PAPER MAKING
OVERVIEW
When required we manufacture our paper cones in house. Our expertise is the manufacture of traditional felted
paper cones made from raw paper stock and processed to achieve the desired characteristics. . Paper cones have
been a preferred base material for loudspeaker diaphragms for many years and are still much sought after for the
following reasons. Paper cones are widely employed because they permit optimization ie.
No real limitations to shape
Wide choice of base materials from softwoods to hardwoods
Weight and thickness infinitely variable and controllable to better than 5%.
A wide choice of additives and treatments to improve strength, stiffness, moisture resistance and
damping.
High tech additives such as (Kevlar fibers, carbon fiber) can be added to improve strength and or
damping.
Coating and/or impregnation options exist to improve performance.
Thickness can be varied over the cone body for optimum performance
Density can be varied over the cone body by pressing or impregnation.
Material properties are under house control.
There will always be discussion regarding the linearity of various cone materials. A thorough finite element
analysis of many of our models was performed by D. Featherston. Analysis of strain data from FE simulations
confirmed our loudspeakers were deforming within their elastic range. It revealed that our paper cones in
demanding applications were undergoing less than 0.001 strain. As stated by Sawyer et al. (1997), paper cycled at
low strains – less than 0.003 – showed little evidence of nonlinear behavior in loading or unloading.
The paper also responds well to after treatments such as lacquer dipping, local strengthening, mould inhibitors,
pesticides wet strength agents, fire retardants and surface treatment to improve qualities such as strength, ageing
and water resistance. In addition, paper offers excellent adhesion qualities and long life. It is these factors
combined with excellent sonic qualities and the ability to optimize a given parameter which make this medium so
popular.
Paper cones are generally made from wood pulp the main components are softwoods i.e. Pine, Spruce and
hardwoods such as Aspen and Eucalyptus. . Usually one species of wood pulp does not meet all requirements and
a blend of selected short fiber (hardwood) and long fiber (softwood pulp) is employed to achieve the stiffness,
density, thickness and damping required for the end application. However often various and sometimes unusual
additives such as Hemp, Kapok, wool, wheat, straw and rice straw flax, rayon cotton glass-fiber Kevlar, carbon
fiber, are added to the mix to achieve the desired properties. The choice and quality of paper materials audible
influence the tonal voicing quality, the myriad of option leads to the numerous claims and benefits. Paper
properties are important in loudspeaker applications and the paper making skills required are unique in this
industry.
PAPER CONE MANUFACTURE
The selection of the raw ingredients paper pulp fiber is very important in achieving high strength paper. The
important considerations are
Pulp type hardwood, softwood and the species
Virgin fiber is preferred to recycled
Country of origin and climatic conditions
Bleached/unbleached
Processing i.e. cooked, mechanical or high-yield thermo-mechanical Kappa number
Age of the trees used in the pulp production
Drying process
The virgin pulp arrives in sheeted bales.
Various stock of selected virgin pulp.
REFINING
Refining (also referred to as beating) is a mechanical process carried out on pulp; the refiner modifies the fibers
properties making them ready for papermaking. Refining the pulp improves paper strength but also increases
paper density and damping properties are diminished with refining. Paper can be made without refining but it is
weak.
Typical Refiner employed by the loudspeaker
Our Valley paper pulp refiner.
In the paper industry, refining is carried out by passing the pulp between grooved plates rotating at high speed.
The Valley beater comprises of a fixed roll with bars (visible in the picture above) and driven by an electric motor.
Beneath the roll is a set of fixed bars called the bedplate which is attached to a lever-arm. A specified load is
applied to the lever arm which brings the bedplate bars in contact with the roll bars. The pulp passes between the
roll and the bedplate and the purpose being to collapse the fibres into flat ribbons and shred the outer layer
without cutting or shortening the fibres. Refining actually performs two key tasks to increase fiber-to-fiber
bonding:
(a) Collapses the fibres into flat ribbons
(b) Shreds the outer layer of the fibres (called fibrillation), making it hairy-looking and producing even
more surface area for bonding
Scanning electron micrograph of the surface of a paper
loudspeaker cone displaying the collapsed fibers as
ribbons.
PAPER PROPERTIES EFFECTED BY BEATING
The effects that beating have on paper can readily be observed by plotting measured paper properties with
respect to the beating time
Strength Values vs Paper Beating Time
Normalised Strength Values
80
70
60
50
40
30
20
10
0
0
20
40
60
80
100
Beating Time
A graph of normalized paper strength values vs. beating time.
Purple graph
Tensile strength vs. beating time
Green graph
Folding strength vs. beating time
Red graph
Bursting strength vs. beating time
Dark blue graph Light blue graph -
Paper density vs. beating time
Tearing strength vs. beating time
The above graph displays that tensile strength, folding strength; busting strength, tearing strength and paper
density are dependent on beating time. Another important parameter “the paper damping properties” are also
modified by beating but not displayed in the above chart.
DESIRED LOUDSPEAKER PAPER PROPERTIES
From Finite Element work the prime objective in a loudspeaker is to produce the lightest and most rigid, most stiff
paper cone. The stiffness and material damping are prime factors in controlling frequency response and cone
break-up behavior. The paper loudspeaker cone must be stiff to act as a rigid piston when driven at its apex.
Loudspeaker cones are made to a fixed dry felted mass. The loudspeaker efficiency is inversely proportional to the
mass squared hence it is desirable to achieve the stiffest diaphragm for a given mass. Also the paper cone must be
rigid to push natural Eigen (vibration) modes as high as possible. As it is undesirable to operate a loudspeaker in its
break-up mode unless these vibration modes are well controlled. Break-up being defined as the region where the
diaphragm experiences bending modes. The bending modes are controlled by damping in the paper hence it is
desirable that stiffness is developed early in the beating cycle as unbeaten pulp offers the best damping without
additives.
If you wish to observe an example of an Eigen mode (a bending resonance) click on M12-AN.AVI to observe a
typical break-up mode greatly magnified to visually display the mode.
As a general rule when a shape is under load deflection then the fundamental vibration frequency is a function of
the Young’s Modulus E and the material densityρ that is we wish to maximize the specific material stiffness
defined by:
Specific stiffness = E/ρ
The higher this ratio, the higher the fundamental frequency and the deflection is less.
Strength Stiffness vs Beating Time
Normalised Strength Values
90
80
70
60
50
40
30
20
10
0
0
20
40
60
80
Beating Time
Graph of tensile strength, paper density and specific stiffness vs. beating time.
100
Purple graph
Dark blue graph Orange graph
-
Tensile strength vs. beating time
Paper density vs. beating time
Specific stiffness vs. beating time
OPTIMIZED FIBER PROPERTIES OPF
We observe from the above graph that there exists an optimum beating time to achieve maximum tensile
strength, however more important is the specific stiffness is a maximum at 46 min and although strength is
developing with additional beating it is at the expense of less damping and less stiffness. The resultant paper is
becoming more dense (thinner) resulting in a loss of specific stiffness, and in our application stiffness and damping
are more important than strength. At Lorantz we process our paper pulp to achieve optimum paper parameters
dependant on the application and desired properties. Usually a blend of optimized pulps is required to achieve the
best performance as each species of pulp have unique properties.
Effect of beating on sound
pressure response.
The above sound pressure response graph clearly displays the effect beating has on the loudspeaker response.
Two identical 10” loudspeakers were constructed with cones of identical fibre and same cone mass without dust
caps and/or other enhancements, the only difference being the beating/refining time.
Red curve – pulp was beaten for 15 min
Blue curve – pulp was beaten for 60 min
We observe in the 15 minute pulp the Eigen modes are at a lower frequency due o low tensile strength however
the resonances are well controlled as the unbeaten pulp has high bulk and high damping. Increasing the beating to
60 minutes has increased paper strength increased stiffness pushed the eign modes to a higher frequency and
extended the frequency response, however the resonances are more pronounced as the denser paper exhibits less
damping. One can see that a skilled engineer has at his disposal many options available with the same pulp; there
exist both art and science in paper making, and the selection of pulps and processing for audio are quite different
to those required for making writing paper.
TESTING OUR PAPER PROPERTIES
The paper strength is largely dependant on:
Pulp fiber selection
Beating
Wet end chemistry/coatings/additives and or lacquering
We produce hand sheets of our paper cone machine for testing
A circular flat hand sheet of the machine
and a trimmed test strip 100x20mm
OPTIMIZING PAPER PROPERTIES –OFP
The paper pulp is processed by beating (paper trade term). As discussed in the previous section beating modifies
the paper properties. The beating process allows the optimization of parameters such as Young’s modulus, density,
and tears strength bursting strength or folding endurance. Important parameters are Young’s modulus, density,
damping and sonic velocity. The extent of beating is measured by the Canadian standard freeness (CSF) test. Paper
cones are usually made from a blend of ingredients. Because it is unlikely that one material alone would produce
high rigidity, high tear, high modulus and optimum damping from a common process. Softwoods generally exhibit
the highest rigidity for a given weight however they exhibit poor local strength; softwoods therefore require
additives and or treatments if used in high power demanding applications. Apart from optimizing the mechanical
properties of a loudspeaker diaphragm we observed the existence of a subjective preferred listener sonic velocity
range within the paper. We employ OFP (Optimized Fiber Property) technology in our in-house paper cone
production.
OFP PULP FOR LOUDSPEAKER CONES
We determine material properties two ways:
We vibrate a paper cantilever and measure the resonance frequencies, to obtain E and damping. Using finite
element model we simulate the acoustical behavior of our loudspeaker, which we match with the measured
acoustic performance, a match only occurs when the following simulated material properties are correct:
Young modulus
Poisons ratio
Density
Damping factor
Material thickness
FE MODELING
Green curve- is the measured SPL
Blue curve – is the FE computed SPL
The above graph displays the measured and finite element simulated sound pressure responses for a test
loudspeaker, the responses have been offset for clarity. A loudspeaker model with a peaked top end response is
deliberately chosen to determine material properties. Due to the peak at 2.2 kHz and sharp cut-off the Young’s
modulus for the cone material can be accurately determined. For more info on modeling loudspeaker
performance with finite element tools go to loudspeaker design >>
As loudspeakers are required to operate at high excitation frequencies it is important to derive the dynamic
properties at or close to the excitation frequency of interest as many of the material properties are frequency
dependent.
CANTILEVER RESONANCE METHOD
A simple cost effective method of measuring paper stiffness and damping is by the cantilever resonance method
described below.
Test paper strip is clamped, excited by the loudspeaker and vibration amplitude recorded by laser.
Our test jig comprises clamp, test paper strip, excitation loudspeaker, tube, microphone, laser displacement
sensor not shown
The microphone is used to measure the excitation force acting on the test paper strip. The vibrations of the test
paper strip (40mm long free-length and 20mm wide) were measured with a laser displacement transducer type
Omeron ZX-LD40. During the frequency scan the sound pressure was held constant, to maintain constant
excitation force, hence the measured vibrations were normalized with respect to the sound pressure. The
following formula was used to calculate Young’s Modulus:
t
f = 0.162 2
L
E
ρ
in SI units
 f 2 L4 
E = 38.1ρ  2 
 t 
This formula is correct only for the first modal vibration frequency only
f = Resonance frequency in Hz
t = Paper thickness in meters
L = Free-length of paper strip in meters
ρ = Density in kg/cubic meter
E = Young’s Modulus in Pascal’s
The above model has limitations. The formula does not include mass air loading on the paper strip nor the
damping attributed to air loading however it is satisfactory to optimize beating and paper treatment for each pulp
type and accurate comparisons can be made. We use this method to select the best pulp and process specification
for our products
SELECTING THE MANUFACTURING PROCESS
Trimming the paper cone body to size.
A variety of paper-manufacturing processes are employed in loudspeaker industry, often the manufacturing
process restricts the choice of usable raw materials, and hence not all paper cones are the same. Many automatic
cone plants are geared for high throughput limiting choices. In the wet state paper is weak and the transfer
process to multiple drying heads can disturb the paper and impair quality and consistency. Our paper cones are
individually air-dried on the felting screen to ensure the paper is not disturbed; our cones therefore exhibit high
bulk, which greatly improves stiffness and bending strength. The production process, tooling, associated process
and optimized fiber property (OFP) technology developed by Lorantz have resulted in the production of superior
paper cones. The down side of paper cones is the high tooling cost, a major contributing factor in loss of market
share to other cheaper alternatives.
HOW MUCH DID THE NEW TOOLING COST
Refer comments from an eminent manufacturer producing polypropylene cones:
“Building and establishing the cone geometry and making a moving system with a good balance takes time. In
house we have all the machinery for producing the tools for P/P cones. A nice tool for production costs around 7000
to 10000 US$. A paper cone tool costs almost 10 times the amount. If you do not hit the right geometry within the
first couple of times, it will very fast become a costly affair. The P/P tools can be adjusted into hitting the right
geometry and you therefore have much more attempts with a relative low cost.”
When the market demand shifted to higher-mass cones new optimized cone profiles were required, paper cone
suppliers were slow to respond due to the high cost of tooling. Our push to employ finite element computer
analysis and CAD-CAM manufacturing techniques opens a new world for optimizing new complex shapes quickly
and efficiently. However the resistance to change has seen the acceptance of some good alternatives.