When imaging, monitoring, or stimulating samples in a scattering medium, even the most powerful optical microscopes and probes are hindered by the diffusion limit, the length scale beyond which the incident light uncontrollably scatters. To overcome that limit, some techniques focus the wavefront as it propagates through the sample; others iteratively shape it to amplify the target signal. Lihong Wang and colleagues at Washington University in St. Louis have developed a new approach that combines time reversal with ultrasound, whose waves scatter weakly in biological tissue, to focus light to a controllable position. (For an introduction to multiwave imaging, see the article by Mathias Fink and Mickael Tanter in Physics Today, February 2010, page 28.) In the team's setup, laser light, frequency-shifted by two acousto-optic modulators in series, entered the sample medium, where the diffused light was further modulated by an ultrasonic wave tuned to the frequency shift. The interaction between the light and the ultrasound produced a virtual point source within the sample. From a holographic record of the modulated diffused light, the researchers generated a time-reversed trajectory that produced optical focusing at the virtual source location. The new technique, known as time-reversed ultrasonically encoded (TRUE) optical focusing, generated a noticeably higher contrast for objects inside a slab of synthetic biological tissue than was attained by conventional ultrasound-modulated optical tomography, which cannot focus light. (X. Xu, H. Liu, L. V. Wang, Nat. Photonics, in press, doi: 10.1038/nphoton.2010.306M.)—Jermey N. A. Matthews
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At the heart of the Balinese percussive orchestra known as a gamelan is the large gong called the gong ageng wadon. It features a large, protruding dome or boss in the middle; when the boss is struck with a padded mallet, the gong produces a pronounced acoustic beating or ombak (meaning "wave"), as can be heard in this sound file. Using acoustical and vibrometric analyses, David Krueger and his colleagues at Brigham Young University have studied the sources of the ombak. Although some beating was found to come from asymmetric vibration modes with closely spaced frequencies, those appear to contribute mostly to the gong's timbre. The more significant contribution arises from the gong ageng wadon's nonlinear structural response. The gong's two dominant vibration modes, both axially symmetric, have nearly a 2:1 frequency ratio. That relationship gives the gong its perceived pitch, but the ratio isn't exact. So when the gong is struck, causing displacements large enough to produce overtones, the fundamental generates harmonics and interacts with the second axisymmetric mode to yield sum and difference frequencies. The resulting sound spectrum features strong peaks of similar amplitudes that are spaced only a few hertz apart and give rise to the distinctive sound of ombak. (D. W. Krueger, K. L. Gee, J. Grimshaw, J. Acoust. Soc. Am. 128, EL8, 2010.)—Richard J. Fitzgerald
A collaboration led by Pascal Martin of the Curie Institute in Paris yoked live and virtual cells to tackle the question, How does the ear amplify faint signals by factors of up to 1000? Hearing relies on converting mechanical vibrations to electrical impulses. The transduction takes place in the cochlea and is carried out by micron-scale hairs that sprout from specialized hair cells. To the tiny hairs, the watery liquid that surrounds them is viscous, just like honey would be to a tuning fork. An active feedback mechanism in the hair cells not only overcomes the viscosity, but also amplifies faint signals. But the feedback can't provide all, or even most, of the ear's prodigious amplification. Two years ago, theorists proposed that elastic coupling among groups of hair cells could make up the shortfall. Coupling, they argued, lowers the detection threshold by averaging out each cell's noise fluctuations. Hair cells in frogs, humans, and other terrestrial vertebrates are indeed coupled to each other by various flexible structures. To prove that coupling boosts amplification, Martin's team extracted a vibration-sensing organ from a bullfrog and attached a flexible whisker to one of the hair cells (see schematic). Through sophisticated computer control, the whisker was made to vibrate in a way that mimicked the net force on the live cell (green) exerted by two neighboring hair cells (gray). The expected enhancement to the amplification showed up. (J. Barral et al., Proc. Natl. Acad. Sci. USA, in press.)—Charles Day
The brain localizes the source direction of a pure tone at low frequency by interaural phase difference (IPD), and at high frequency by interaural level difference (ILD), a logarithmic measure of the ratio of sound intensities at the two ears. (See Physics Today, November 1999, page 24.) Localization by IPD shuts off abruptly around 1 kHz, where phase ambiguity could cause a disastrous 180° mistake. But nature doesn’t protect us from all acoustic misinformation. At frequencies up to 4 kHz, wavelengths are still comparable to the size of the head, so diffraction around the head might be misleading. At much higher frequencies, where diffraction is negligible, the head casts a proper acoustic shadow and ILD is a reliable clue to how far the source is off to the left or right. A new paper by Eric Macaulay and coworkers in the Psychoacoustics Group at Michigan State University compares sound-localization attempts of test subjects at 1.5 kHz with wave-propagation calculations that predicted they should often be badly misled by a diffractive phenomenon analogous to Fresnel’s optical bright spot. And indeed they were. The acoustic bright spot is a diffractive enhancement in the middle of the shadow cast by the head. The MSU results show that the effect consistently misleads hearers by spoiling the monotonic growth of ILD with increasing departure of the source from the forward direction. The photo shows a tiny unobtrusive microphone being put in a subject’s ear in the group’s anechoic test room to measure ILDs and correlate them with his guesses about source location. (E. J. Macaulay, W. M. Hartmann, B. Rakerd, J. Acoust. Soc. Am., in press.) —Bertram Schwarzschild
Loud, fast, rhythmic music is exciting. It mirrors aspects of human behavior—we are loud and active when excited. Likewise slow, soft, static music mimics how we behave when subdued. That congruence might explain why music affects us as it does. Also, melodies in a major key generally come across as happy; minor melodies are sad. But why those tonalities have their particular emotive effects is not clear. Now, Duke University neuroscientist Dale Purves, his graduate student Daniel Bowling, and colleagues report that qualities of major- or minor-key melodies also mirror human behavior—specifically, speech—according to the mood of the speaker. One of their analyses focuses on the intervals (tone pairs) implied by melodies. Major-key melodies emphasize the major-third interval, whose two notes have a frequency ratio of about 5:4. Minor-key melodies feature an interval, the minor third, with a 6:5 frequency ratio. How is that interval dichotomy mirrored in speech? The Duke team asked 20 subjects to read a word or short passage in an excited or subdued manner (click the figure below to enlarge). They then analyzed the ratios of the two lowest (and strongest) frequencies of vocal-tract resonance associated with each vowel sound. The prevalence of major-third intervals (5:4 ratios) as compared with minor thirds (6:5) was much greater in excited than in subdued speech. Musically speaking, at least, our thrills are major; our disappointments, minor. (D. L. Bowling et al., J. Acoust. Soc. Am., in press.) —Steven K. Blau
The phenomenon of dynamic stabilization can be demonstrated with an inverted pendulum: If the pivot point vibrates fast enough and strongly enough, the pendulum aligns with the vibration direction and can stably stand upside down, even at an angle, seeming to defy gravity. Physicists Greg Swift and Scott Backhaus (Los Alamos National Laboratory) looked at an analogous situation with gas in a so-called pulse tube that has one end much hotter than the other. Colder gas is denser and therefore sinks below the hotter gas; a vertical tube with the cold end down is like an undisturbed pendulum with the heavy bob at the bottom. However, raise the cold end above the hot end and convection sets in—the cold gas falls due to gravity and the hot gas rises in a natural convective flow. Such orientation-dependent effects are undesirable for cryogenic thermoacoustic pulse-tube refrigerators, like the commercial one shown here, in which the gas is used to transmit acoustic power but not heat. (For more on thermoacoustics, see Physics Today, July 1995, page 22.) Swift and Backhaus found that suppression of convection when these refrigerators run at high enough frequency and amplitude is related to the well-understood stabilization of the inverted pendulum. Although their experiments and theoretical analysis are beginning to unravel the essentially nonlinear physics at the core of the system, many mysteries remain, including the actual role of the oscillating pressure. (G. W. Swift, S. Backhaus, J. Acoust. Soc. Am. 126, 2273, 2009.) —Stephen G. Benka
When a person’s head strikes, or is struck by, another object, it accelerates. As it begins to decelerate, the brain slams into the skull, then bounces off and oscillates until the impact energy dissipates. The resulting shear and compressive strains can lead to brain damage. But in battlefield explosions, just the acoustic waves alone can cause soldiers traumatic brain injuries. To better understand that process, Lawrence Livermore National Laboratory's William Moss and Michael King and the University of Rochester’s Eric Blackman compared numerical simulations of a head colliding with a wall to one being struck by an explosion’s blast waves. Despite accelerating the head at less than half the rate of the wall collision, the simulated blast produced on the brain surprisingly comparable pressure spikes—ranging up to 3 bars—and even larger pressure gradients. Apparently, those mechanical loads are delivered by the skull, which ripples under the pressure of blast waves—the rippling motion is indicated in the image by velocity vectors. The researchers confirmed the role of the skull’s elasticity by making it 1000 times stiffer in their simulations and observing a fivefold drop in the pressure spikes. The simulations also revealed that helmets in contact with the head can impart an additional mechanical load to the skull and helmets that allow for an air cushion geometrically focus and increase the magnitude of blast waves. (W. C. Moss et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews
In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews
To understand how babies and children learn to process music and other sounds, it's important to know what they can do at birth. Researchers in Hungary and the Netherlands, led by István Winkler of the Hungarian Academy of Science in Budapest, have found that three-day-old infants can distinguish the downbeat in a musical rhythm — that is, the "one" in "one, two, three, four." They used electroencephalography — the detecting of electrical activity in the brain via electrodes affixed to the head, as shown in the photo — to monitor the babies' reactions to a repeating synthesized drum rhythm from which notes were sporadically left out. When the omitted sound was a downbeat, the electrodes picked up a strong discriminative response, but when a note in any other position was left out, the infants' response was much weaker. In music played by human musicians, the downbeat is often longer or louder than the surrounding notes, but that was not the case for the computer-generated sound sequence used in the experiment: The downbeat was distinguished by the arrangement of sounds alone. The result suggests that beat perception is either innate or learned in the womb. (I. Winkler et al., Proc. Nat. Acad. Sci. USA, in press; photo courtesy of Gábor Stefanics, Hungarian Academy of Sciences.) — Johanna L. Miller
Several experiments are operating or being built to detect astrophysical neutrinos. Ranging up to about a cubic kilometer in size, those experiments are embedded in ice or in a liquid such as water, where they watch for telltale flashes of Cherenkov radiation. (See the article by Francis Halzen and Spencer Klein in Physics Today, May 2008, page 29.) But the highest-energy neutrinos, with energies of an exaelectron volt (1EeV = 1018 eV) or higher, are so scarce that installations spanning 100 km3, along with massive numbers of expensive photomultiplier tubes, would be needed to collect adequate event statistics in a reasonable time. So other detection schemes are being explored, one of which involves acoustics: When a very–high-energy neutrino interacts with water or ice, a sudden localized thermal expansion occurs and the resulting wave propagates farther than the light flashes. To explore that method, the Aachen Acoustic Laboratory was set up in late 2007 and its first experiment made a precise measurement of the speed of sound in ice that is entirely devoid of bubbles and cracks. The Aachen physicists carefully positioned an array of sensors—six detectors and one emitter—in a 3-m3 water tank (shown here) equipped with a freeze-control unit and a degassing system. The difference in arrival times of an acoustic pulse at adjacent receivers determined the speed of sound. Between 0 °C and −17 °C, where they took measurements, the speed ranged from about 3840 m/s to 3890 m/s, agreeing well with earlier laboratory experiments. The team is also part of SPATS (the South Pole Acoustic Test Setup), which is currently obtaining complementary in situ measurements. (C. Vogt, K. Laihem, C. Wiebusch, J. Acoust. Soc. Am. 124, 3613, 2008.) —Stephen G. Benka
In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from Tsinghua University and Beijing Normal University have demonstrated a radically simpler loudspeaker design based on nanotubes: They showed that a thin film of nanotubes can reproduce sounds over a wide frequency range--including the full human audible range--with high sound pressure level, low total harmonic distortion, and no magnets. The team created the film by drawing nanotubes from a so-called superaligned array grown on a wafer, a technique the group introduced six years ago (see also PHYSICS TODAY, October 2005, page 23). The resulting film, only tens of nanometers thick but up to 10 cm wide, is transparent and has a nearly purely resistive impedance. When electrodes are placed along its ends and an alternating current is applied, the film produces clear tones that can be as loud as a conventional speaker. Moreover, since the film is flexible, the nanotube speaker can be configured into arbitrary shapes or mounted onto curved substrates; the figure shows an omnidirectional cylindrical loudspeaker 9 cm in diameter and 8.5 cm high. The film can even be stretched with essentially no degradation of the sound reproduction. The researchers attribute the sound generation not to vibration but to a thermoacoustic effect first proposed nearly a century ago: Thanks to the nanotube film's extremely low heat capacity per unit area, changes in the current flowing through the film are reflected in the film's temperature, and those temperature changes excite pressure waves in the surrounding air. The mechanism is independent of the sign of the current, which leads to a frequency doubling of the input signal, but that drawback can be overcome by applying a constant current bias. The movie shows a nanotube loudspeaker being periodically stretched with almost no noticeable effect on the sound intensity. (L. Xiao et al., Nano Lett., in press, doi:10.1021/nl802750z.) -- Richard J. Fitzgerald
The Vitthala Temple in the south Indian city of Hampi is more than 500 years old. The image here shows its most curious feature—numerous pillars, each of which includes separate columns that sound musical notes when struck with a finger. Different columns in a pillar produce sounds of different frequencies. Moreover, several multi-columned pillars make sounds similar to specific Indian musical instruments such as the ghanta (bell), mridanga (percussion), or veena (strings). Well known for centuries, the musical pillars are only now beginning to be studied scientifically. Anish Kumar of the Indira Gandhi Center for Atomic Research in Kalpakkam, India, and colleagues took the first steps to characterize the columns: The physicists applied three techniques to learn about the structure of the columns and also analyzed recordings of generated sound. In situ metallography showed the granite to have typical microstructures; both low-frequency ultrasound and impact-echo testing revealed all the columns to be solid shafts. From those studies and spectral analyses, the researchers conclude that the pillars' sounds arise from the flexural mode of vibrations. Next on their agenda is to study how the columns can be excited by just the tap of a finger. (A. Kumar et al., J. Acoust. Soc. Amer. 124, 911, 2008.) — Stephen G. Benka
