Singing Ice: A Star Wars Story
David T. Bradley, Erica Ryherd, and Lauren Ronsse (editors)
Acoustics of Worship Spaces: three decades of design (Springer-Verlag, 2016)
Sound Materials: A Compendium of Sound Absorbing Materials for Architecture and Design (Frame Publishers, November 2016)
2013 – 2014
Eavesdropping on Echosystems
Science 21 Feb 2014
Vol. 343, Issue 6173, pp. 834-837
The Science of the Sonic Wonders of the World
W. W. Norton & Co.
Your ears never sleep: auditory processing of nonwords during sleep in children
Adrienne Roman – firstname.lastname@example.org
Carlos Benitez – email@example.com
Alexandra Key – firstname.lastname@example.org
Anne Marie Tharpe – email@example.com
The brain needs a variety of stimulation from the environment to develop and grow. The ability for the brain to change as a result of sensory input and experiences is often referred to as experience-dependent plasticity. When children are young, their brains are more susceptible to experience-dependent plasticity (e.g., Kral, 2013) so the quantity and quality of input is important. Because our ears are always “on”, our auditory system receives a lot of input to process, especially while we are awake. But, what can we “hear” when we are asleep? And, does what we hear while we are asleep help our brains develop?
Although there has been research in infants and adults examining the extent to which our brains process sounds during sleep, very little research has focused on young children, a group that sleeps a significant portion of their day (Paruthi et al., 2016). We decided to start our investigation by trying to answer the question, do children process and retain information heard during sleep? To investigate this question, we used electroencephalography (EEG) to measure the electrical activity of children’s brains in response to different sounds – sounds they heard when asleep and sounds they heard when awake.
First, during the child’s regular naptime, each child was hooked up to a portable EEG. Using EEG, a technician could tell us when the child went to sleep. Once asleep, we played the child three made-up words over and over in random order for ten minutes. Then, we let the child continue to sleep until he or she woke up.
When the children awoke from their naps, we took them to our EEG lab for event-related potential (ERP) testing. ERPs are segments of on-going EEG recordings appearing as waveforms that reflect the brain’s response to events or stimulation (such as a sound played).
The children wore “hats” consisting of 128 spongy electrodes while listening to the same three made-up words heard during the nap mixed in with new made-up words that the children never heard before. We then analyzed the ERPs, to determine if the children’s brains responded differently to the words played during sleep than to the new words the children had not heard before. We were looking for ‘memory traces’ in the EEG that would indicate that the children ‘remembered’ the words heard while sleeping.
We found that children’s brains were able to differentiate the nonsensical words “heard” during the nap from the brand new words played during the ERP testing. This means that the brain did not just filter the information coming in, but also retained it long enough to recognize it after they woke up. This is the first step in understanding the impact of a child’s auditory environment during sleep on the brain.
Kral, A. (2013). Auditory critical periods: a review from system’s perspective.
Neuroscience, 247, 117-133.
Paruthi, S., Brooks, L. J., D’Ambrosio, C., Hall, W. A., Kotagal, S., Lloyd, R. M.,
Malow, B. A., Maski, K., Nichols, C., Quan, S. F., Rosen, C. L., Troester, M. M., & Wise, M.S. (2016). Recommended amount of sleep for pediatric populations: a consensus statement of the American Academy of Sleep Medicine. Journal of clinical sleep medicine: JCSM: official publication of the American Academy of Sleep Medicine, 12(6), 785.
Many centuries ago glass blowers observed that sound could be generated when blowing through a hot bulb from the cold end of a narrow tube. This phenomenon is a result of thermoacoustic oscillations: a pressure wave propagating in a compressible fluid (e.g. air) can sustain or amplify itself when being provided heat. To date, thermoacoustic engines and refrigerators have had remarkable impacts on many industrial applications.
After many centuries of thermoacoustic science in fluids, it seems natural to wonder if such a mechanism could also exist in solids. Is it reasonable to conceive thermoacoustics of solids? Can a metal bar start vibrating when provided heat?
The study of the effects of heat on the dynamics of solids has a long and distinguished history. The theory of thermoelasticity, which explains the mutual interaction between elastic and thermal waves, has been an active field of research since the 1950s. However, the classical theory of thermoelasticity does not address instability phenomena that can arise when considering the motion of a solid in the presence of a thermal gradient. In an analogous way to fluids, a solid element contracts when it cools down and expands when it is heated up. If the solid contracts less when cooled and expands more when heated, the resulting motion will grow with time. In other terms, self-sustained vibratory response of a solid could be achieved due to the application of heat. Such a phenomenon would represent the exact counterpart in solids of the well-known thermoacoustic effect in fluids.
By using theoretical models and numerical simulations, our study indicates that a small mechanical perturbation in a thin metal rod can give rise to sustained vibrations if a small segment of the rod is subject to a controlled temperature gradient. The existence of this physical phenomenon in solids is quite remarkable, so one might ask why it was not observed before despite the science of thermoacoustics have been known for centuries.
“Figure 1. The sketch of the solid-state thermoacoustic device and the plot of the self-amplifying vibratory response.”
It appears that, under the same conditions of mechanical excitation and temperature, a solid tends to be more “stable” than a fluid. The combination of smaller pressure oscillations and higher dissipative effects (due to structural damping) in solids tends to suppress the dynamic instability that is at the origin of the thermoacoustic response. Our study shows that, with a proper design of the thermoacoustic device, these adverse conditions can be overcome and a self-sustained response can be obtained. The interface conditions are also more complicated to achieve in a solid device and dictates a more elaborate design.
Nonetheless, this study shows clear theoretical evidence of the existence of the thermoacoustic oscillations in solids and suggests that applications of solid-state engines and refrigerators could be in reach within the next few years.