Method of and apparatus for dynamically controlling operating characteristics of a microphone

A microphone has a diaphragm which vibrates in response to sound waves and a proximately positioned charge carrying circuit for dynamically controlling a compliance of the diaphragm. A micromachined microphone has a substrate, a compliant silicon membrane, and a charge carrying circuit positioned between the substrate and the membrane. A charge applied to the charge carrying circuit controls the compliance of the membrane thereby controlling operating characteristics of the microphone, such as dynamic range and frequency response.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to sound transducer devices, and more particularly to 
microphones with dynamically controllable operating characteristics. The 
invention further relates to a method of making and using such devices. 
2. Related Art 
Sound is propagated through a medium such as air by means of wave motion. 
Sound pressure level is the incremental variation from the static pressure 
in the air absent a sound wave. Each sound corresponds to a unique 
frequency which is the number of times a sound pressure varies from the 
static pressure level during a given time period. The cycle typically used 
is the Hertz (Hz) in which 1 Hz is equal to 1 cycle per second. A cycle is 
thus defined as a variation from equilibrium, a return to equilibrium, a 
negative variation from equilibrium, and a return to equilibrium. A sound 
travels through air as a wave at its corresponding frequency and sound 
pressure level. 
A microphone is a sound wave transducer. A microphone typically includes a 
surface called a diaphragm which vibrates in response to sound waves 
incident thereon. The diaphragm is coupled to circuitry which translates 
the diaphragm vibrations into electrical signals which are proportional to 
the sound waves. In an electrostatic microphone the electrical signals are 
generated by detecting a variation of capacitance between the vibrating 
diaphragm and a fixed surface. For example, in a dc-biased electrostatic 
microphone, a capacitor is formed by two electrically conductive surfaces 
(a fixed backplate and the diaphragm) having an air gap between them and a 
voltage applied across them. An electret-biased electrostatic microphone 
is another example. This type of microphone also utilizes two surfaces to 
form a capacitor, but a permanently charged dielectric, such as 
Teflon.RTM., is attached to the one of the surfaces. In one example, the 
dielectric is cemented onto the backplate and forms a capacitance with the 
diaphragm. In another example, the dielectric is metalized on one side and 
used as the diaphragm to form a capacitance with the backplate. 
In both the dc-biased and electret-biased electrostatic microphones, sound 
waves produce a vibration of the diaphragm. Circuitry connected to the 
diaphragm then generates an output electrical signal corresponding to the 
variation of the capacitance. These signals are typically then further 
amplified and processed as required. Such microphones are used in numerous 
devices, such as telephones, tape recorders, and intercoms. More recently, 
computer-based devices have utilized microphones for speech recognition, 
tele-conferencing, and in multi-media systems. 
The diaphragm of a conventional microphone has a fixed compliance defined 
by its mass, the stiffness of its material, and by the restoring forces 
applied to the diaphragm. The restoring forces include the resiliency of 
the diaphragm material, the mechanical tension on the diaphragm, and the 
acoustic stiffness of the air gap. In some microphones, the backplate is 
perforated with holes to decrease the acoustic stiffness of the air gap. 
As used herein, more compliant means more flexible and less compliant 
means less flexible. 
To a large extent, the operating characteristics of a microphone depend 
upon the compliance of the diaphragm. For example, if sound waves in the 
vicinity of the microphone are not of sufficient sound pressure level to 
move the diaphragm against the restoring forces, the diaphragm will not 
move. Similarly, if the sound wave is not of sufficient frequency to 
overcome the restoring forces on the diaphragm, the diaphragm will not 
vibrate. On the other hand, if sound waves in the vicinity of the 
microphone have a sound pressure level that greatly exceeds the restoring 
forces on the diaphragm, the diaphragm will flex in response to the sound 
pressure but the position of the diaphragm will not accurately correspond 
to the instantaneous sound waves, and clipping distortion will occur. 
Similarly, if the sound wave has a frequency in excess of the diaphragm's 
ability to flex and return, the frequency of the diaphragm's vibration 
will not correspond to the frequency of the sound wave. 
Two characteristics of a microphone which are based on the compliance of 
the diaphragm are its "dynamic range" and its "frequency response." The 
dynamic range is defined by the difference between the microphone's 
minimum sound pressure level (SPL) (the most quiet sounds detectable by 
the microphone), and its maximum SPL (the loudest sounds the microphone 
can convert to electrical signals without distortion). The frequency 
response is defined by the range of sounds the microphone can detect. For 
a typical microphone, these are within the spectrum of human hearing. For 
example, a silicon micromachined microphone such as the one provided by 
Noise Cancellation Technologies, Inc. has a dynamic range of 160 decibels 
(dB) (based on a minimum SPL -40 dB and a maximum SPL of 120 dB) and a 
frequency response of 20 Hz (the low end of the human hearing spectrum) to 
10,000 Hz (the high end of human speech). 
Within this general spectrum of characteristics, special purpose 
microphones are commercially available with specific operating 
characteristics optimized for use in various applications. However, a 
microphone which is optimized for one application may not be suitable for 
another application. For example, a spectrum of typical sound pressure 
levels measured in dBs can include a quiet office, with a SPL of 
approximately 30 dB; ordinary human conversation, with an SPL of 
approximately 40-50 dB; and factory machinery, with an SPL of 
approximately 80 dB. Therefore, a microphone with a maximum SPL of 60 dB 
will be a good microphone for a speakerphone in a quiet office, but will 
be a poor microphone for an intercom on a factory floor. On the other 
hand, a microphone with a minimum SPL of 60 dB and a maximum SPL of 120 dB 
will be a good microphone for an intercom on a factory floor, but will be 
a poor microphone for use in a speakerphone in a quiet office. 
Many microphones may be subject to a variety of conditions. A conventional 
microphone with fixed operating characteristics may not operate 
effectively across the entire range of conditions for a particular 
application. For example, an environment such as a factory floor has 
generally noisy conditions, but may also be occasionally quiet such as 
during breaks or after working hours. Thus, an intercom might be optimized 
to communicate between the factory floor and other areas of the factory 
when the factory is noisy. Such an intercom, which typically comprises a 
microphone, a speaker, and associated circuitry to facilitate 
communication with other intercoms, may operate poorly when the factory is 
quiet, due to its fixed operating characteristics. 
In operation, a person wishing to communicate via the intercom speaks in 
the vicinity of the microphone. The microphone operating as a conventional 
microphone as described above, produces an electrical signal which is 
transmitted to another intercom where it is amplified to drive a speaker 
and be heard by a listener. 
When the factory floor is noisy, a person trying to communicate therefrom 
must shout to be heard above the noise. Because the person speaking must 
shout, an intercom optimized for the factory floor will have a microphone 
with a relatively high maximum SPL. As discussed above, to achieve a high 
maximum SPL the compliance of the diaphragm must be relatively stiff. This 
is satisfactory for when the factory floor is noisy, however, a stiffer 
compliance also results in a higher minimum SPL. The higher minimum SPL, 
which is the lowest sound pressure level which will cause the diaphragm to 
vibrate, requires a greater sound pressure level to vibrate the diaphragm. 
With a microphone as described above, when the factory floor is quiet, a 
person would still have to speak loudly to effectively use the intercom 
which was designed to operate optimally in a noisy factory environment. 
Therefore, such a microphone cannot operate optimally across the range of 
operating conditions. Furthermore, other areas of the factory which 
communicate through the intercom system may include a relatively quiet 
office. Since the microphone for the office will optimally have a lower 
maximum and minimum SPL, the same type of intercom unit cannot be used 
optimally in both the factory floor and the office environments. 
Another example of a variable environment is in the area of computer-based 
microphone applications. It is sometimes desirable, such as in a speech 
recognition application, to have the microphone optimized for the speech 
of a specific person. Alternatively, it is sometimes desirable to have the 
microphone respond to a broader range of sounds, such as in a multi-party 
tele-conferencing application. In order to have the microphone respond to 
the speech of a specific person, a highly directional microphone is used 
and the person should speak in close proximity to the microphone. An 
omni-directional microphone, on the other hand, cannot typically focus on 
one specific person without also picking up sounds from other directions, 
such as in the vicinity of the speaker. 
When sound conditions in an environment vary, a microphone with fixed 
operating characteristics may be unable to operate effectively in certain 
variations of the environment. A microphone with dynamically controllable 
operating characteristics is therefore needed to provide the flexibility 
to optimize the microphone to operate effectively in each variation of the 
environment. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is an object of the invention to provide a microphone with dynamically 
controllable operating characteristics. These include, among other 
characteristics, the maximum SPL, the minimum SPL, the dynamic range, and 
the frequency response of the microphone. 
More generally, it is an object of the invention to control a compliance of 
a diaphragm in a sound transducer device. 
These and other objects of the invention are accomplished by a microphone, 
including a diaphragm mounted therein which vibrates in response to sound 
waves, with means for dynamically controlling a compliance of the 
diaphragm. 
In one embodiment according to the invention, a charge carrying circuit is 
positioned proximate to the diaphragm to dynamically control the 
compliance of the diaphragm and thereby control the operating 
characteristics of the microphone. 
In another embodiment according to the invention, a micromachined 
microphone has a charge carrying circuit positioned in a depression 
between a compliant silicon membrane and a substrate. The charge carrying 
circuit may comprise, for example, concentric charge carrying rings. In 
another variation, the circuit may be embedded within the substrate.

DETAILED DESCRIPTION 
A conventional micromachined microphone is shown in FIGS. 1a and 1b. Such a 
microphone consists of a compliant silicon membrane 105 non-conductively 
attached to a substrate 107. The membrane 105 is coupled to circuitry 109. 
The substrate 107 forms a depression 111 underneath the membrane 105 which 
allows the membrane 105 to vibrate in response to sound waves. Thus, the 
membrane 105 functions as a diaphragm. The substrate 107 has conductive 
and non-conductive areas. In the area of the depression 111, for example, 
the bottom surface 113 may be a conductive surface, while the side walls 
115 and the top surface 117 may be non-conductive. Other conductive areas 
of the substrate 107 may comprise the circuitry 109. 
The membrane 105 and substrate 107 function together as plates of a 
capacitor. The vibration of the membrane 105 causes the capacitance to 
vary. The circuitry 109 responds to the variable capacitance and produces 
an electrical signal which is proportional to the sound waves vibrating 
the membrane 105. The operating characteristics of such a microphone are 
fixed once the microphone has been manufactured. 
A microphone according to the invention, as disclosed herein includes a 
membrane and a substrate and further includes a charge carrying circuit 
which acts to apply a force on the membrane. A principal feature of the 
microphone according to the invention is that the microphone can be 
dynamically adapted to many different applications by controlling the 
charge applied to the charge carrying circuit. 
In FIG. 2a a block diagram of a microphone according to the invention shows 
a flexible charge carrying surface 226 (the diaphragm) and a fixed charge 
carrying surface 228 coupled to a capacitance sensing circuit 232. This 
microphone further includes a charge carrying circuit 224 coupled to a 
charge control circuit 230. The charge carrying circuit 224 is positioned 
proximate to the flexible charge carrying surface 226, such that a charge 
applied by the charge control circuit 230 to the charge carrying circuit 
224 imparts a force on the flexible charge carrying surface 226. Sound 
waves incident upon flexible charge carrying surface 226 cause the surface 
226 to vibrate thereby causing a variation in the capacitance between the 
flexible surface 226 and the fixed surface 228. Capacitance sensing 
circuit 232 produces output electrical signal 234 which corresponds to the 
variance of the capacitance. 
Referring to FIG. 2b, a first embodiment of a microphone according to the 
invention has a membrane 204, a substrate 206, a depression 208 formed in 
substrate 206, and a charge carrying circuit 202 positioned in the 
depression 208 between the membrane 204 and the substrate 206. The charge 
carrying circuit 202 is placed proximate to the membrane 204 such that a 
charge applied to the circuit 202 applies either a repulsive or attractive 
force on the membrane 204. The compliance of the membrane 204 can thus be 
effectively controlled by varying the amount of charge on circuit 202. The 
operating characteristics of a microphone of the invention are therefore 
dynamic and can be controlled by a user after the microphone is 
manufactured. 
In the embodiment of FIG. 2b, the circuit 202 is shown immediately adjacent 
to the substrate 206. It will be appreciated, however, that this is one 
exemplary position for circuit 202. Alternatively, circuit 202 could be 
located, for example, at other positions within the depression 208, at 
positions within the substrate 206, or at positions outside the 
microphone, such as above membrane 204 or below substrate 206. For 
example, FIG. 2c shows a second embodiment according to the invention 
having a membrane 254, a substrate 256, and a charge carrying circuit 252 
which is embedded within substrate 256. 
By controlling the compliance of the membrane, the microphone according to 
invention controls the minimum and maximum sound pressure level that the 
membrane can effectively respond to and can thereby tune a dynamic of the 
microphone to a desired range. An advantage of the invention is that such 
tuning is achieved without changing the frequency response of the 
microphone. 
FIG. 3 shows a third embodiment of a microphone according to the invention. 
This embodiment includes a membrane 303, a substrate 305, and a charge 
carrying circuit 301 positioned in a depression 307. The charge carrying 
circuit 301 includes concentric charge carrying rings 311, 313, and 315. 
The charge carrying circuit of FIG.3 has three rings, 311, 313, and 315, 
which are of substantially equal width and spacing. Other numbers of rings 
and variations of widths and spacings are of course possible. 
An advantage is provided by the ability to selectively charge individual 
rings with various degrees of charge. The charge carrying circuit 301 
offers more precise control over the compliance of the membrane compared, 
for example, with a charge carrying circuit having a single surface such 
as an oval. In a single charge carrying surface, the charge is uniformly 
spread across the entire surface of the charge carrying circuit, thus 
applying a uniform force to the entire membrane. With the concentric 
charge carrying rings 301 according to the invention, the charge on each 
ring can be individually controlled. A charge applied to outermost ring 
315 applies a force principally to the outermost portion of the membrane 
303. The inner portion of the membrane 303 remains relatively compliant 
compared to the compliance of the inner portion when a uniform charge is 
applied across the entire membrane. Alternatively, a first charge could be 
applied to outermost ring 315, while a second charge is applied to middle 
ring 313, and a third charge is applied to innermost ring 311. For 
example, applying progressively smaller charges to the rings from the 
outermost ring 315 to the inner most ring 311 renders the inner portions 
of the membrane less compliant than they would be with no charge applied, 
but more compliant than the outer portions of the membrane. 
By controlling the compliance of the membrane, the microphone according to 
invention controls the effective geometry of the membrane and can thereby 
tune a frequency response of the microphone to a desired range. An 
advantage of the invention is that such tuning is achieved without 
additional circuitry required for a conventional band-pass filter. 
FIG. 4a shows operation, within a specified dynamic range, of a 
conventional microphone, having a membrane 404 and a substrate 406. A 
sound source 402 creates sound waves which cause the membrane 404 to 
vibrate within area 405. An output electrical signal is generated by the 
microphone. The signal is transmitted to an apparatus such as an amplifier 
driving a loudspeaker (Not shown). The amplifier and loudspeaker convert 
the electrical signal to sound waves corresponding to those incident on 
the membrane 404. 
FIG. 4b illustrates the response of the membrane 404 to a sound source 408 
when the sound waves exceed the maximum SPL of the microphone. The effect 
shown is that of the membrane 404 being over-driven and contacting the 
substrate 406, thus creating a phenomenon known as "clipping." A graphical 
representation of clipping is shown in FIG. 4c. Clipping is also possible 
without physical contact between the membrane 404 and the substrate 406. 
Such clipping occurs when the membrane 404 is fully flexed. In this case 
the capacitance between the membrane 404 and the substrate 406 remains 
constant and can not vary until the sound pressure level drops below the 
clipping threshold. As shown in FIG. 4c, during clipping the electrical 
signal produced is clipped at the maximum possible output level. The 
effect on the listener is distortion of amplitude, noise-level, and 
harmonic content of the signal, resulting in reduced intelligibility. 
FIGS. 5a and 5b show a microphone according to the invention in normal 
operation having a membrane 503, a substrate 505, and a charge carrying 
circuit 501. Two charged surfaces spaced apart, one having a positive 
charge relative to the other, form a capacitor. Thus, membrane 503 and 
substrate 505, when charged this way, form a capacitor. It is known that 
charge carrying circuits with opposite polarization in close proximity 
have an attractive force with respect to each other while circuits with 
like polarization have a repulsive force. According to the invention, a 
positive charge or a negative charge can be applied to the circuit 501. 
Such a charge can be applied, for example, by connecting charge carrying 
circuit 501, through appropriate circuitry, to a power source. 
Representative appropriate circuitry is shown in FIG. 5c wherein a voltage 
source 507 is connected in series with a variable resistor 509. A 
variation of the compliance is achieved by varying a resistance of the 
variable resistor 509. 
FIG. 5a shows the effect of applying a charge to the charge carrying 
circuit of like polarization with respect to the charge on the diaphragm. 
The circuit 501 creates a repulsive force on membrane 503, thereby 
stressing membrane 503 and rendering the membrane 503 less compliant or 
more resistant to deflection from impinging sound waves than the membrane 
503 would be without the charge applied. 
FIG. 5b shows the response of the microphone of FIG. 5a to a sound source 
408 with sound waves identical to the sound waves of FIG. 4b. Unlike the 
case shown in FIG. 4b, in FIG. 5b the stressed membrane 503 does not 
contact the substrate 505 because the charge applied to the charge 
carrying circuit 501 increases the resistance of the membrane to movement 
in response to impinging sound waves. This increased stiffness provides a 
higher maximum SPL thereby reducing clipping and distortion. 
A further advantage of a microphone according to the invention is that the 
maximum SPL can be varied by varying the charge on the charge carrying 
circuit 501. The microphone of the invention in FIG. 5a can operate 
identically to the microphone of FIG. 4a with no charge applied to the 
charge carrying circuit. This absence of charge might be useful in low 
volume situations. However, the microphone of FIG. 4a has fixed operating 
characteristics. The microphone according to the invention can further 
operate as in FIG. 5b with an appropriate charge applied to dynamically 
change the operating characteristics, for example, to avoid clipping in 
response to loud sounds. 
The amount of charge applied to the charge carrying circuits of any of the 
embodiments herein can be tailored for particular situations and can be 
dynamically altered so that the same microphone can be optimized, by 
varying the charge applied to the charge carrying circuit, for use in an 
environment with variable conditions. In the previous example of a noisy 
factory floor which is occasionally quiet, a microphone according to the 
invention could be utilized such that no charge is applied under quiet 
conditions, optimizing the microphone for a user speaking in a normal 
voice, while a charge is selectively applied when the factory floor is 
noisy, thereby raising the maximum SPL characteristic of the microphone 
and optimizing the microphone for a user who is shouting. 
It should also be understood that certain trade-offs are involved in 
rendering the diaphragm less compliant. While a stiffer diaphragm can 
handle louder sounds without distortion, it also requires louder sounds to 
move the diaphragm against the stronger restoring forces. Thus increasing 
the maximum SPL characteristic of the microphone creates a corresponding, 
although not necessarily proportional, increase in the minimum SPL 
characteristic. 
FIG. 6a shows a graph of the general relationship between maximum SPL of 
the microphone and the amount of charge applied to the charge carrying 
circuit, such as 202, 252, 301, and 501, of FIGS. 2b, 2c, 3, and 5a 
respectively. FIG. 6a shows that the maximum SPL characteristic of the 
microphone can be increased by increasing the charge applied to the charge 
carrying circuit. Furthermore, the maximum SPL characteristic can be 
controlled by coupling the microphone with circuitry which can dynamically 
vary the charge applied to the charge carrying circuit based on the 
particular application and conditions. The operating characteristics can 
be controlled by any of a variety of means. For example, a user via a user 
interface, such as a dial connected to a variable resistor can vary the 
voltage applied to the charge carrying circuit by varying the resistance. 
Another method of controlling the microphone is coupling to a separate 
microphone that detects changes in operating conditions or is tuned to 
specific ranges of volume and/or frequency. 
Alternatively, the microphone itself can use a feedback loop or other 
control process to set the microphone characteristics. A more 
sophisticated system could use predictive techniques to dynamically vary 
the characteristics. For example, in an industrial environment where there 
is a repetitive banging of machinery, the control circuit could vary the 
characteristics of the microphone in accordance with the predicted 
repetition of the sound. 
As described above, increasing the maximum SPL typically involves 
stiffening the compliance of the membrane with a corresponding increase in 
the minimum SPL characteristic of the microphone. The difference between 
the maximum SPL and the minimum SPL defines the dynamic range 
characteristic of the microphone. FIGS. 6a and 6b which also show the 
relationship between maximum SPL and minimum SPL for various levels of 
charge applied to the charge carrying circuit, thereby show the varying 
dynamic range. As shown in FIGS. 6a and 6b, the maximum SPL and minimum 
SPL do not necessarily vary to the same degree for a given change in the 
charge applied. 
Further, as is shown in FIGS. 6b and 6c, the minimum SPL and dynamic range 
characteristics of the microphone can also be controlled using methods 
similar to those described above with respect to controlling the maximum 
SPL characteristic. Of course, each of the characteristics cannot be 
varied independently of the others, as all of these characteristics are a 
function of the compliance of the diaphragm and therefore vary in 
accordance with a variance of the charge applied to the charge carrying 
circuit. 
The frequency response of a microphone according to the invention is an 
additional operating characteristic that can be controlled by applying a 
charge to the charge carrying circuit in accordance with the invention. 
The length of the diaphragm is related to the frequency response of the 
microphone. FIG. 7 shows a fourth embodiment according to the invention 
particularly suited to dynamically control the frequency response of the 
microphone. This embodiment includes a substrate 705, a membrane 703, and 
a charge carrying circuit 701 positioned in a depression 707. The charge 
carrying circuit 701 includes charge carrying strips 711, 713, 715, 717, 
719, 721, 723, 725, and 727 which are of substantially equal width and 
spacing. Other numbers of strips and variations of widths and spacing are 
of course possible. 
An advantage is provided by the ability to selectively charge individual 
strips with various degrees of charge. A charge applied only to strip 711 
applies a force principally to the leftmost edge of the membrane 703. The 
leftmost edge of the membrane 703 would be relatively stiff compared the 
portion to the right of the charged strip 711. Thus, the effective length 
of the membrane 703 which is not restricted from vibration would be 
decreased. The frequency response can thereby be controlled by selectively 
charging the strips to control the effective length of the membrane 703 
which is left free to vibrate. 
In general, FIGS. 7a-7d represent various frequency responses of a 
microphone according to the invention. In each of these figures, the 
x-axis represents frequency shown logarithmically from 20-10,000 Hz. The 
y-axis represents the output electrical signal of the microphone in 
response to a broad band input signal containing representative sound wave 
frequencies from 20-10,000 Hz at a constant sound pressure level. As 
shown, the microphone functions essentially as a band pass filter wherein 
only certain frequencies of all those present in the input sound wave are 
present in the output electrical signal. 
FIG. 7a shows the full frequency response projected for a microphone formed 
according to the invention in the absence of any charge applied to the 
charge carrying circuit. Such a frequency response characteristic is 
similar to that of a prior art microphone, such as the microphone of FIGS. 
4a and 4b. 
It is sometimes desirable to tailor the frequency response of a microphone. 
FIG. 7b shows an example of the projected frequency response for a 
microphone formed according to the invention when a charge is applied to 
the charge carrying circuit. The frequency response of FIG. 7b is tuned to 
the normal range of human speech. FIG. 7c shows the frequency response 
further limited to a narrower band corresponding to a speaker with a low 
voice. FIG. 7d shows the frequency response limited to a speaker with a 
high voice. 
FIG. 8 shows a microphone according to the invention connected to a 
feedback control loop for adjusting its frequency response. The microphone 
802 has an output signal 810. The output signal 810 can be sampled by a 
frequency analyzer 804 to determine the frequency range of the sampled 
sound. Information from the frequency analyzer 804 is supplied to control 
logic 806 which is connected to a charge control circuit 808. The charge 
control circuit 808 can adjust the frequency response of the microphone 
802 according to the methods described above. 
The feedback loop in FIG. 8 would typically sample at discrete intervals 
under user control. For example, a user would want to set the tuned 
frequency response when the office is quiet with few or no other sounds 
present. 
A user can thus vary the frequency response characteristics of the 
microphone by varying the charge applied to the charge carrying circuits 
202, 252, 301, and 501 in each of the above embodiments of the invention. 
Of course, as mentioned earlier, each change in a frequency response would 
yield a change to the maximum SPL, minimum SPL, and dynamic range 
characteristics of the microphone, since each of these characteristics is 
affected to some degree by the a change in the charge applied by the 
charge carrying circuit. Thus, a microphone according to the invention 
enables an operator to optimally set the microphone according to the 
operational environment and objectives, recognizing that optimizing one 
characteristic may yield trade-offs in other characteristics. 
It will be understood that various modifications in the form of the 
invention as described herein and its preferred embodiments may be made 
without departing from the spirit thereof and of the scope of the claims 
which follow.