The law of reciprocity in acoustics and elastodynamics codifies a relation of symmetry between action and reaction in fluids and solids. In its simplest form, it states that the frequency-response functions between any two material points remain the same after swapping source and receiver, regardless of the presence of inhomogeneities and losses. As such, reciprocity has enabled numerous applications that make use of acoustic and elastic wave propagation. A recent change in paradigm has prompted us to see reciprocity under a new light: as an obstruction to the realization of wave-bearing media in which the source and receiver are not interchangeable. Such materials may enable the creation of devices such as acoustic one-way mirrors, isolators and topological insulators. Here, we review how reciprocity breaks down in materials with momentum bias, structured space-dependent and time-dependent constitutive properties, and constitutive nonlinearity, and report on recent advances in the modelling and fabrication of these materials, as well as on experiments demonstrating nonreciprocal acoustic and elastic wave propagation therein. The success of these efforts holds promise to enable robust, unidirectional acoustic and elastic wave-steering capabilities that exceed what is currently possible in conventional materials, metamaterials or phononic crystals. Nonreciprocal acoustic and elastic wave propagation may enable the creation of devices such as acoustic one-way mirrors, isolators and topological insulators. This Review presents advances in the creation of materials that break reciprocity and realize robust, unidirectional acoustic and elastic wave steering.

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Nonreciprocity in acoustic and elastic materials

A Spider’s World

Methods and systems for providing secondary power to aircraft systems. In one embodiment, an aircraft system architecture for providing power to an environmental control system includes an electric generator operably coupled to a jet engine. The jet engine can be configured to provide propulsive thrust to the aircraft, and the electric generator can be configured to receive shaft power from the jet engine. The environmental control system can be configured to provide outside air to a passenger cabin of the aircraft in the absence of bleed air from the jet engine.

Electric-based secondary power system architectures for aircraft

A The Central Nervous System: Structure and Development.- I Patterns of Arrangement and Connectivity in the Central Nervous System of Arachnids.- II Ontogeny of the Arachnid Central Nervous System.- III The Stomatogastric Nervous System and Neurosecretion.- B Structure and Function of Sensory Systems.- Vision.- IV The Morphology and Optics of Spider Eyes.- V The Fine Structure of Spider Photoreceptors in Relation to Function.- VI Photoreceptor Cells in the Spider Eye: Spectral Sensitivity and Efferent Control.- Mechano- and Chemoreception.- VII Mechano- and Chemoreceptive Sensilla.- VIII Trichobothria.- IX Slit Sensilla and the Measurement of Cuticular Strains.- Sensory Nerves and Peripheral Synapses.- X Sensory Nerves and Peripheral Synapses.- C Senses and Behavior.- XI Neuroethology of the Spider Vibration Sense.- XII Spider Proprioception: Receptors, Reflexes, and Control of Locomotion.- XIII Target Discrimination in Jumping Spiders (Araneae: Salticidae).- XIV Homing Behavior and Orientation in the Funnel-Web Spider, Agelena labyrinthica Clerck.- XV Analytical Cybernetics of Spider Navigation.- D The Motor System.- XVI Neural Control of the Heartbeat and Skeletal Muscle in Spiders and Scorpions.- XVII Central and Peripheral Organization of Scorpion Locomotion.- E Neurobiology of a Biological Clock.- XVIII Neurobiology of a Circadian Clock in the Visual System of Scorpions.

Neurobiology of Arachnids

http://www2.eng.cam.ac.uk/~jw12/JW%20PDFs/partitions.pdf

Sound transmission through lightweight double-leaf partitions: theoretical modelling

A Little Dynamics- Survey of Wave Types and Characteristics- Damping- Impedance and Mobility- Attenuation of Structure-Borne Sound- Sound Radiation from Structures- Generation and Measurement of Structure-Borne Sound

Structure-Borne Sound: Structural Vibrations and Sound Radiation at Audio Frequencies

1,081,658. Silencers. L. CREMER. Dec.7, 1964 [Dec.9, 1963], No.49797/64. Heading F1B. In a silencer having chambers 12, Fig.1, branching from a flow conduit 11 from which the chambers have separated by perforate walls 13, the chambers are sub-divided into compartments by narrow layers of second-absorbing material 15. Walls 13 may be perforated metal sheets, gauze or foil. In other embodiments the chambers may have more than one layer of sound absorbing material or be of varying depth or have inclined side walls. Alternate chambers may be filled homogeneously with sound-absorbing material. The narrow layer of sound-absorbing material may be inclined to the axis of the flow conduit. In other embodiments the chambers are arranged between neighbouring passages with each second chamber alternately closed relative to one or other passage. The latter embodiments may have chamber walls which are inclined in pairs towards one another or the rear wall of the chambers may be omitted or the chambers are spaced from the wall of the adjacent flow conduit by a space filled with sound absorbing material.

Sound absorbing ventilation conduit with side branch chambers

The different distributions of the audience

The previous chapter dealt with the most important aspects of wave excitation in structures; one now needs to consider what happens to these waves once they are excited. Waves propagating along practical structures soon encounter locations where there occur changes in the material, in the structural configuration, or in both. Each such discontinuity reflects the incident wave to some degree, and thus reduces (or attenuates) the energy propagating past the discontinuity. The present chapter deals with these attenuating effects, which are of particular practical importance for building structures, in that it addresses not only the problem of predicting the attenuation inherent in a given structure, but also that of designing configurations with desired attenuation properties.

Lothar Cremer

Papers

Structure-Borne Sound: Structural Vibrations and Sound Radiation at Audio Frequencies

Sound absorbing ventilation conduit with side branch chambers

The different distributions of the audience

Attenuation of Structure-Borne Sound

Generation and Measurement of Structure-Borne Sound