 WOOFER
2
2. KEF BLADE: Retail price US$30,000 / pair (passive loudspeaker) with Al tweeter
dome and Al mid cone
www.my-hiend.com/leoyeh/2009c/White%20Paper.pdf
FIGURE 3A. SECTIONAL
ILLUSTRATION OF THE KEF BLADE
MIDRANGE AND TWEETER COAXIAL
TRANSDUCERS
FIGURE 3B. SECTIONAL ILLUSTRATION
OF THE KEF BLADE WOOFER
FIGURE 4A. SECTIONAL
ILLUSTRATION OF THE KEF
BLADE TWEETER
GP Acoustics filed a US Design Patent
application for a Loudspeaker on 1 April
2011,
www.freepatentsonline.com/D653237.pdf.
FIGURE 4B. PICTURES OF THE KEF
BLADE LOUDSPEAKER
There is manufacturability issue related to gauging of the BLADE midrange voice coil.
The midrange soft-part shim gauge must be pulled with only a single plane half-roll
suspension attachment. The tweeter and the midrange are effectively individual
3
transducers mounted coaxially. The BLADE woofer uses 2 x spiders, which provides a
two plane suspension attachment before the soft-part gauge is pulled.
3. THIEL 3.7CS: Retail price US$14,000 / pair (passive loudspeaker) with Al tweeter
dome and Al mid diaphragm
www.thielaudio.com/wp-content/docs/CS3.7-prem-tech-paper.pdf
FIGURE 5. EXPLODED SECTIONAL AND ISO ILLUSTRATIONS OF
THE THIEL 3.7CS MIDRAGE AND COAXIAL TRANSDUCERS
There are manufacturability issues related to gauging. The soft-part shim gauge must be
pulled with only a single plane suspension attachment, the spider. A more repeatable and
robust topology would use 2 x spiders. See the BLADE woofer section illustration in
figure 3B. The butt joint of voice coil bobbin to convoluted diaphragm could use a
coupler to facilitate a more robust neck joint. The tweeter and midrange are effectively
individual transducers. The tweeter is shown is figure 6. The assembly procedure for
this transducer seems complicated and a bit mystical!
FIGURE 6. ISO ILLUSTRATION OF THE THIEL TWEETER ON THE
LEFT AND WITH THE FIVE MAGNET ARRAY ON THE RIGHT
The 3.7CS tweeter illustrated in figure 6 uses five (5) NdFeB magnets, four (4) radially
magnetized primary arc segment magnets and one (1) axially magnetize secondary ring
magnet.
4
FIGURE 7. SECTIONAL
ILLUSTRATION OF THE THIEL 3.7CS
WOOFERS
The THIEL 3.7CS woofer seems to be manufacture
unfriendly. The soft-part gauge must be pulled with
only the single spider serving to maintain the voice coil
alignment while the cone-coupler and surround
subassembly is attached. The topology is similar to a
woofer that I did for Philips in 1999. See figure 9
below. The Al diaphragm can be treated as a dust cap
and the cone is a coupler.
SECTION VIEW
FIGURE 8. PICTURE OF THE THIEL
3.7CS LOUDSPEAKER
TOP VIEW
Please note that the transducer
illustrated in figure 9 had a unit cost
of less than US$4.00 in 1999.
FIGURE 9. CONE COUPLED Al DOMEDUST CAP WOOFER ASSEMBLY
5
4. GENELEC 8260A: Retail price US$14,000 / pair (active loudspeaker system with
DSP hardware and software and with calibration microphone) with Al tweeter dome
and Al mid cone
www.genelec.com/documents/other/Genelec%208260A%20Technical%20Paper.pdf
GENELEC OY filed a US Utility Patent
application for a Nested Compound Loudspeaker
Drive Unit on 19 July 2002,
www.freepatentsonline.com/8660279.pdf .
FIGURE 10. PICTURE OF THE GENELEC 8260A LOUDSPEAKER
WITH A SECTIONAL DETAILS OF THE COAXIAL MIDRANGE AND
TWEETER TRANSDUCERS IN AN INTEGRAL ISOLATION POT
There are also manufacturability issues with the GENELEC 8260A. There is no
spider(s). The soft-part assembly maybe manually buzzed-in where the assembly is
moved around until no buzz, rub, or tic can be detected then the screws are tightened.
Unfortunately, this is a random process and in the limit, the relative location of the voice
coil is unknown. This is the only coaxial that uses quasi-butterfly self-alignment of the
midrange voice coil without any shim gauging. Only shim gauging can guarantee a
minimum clearance between the pole and the voice coil bobbin. Remember that
“Quality” is defined as the absence of variability. The 19 mm button tweeter is consistent
with a US$5.00 or less transducer in reasonable quantities.
In general, when it is difficult to identify the transducer assembly procedure, then
chances are it is difficult to assemble that transducer.
6
EMI TWEETER MODEL
PRIMARY
Ve
Rp(i ) Lp(i )
j2f t
Rem(i )
(1-
I(t,i )
Lem(i )
I(t,i )
N :1
SECONDARY B2 rs(i ):1
Ls(i )
Rs(i )
.
Blx(t,i )
0.5BlI(t,i )
.
x(t,i )
1:Sd
Rms(i) Mms Cms(i )
BlI(t,i )
NI(t,i )
FIGURE 11. NONLINEAR MODEL OF AN EMI TWEETER
Where V is the AC input voltage amplitude at the transducer terminals (V).
I is the input AC current amplitude (A).
t is time (s) and f is frequency (Hz or 1/s).
j is the unit-less complex operator, where j  1 .
i is complex AC current (A).
e is Euler’s Number.
Re is the DC resistance of the voice coil ().
Le is the AC self-inductance of the voice coil (H).
Reb is the additional AC resistance related to eddy current ().
Leb is the additional AC inductance related to eddy current ().
RP is the DC resistance of the primary coil ().
RS is the DC resistance of the secondary turn ().
LP is the AC self-inductance of the primary ().
LS is the AC self-inductance of the secondary ().
Rem is the real part of induction loss ().
Lem is the imaginary part of induction loss ().
T is temperature (C) where 20C is the ambient.
I is the induced AC current amplitude that couples to the secondary (A)
Where  is the loss factor, such that 0 <  < 1.0.
N is the number of turns in the primary coil.
rs is the radius of the secondary (m).
x is the position of the moving assembly (m).
x is the change in position with respect to time, dx/dt or the velocity (m/s).
B is the static DC magnetic flux density (T).
l is the effective of length of the windings, where l = N2pr (m).
.
B(t) is the AC magnetic flux density (T).
Rms is the mechanical loss term (s/m).
Mms is the moving mass including air load (kg).
Cms is the compliance of the suspension (mm/N).
Sd is the effective piston area of the diaphragm (m2).
p is the acoustic pressure (N/m2).
A major design challenge for an EMI tweeter is the induction efficiency (a < 1.0) and the
related sensitivity of the EMI tweeter implementation.
7
p(t,i )
FARADY’S, OHM’S, AND LENTZ’S LAWS
Faraday was credited with the discovery of EMI in 1831. He found that the time, t (s),
varying Electromotive Force (EMF), v(t) (V), produced around a closed path is
proportional to the number of turns of coil wire, N, and the rate of change of the AC
magnetic flux,  (t) (Tm2), through any surface bounded by that path but in the opposite
direction (), Lentz’s Law.
d (t)
v (t )  N
(V) Faraday’s Law of Induction
(1)
dt
d
 j 2 f (1/s)
Assume
(2)
dt
Then V   j 2f N (V)
(3)

The impedance of a coil that is related to inductance is as follows.
Z = j2 f Le  Where Le is the self-inductance of the coil (H).
V


 j 2π f N Where Le  N and V = ZI Ohm’s Law.
I
I
I
VDROP  j 2π f N
VINDUCED   j 2π f N
Then in the limit,
(4)
(5,6)
(7)
(8)
VDROP
 1 Lentz’s Law.
VINDUCED
(9)
EMI PATENTS
Sony has spent much time on R&D and they have several patents that discuss and
develop the theory for the above.
www.freepatentsonline.com/5062140.pdf
www.freepatentsonline.com/6904158.pdf
www.freepatentsonline.com/7016515.pdf
Klipsch also has two Euro Patents on EMI drivers.
www.freepatentsonline.com/EP0453130A2.pdf
www.freepatentsonline.com/EP0453130A3.pdf
Then there is Tannoy’s co-axial.
www.freepatentsonline.com/4965839.pdf
I found a US Patent issued to W. K. Volkers in 1950.
www.freepatentsonline.com/2494918.pdf
I found another US Patent issued to S. E. Karlsson et al. in 1950.
www.freepatentsonline.com/2621261.pdf
Finally, the oldest Patent that I found was issued to A. Nyman in 1927.
www.freepatentsonline.com/1643169.pdf
Wisdom Audio has the latest EMI Patent that is based on Marshall Leach et al. R&D.
www.freepatentsonline.com/US8009857.pdf
8
EMI PAPER
www.redrockacoustics.com/induction_%20drive_%20loudspeakers_alma.pdf
With regards to all the previous listed Patents, only US8009857 Bohlender et al.
describes EMI motor assemblies that contain low reluctance AC flux paths. Sony,
Klipsch, and Tannoy all utilize typical OD ring magnet motor topology (see figure 9
below); however, Tannoy’s transducer is an integrated coaxial where the low frequency
driver’s voice coil is also the primary coil for the EMI high frequency transducer. Then
in Tannoy’s case both the primary and secondary are suspended in the magnetic gap.
Although interesting, the resultant acoustic output is not really hi-fi. Tannoy’s
implementation seems more cost driven than performance driven.
MOVING COIL DRIVES
Cu SHORTING
Al WIRE
RING
VOICE COIL
Cu SHORTING
RING
Al WIRE
VOICE COIL
INDUCTION DRIVES
Cu WIRE
PRIMARY
HIGH
RELUCTANCE
(OPEN CIRCUIT)
ALUMINUM
SECONDARY
Cu WIRE
PRIMARY
ALUMINUM
SECONDARY
HIGH
RELUCTANCE
(OPEN CIRCUIT)
(SONY & KLIPSCH TOPOLOGY)
INTEGRATED HYBRID COAXIAL DRIVES
FIGURE 12. SECTIONAL ILLUSTRATIONS OF THE TYPICAL
MOVING COIL AND INDUCTION DRIVE TOPOLOGIES WITH AN
INTEGRATED HYBRID IMPLEMENTATION
The illustrations in figure 12 show three topologies. The induction drive section shows
the primary coil is held stationary, while the secondary (shorting ring) is suspended in the
magnetic gap. However, the moving coil drive section shows the shorting ring held
stationary, while the voice coil is suspended in the magnetic gap. The hybrid coaxial
topology shows an implementation of both moving coil and induction drives.
9
Equation 8 shows that the induced signal goes with frequency. This tells us two things.
Shorting rings are typically used in high end tweeters. Inductive dive works best for high
frequency transducers. In both cases, the load inductance is reduced or the load is less
reactive with either a shorting ring or induction drive.
SM Audio Coaxial Transducer Concept: Target retail price US$2,000 / pair with Be
tweeter dome and sandwich composite cone
SECTION
VIEW
BACK
VIEW
FRONT
VIEW
OD
Ø180 mm
NOMINAL
MIDRANGE
CONE-SURROUND
ASSEMBLY
VOICE COILSPIDERS
ASSEMBLY
WITH GAUGE
TWEETER
DIAPHRAGM
ASSEMBLY
EXPLODED
SECTION
VIEW
HARD-PART
ASSEMBLY
BASKETTERMINALS
ASSEMBLY
FIGURE 13. ILLUSTRATIONS OF THE SM AUDIO INTEGRATED
MIDRANGE AND TWEETER COAXIAL TRANSDUCER CONCEPT
A listing of module subassembly specifications and illustrations follow.
C.1
Basket materials / process / finish / terminals:
Glass filled Nylon / injection mold / none / push type midrange terminals attached
10
SECTION
VIEW
BACK
VIEW
FRONT
VIEW
FIGURE 14. ILLUSTARTIONS OF THE BASKET-TERMINALS ASSEMBLY
Glass filled Nylon was the preferred basket material for the following reasons.
1. Reduced transducer mass
2. Reduced basket cost
3. Reduced tooling cost
4. Reduced induction loss
5. Increased material damping
C.2 Hard-part assembly materials / assembly process:
The cup-pole and the gaps plate parts are machined Carpenter Chrome Core 13XP stainless steel or equivalent. The two secondary magnet keeper plates are
machined and/or blanked AISI 1010 annealed steel or better with zinc plate. The
primary tweeter coil is wound with round copper magnet wire on a Kapton HN
film bobbin/integral height gauge and terminated with push type terminals.
Nickel plated sintered neodymium iron boron magnets with ground surfaces are a
must. The assembly is adhesive bonded together with an alignment gauge and/or
fixture. Due to the complexity of this assembly, it should be purchased assembled
and magnetized.
11
SECTION VIEW
SHARED
PRIMARY
NdFeB
MAGNET
TWEETER
SECONDARY
NdFeB
MAGNET
AISI
1010
STEEL
AISI
1010
STEEL
FRONT
VIEW
CHROME
CORE 13-XP
LOW
RELUCTANCE
AC PATH
CHROME
CORE 13-XP
MIDRANGE
SECONDARY
NdFeB
MAGNET
BACK
VIEW
FIGURE 15. ILLUSTRATIONS OF THE HARD-PART ASSEMBLY WITH
THE TWEETER PRIMARY COIL ATTACHED AND TERMINATED
MATERIALS
MAXIMUM
PERMEABILITY
SATURATION
FLUX DENSITY
ELECTRICAL
CONDUCTIVITY
ANNEALED AISI 1010
2750m0
2.0 T
6.7 x 106 1/W/m
CHROME CORE 13-XP
3200m0
1.7 T
1.3 x 106 1/W/m
NdFeB MAGNET
~m0
NA
0.7 x 106 1/W/m
TABLE 1. MATERIAL PROPERTIES OF HARD-PARTS
C.3
Tweeter diaphragm materials / assembly process:
Acoustic grade Beryllium foil dome with unalloyed aluminum secondary,
acoustic grade poly-foam surround and Kapton JP film hot formed spider
mounted on a black glass filled Nylon frame / adhesive bond with alignment
gauge and fixture.
SECTION
FOAM
KAPTON JP VIEW SURROUND
SPIDER
Be
FOIL
DOME
FRONT
VIEW
GLASS
FILLED
NYLON
FRAME
Al 1199
SECONDARY
BACK
VIEW
FIGURE 16. ILLUSTRATIONS OF THE DIAPHRAGM ASSEMBLY
12
C.4
Voice coil – Spiders materials / assembly process:
Anodized aluminum foil bobbin with 15% copper clad aluminum round magnet
wire coil and Nomex paper collar – hot formed and trimmed cloth blend spiders
with molded nylon spacer / adhesive bond spiders to voice coil bobbin and spacer
ring with alignment fixture.
SECTION MIRROR
CLOTH
VIEW
Al FOIL
BOBBIN
FRONT
VIEW
BLEND
SPIDERS
Cu
CCAl
TINSELS
MAGNET
ATTACHED
WIRE
& TINNED
POLYMER
RING
BACK
VIEW
FIGURE 17. ILLUSTRATIONS OF THE MIDRANGE VOICE COILSPIDERS ASSEMBLY
C.5
Midrange sandwich cone materials / process:
Woven Kevlar fiber with epoxy skins on a Rohacell foam core with integral
thermally sealed poly-foam surrounds and aluminum alloy foil cone to voice coil
coupler / adhesive bond in heated mold and trim
13
SECTION VIEW
2xSOFT-FOAM
SURROUNDS
HARD-FOAM
CORE
Al FOIL
COUPLER
2xKEVLAR
FIBER-EPOXY
SKINS
FRONT
VIEW
BACK
VIEW
FIGURE 18. ILLUSTRATIONS OF THE SANDWICH COMPOSITE
CONE-SURROUND ASSEMBLY
Asian specialty transducer and/or part manufacturers can be identified and qualified to
make the five (5) transducer subassembly modules. Then final assembly of the modules
is convenient and can be made efficient and effective regardless of location, if and only
if, the geometry dimensioning and tolerancing are correct and well documented.
CLARASONIC in Thailand (www.clarasonic.com/) is presently working on the
development of the cone-surround assembly illustrated in figure 18.
INDUCTION DRIVE TWEETER DRIVER DESIGN CONSIDERATIONS
1.
2.
3.
4.
5.
6.
7.
Low reluctance AC flux path
Low moving mass with Be dome and 25mm ID Al secondary and no lead dress
Large dome 35mm OD
BETA booster secondary magnet
Utilize low conductivity ferromagnetic material to reduce induction loss
Eliminate typical electrically conductive materials where possible
Inherent loading of the tweeter diaphragm by the midrange cone geometry et al.
MOVING COIL DRIVE MIDRANGE DRIVER DESIGN CONSIDERATIONS
1.
2.
3.
4.
5.
6.
A stable and repeatable bottom-up gauged assembly procedure
Continuous geometric integration with tweeter and enclosure baffle
BETA booster complementary secondary magnet
Cu shorting ring to reduce inductance nonlinearity and tweeter crosstalk
Under-hung voice coil topology
Polymer basket
14
The resultant transducer concept is a no nonsense integrated midrange and tweeter
coaxial implementation that is well suited for manufacture by a small knowledge based
company. The intension is to implement loudspeakers utilizing one coaxial transducer
and one or more KOMODO 9 EM woofers per side.
SECTIONAL
VIEW
85 cm
NOMINAL
FIGURE 19. SECTIONAL ILLUSTATION OF THE LOUDSPEAKER’S
TRANSDUCER ARRAY WITH FRONT BAFFLE
15
The Author: Steve Mowry is in his 20th consecutive year of exclusively practicing audio
transducer engineering that all began within the BOSE transducer research group (TRES)
in mid 1995 after obtaining BS and MS degrees in Electrical Engineering with highest
distinction from URI. He has worked with audio transducer and system development
teams worldwide in diverse environments. With respect to transducer manufacturers and
part suppliers, Steve is conveniently stationed in Southeast Asia. He is currently an
independent transducer researcher and consultant to the loudspeaker industry with strong
interests in Design for Manufacture, Supply Chain Design, Documentation Control,
Concurrent Product Development, and Concurrent Project Management.
Steve Mowry
18 September 2014
16
THE DEVELOPMENT OF A DINOSAUR INTO DRAGONS
BY STEVE MOWRY
The following is a discussion of the long term development of a high
performance low frequency transducer topology and woofer implementation.
The focus of the transducer R&D during the development period was multimagnet symmetrical drive assemblies as a way to improve low frequency
transducer performance. Overall the developmental transducer topologies
remained relatively consistent with incremental improvements with regards
to the initial push-pull complementary symmetrical drive topology that was
implemented in 2001. The high moving mass of low frequency transducers
makes the typical cantilevered drive topologies a far cry from the ideal. The
motivating consideration for the cantilevered drive topology seems to be at
least in part convenience based. Two dimensional illustrations and linear
parameter models were recreated from engineering notebooks and show
examples of design milestones. Two high end manufactures’ low frequency
transducer implementations are also illustrated with approximated
dimensions. Common transducer topology elements can be observed within
all of these illustrations. Additionally, the perceived pros and cons of each
transducer implementation will be briefly discussed; however, the
fundamental mechanical advantages of complementary symmetric topology
should be at least somewhat obvious.
In 2001 my employer asked me to design a new 6 in woofer that was
intended to be a member of the “TM” (Top Most) product family. At that
time, P.Audio was manufacturing and marketing their TM-12 woofer. The
TM-12 was essentially reverse engineered by inspection of the Velodyne 12
(in) “Digital Drive” active subwoofer (http://velodyne.com/pdf/hgs/hgsx_series_datasheet.pdf) before I joined P.Audio’s transducer development
team. A-TON Thailand, P.Audio’s sister company was the manufacturer of
the Velodyne 12’s voice coil, at that time. Velodyne’s transducer
implementation utilized common sized hard ferrite magnets with open
tooling for much of the other parts. They used the popular NuWay large
progressive roll spider with open tooling. The primary basket was the
common “Venezuela” cast alloy basket, again with open tooling; however,
the secondary basket required custom tooling. The additional parts were
either selected and/or trimmed from open tooling parts or custom tooled.
Both the Velodyne 12 and the TM-12 were of significantly different
topology than anything that I had seen before. Finally there was a low
frequency transducer topology that seemed to be “correct”. I used a high
performance automotive analogy in code naming this topology as “Midengine”, with the “Wheelbase” defined as the peak to peak distance between
suspension components.
Being an electrical engineer by training, a complementary symmetrical
configuration was something that I was familiar with and the analogy to
modern electronics design was clear. I thought what would happen to their
performance if amplifiers where designed asymmetrically like cantilevered
drive audio transducers? The only complementary symmetrical drive
transducers that I could identify then were either some planar magnetic or
electrostatic panels. Their sonic characteristics are legendary but with
limited low end output. Figure 1 contains an illustration of the TM-12 and a
linear parameter model that were recreated from my engineering notebook.
In all the following transducer illustrations (figures 1, 2, 3, 4, 5, 6, 9, and 11)
the blue sections are the ferromagnetic low carbon steel parts and the cyan
sections are the permanent magnets.
Ø238mm
2xNuWay
Progressive
Spider
12in
Venezuela
Basket
2xMagnet
OD 200mm
ID 85mm
Thick 20mm
SECTIONAL
VIEW
A-TON
Voice Coil
TOP
VIEW
FIGURE 1. TM-12 aka RAPTOR 12 Transducer Assembly Drawing with
a LspCAD Linear Parameter Model
2
Whereas the Velodyne 12 was used in a closed loop servo control system
with an accelerometer on board the moving assembly, the TM-12 was
marketed as a subwoofer driver without the accelerometer and/or control
system. The TM-12’s parameters seemed to be suitable for car audio and
there was consumer interest for that application.
With respect to the LspCAD model in figure 1, it can be seen that mass and
stiffness were high, while the ratio of BETA/mass/cone area was moderate,
where b= (Bl)2/Re (N2/W). Using a single spider would reduce mass and
stiffness. Plus using a smaller voice coil diameter would reduce mass and
allow for a larger annular, less stiff, spider.
With that information under consideration and with the typical cost, package
size, and capability constraints, I designed an interesting small woofer. All
parts except the terminals were new and required simulation, design, and
documentation. HKS ABAQUS was used for soft-part design simulations,
including spider, surround, cone, and dust cap, while VECTOR FIELDS
OPERA was used for DC, AC, and Steady State Thermal motor design
simulations. P.Audio’s management licensed that software for me in early
2000 to facilitate new audio transducer development in SE Asia. These were
the same tools that were used at BOSE at that time. A transducer engineer is
really only as good as his tools and his attitude.
The primary customer for the TM-6 was MONACOR. They renamed this
woofer “RAPTOR 6” (www.monacor.co.uk/products/carfisubwoofer/vnr/104350/) and this woofer is still sold today. An illustration
of my transducer concept assembly from 2001 and a linear parameter
LspCAD model that were recreated from my engineering notebook are
shown in figure 2.
3
BOTTOM
VIEW
Ø120
VCID Ø26
2x23
2x8
30
17
SECTIONAL
VIEW
Ø118
TOP
VIEW
Dimensions are at nominal and in
millimeters (mm) unless specified otherwise.
FIGURE 2. TM-6 a.k.a. RAPTOR 6 Transducer Concept Assembly
Drawing with a LspCAD Linear Parameter Model
TM-6/RAPTOR 6 was well received by users especially in Europe. There
are still a few videos on YouTube that contain demonstrations of the large
displacement capability of this woofer. Here’s a link to an example,
www.youtube.com/watch?v=9o32OsgSyPc.
4
A detailed listing of RAPTOR 6 part specifications follows.
R.1
Gap plates material / process / finish:
Low carbon annealed steel / stamp or blank / black zinc plate
R.2
Magnets material / process / finish:
Hard ferrite / sintered with ground surfaces / none
R.3
Pole material / process / finish:
Low carbon annealed steel / cold forge or extrude with secondary
machining / black zinc plate
R.4
Primary basket material / process / finish:
Aluminum alloy / die cast with secondary machining / black paint
R.5
Secondary basket material / process / finish:
Aluminum alloy / die cast / black paint
R.6
Voice coil assembly:
2 x 2-layer heavy build copper magnet wire coils wound on a
polyimide film former with Nomex paper or equivalent collars and
braided copper tinsels attached and tinned
R.7
Spider material / process / color:
Treated cloth / hot form and trim / tan
R.8
Surround material / process:
NBR with carbon black or equivalent / injection mold
R.9
Cone material / process:
Treated paper / wet formed, semi-pressed and trim
R.10 Dust cap material / process:
Matte coated paper / wet formed, hot pressed, and trim
R.11 Terminals:
Push type
R.12 Fasteners:
Blackened steel hex head or equivalent
Sometime after leaving P.Audio to pursue independent transducer R&D, I
developed a strong interest in multi-magnet motor assemblies including but
not limited to the STEALLUS topology (STEALLUS, Part 1, Voice Coil,
Nov. 2007 and STEALLUS, Part 2, Voice Coil, Jan. 2008), www.pearlhifi.com/06_Lit_Archive/14_Books_Tech_Papers/Mowry_Steve/Steallus_M
otor_Design.pdf. Figure 3 contains a low frequency transducer
implementation example illustration of the STEALLUS topology.
5
Ø223
VCID
Ø64
4x12
43
2xQUASIINFINITE
GAP
2x40
SECTIONAL
VIEW
Ø218
TOP
VIEW
Dimensions are at nominal and in
millimeters (mm) unless specified otherwise.
FIGURE 3. STEALLUS 11 Woofer Concept Assembly Drawing
It should be clear that the RAPTOR 6 and the STEALLS 11 transducer
topologies are related with the voice coils positioned between the surround
and spider, mid-engine topology. STEALLS 11 began simply as an attempt
to implement the “best” low frequency transducer possible. In retrospect,
the resultant implementation could be considered an upgrade to the
RAPTOR topology; however, this was not intentional. The fundamental
difference between RAPTOR 6 and STEALLS 11 is related to the
respective motor topologies, whereas RAPTOR 6 utilizes a push-pull
topology, while STEALLUS 11 utilizes a simple stacked array of two
magnetic return-less assemblies spaced by a nonferrous material. By using a
spacer between the two magnetic assemblies, the resultant voice coil
6
configuration is Quasi-XBL2
(www.adireaudio.com/Files/XBL2TechPaper.pdf). The STEALLUS 11 is a
simplified minimalist low frequency transducer implementation with very
large and costly NdFeB magnets; however, performance potential is high for
this topology. If the magnetic assembly is purchased as a single assembled
unit and pre-magnetized, manufacture is just about as friendly as possible.
A detailed listing of STEALLUS 11 part specifications follows.
S.1
Gap plates material / process / finish:
Low carbon annealed steel / stamp or blank / black zinc plate
S.2
Magnets material / process / finish:
Neodymium Iron Boron / sintered with ground surfaces / nickel plate
S.3
Motor spacer material / process / finish:
Nonferrous stainless steel / cold forge or extrude with secondary
machining / none
S.4
Primary basket material / process / finish:
Aluminum alloy / die cast with secondary machining / black e-cote
S.5
Voice coil assembly:
8-layer heavy build aluminum magnet wire coils wound on a
polyimide film former with braided copper tinsels attached and tinned
S.6
Spider material / process / color:
Treated cloth blend / hot form and trim / tan
S.7
Surround material / process:
Polyether acoustic foam / hot form and trim
S.8
Cone material / process:
Carbon fiber-epoxy skins with Rohacell IG hard foam core or equivalent /
join in mold with adhesive and trim
S.9
Dust cap material / process:
Carbon fiber-epoxy skins with Rohacell IG hard foam core or equivalent /
join in mold with adhesive and trim
S.10 Terminals:
Push type
S.11 Fastener:
Blackened steel hex socket head or equivalent
7
Shortly after the STEALLUS discussion was published in Voice Coil, I
noticed that other high end loudspeaker manufacturers were also developing
interests in multi-magnet motor assemblies. Some of the top rated
loudspeakers had also begun to utilize multi-magnet motors. Figure 4
contains an assembly drawing with estimated dimensions that illustrates the
B&W 802D woofer’s topology, while figure 5 contains an assembly
drawing with estimated dimensions that illustrates the KEF BLADE
woofer’s topology. The transducers illustrated in figures 4 and 5 will serve
as points of reference for the purpose of a relative transducer evaluation.
The B&W 802D loudspeakers retail for US$15,000 / pair, while the KEF
BLADE loudspeakers retail for US$30,000 / pair. These are not active
loudspeakers; both systems are passive loudspeakers and require power
amplifiers.
The B&W 802D woofer has a topology that uses Neodymium Iron Boron
magnets and resembles the STEALLUS topology but with a magnetic return
path that is magnetically shorted at the very bottom of the motor,
www.bowers-wilkins.com/Discover/Discover/Technologies/Dual-MagnetBass-Motor-System.html. However, that’s about all that is similar. This is
obviously an asymmetric topology; the voice coil is overhung and
cantilevered; and the voice coil bobbin is large and very long. The spider
also seems undersized. There is a thick composite cone with a large dome
dust cap. All in all, this woofer appears to be an attempt to take the typical
low frequency transducer topology to a higher level of performance by using
limited motor drive symmetry.
8
Ø200mm
EMPTY
SECONDARY
GAP
SECTIONAL
VIEW
SHORTED
GAP
TOP
VIEW
FIGURE 4. B&W 802D Woofer Assembly Drawing
The KEF BLADE woofer shown in figure 5 uses opposed hard ferrite
magnets about a gap plate in a configuration not too different from B&W
802D woofer; however the inside secondary magnet has ferrous material on
only one surface, like a missing magnet keeper. This results in a quasiinfinite return with a very low magnet DC operating point for a magnet with
moderate thickness and thus a less than robust motor implementation. Cold
northern temperatures could cause some permanent demagnetization of that
secondary magnet. There is again obvious asymmetric topology with an
overhung large voice coil configuration.
9
Ø225mm
QUASIINFINITE
GAP
VENTED COUPLER
SECTIONAL
VIEW
SHORTED
GAP
ANALOGY
TOP
VIEW
FIGURE 5. KEF BLADE Woofer Assembly Drawing
With respect to figure 5, the KEF BLADE woofer’s soft-parts are quit
extraordinary. The inverted shallow dome diaphragm will have limited
geometric stiffness. So unless the diaphragm is a super material such
Beryllium or Silicon Carbide, there is little hope for a wide band piston
mode. Additionally the “Diving Board” diaphragm to surround termination
is of concern. Diaphragms typically begin to bend at the OD as frequency is
increased.
Two spiders double the effective spider stiffness and require an additional
adhesive neck joint. I believe that KEF uses two small spiders to stabilize
the voice coil location during assembly so the soft-part shim gauge can be
removed (pulled) before the cone-surround assembly is attached. This
allows elimination of the dust cap but displacement will be constrained by
the small spiders’ free length. The diaphragm is also the dust cap in this
case. When I was a transducer engineer at BOSE, there was a rumor that an
engineer was fired for implementing a reverse roll surround in a small
woofer. This was only a rumor; however, reverse roll surrounds were
simply banned at BOSE at that time.
10
KEF touts a “Vented Coupler” that couples the voice coil bobbin to the
diaphragm. Their claims are narrated in a short video that can be view at
YouTube, www.youtube.com/watch?v=CiAVX8_QWxI. Venting under the
dust cap or in this case diaphragm seems like a good idea. Several
manufactures implement similar venting with holes in the voice coil bobbin
or the cone/diaphragm. Care must be taken because holes in the voice coil
bobbin will encourage bending or in some cases buckling and/or yield.
Finally and obviously the KEF BLADE woofer’s transducer topology is
asymmetric and the voice coil is cantilevered. I just cannot understand the
design objectives for such a woofer that is used in a US$30,000 loudspeaker
system. Clearly my low frequency transducer design objectives are
inconsistent with KEF’s.
Having said that, the design objective for a new woofer concept was to
maximize electromagnetic, mechanical, and acoustical performance with a
hard ferrite magnet based motor assembly. Figure 6 contains the concept
transducer assembly drawing of KOMODO 9 and a simulated linear
parameter LspCAD model. All KOMODO 9 parts are unique except the
spider and terminals. The spider geometry is equivalent to the TM6/RAPTOR 6 spider but it is flipped over to facilitate attachment, the
bottom is now the top. It seems that I got the spider right back in 2001.
11
CENTER
(PRIMARY)
VOICE
COIL
BOTTOM
(SECONDARY)
VOICE
COIL
TOP
(SECONDARY)
VOICE
COIL
BOTTOM
VIEW
Ø180
23
2x25
25
4x4
2x12.5
VCID Ø26
SECTIONAL
VIEW
Ø118
TOP
VIEW
Dimensions are at nominal and in
millimeters (mm) unless specified otherwise.
Here’s Alan Babb’s US Utility Patent for an
asymmetric dive cantilevered single voice
coil implementation with multi-permanent
magnets, Multiple Magnetic Air Gap Motor,
filed 14 February 2011,
www.freepatentsonline.com/8781150.pdf.
FIGURE 6. KOMODO 9 Woofer Concept Assembly Drawing with a
LspCAD Linear Parameter Model
12
KOMODO 9 is another attempt to implement the “best” low frequency
transducer possible. Ironically, the resultant implementation can also be
considered an upgrade to the RAPTOR 6; however, again this was
unintentional. KOMODO 9 utilizes two of the push-pull type motors but in
a stacked symmetrical array such that a primary magnetic gap is
implemented along with two secondary magnetic gaps about that primary
magnetic gap.
Figure 7 contains a one half (right hand side) sectional contour plot of the
DC flux distribution simulation for the KOMODO 9 hard-part assembly.
The two magnets are magnetized in opposing directions. The primary and
secondary magnetic gaps show the focus of DC flux density. Notice that the
center pole carries only the DC flux from a single magnet in the stacked
topology mirror array. This facilitates a small diameter voice coil. This is a
critical design aspect and allows larger annular spider geometry. The bobbin
must be long to facilitate the axial stack-up. A large diameter voice coil
would thus be very high mass and would limit sensitivity.
FIGURE 7. VF OPERA DC FEA Flux Contour Plot of the KOMODO 9
Permanent Magnetic Assembly
Figure 8 contains a plot of a DC simulation of the nonlinear parameter Bl(x)
for the KOMODO 9 motor assembly; where B(x) is the position dependent
magnitude of the flux density (T); l is the length of the voice coil conductor
and l = N2pr (m) where N is the number of voice coil winding turns and r
is the radius of the voice coil. Then x is the position of the voice coil (m).
13
The data used to generate the Bl(x) plot shown is figure 8 was acquired
using proprietary voice coil emulating command files that run within the
Finite Element Model. The voice coil is segmented into three (3) segments
and the motor has three (3) magnetic gaps. The voice coil could be
considered to be a linear commutation; however, any switching must be
performed electronically. The obvious control system methodology is DSP
where each voice coil could have its own power amp.
FIGURE 8. KOMODO 9 Large Signal DC Simulation of Bl(x)
With regards to figure 8, the solid blue trace is the resultant DC magnetic
flux linkage to the primary voice coil. While the dashed blue traces are the
respective DC magnetic flux linkage to the two secondary voice coils. Then
the solid black trace is just the sum of the three blue traces or the three coils
driven with the same signal. The area under the black trace is then the work
done per ampere.
Wmax  I 
Xmax
-Xmax
Bl(x) dx (Nm)
Where I is the input AC current amplitude (A).
What I call the “motor bias” is controlled by the distance between the
primary and secondary voice coils. The bias for KOMODO 9 was selected
for maximum linearity with minimum ripple. It should be noted that the
solid blue trace related to the primary voice coil is typical for an overhung
voice coil motor topology and is inherently nonlinear. It should also be
noted that the roll-off slope of the black trace beyond the linear region (~
12.5 mm) helps to control excessive displacement. This phenomenon is the
result of the primary magnetic gap flux linkage plus the secondary anti14
magnetic gaps flux linkage. With regards to the primary voice coil (solid
blue trace), when |x| > 17 mm, then Bl < 0!
Then it can be observed that the resultant Bl(x) characteristic (the solid black
trace) resembles what would be expected from an under hung bias. With all
voice coil driven Bl(x) is linear and symmetrical about the rest position (x =
0) with just a bit of ripple. The objective is for 25 mm peak to peak
displacement with reasonably low distortion at the lowest frequencies, 30 50 Hz.
Several other bias options are convenient. For example, the magnetic
assemblies could be spaced similar to STEALLUS 11 and an enhanced
quasi-XBL2 bias or even enhanced under hung bias could be realized.
However, the length of the voice coil bobbin and the overall axial stack-up
would increase by the thickness of the spacer or additional gap plate
thickness.
KOMODO 9 is intended to be used in 3-way loudspeaker systems with
midrange and tweeter transducers. A detailed listing of KOMODO 9 part
specifications follows.
K.1
Gap plates material / process / finish:
Low carbon annealed steel / stamp or blank / black zinc plate
K.2
Magnets material / process / finish:
Hard ferrite / sintered with ground surfaces / none
K.3
Pole material / process / finish:
Low carbon annealed steel / cold forge or extrude with secondary
machining / black zinc plate
K.4
Basket material / process / finish:
Aluminum alloy / die cast with secondary machining / black electro-plate
K.5
Clamp ring / process / finish:
Aluminum alloy / stamp or blank / black electro-plate
K.6
Voice coil assembly:
3 x 2-layer heavy build copper clad aluminum magnet wire coils wound on
an anodized aluminum foil former with Nomex paper or equivalent collars
and insulated braided copper tinsels attached and tinned
K.7
Spider material / process / color:
Treated cloth blend / hot form and trim / don’t care
K.8
Surround material / process:
NBR with carbon black or equivalent / injection mold
15
K.9
Diaphragm material / process:
Flat sandwich composite with carbon fiber-epoxy skins with Rohacell IG
hard foam core or equivalent / join in mold with adhesive and trim
K.10 Dust cap material / process:
Matte coated paper / wet formed and hot pressed
K.11 Terminals:
Push type
K.12 Fasteners:
Blackened nonferrous countersunk hex socket head or equivalent
K.13 Pole cap material / process / finish:
Aluminum alloy / forge or extrude / black electro-plate
Now in retrospect, the TM-6 was one of the “best” production woofers that I
ever designed! The test of time serves as a testament to this claim.
However, after more than a decade of independent transducer R&D, there is
room for improvement and an upgrade could be interesting. My claim is
then that KOMODO 9 is a serious upgrade to RAPTOR 6 with regards to
almost all aspects of performance and sets a new standard for low frequency
transducer topology.
The KOMODO topology utilizes a flat composite diaphragm with coupling
vents under the dust cap. Ironically, it was KEF that pioneered the flat
diaphragm for low frequency transducers
(www.kef.com/uploads/files/en/kef_units/A%20History%20of%20Kef%20
Drive%20Units%20issue%203.pdf). However, the flat panel composite
technology has significantly advanced since the B139 woofer et al. and
composite panels are now commonly utilized in the aerospace industry and
in F1 auto racing applications. The flat diaphragm assembly significantly
reduces the baffle cavity created by cone drivers. The basket and the
surround termination are designed to facilitate flush mounting of the
transducer from the backside of the baffle. The KEF BLADE woofer also
seems to be designed to address the cone cavity et al. The KOMODO
topology transducers utilize a trick molded surround that allows for high
displacement while relatively increasing effective diaphragm area. The
KEF BLADE and the B&W 802D woofers appear to use well dated half
roll surround topology. I am presently working with Clarasonic in Thailand
to develop flat composite diaphragms and attach trick surrounds for low
frequency transducer applications (www.clarasonic.com/).
The center of moving mass with the KOMODO topology is as close to ideal
as currently possible. While the resultant inherent mechanical stability
related to the “ultra wide wheel base” is second to no topology that I know
16
of. The backdoor lead-out eliminates through the cone lead dress. The
KOMODO 9 transducer concept is designed for manufacture and assembly
and is well suited for manufacture within Asia.
Another almost coincidental feature of KOMODO/RAPTOR transducer
topology is that DC Electromagnets
(http://en.wikipedia.org/wiki/Electromagnet) can be conveniently utilized as
a replacement for the hard ferrite permanent magnet assemblies. In car
audio applications, a clean 12 (14) VDC source is available; otherwise a DC
voltage source is required. My research has shown that a size requirement
guideline for aluminum magnet wire field coils is approximately the same as
for hard ferrite magnets for a given b(x) (N2/W) target. Aluminum magnet
wire is the material of choice for two reasons, mass and cost. The mass
density of hard ferrite magnet is approximately 4,850 kg/m3. Whereas, the
mass density of copper is 7,760 kg/m3 and the mass density of aluminum is
2,700 kg/m3. So the aluminum magnet wire electromagnets can actually
reduce transducer mass. Although, the cost of the aluminum magnet wire
field coils will be significantly less than the equivalent copper magnet wire
field coils they will still be significantly higher cost than hard ferrite
magnets.
In the cases with opposing magnet directions of magnetization such as
KOMODO 9, the assembly procedure is simplified and obviously there is
no need for a costly magnetizing system. The voice coil is shim gauged on
both the inside diameter, ID, and the outside diameter, OD, during the
assembly process to facilitate high quality and high reliability. Quality is
defined as the absence of variability.
Both the RAPTOR 6 and KOMODO 9 motor assemblies are lossy. The
DC flux leakage about the magnet(s) OD is quite high. In contrast both the
KOMODO EM 6 and KOMODO EM 9 are inherently magnetically
shielded and the external to the magnetic assembly DC flux leakage will be
low. This can be observed within the contour plots shown in figures 7 and
10. Figure 9 contains a concept assembly drawing of the KOMODO 9 EM
low frequency transducer.
I refer to KOMODO 9 EM as “DSP Ready”. There is the potential to
actively control the electromagnets and the voice coils inputs independently
and dynamically; however, that’s a problem for a digital signal processing
specialist.
17
BOTTOM
FIELD
COIL
TOP
FIELD
COIL
BOTTOM
VIEW
SECTIONAL
VIEW
TOP
VIEW
FIGURE 9. KOMODO EM 9 Woofer Concept Assembly Drawing
18
FIGURE 10. VF OPERA DC FEA Flux Contour Plot of the KOMODO 9
EM Electro-magnetic Assembly
Figure 10 contains a DC simulation contour plot of the magnetic flux. The
DC magnetic flux goes as NI (ampere turns) from the field coils, where N is
the number of magnet wire turns and I is the DC current.
Figure 11 contains a concept assembly drawing of a TM-6/RAPTOR 6 that
has been retrofitted with the KOMODO EM transducer topology with a 12
VDC electromagnet drive, an “Engine Swap” analogy. Two terminals for
AC input signal are added to the bottom of the assembly, then the existing
two terminals are utilized for the 12 VDC field coils supply.
19
BOTTOM
VIEW
Ø125mm
SECTIONAL
VIEW
TOP
VIEW
FIGURE 11. KOMODO EM 6 Woofer Concept Assembly Drawing
This discussion has briefly traced the R&D milestones that have taken low
frequency transducer topology development from the Velodyne’s Digital
Drive complementary symmetrical drive example to the KOMODO
topology. Are there any serious claims that asymmetric transducer
topologies with cantilevered voice coils represent an advantage in low
frequency transducer implementations? I can only identify cost and/or
inconvenience as the dominate obstacle to complementary symmetrical drive
low frequency transducer implementations.
20
Finally, both STEALLUS and KOMODO transducer topologies resulted
from independent R&D. Working independently allows for greater design
flexibility, which can sometimes map into innovation; however, the team
element of development is lost or constrained. Serious inquiries, comments,
and/or questions are welcome and encouraged. They can be sent to Steve’s
attention at [email protected].
The Author: Steve Mowry is in his 20th consecutive year of exclusively
practicing audio transducer engineering that all began within the BOSE
transducer research group (TRES) in mid 1995 after obtaining BS and MS
degrees in Electrical Engineering with highest distinction from URI. He has
worked with audio transducer and system development teams worldwide in
diverse development environments. With respect to transducer
manufacturers and part suppliers, Steve is conveniently stationed in
Southeast Asia. He is currently an independent transducer researcher and
consultant to the loudspeaker industry with strong interests in Design for
Manufacture, Supply Chain Design, Concurrent Product Development, and
Concurrent Project Management.
Steve Mowry
18 September 2014
21