(19 Aug 04)

In the section on page 3 on Continuous Flow Cells, reference is made to a Flow Through Horn, and one is included in the illustration, but let me expand upon that slightly here.  First, here is a stand-alone sketch of a flow through horn:

(19 Aug 2004 sketch by and © 2004 S. Berliner, III all rights reserved)
Flow Through Horn

Flow-through horns can be used to inject a fluid into a sample prior to or during sonication, as well as to withdraw sample during or after sonication.  They can also be used in sparging or aerosoling; contact your equipment supplier (or the author).

Explosion Resistance

(19 Aug 04)

The author has been able to get "explosion resistant" certifications for numerous installations following these simple precautions.  Schematically, a convertor has to be fabricated or modified to allow for a low-pressure inert gas feed:

(19 Aug 2004 sketch by and © 2004 S. Berliner, III all rights reserved)
Explosion-Resistant Convertor

The generator must be isolated from the work area and the intervening wall or cabinet should be fitted with a gas-tight bulkhead connector rated for the same current and voltage (or higher) as the high-frequency cable itself.  A second high-frequency cable is used in series with the bulkhead connector and the regular cable:

(19 Aug 2004 sketch by and © 2004 S. Berliner, III all rights reserved)
Explosion-Resistant Set-Up

ULTRASOUND - Sonar, Imaging, NDI/NDE, and HIFU

Let us spend a moment delving into the areas of "ULTRASOUND" that are NOT covered under the umbrella of "Ultrasonics", the use of high-intensity acoustic energy to change materials (this topic is often requested when I lecture on ultrasonic processing).  I use the arbitrary semantic distinction between "ultrasonics" and "ultrasound" to cover these related, but vastly different, fields.  In both, acoustic energy is propagated by a transducer (electrostrictive or magnetostrictive) driven (most commonly) by an electronic generator or power supply.  The big difference comes in how that energy field is created and what it does.

In both ultrasonics and ultrasound, the energy is radiated outward at frequencies above human hearing, generally 20KHz (20,0000 cycles per second) and in both that frequency can range upwards into megahertz (MHz - a million or more cycles per second).

In ultrasonics, the energy field is created at very high intensity to change materials, deliberately to create cavitation in a liquid (for processing or cleaning) or to create friction to melt materials (welding and bonding).

In ultrasound, the energy field is created at low intensity to examine materials, without cavitation.  Among the many uses of ultrasound, the most common are sonar (Sound Navigation and Ranging, also known as echo-location), imaging, and NDI/NDE (non-destructive inspection or evaluation).  A signal is sent out, reflects back, and is detected.

In its most basic form, the time it takes for the signal to propagate, reflect, and return, can be interpreted as the distance from the radiating surface to the reflector.  In sonar, as used both by marine mammals, bats, and ships and submarines, the reflector can be food, enemies, or obstacles.  In fact, dolphins can actually determine the contents of another animal's stomach, so finely tuned are their echo-location and reception.  Changes in the medium (salt water vs. fresh, temperature gradients, static pressure, particulate suspensions, etc.) all contribute to background noise and degradation of signal but techniques have been developed to minimize their effects.

Two other widely-used applications of sonar are depth-finding and fish-finding; every commercial vessel and many private vessels carry one or both devices.

Similarly, the acoustic signal can be used to probe tissue (flesh) to generate multiple reflections which can then be assembled by computer to yield a two-dimensional, and even three-dimensional, picture of the area under examination.  The most commonly-known use is for mammography, examining the breast for tumors.  Tumors and other abnormalities yield images which differ from normal, healthy tissue and so can be detected.  Another ultrasound technique rapidly becoming well-known is the CAT scan (Computerized Axial Tomography), in which "slices" of the body are made by ultrasound and then assembled in the computer to generate a whole-body image.

Lastly, and least known to the general public, NDI/NDE (non-destructive inspection or evaluation) has long been used to find flaws in materials, especially in metals (NDI has long been denigrated; NDE is the current term of art).  One very visible evidence of this is the ubiquitous yellow Sperry Rail Services cars which run all over the railroads of the United States and Canada, probing the rails for defects by sending an ultrasound pulse out and measuring the distance to the crack or void from which the energy reflects.  Ultrasound testing is also used in all steel mills, in structural engineering, and in pipeline work.

There is a "grey" area where the two disciplines overlap; this is in medical applications.  Therapeutic ultrasound is the use of ultrasonics to heat tissue for relief from pain and inflammation.  Minimally-invasive surgery uses both imaging to locate the work and ultrasonic ablation and cutting to remove tissue and also for fat removal (liposuction).  HIFU (High-Intensity Focused Ultrasound) is a burgeoning discipline in which acoustic energy is focused to a point (much as a burning glass focuses light) to destroy tumors, prostate cancer, and kidney stones and gallstones, usually in conjunction with imaging.  In HIFU, the energy traverses intervening tissue without causing damage and then becomes concentrated and heats tissue at the focal point to destroy it.

A combination of radar and ultrasound was used early on to treat kidney stones; phased-array radar used many widely-spaced smaller radar stations to give broad coverage and yet very high resolution by focusing them on a common target.  Applying this technique to ultrasonics, kidney stones are exploded by immersing the body in a tank of water and focusing a large array of small transducers on the stone; convergence of the energy in the stone shatters it into pieces small enough to be passed naturally.

While these applications may use large amounts of energy (a large sonar array can use as much power as a small city), they generate the radiation at low intensity.  Even in HIFU, the energy leaves the radiating surface at low intensity.  The radiator ocscillates at low amplitude.  Quite to the contrary, ultrasonic processing equipment causes the radiating surface to oscillate at high amplitude.

To truly understand the full ramifications of acoustic energy, these fine distinctions should be noted.

In addition to the medical applications of ultrasound which overlap into ultrasonics, there are two widely-used areas of pure medical ultrasonics - these are dental prophylaxis (tartar removal by direct probe impact and gum treatment by ablation) and phæco-emulsification (removal of the lens by ablation in cataract surgery).  Not as widely used but equally effective is direct probe application in cautery and débridement of wounds and skin cancers.  Direct probe impact has also been used to break liver, kidney and gall stones as a minimally-invasive technique.

Quacks and Failures in Ultrasonics.

There is a lighter, as well as a sadder, side to ultrasonics; I speak elsewhere of the ultrasonic oil burner, the ultrasonic carburet(t)or, and the ultrasonic dishwasher, all perfectly feasible, but all economically disastrous. However, the ultrasonic venetian blind cleaner, at which many of us had the galloping hee-haws, is very much a reality, often truck mounted to go to the point of use quickly and easily.  For all the great men, such as Lord Rayleigh and Norm Branson, who each made important contributions in their ways that changed our corner of history, there are ten or more that blew it.  One of those was the reclusive John Ernst Worrel Keeley (1827-1898), whose work on Sympathetic Vibration brought him more scorn than praise; yet, one wonders what he was up to.

Worrell and others like him are given brief bios at the Science Section of the Engine Room, a quirky site, but fun.

Dr. Wilhelm Reich, of Orgone Box fame/infamy, also seems to have his fling with acoustics.

Similarly, the famous Hermann Ludwig Ferdinand von Helmholtz oscillated (if you'll pardon the pun) from a brilliant world-renowned scientist to a reviled quack; he invented and built an acoustic resonator that also functioned as a motor and built a speedboat that functioned well on that principle but it never got anywhere (ooh, another pun).

And so it goes - one man's hero is another man's crackpot, one's genius another's idiot savant.

You may wish to visit the main ULTRASONICS page, et seq., with more on ultrasonics, as well as the Ultrasonics Cleaning page {in process} and the Ultrasonics Glossary page {also in process}.

Those persons interested in SONOCHEMISTRY might wish to look at the sonochemistry pages of: