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Reliable and consistent measurement of various gas molecules at low parts per million (low-ppm) concentration is an essential intermediary step towards low-cost versatile gas analysers. Here we demonstrate physical measurements of various low-ppm molecules by means of an acoustic standing wave-based resonating device with a speaker and a microelectromechanical systems (MEMS) microphone, using either the sound pressure level (SPL) at a fixed frequency, or alternatively, tracking the resonance frequency as gas molecules enter the acoustic device. Low-ppm gas molecules are measurable at frequencies near standing wave resonances, as slight shifts in the speed of sound result in a shift of the acoustic frequency response function. Since a chemical receptor layer on the sensing component is not required for the measurements, absolute physical quantities of the molecules are measurable, which coincide well theoretically with expected values for pure chemicals. Various conditions are tested using different low-ppm molecules to demonstrate the limit of detection (LOD), using a low-cost and portable acoustic setup. An LOD is determined to be in the order of several ppm for most molecules. Gas analysers based on a physical approach present new opportunities for mobile gas sensing in practical applications. [Display omitted] • A physical approach for measuring low-ppm molecules using audible range acoustics is proposed. • A variety of different low-ppm molecules can be detected in either the frequency or time domains. • Acoustic amplitude and frequency are investigated in the time domain, revealing unique signal attributes. • A limit of detection in the order of 5−10 ppm is observed for most molecules. [ABSTRACT FROM AUTHOR]

Measuring vapour-phase properties including pressure, density, concentration and molecular weight is an important step towards integrating mobile olfactory devices into smart societies for monitoring chemical processes. However, there are few portable and low-cost devices for realising dynamic headspace vapour-phase measurements, which work for various molecules. Herein, the concept of utilising audible-range sound waves, in the form of an acoustic resonator, is introduced for extracting vapour-phase properties. As different headspace molecules enter the acoustic resonator, the weight and number of molecules change, and molecular vibrations are perturbed, manifesting in modified local acoustic waves due to changing vapour density and specific heat ratio. Physical waves enable the accurate determination of vapour properties for different molecules, agreeing well with reference values. A minimum vapour pressure down to n -decane of 0.2 kPa is detectable using an audible-range acoustical approach, either by flowing N 2 to force molecules into the headspace; or using a simple cap-based design, which securely fastens onto a vial without any external flow. Low-cost speaker and microphone technologies are an innovative physical approach for determining vapour characteristics, whilst simultaneously demonstrating the inherently complex dynamic fluid space over time. Audible-range acoustical techniques can easily integrate into sensor networks and may yield new alternatives in mobile olfactory technology, due to its physical nature and irrelevance to chemical affinity, which may be useful for portable devices in various applications. [Display omitted] • An acoustic-olfactory sensor accurately determines vapour-phase densities in the range of 1.1–1.7 kg/m3. • Vapour pressure is accurately determined for a wide set of chemicals, including, alcohols, alkanes, ketones and acetates. • The fluid space of vapours enclosed in vials exhibits a first order response and take 15 min to reach steady state. • The vapour pressures of different chemicals agree well with different reference values at various conditions. • Audio-visual spectrograms are generated, displaying an interactive odour display. [ABSTRACT FROM AUTHOR]

Acoustic transduction combining a quarter wavelength resonator, a speaker and a microphone is used for the detection and identification of gas properties such as n -hexane, acetone and ethanol. As a target gas flows into the resonator, the density and the speed of sound of the gas in the resonator change, causing a shift in the acoustic pressure waves manifesting from the speaker. Resonance frequency curves of each gas were experimentally obtained using a standard 1/ f equal octave pink noise test over the audible frequency range. The speed of sound of each gas was analytically determined from the obtained resonance frequency. As the flow concentration of a target gas increases, the speed of sound decreases as the gas density increases. Time series signals at a fixed frequency exhibit unique profiles for each gas and concentration. The theoretical limit of detection for n -hexane in the time domain was calculated to be in the order of several tens of ppm. Whilst the frequency domain data obtains a direct physical parameter, time domain data enables multi-dimensional data analysis, relaying decisive data for artificial olfaction. Principal component analysis (PCA) reveals unique attributes and discrimination per gas species and concentration based on multi-dimensional data obtained through the dual domain measurements. This study demonstrates that the device can adequately identify gases at concentrations of at least several thousand ppm. This approach may provide a new platform as a mobile gas discriminator coupled to artificial olfaction with the added benefit of audio capabilities and signal processing techniques. [Display omitted] • A dual domain acoustic olfactory sensor detection mechanism is fabricated. • The speed of sound of different headspace gases are measured during dynamic sampling. • Time domain testing at a fixed frequency revealed repeatable and distinct signals per gas species. • Principal component analysis is conducted using multi-dimensional data from the time domain and the speed of sound. • A linear relationship between output voltage and n -hexane concentration is obtained. [ABSTRACT FROM AUTHOR]