Method and apparatus for measuring acoustic parameters in liquids using cylindrical ultrasonic standing waves

Methods and apparatus are disclosed for analyzing liquids utilizing cylindrical acoustic standing waves, generally in the ultrasonic region. The invention facilitates measurement of acoustic parameters of a fluid, such as sound velocity and attenuation, which themselves serve as indicators of solute concentrations and various ongoing chemical processes occurring. In preferred embodiments of the invention, cylindrical ultrasonic standing waves are generated in a liquid contained within a cylindrical housing by causing coherent oscillation of the entire cylinder, or a circumferential segment thereof, or multiple circumferential segments thereof. The invention is amenable to a variety of applications and implementations, most involving pairs of resonators, one containing a sample of the liquid under study, and the other containing a reference liquid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to acoustic analysis of liquids, and more 
particularly to the use of resonant ultrasonic fields to determine the 
concentration of a dissolved species. 
2. Description of the Related Art 
Just as electromagnetic energy has been widely exploited in the measurement 
of physicochemical properties of gases, liquids and solids, so too have 
sound waves. Ultrasound, in the upper kilohertz and megahertz frequency 
bands, has proven especially useful for studying liquids. The acoustic 
properties of a liquid -- in particular, the velocity and attenuation of 
an ultrasonic pressure wave through the liquid -- depend on, and therefore 
can be used to measure, various thermodynamic and kinetic characteristics. 
Sound velocity, for example, provide information about adiabatic 
compressibility and density. Attenuation of sound in the medium provides 
information about the kinetic and thermodynamic parameters of relaxation 
processes. Both the velocity and attenuation of ultrasound are frequently 
observed to monitor chemical processes occurring in solution and to 
determine solute concentrations. 
A wide variety of ultrasonic instrumentation has been developed for 
specialized research purposes. Essentially, these instruments include 
means for generating a sound wave in the material to be studied, and means 
for measuring reporting changes in the sound wave as it propagates through 
and interacts with the material. Devices intended to analyze liquids 
generally make use of a pair of piezoelectric transducers, one of which 
generates the acoustic signal and the other of which detects the signal 
after it has traveled through the liquid under study. The acoustic signal 
may take many forms, e.g., a pulse wave, a continuous traveling wave or a 
continuous standing wave; the frequency of the applied field may be varied 
or kept constant; and the measured parameters may include amplitude, phase 
and/or frequency. 
Plane-wave resonators are a common type of instrument for ultrasonic 
analysis of liquids. These devices may comprise a chamber having plane 
piezotransducers along two opposed, precisely parallel walls. A plane 
pressure wave is generated by one transducer and progresses through the 
liquid to the other transducer, where it is detected and reflected back to 
the first transducer. At certain fundamental frequencies determined 
primarily by the distance between transducers and the acoustic properties 
of the contained liquid, the traveling transmitted and reflected waves 
combine into the stationary pattern characteristic of a standing wave. The 
standing wave condition results in delivery by the detection transducer of 
large voltage peaks. 
In operation, once the standing wave is achieved, one changes the applied 
frequency and plots (or otherwise monitors) the amplitude and phase at the 
detecting transducer as a function of applied frequency. This information 
facilitates calculation of the primary acoustic parameters of the liquid, 
namely, the velocity and attenuation of sound. These parameters, in turn, 
can provide information on characteristics such as concentration. The 
plane-wave resonator has also been used to measure the thermodynamic 
properties of a liquid (since the velocity of sound is a simple function 
of the second derivative of free energy with respect to pressure, and 
therefore the profile of sound velocity at different temperatures and 
pressures can be used to derive the equation of state). 
Acoustic absorption occurs as a result of irreversible interaction of 
ultrasonic pressure waves with a liquid and/or with a chemical species in 
the liquid. To distinguish between the absorption due to the pure liquid 
and to a dissolved species, one compares the absorption characteristics of 
the solution against that of the pure solvent, both measured at the same 
temperature and in the same resonator cell. The degree to which absorption 
of the solution exceeds that of the pure solvent reflects the contribution 
of the solute, and therefore its concentration. 
To measure absorption using the plane-wave resonator, one typically 
activates the driven transducer and adjusts the frequency until a standing 
wave is observed. The amplitude and resonance frequency fn are measured at 
peak output voltage (resonance) and at oscillation frequencies above and 
below resonance where the amplitude falls 3 db below peak (the half-power 
level). This procedure is executed for the pure solvent and, separately, 
for the sample under study. 
An important characteristic of a resonator is its quality factor, Q, 
defined as the ratio of the resonance frequency to the half-power 
frequency band, f.sub.n /.DELTA.f.sub.n. Q is inversely proportional to 
the total energy loss in the resonator system, which includes, in addition 
to attenuation due to the liquid, losses from beam divergence, scattering, 
friction, imperfect reflection, and transducer mounting and coupling. High 
Q-factors are associated with symmetry and smoothness of sharp resonance 
peaks and definite separations of resonance peaks in the frequency scale. 
Solute concentration may be derived from comparison of the measured 
Q-factors of the pure solvent and that of the solution. Investigations of 
fast chemical reactions and relaxation processes occurring in solution, by 
contrast, generally involve measurement of the absorption over a range of 
frequencies. 
Measurements of acoustic velocity in a liquid are made primarily to 
evaluate elastic properties, such as compressibility. The natural 
resonance frequencies of a liquid-containing resonator are linearly 
related to the ultrasound velocity. These frequencies may be determined by 
identifying output-voltage maxima (as described above) or by determining 
the inflection points of a phase-frequency plot. For solutions, the 
relative difference between sound velocities in a reference liquid (e.g., 
a pure solvent) and a sample liquid (e.g., a solution) is a linear 
function of the relative difference between resonance frequencies of the 
liquids according to the relation 
EQU (V.sub.s -V.sub.r)/V.sub.r =(f.sub.ns -f.sub.nr)/f.sub.nr 
where V.sub.r is the velocity of sound in the reference liquid, V.sub.s is 
the velocity in the sample liquid, and f.sub.nr and f.sub.ns are resonance 
frequencies of the reference and sample liquids, respectively. The sound 
velocity of a sample is calculated using resonance-frequency measurements 
and knowledge of the sound velocity in the reference liquid. 
Plane-wave resonators, while common, suffer from a number of disadvantages, 
one of which is the necessity for complex constructions to achieve and 
maintain the parallelism conditions required to support standing waves. 
Plots of amplitude as a function of frequency obtained with improperly 
adjusted plane-wave resonators often exhibit field distortions, which may 
be manifested as "humps" indicative of the presence of unwanted 
interference effects, spurious modes, reflective side walls, or 
misalignment of the plane transducers. This is due in large part to the 
mechanical difficulty of achieving and maintaining precise alignment among 
the various resonator components. Also, the production of adequate 
standing-wave patterns requires transducer diameters that are much larger 
than the wavelength (typically, the ratio of diameter to wavelength 
exceeds 20), thus placing relatively large lower limits on resonator 
volumes. 
Resonators of all types are vulnerable to temperature drift, since the 
fluid wavelength of sound in the fluid is highly temperature-dependent. 
Thermostating capability, therefore, is frequently crucial. For example, 
in water, a change of 1.degree. C. alters the speed of sound by 
approximately 0.15%, altering the resonance wavelength by the same 
proportion; this shift is significantly greater than the resonance range, 
and will therefore drive the system out of resonance. For example, using a 
water-filled resonator operating at a resonance frequency of 10 MHz, the 
half-power bandwidth (i.e., the effective resonance range) is 
approximately 1 kHz; a change in temperature of as little as 0.066.degree. 
C. is sufficient to drive the system outside this bandwidth. See Eggers et 
al., "Ultrasonic Measurements with Milliliter Liquid Samples in the 
0.5-100 MHz Range," 44 Rev. Sci. Instr. 969 (1973). 
DESCRIPTION OF THE INVENTION 
Objects of the Invention 
It is, therefore, an object of the invention to facilitate measurement of 
the acoustic parameters of a liquid using apparatus that is simple in 
design and which minimizes field distortions. 
It is another object of the invention to facilitate measurement of the 
acoustic parameters of a liquid using apparatus that does not require 
mechanical adjustment of the resonator. 
It is still another object of the invention to facilitate measurement of 
the acoustic parameters of a liquid in small volumes unachievable with 
existing equipment. 
It is yet a further object of the invention to provide an ultrasonic 
measurement apparatus that delivers stable standing waves without the need 
for high-precision thermostating capability. 
Other objects will, in part, be obvious and will, in part, appear 
hereinafter. The invention accordingly comprises an article of manufacture 
possessing the features and properties exemplified in the constructions 
described herein and the several steps and the relation of one or more of 
such steps with respect to the others and the apparatus embodying the 
features of construction, combination of elements and the arrangement of 
parts that are adapted to effect such steps, all as exemplified in the 
following summary and detailed description, and the scope of the invention 
will be indicated in the claims. 
BRIEF SUMMARY OF THE INVENTION 
The invention utilizes cylindrical acoustic standing waves, generally in 
the ultrasonic region of frequencies, to measure acoustic parameters of a 
fluid (a liquid, most commonly, or a gas) such as sound velocity and 
attenuation. These parameters facilitate determination of solute 
concentrations and allow various physical and chemical processes occurring 
in a fluid to be monitored. 
In preferred embodiments of the invention, cylindrical ultrasonic standing 
waves are generated in a fluid contained within a cylindrical housing by 
causing coherent oscillation of the entire cylinder, or a circumferential 
segment thereof, or multiple circumferential segments thereof. 
A central component of the present invention is the cylindrical resonator 
used to contain fluid and generate cylindrical standing waves therein. The 
resonator may be a radially polarized, cylindrical piezoelectric tube 
having a set of associated electrodes, or a nonpiezoelectric (e.g., glass, 
plastic or steel) tube acoustically coupled to a source of oscillation. 
Either type of resonator is easily manufactured, and provides the symmetry 
necessary for generation of cylindrical standing waves without the need 
for delicate adjustment assemblies. That symmetry also largely avoids the 
field distortions that occur at the edges of planar resonators. The 
electrodes associated with our resonator are connected to electrical 
circuitry to both cause and detect oscillation, and to report acoustic 
parameters. The invention also features a feedback system that ensures 
maintenance of the standing wave condition notwithstanding temperature and 
other environmental variations that cause drift of the resonance 
frequency. 
The apparatus of the invention can be constructed to admit and operate on 
very small volumes, thereby facilitating a wide range of biological 
manipulations and assays for which only very small samples are available. 
For example, volumes of 10 .mu.l or less are readily achieved. The 
invention is also amenable to a variety of applications and 
implementations. Most of these involve pairs of resonators, one containing 
a sample of the liquid under study, and the other containing a reference 
liquid. 
In one implementation, a "dip-in" probe includes a sealed resonator chamber 
containing the reference liquid and a carrier for a second, identical 
resonator that may be immersed directly in a reservoir of the sample 
liquid. In a related "fill-in" implementation, configured as a syringe, 
facilitates suction withdrawal of liquid from a reservoir into the 
resonator. Other implementations facilitate acoustic analysis of liquids 
at elevated surrounding pressures, at different temperatures and on a 
continuous-flow basis. The invention also facilitates simultaneous 
analysis of particle suspensions and the particle-free liquid carrier. 
The invention is also amenable to a variety of control and reporting 
configurations. These may be as simple as meters that indicate amplitude 
and/or frequency, but may also extend to computer-executed algorithms for 
calculating user-specified acoustic parameters from measured quantities. 
The invention can also include programmable software that, in response to 
user selection of a desired parameter, directs the execution of the 
various appropriate measurements as well as processing of the data 
obtained therefrom to report a final value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
1. Basic Apparatus and Operation 
Refer first to FIG. 1, which illustrates the components of the first 
resonator embodiment of the present invention connected to simple 
oscillation and feedback circuitry. The depicted circuit contains a 
tubular transducer, denoted generally by reference numeral 20, that 
includes a cylinder 22 of piezoelectric material (e.g., radially polarized 
barium titanate ceramic, suitable preformed cylinders of which are 
available, for example, from Staveley Sensors Inc., Hartford, Conn.), or a 
non-piezoelectric (e.g., glass or plastic) tube in contact with or at 
least partially surrounded by a piezoelectric film layer, plated with 
metal (such as nickel) to form surface electrodes. Specifically, the 
entire interior surface of sensor 20 is plated to form an inner electrode 
24, which is grounded; and the outer surface of transducer 20 includes at 
least two circumferential electrodes 26, 28. These outer electrodes are 
spaced apart from one another on the surface of cylinder 22. The 
transducer may be provided with electrodes simply by plating the interior 
and exterior surfaces of cylinder 22, then removing narrow exterior lanes 
of plating to create the depicted pattern. 
The illustrated circuit also includes a phase shifter 35 and a phase-locked 
loop 36, which consists of a phase comparator 40, a low-pass filter 42, 
and a voltage-controlled oscillator (VCO) 44. Electrode 28, which serves 
as the detection electrode, is connected to an amplitude-measurement 
device or detector 34, such as a meter, an oscilloscope or a computer 
(through suitable analog-to-digital conversion circuitry) and to a first 
input terminal of comparator 40 via phase shifter 35. Measurement device 
34 includes appropriate low-pass or smoothing filter circuitry to ensure a 
reliable signal. An output terminal of comparator 40 is connected to the 
input terminal of low-pass filter 42, and the output terminal of the 
latter component is connected VCO 44. The output terminal of VCO 44, in 
turn, is connected both to a second input terminal of comparator 40 and to 
electrode 26, which functions as the transmission electrode. A 
frequency-measurement device 45 provides an output indicative of the 
frequency at which VCO 44 operates; this output may be used for parameter 
calculation, to drive a display device, etc. 
Operation of the circuit is based on the fact that the standing wave 
condition is characterized by a certain phase relationship between 
transmitted and reflected waves. This condition is maintained by the 
phase-locked loop 36. The approximate output frequency of VCO 44 is set 
for a particular application when the system is manufactured or selected 
by the user. The precise output frequency, however, is determined by the 
filtered response of comparator 40. A cylindrical standing wave is 
established by adjusting the frequency of VCO 44 until a peak output 
voltage from detection electrode 28 is obtained, or until an inflection 
point of a phase-frequency plot is reached. Either condition arises as a 
result of resonance, which is associated with the standing wave condition 
and therefore indicates its presence. Phase-locked loop 36 is operated to 
maintain the phase existing at detection electrode 28 when resonance is 
reached. The circuit is configured to maintain the appropriate phase 
relationship despite variations in temperature or other conditions that 
alter the sound velocity, and therefore the resonance wavelength, of the 
liquid. Such variations produce compensating changes in the frequency 
output of VCO 44. 
Once the resonance point has been obtained, phase shifter 35 is used to 
alter the output frequency of VCO to bring the resonator to the two 
frequencies corresponding to the half-power levels that flank each 
resonance frequency. The half-power level is reached when the phase 
difference between transmitted and received signals are shifted 
.+-.45.degree. with respect to the center of resonance. Accordingly, the 
phase shifter is operated so as to create these conditions by providing to 
phase-locked loop 36 successive outputs that are appropriately shifted 
with respect to the phase corresponding to the center of resonance. 
It is also possible to utilize a transducer design featuring a single outer 
electrode 47 as part of a feedback oscillator 48, as illustrated in FIG. 
2. The transducer functions as the frequency-determining element of the 
oscillator. Constraining the oscillator to operate in the frequency region 
of the desired resonance condition (e.g., by filter circuitry, as 
discussed below) will result in production of a cylindrical standing wave. 
Because the transducer forms a part of the oscillator circuit, the 
standing wave will be maintained notwithstanding drift of the resonance 
wavelength with temperature. Suitable oscillation circuitry is well-known 
in the art, as exemplified in Sarvazyan, "Development of Methods of 
Precise Ultrasonic Measurements in Small Volumes of Liquids," Ultrasonics, 
July 1982, at 151-54 (the disclosure of which is hereby incorporated by 
reference). 
Refer now to FIGS. 3A-3D, which illustrate various configurations of the 
above resonator embodiments in accordance with the present invention. Each 
configuration utilizes at least two resonators connected in parallel. The 
parallel resonators not only fail to interfere with one another, but also 
provide mutual compensation that obviates the need for the correction 
circuits ordinarily required for single-cell designs. Ordinarily, one 
resonator contains the solution under study and the other contains a 
reference liquid (typically the pure solvent). So long as the resonant 
frequencies of the two resonators are well-separated (e.g., by 10 or more 
times the half-power frequency bandwidth), their parallel connection will 
not interfere with measurement. The second resonator, even if not filled 
with a reference liquid, functions as a balancing element in the circuit 
that compensates for the frequency dependences of the electromechanical 
parameters of the measuring resonator. 
In FIGS. 3A and 3B, the detection electrodes of two to four resonator cells 
20.sub.1, 20.sub.2, 20.sub.3, 20.sub.4 are connected to the phase shifter 
35 and the phase-locked loop 36 described earlier (but which should 
provide integration capability). The resonance condition is ordinarily 
established for a given application and set during manufacture by 
selection of phase conditions corresponding to the appropriate frequency 
of oscillation. Alternatively, the device may be constructed to permit the 
user to adjust oscillation frequency until a peak output voltage is 
obtained, or until an inflection point of a phase-frequency plot is 
reached; phase shifter 35 is set so that phase-locked loop 36 maintains 
the phase associated with the resonance frequency. Half-power frequency 
measurements can be made by varying the phase to .+-.45.degree. with 
respect to the resonance phase, and noting the output frequencies of VCO 
44. 
FIG. 3B illustrates the flexibility of this design, which can be extended 
to two or more pairs of resonator cells 20.sub.1, 20.sub.2, 20.sub.3, 
20.sub.4 so long as sufficient separation between the resonance peaks is 
assured. If the resonators are to contain acoustically similar liquids, it 
is possible to manufacture the cells with slightly different geometries to 
ensure sufficient separation of resonance frequencies. 
FIGS. 3C and 3D illustrate parallel implementations of the resonator 
embodiment shown in FIG. 2. The resonator cells 20.sub.1, 20.sub.2, 
20.sub.3, 20.sub.4 each contain a single outer electrode and a grounded 
inner electrode. The outer electrodes are connected, via a series of 
capacitors denoted generally by reference numeral 60, to the input 
terminal of an amplifier 64 and to the inverting and non-inverting output 
terminals of a second amplifier 66. The output of amplifier 64 is fed back 
to amplifier 66 by way of a phase-locked loop 36 and a phase shifter 35. 
With particular reference to FIG. 3C, leaving both resonators 20.sub.1, 
20.sub.2 empty or filling them with the same fluid results in precise 
cancellation of the electrical signals from amplifier 66, since their 
magnitudes will be equivalent and their phases opposite; the result is 
zero voltage at the input of amplifier 64. 
When one of the cells resonates, the resulting voltage at the input to 
amplifier 64 is very large and due almost entirely to the resonating cell. 
Once again, phase conditions corresponding to the resonance condition are 
established and fixed during manufacture or by the user. Phase-locked loop 
36 maintains this condition, and the frequency indicated by the 
measurement device 45 will then be the resonant frequency of the sample 
contained in the resonating cell. Phase shifter 35, which may be located 
on either side of phase-locked loop 36, is used to confine circuit 
operation to the resonance condition. 
2. Preferred Device Implementations 
A representative dip-in probe embodying dual matched transducers, as shown 
in FIGS. 3A and 3C, is illustrated in FIG. 4. The probe comprises a 
housing 102 having therein cylindrical cavities that each accommodate a 
resonator and open into a window therebetween. A first cavity 104 contains 
a first resonator 20.sub.1, and is collared over the resonator ends to 
create a pair of apertures 106, 108 that, together with resonator 
20.sub.1, define a flow channel. Accordingly, when immersed in a liquid as 
shown in the figure, the liquid is free to flow through the resonator as 
indicated by the arrows. A second cavity 110 contains a second resonator 
20.sub.2. Unlike the first resonator, resonator 20.sub.2 is fluidically 
sealed within cavity 110 by means of a gasketed upper wall and a threaded, 
removable cap 112. Cap 112 screws into the lower extremity of cavity 110 
and seals by means of a gasket or the like. 
Housing 102 is sealably joined to an elongated conduit 115, which carries 
one or more cables 117 that connect resonators 20.sub.1, 20.sub.2 to the 
electrical circuitry described previously. Cable 117 passes through 
housing 102 into the window therebetween, where it is electrically 
connected to the electrodes of both resonators. 
In operation, with cap 112 removed, resonator 20.sub.2 is filled with a 
reference solution and cap 112 then replaced. The probe 100 is immersed in 
a sample solution, and the circuitry of FIG. 3A or 3C operated to create 
cylindrical standing waves in each of the resonators. For attenuation and 
velocimetric studies, the resonance and the half-power frequencies are 
measured for one or more resonances. It should be emphasized that while 
this and ensuing figures depict implementations that feature a single pair 
of resonators, this is for convenience of illustration only; multiple 
pairs of resonators can be introduced by straightforward modification of 
the depicted designs. 
FIG. 5A illustrates incorporation of the resonators of the present 
invention into a sealed vessel that accommodates measurement of fluid 
characteristics at high pressures. The measurement device 150 includes a 
rigid shell 152 whose interior holds at least one pair of resonator 
assemblies, whose construction is detailed in FIG. 5B. Each resonator 
assembly 160 includes a resonator cell 20 whose ends are fitted tightly 
within a pair of elastomeric sleeves 162, 164. Each sleeve is itself 
surrounded by a protective metal jacket 166, 168. A slot 170 through part 
of the perimeter of jacket 168 exposes a portion of sleeve 164 to the 
surrounding high-pressure atmosphere within shell 152. The space between 
sleeves 162, 164 exposes a portion of resonator 20 to accommodate 
electrical connection. The ends of sleeves 162, 164 opposite the resonator 
20 are sealed by a pair of tightly fitting plugs 172, 174. 
With reference to FIG. 5A, the sealed resonator assemblies are carried 
within an opposed pair of cups 176, 177, which receive the ends of metal 
jackets 166, 168. Projecting outwardly from the base of each cup 176, 177 
is a mounting pin 178, 179. The cups are borne on a mounting bracket that 
includes a pair of opposed retaining platforms 180, 182, each of which 
contains a bore for receiving a pin 178, 179. A shaft 184 projects from 
platform 180 and fits through a central bore in opposed platform 182, 
terminating in a series of threads that are engaged by a nut 186. 
Tightening nut 186 anchors the resonator assemblies, whose ends are 
contained within cups 176, 177, between the platforms. The pin 179 from 
each lower cup 177 protrudes through platform 180 sufficiently to engage 
bores through a table 188. 
One end of shell 152 contains an inlet 190 that admits pressurized fluid 
(ordinarily a liquid) into the interior of the shell. The opposite end of 
shell 152 receives a threaded plug 192 that bears a series of electrical 
contacts 195 to establish connection between the resonators and external 
circuitry. The contacts 195 reside within lined, tapered channels that 
prevent ejection of the contacts as a result of the high-pressure 
environment. A central depression in the interior face of plug 192 
receives one end of a collared shaft, the other end of which projects into 
a central bore in table 188. The collar spaces the underside of table 188 
from the top surfaces of electrical contacts 195, which are wired or 
otherwise connected to the resonator electrodes. Plug 192 includes an 
0-ring or other gasket to provide a pressure seal when the plug is 
threaded into shell 152. 
The foregoing mechanical arrangement allows the resonator assemblies 160 to 
be secured within the mounting bracket before the latter is coupled to the 
plug 192. In operation, plugs 172, 174 are removed from each resonator 
assembly, facilitating the introduction of a liquid sample therein. The 
removed plugs are then replaced, the resonators are secured within the 
mounting bracket, electrical connections between contacts 195 and the 
resonators are established, and the mounting bracket is coupled to plug 
192. After plug 192 is threaded into shell 152, pressure-transmitting 
fluid is admitted into the interior of shell 152 via inlet 190. As the 
pressure increases, elastic tube 164, which is exposed to the interior 
atmosphere through slot 170, bows inwardly and thereby communicates the 
surrounding pressure to the liquid. Resonance acoustic measurements 
through the liquid under pressure may then be taken. 
FIGS. 6A and 6B illustrate a resonator assembly adapted for fill-in 
applications, where a sample of liquid is suctioned from a reservoir into 
the resonator channel. The resonator assembly 200 shown in the figures 
includes a sealed body 205 that carries therein a pair of resonator cells, 
one of which is connected to a flow tube 207 and the other to a 
reference-liquid carrier 209. Both of these elements project radially 
through body 205. 
As shown in greater detail in FIG. 6B, flow tube 207 actually consists of a 
pair of fluid connectors 207a, 207b sealed with respect to the exterior of 
body 205 and sealably joined to a resonator cell 20 to form a fluid 
channel therethrough. In the illustrated embodiment, the resonator 20 is 
the three-electrode embodiment shown in FIG. 2, with inner and outer 
electrodes connected to a pair of prongs 210, 212 that span body 205. 
Reference-liquid carrier 209 is a hollow tube that also includes a 
resonator cell 20 (not shown) contiguous therewith inside body 205; that 
resonator, too, is connected to a pair of prongs, one of which is shown at 
214, that protrude through body 205 for external connection. 
Reference-liquid carrier 209, for reasons discussed below, is preferably 
oriented perpendicularly to flow tube 207, and includes a removable cap at 
one end. The cap is withdrawn to permit introduction of a sample liquid 
therein. When replaced, the cap forms a seal that prevents entry into 
reference-liquid carrier 209 of liquid from the reservoir in which the 
resonator assembly 200 may be immersed. 
FIG. 7A illustrates a dip-in syringe-type device incorporating the sensor 
assembly 200. The device 230 includes a first generally tubular segment 
235 whose interior bore 237 mates with flow tube 207 of the resonator 
assembly at a first end, and whose exterior at that end flares outward and 
forms a ridge to accept the upper edge of a cylindrical wall 239. The 
lower edge of wall 239 is sealably joined to a similar ridge in a second, 
tapered tubular segment 241, the bore of which mates with the other end of 
flow tube 207. The fluid-tight chamber thus formed within wall 239 
contains the resonator assembly 200. 
For this application, the resonator design of FIG. 3A or 3B is preferred. 
In this case the prongs connected to the common resonator electrodes are 
connected together, and these prong sets are soldered to a pair of opposed 
contact plates 245, 247 that clip through slots in wall 239. At least the 
removably capped end of reference-liquid carrier 239 is long enough to 
protrude through an aperture 251 in wall 239. 
The other end of tube segment 235 widens in its interior to form a 
compressible bladder 253. The top of tube segment 235 is closed by a 
removal cap 255. 
By means of its exposed electrical contacts, the illustrated configuration 
may be joined directly to a housing 260 that contains the circuitry 
discussed above in connection with the resonator cells, as well as control 
and display elements tailored to a particular application. For example, 
using programmable microcomputer circuitry, it is possible to implement 
selectable control protocols that perform user-specified measurements and 
calculate desired acoustic parameters. Thus, using keyboard 262, the user 
may first obtain readings from one or more samples of known concentration 
and electronically store these, along with the associated concentrations, 
as calibration points; then, switching modes from calibration to 
measurement, the user may obtain a reading from an unknown sample and, 
based on the stored calibration points, compute the concentration of the 
sample for display. In other measurement modes of operation, the user may 
select attenuation (in which case the display 264, in conjunction with 
appropriate circuitry, serves the function of measurement device 34) or 
acoustic velocity (in which case peak and half-power frequency 
measurements are taken from the sample and the reference solutions, and a 
velocity calculated as discussed above) or resonance frequency (in which 
case display 264, in conjunction with appropriate circuitry, serves the 
function of measurement device 45). The programming and circuitry to 
implement the foregoing functions are straightforwardly realized without 
undue experimentation by those skilled in the art. 
Electrical connection between contacts 245, 247 and the circuitry within 
housing 260 is made via complementary contacts on the interior surface of 
a clamp 266, facilitating convenient docking of the device 230. A second 
clamp 268 grips upper portion of device 230. In operation, the user first 
introduces a reference liquid into carrier 209. The user then compresses 
bladder 253 and immerses the inlet to tube 241 into a reservoir of the 
liquid under study. Releasing the bladder draws liquid into flow tube 207, 
and the vacuum thereby created prevents its release. The filled device may 
then coupled with housing 260 as described above. 
A variety of modifications to this basic configuration are possible. A more 
rugged, precise syringe design can include a spring-loaded plunger that 
retracts to withdraw precisely the amount of sample liquid necessary to 
fill the resonator cell. Composite devices including multiple syringe 
elements, simultaneously operated by plungers joined to a common yoke, are 
useful in performing analyses of biological samples contained in 
microtiter plates. The resonator embodiments illustrated in FIGS. 3B and 
3D are especially useful with such composite devices. 
Another useful configuration embodying the present invention is illustrated 
in FIGS. 8A-8C. This flow-through embodiment permits continuous monitoring 
of a liquid stream whose composition changes over time. Such capability is 
of particular value, for example, in analyzing effluent from separation 
columns, liquid chromatography columns, and flow streams from chemical and 
pharmaceutical manufacturing processes, where even small compositional 
changes can prove critical. The apparatus is mounted on a support block or 
structure 300, and includes a first flow tube 302 having ends 304, 306; a 
second flow tube 310 having ends 312, 314; a four-port valve 320 for 
selectably connecting the tube ends; and a pair of sensor elements 322, 
324, each associated with one of the flow tubes; and an outlet tube 326. 
FIG. 8B shows a sensor element in greater detail. The element includes a 
resonator cell 20, the electrodes of which are wired to external circuitry 
as described previously. The resonator 20 intervenes along and is 
contiguous with its associated flow tube to maintain a continuous fluidic 
pathway therethrough, and is sealably joined at each end to spaced-apart 
sections of the tube. 
The four-port valve, as shown in FIG. 8C, contains interior elbow joints 
and controls the connections among tube ends 306, 312 and 314, and outlet 
tube 326. In a first position 330 (also illustrated in FIG. 8A), end 306 
of flow tube 302 is connected to outlet tube 326, while tube 3 10 forms a 
closed loop. In a second position 332, flow tubes 302 and 310 join one 
another to form a continuous fluid path from inlet 304 to outlet tube 326. 
In operation, valve 320 is initially set to the second position 332, and a 
reference liquid introduced into inlet 304 until it is observed exiting 
outlet tube 326. At this point, liquid occupies the entire flow path. 
Valve 320 is then shifted to position 332, trapping the reference liquid 
within the closed loop of tube 310. Sample liquid entering inlet 304 
eventually displaces the reference liquid. The acoustic characteristics of 
the sample liquid can be continuously monitored against the trapped 
reference liquid. 
FIG. 9 illustrates an implementation of the invention that facilitates 
acoustic analysis of a liquid at multiple temperatures. This capability 
permits, for example, simultaneous analysis of multiple physical 
characteristics that each exert an acoustic effect. For example, the fat 
content of milk cannot be assessed using measurements of the speed of 
sound and attenuation at a single temperature, since both nonfat solids 
and fat independently contribute to changes in these acoustic parameters. 
However, the temperature dependence of these parameters are different for 
nonfat solids and fat (the former being more strongly dependent on 
temperature), so measurements of both the overall speed of sound at 
different temperatures specify the individual levels of each type of 
material. 
As shown in the figure, the device 400 includes a sturdy, thermally 
conductive (e.g., metal) outer housing 402, partially filled with an 
insulating material 404 (such as rubber or fiberglass) to form a cavity 
therein. Contained within the cavity are first and second resonator cells 
410, 412, each housed within a thermally conductive casing 414, 416. The 
individual resonator cells 410, 412 each include a resonator cell 20 
packaged so as to be in thermal communication with the associated casing 
414, 416. The resonator cells 20 intervene along and are contiguous with a 
flow tube 420, which draws liquid from a three-way stopcock valve 422 and 
ejects it from an outlet 425. Sandwiched between casings 414 and 416 is a 
first Peltier element 430. A second Peltier element 432 is disposed above 
casing 414, sandwiched between this casing and an inner wall of housing 
402. The Peltier elements are thermoelectric devices comprising a junction 
of two dissimilar metals; a current flowing through the junction causes 
either absorption or liberation of heat, depending on the direction of the 
current, in approximate proportion to its magnitude. In the present 
invention, the devices are used in combination to maintain a controllable, 
fixed temperature difference between casing 414 and 416. Specifically, 
Peltier element 430 is controlled to maintain the fixed temperature 
differential (for example, 5.degree. C., in the case of milk analysis) 
between casing 414 and 416, with 416 maintained at the hotter temperature; 
and Peltier element 432 is operated as a heat pump to maintain casing 414 
at a selected temperature, conducting excess heat to housing 402 and a 
series of cooling fins 440 thereon. The temperature monitoring and 
feedback circuitry necessary to accomplish these actions is well-known by 
those skilled in the art. 
The efficiency of convective heat removal from the cooling fins 440 may be 
further enhanced by locating a fan 442 thereabove and directing its 
airflow against the fins. 
Valve 422 accepts incoming liquid from either of two inlet tubes 445, 447. 
In operation, one of these tubes carries the sample liquid, and the other 
a flush liquid. Valve 442 is initially set to admit the sample liquid, 
which is provided until both resonator cells are filled. Before 
measurements are made, the liquid is allowed to reside in the cells until 
Peltier elements 430, 432 re-establish the desired temperature. The 
resonators are then controlled, as discussed previously, to measure one or 
more acoustic parameters of the sample liquid. After the measurements have 
been taken, valve 422 is switched to admit flush liquid, which cleans the 
system. 
In some cases it is useful to measure the concentration not of dissolved 
substances, but of particles in suspension (e.g., biological cells in a 
culturing medium). In such cases, the total concentration of particles is 
ordinarily small compared with the concentration of molecules or ions in a 
typical solution, and their contribution to the speed of sound is 
therefore one or two orders of magnitude lower than that of dissolved 
substances. Accordingly, to use acoustic techniques to measure the 
concentration of particles suspended in a liquid that itself contains 
dissolved substances, it is necessary to subtract the much larger acoustic 
contribution of the dissolved substances themselves. The embodiment 
illustrated in FIG. 10 is a fixture that facilitates this procedure by 
allowing simultaneous acoustic analysis of a particle suspension and the 
pure, particle-free carrier liquid. 
The fixture 500 includes oppositely oriented resonator enclosures 502, 504 
affixed by means of a hinge 506 and a releasable fastening clip 508. Hinge 
506 and clip 508 are mounted so as to accommodate but firmly retain, upon 
closure, a membrane filter 510 between enclosures 502, 504. Each enclosure 
includes an inlet port 512a, 512b and a cavity 514a, 514b. Mounted within 
each cavity 514a, 514b is a resonator cell 20. Electrical connection to 
the electrodes of the resonator cells 20 can be facilitated, for example, 
by wiring them to external plate contacts or plugs on the outer surface of 
enclosures 502, 504. 
Each cavity 514a, 514b widens past the side opposite the inlet port to 
define a conical reservoir 516a, 516b. The resonators 20 are mounted 
within the cavities so as to define a continuous fluidic pathway, 
interrupted only by membrane filter 510, between inlet ports. In 
operation, the particle-containing liquid is introduced into one of the 
inlets 512a, 512b. Particles cannot traverse membrane filter 510, and are 
thereby prevented from entering the other resonator enclosure. The carrier 
liquid, however, passes freely through filter 510, eventually filling 
resonator associated with the other inlet, where it exits. At this point 
the flow is stopped so that particles, which build up within the reservoir 
associated with the first inlet, do not back up into the associated 
resonator and distort acoustic measurements. Those measurements are taken 
in the manner heretofore described. 
As an alternative to the membrane, one or both of the resonators can be 
configured as shown in FIG. 2 of copending application Ser. No. 08/241,296 
filed on May 11, 1994, entitled METHOD AND APATUS FOR MANIPULATING, 
ANALYZING AND SELECTIVELY ISOLATING SUSPENDED TICLES USING CYLINDRICAL 
ULTRASONIC STANDING WAVES (filed contemporaneously herewith and hereby 
incorporated by reference). In this case a first set of resonator 
electrodes is used to retain particles within a cylindrical standing wave, 
preventing their travel to the other resonator, while a second set of 
electrodes produce a cylindrical standing wave used to measure the 
acoustic parameters under investigation. 
It will therefore be seen that we have developed highly versatile methods 
and apparatus for the evaluation of acoustic parameters associated with 
liquids under various conditions. The terms and expressions employed 
herein are used as terms of description and not of limitation, and there 
is no intention, in the use of such terms and expressions, of excluding 
any equivalents of the features shown and described or portions thereof, 
but it is recognized that various modifications are possible within the 
scope of the invention claimed.