Natural Science Forum / Physics / Acoustics / August 2005

Generally speaking, because of the way sound is generated, it is
coherent without further manipulation. This is why room acoustics are
such a mess of standing waves and modes.

d

Pearce Consulting
http://www.pearce.uk.com

BJ Nash schrieb:

The problem with coherency of sound beams is existing only at very high frequencies (significantly more than 100GHz). As far as I know, no method is known, how to produce such sound in the way, that it is coherent. This kind of sound waves is called phonons.

In all other cases it is fully different than in the case of light: it is difficult to produce sound, that is not coherent. It can be propbably made only with such sources, as waterfall, large machines with many sound sources, crowds of people, that are doing something, that creates sounds, etc.

There is another interesting difference between sound and light waves: Sound is not only coherent, but also its phase is determined - especially if the generator is trigerred. This is not possible with light.

Wieslaw Bicz

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http://www.ingentaconnect.com/content/ap/sm/1997/00000022/00000003/art00372
 Double-barrier coherent sound generator: a new device

Authors: Tuyarot D.E. 1; Makler S.S. 1; Anda E.V. 1; Vasilevskiy M.I. 2

Source: Superlattices and Microstructures, October 1997, vol. 22, no. 3,
pp. 427-430(4)

Publisher: Academic Press
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Abstract:

We investigate a new device, a SASER, consisting of a double-barrier
heterostructure (DBH) designed to generate ultra-high-frequency coherent
sound. The device is tailored such that under the influence of an
external bias, some of the electrons injected into the first excited
level decay to the ground state by emitting LO-phonons. Due to the low
energy and short wavelength of the phonon beam, this device can be used
for imaging, for non-destructive characterization of nanostructures and
to construct phonoelectronic systems (analogous to optoelectronics). In
this paper we use a simple model to calculate the electronic current
which takes into account the electron–phonon and electron–electron
interactions. The electronic part is described in terms of a
tight-binding Hamiltonian. Lattice dynamics are presented by a single
LO-phonon mode confined inside the well for the primary beam and another
single TA-phonon mode for the secondary one (F. Vallée, Phys. Rev. B49,
2460 (1994)). The electron--phonon interaction is described by a single
transition matrix element between the two lowest states localized at the
well. The electron and phonon populations, the current and the potential
profile are calculated self-consistently. The results confirm the
viability of the device, predicted in previous simplified calculations
(S. S. Makler et al., Surf. Sci. 361, 239 (1996)).Copyright 1997
Academic Press Limited

Language: English

Document Type: Research article

Affiliations: 1: Instituto de Física, Universidade Federal Fluminense,
Niteroi-RJ, 24210-340, Brazil 2: Faculty of Applied Physics, N. Novgorod
University, Nizhni Novgorod, 603600, Russia

http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:cond-mat/0101041
Self-organization in a phonon laser

Authors: Camps, I ; Makler, S S

      We make an adaptation of laser modelling equations to describe
the behavior of a phonon laser (saser). Our saser consists of an
AlGaAs/GaAs double barrier heterostructure designed to generate an
intense beam of transversal acoustic (TA) phonons. To study our system,
we begin with a Hamiltonian that describes the decay of primary
longitudinal optical phonons (LO_1) into secondary (LO_2) and TA (LO_1
-> LO_2 + TA) and its inverse process (recombination). Using this
Hamiltonian, a set of coupled equations of motion for the phonons is
obtained. We also consider the interaction between the phonons and its
reservoirs. These interactions are introduced in the equations of motion
leading to a set of coupled Langevin equations. In order to obtain an
expression to describe our saser we apply, in the Langevin equations, an
adiabatic elimination of some variables of the subsystem. Following the
method above we obtain the value of the injection threshold for the
operation of our phonon laser. At this threshold occurs a phase
transition from a disordered to a coherent state. It is shown that it is
not necessary a big "optical" pumping to get a sasing region.

      Comment: 4 figures

On 6/29/05 5:14 PM, in article 6fe6c1tr3fda9m1mpirbfdr92n1qmuuc84@4ax.com,

You first have to know what coherent sound is. What do you think it is.

To my mind it means that you can get acoustic interference. Think of how you
might make an acoustic Michelson interferometer.

Consider trying to do an acoustic two slit experiment. Two separated
speakers driven by the SAME audio oscillator will produce a stationary
pattern of fringes. On the other hand, two separated flute players playing a
pure note will also produce fringes, but the fringes will not be stationary.
The two flutes do not maintain coherence over a long period of time because
they are independent sources.

Bill