Two plates placed in tandem in a duct flow shed vortices, which can excite and sustain an acoustic resonance associated with the duct. The sound can in turn ‘feed back’ and ‘lock’ the vortex shedding rate to the sound frequency. The experimental conditions under which loud resonant sound is sustained are described in this paper. The acoustic sources are predicted by combining a vortex model of the flow field with a finite-element solution of the sound field, and then using Howe's theory of aerodynamic sound to calculate the energy exchange between the flow and the sound field. Only in certain regions near the plates is substantial net energy exchange possible; the direction of energy transfer depends on the spacing of the plates. The region around the trailing edge of the upstream plate is found to be always a net acoustic source during resonance, while the region around the downstream plate is a net source or sink depending on the phase of the acoustic cycle at which vortices arrive there, which in turn depends on plate spacing and flow velocity. The net source region around the downstream plate is suppressed over a wide range of plate spacings by splitting this plate at midspan and rejoining it so that one half is offset in the flow direction by the distance a vortex travels in half a sound cycle.

A survey is made of enclosed flow systems in which acoustic and/or mechanical oscillations can be excited by vortex shedding from bodies immersed in the flow which becomes ‘locked’ to an acoustic resonance.

Acoustic analysis of cough both in the time and frequency domain has been reported using voluntary and spontaneous cough. The main aim of this study was to discover whether such analysis of capsaicin-induced cough enables differences between normal subjects to be recognized.We present data from 13 healthy subjects (with normal lung function and no history of respiratory disease) using a new method of acoustic analysis, which presents the data in three graphical forms: 1) spectrogram; 2) overall spectral energy, 3) root mean square (RMS) pressure plots.Using the RMS sound pressure traces, different subjects had either two peaks, a single peak or multiple peaks. The occurrence of single and multiple peaks has previously been associated with disease states but we found them in normal subjects. The number of peaks and the visual pattern of the spectrogram was reproducible within and specific to each individual over time. During a peal of coughs in a single expiration, the peak amplitude of successive coughs decreased as lung volume reduced. Despite similarities in the overall spectral energy between individuals, there were marked differences in the small visual details of the spectrograms. However, in an individual, these small details were remarkably constant both within and between days, and can be regarded as a "cough signature".This type of spectrographic analysis provides a new approach to the analysis both of normal and abnormal cough sounds, and has identified similarities and differences in capsaicin-induced cough in normal individuals. It has potential as a tool with which to study the pathophysiology of cough. Eur Respir J., 1997; 10: 202-207 Cough is a powerful response to the stimulation of sensory nerves predominantly in the larynx, trachea and central airways. It is characterized by an initial contraction of the expiratory muscles against a closed glottis, followed by a violent expiration as the glottis opens suddenly [1]. In healthy subjects, cough is infrequent, is usually nonproductive, and is self-limiting. However, in patients with chronic chest disease, particularly when associated with cigarette smoking, cough is a very common and troublesome symptom [2,3]. The sensitivity of the reflex can be tested by inhaling a tussigen. One such agent is the extract of red pepper, capsaicin, which when inhaled at low concentrations produces both doserelated and reproducible coughing [4].Cough produces an easily recognizable sound, which can be recorded and used to count the number of coughs [3,5]. However, there has been little work published on the acoustic analysis of the cough sound [6,7]. In this study, we describe a method of analysing cough sounds using techniques usually employed for lung sound analysis [8], and apply this method of analysis to capsaicin-induced cough.Previous studies have shown that cough in normal individuals can be distinguished from cough in disease states [6]. This suggests that there is considerable similarity between the acoustic features of cough in normals. Anecdota...

An investigation has been undertaken to determine whether there is any correlation between the spectral analyses of breath sounds at the mouth and the diagnosis of lung dysfunction in a small number of patients. The results of the investigation have shown that it is possible to differentiate between the spectral characteristics of the breath sounds of people who are: (a) normal; (b) asthmatic adult, both before and after the administration of a bronchodilator drug; (c) asthmatic adult, treated with an inhaled steroid, both before and after the administration of a bronchodilator drug; (d) asthmatic child, both before and after the administration of a bronchodilator drug. The possible applications of the technique are discussed.

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