Parametric Analysis of the Improved Open Photoacoustic Helmholtz Cell

A Geras,T Starecki

doi:10.1007/s10765-013-1497-9

Abstract

Many photoacoustic cells use a Helmholtz structure for amplification of the photoacoustic signal, but very few of them are open cells. Open Helmholtz cells known from the literature exhibit very poor attenuation of the external acoustic noise, which severely limits their applications. Recent research showed that this problem can be overcome in a so-called improved open photoacoustic Helmholtz cell structure. The paper describes how mechanical dimensions of such a cell affect the frequency response of the cell and its penetration by the external acoustic noise. Properties of the cell were investigated by means of computer simulations based on the loss-improved transmission line model. The analysis showed that appropriate selection of the duct dimensions and buffer volumes should result in over 50 dB attenuation of the external acoustic noise, without noticeably affecting the Q-factor and resonance frequency of the cell.

Highlights

Open photoacoustic cells are often used in gas measurement equipment [1,2,3,4,5]
In the case of an improved open Helmholtz cell (Fig. 1), which is a modification of an earlier design (Fig. 2) presented by Starecki [8], use of additional buffers reduces infiltration of the external acoustical noise by over 40 dB, without affecting the frequency response at frequencies above 200 Hz (Fig. 3) [9]
Preliminary simulations of the frequency response and external acoustic noise infiltration presented in Fig. 3 were obtained for a cell with the structure as in Fig. 1, in which the size of the sample and microphone cavities, as well as diameters and lengths of the main and internal ducts were identical as in the previously studied cell structure from Fig. 2 [8]

Summary

Introduction

Open photoacoustic cells are often used in gas measurement equipment [1,2,3,4,5]. They are suitable for continuous flow measurements, due to easy gas exchange and mechanical simplicity. In the case of an improved open Helmholtz cell (Fig. 1), which is a modification of an earlier design (Fig. 2) presented by Starecki [8], use of additional buffers reduces infiltration of the external acoustical noise by over 40 dB, without affecting the frequency response at frequencies above 200 Hz (Fig. 3) [9]. Such properties mean that the cell can be considered for use in practical applications

Preliminary Considerations

Modeling Results

Conclusions