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Converting Sunlight Into Audible Sound by Means of the Photoacoustic Effect: the Heliophone Nicolaas Bernardus Roozen, Christ Glorieux, L

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Converting Sunlight Into Audible Sound by Means of the Photoacoustic Effect: the Heliophone Nicolaas Bernardus Roozen, Christ Glorieux, L Converting sunlight into audible sound by means of the photoacoustic effect: The Heliophone Nicolaas Bernardus Roozen, Christ Glorieux, L. Liu, Monika Rychtarikova, T. van der Donck, A. Jacobs To cite this version: Nicolaas Bernardus Roozen, Christ Glorieux, L. Liu, Monika Rychtarikova, T. van der Donck, et al.. Converting sunlight into audible sound by means of the photoacoustic effect: The Heliophone. Journal of the Acoustical Society of America, Acoustical Society of America, 2016, 140 (3), pp.1697- 1706. 10.1121/1.4962493. hal-02895918 HAL Id: hal-02895918 https://hal.archives-ouvertes.fr/hal-02895918 Submitted on 10 Jul 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Typeset by REVTEX 4 for JASA Converting sunlight into audible sound by means of the photoacoustic effect: The Heliophone N.B. Roozen, C. Glorieux, and L. Liu Laboratory of Acoustics, Division Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium M. Rychtarikov´ a´ a) STU Bratislava, Faculty of Civil Engineering, Department of Building Structures, Radlinskeho 11, Bratislava, 810 05, Slovak Republic T. Van der Donck Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium A. Jacobs Overtoon, Platform for Sound Art, Luchtvaartsquare 19, 1070, Brussels, Belgium (Dated: September 15, 2016) One hundred and thirty-five years after Alexander Graham Bell and his assistant Charles Sumner Tainter explored the photoacoustic effect, and about 40 years after Rosencwaig and Gersho modeled the effect in a photoacoustic cell configuration, we revisit the phenomenon, in a "heliophone" device that converts sunlight into sound. The light is focused on a carbon blackened copper coated Kapton foil in an acoustic cell by means of a compound parabolic collimator, and its intensity is modulated by a mechanical chopper. A horn is employed to make the sound audible without electronic amplification. The description of the pho- toacoustic effect that was introduced by Rosencwaig and Gersho is extended to a cell-horn configuration, in which the periodically heated air above the foil acts as an oscillating piston, driving acoustic waves in the horn. The pressure in the cavity-horn assembly is calculated by considering the air layer piston as an equivalent volume velocity source. The importance of the carbon black (soot) layer to enhance light absorption, but above all to enhance the photothermal excitation efficiency, is elucidated by means of an experimentally supported physical model. PACS numbers: 43.20.+g, 43.90.+v, 44.90.+c I. INTRODUCTION of lampblack put inside the cavity significantly reinforced the sound produced by the device.2 In the early days of telecommunication, Alexander Just under 100 years later the theoretical basis of the Graham Bell experimented with the transmission of photoacoustic effect was developed by Rosenswaig and speech on a beam of light. The initial version of the Gersho.3 The effect is based on the intermittent thermal device, called the photophone, was non-electric. At the heating of a medium that absorbes the energy of inten- transmitter side the speaker's voice put a mirror into vi- sity modulated light. This causes an intermittent expan- bration. A light beam was modulated by the vibrating sion of the absorbing medium. As a result, the medium mirror. At the receiver side, the intensity variations of launches an acoustic wave into its surroundings. the light beam were converted into sound as a result of the photoacoustic effect. The device consisted of a con- Developing simple classroom demonstrations of ad- ical cavity in a brass housing closed by a flat plate of vanced concepts in physics is one of the missions of many 4 glass. The brass housing was connected to a hearing professors and teachers in physics. Rush et al. described tube, which was to be held close to the ear of the re- an apparatus to demonstrate the photoacoustic effect us- ceiver. The modulated light beam entered the cavity ing a slide projector, a mechanical chopper to periodically through the flat plate of glass. Alexander Graham Bell modulate the light beam, and a Fresnel lens to bundle and his co-worker Mr. Tainter found that a teaspoonful light onto a funnel. The interior of the (closed) funnel was blackened by a flame. The purpose of the blackening is not mentioned in the article, although it is obviously required to absorb the light energy. The researchers made a) Also at Laboratory of Acoustics, Division Soft Matter and Bio- the sound audible by means of a hearing tube, which was physics, Department of Physics and Astronomy, KU Leuven, Ce- connected to the funnel. The hearing tube had to be held lestijnenlaan 200D, 3001 Leuven, Belgium. close to the ear to hear a distinct sound caused by the J. Acoust. Soc. Am. Roozen et al.: the Heliophone 1 photoacoustic effect. Very recently (2015), Zhu et al.5 described a photoa- coustic device to turn the oscillating light from an incan- descent light bulb into audible sound. The light bulb was driven by a sinusoidal AC power source with constant fre- quency of 50 Hz, as well as by a square-wave chopped DC source with adjustable frequencies. The light of the bulb was incident upon a sealed cylindrical glass beaker. Half of the inner side of the beaker was coated by a soot layer of black smoke from burning kerosene. The sound was picked up by means of a microphone and electronically amplified. The authors reported that the sound level gen- erated by the photoacoustic effect decayed over time, as a result of an increase of the mean temperature of the soot layer. This is physically explained by Rosencwaig and Gersho' s model, in which the pressure fluctuation is inversely proportional to the absolute temperature. Be- cause of the continuous rise of the temperature of soot, the gas layer inside and just above the soot layer responds less easily to temperature fluctuations, causing an atten- uation effect. In the present work a heliophone device1 is proposed to convert sunlight into audible noise using the photoa- coustic effect. The device proposed here uses a horn to amplify the sound in a passive manner. Numerical simu- FIG. 1. (color online) Final realization of the Heliophone. lations are used to optimize the geometry of the device. Similar to the findings of Bell,2 Rush et al.4 and Zhu et 5 al., experiments indicate that the photoacoustic effect the site and the solar time as input parameters. The ele- is strongly increased by means of a layer of soot (car- vation is manually set to a fixed angle, in according with bon black). The layer of soot has two functions. First the season and latitude of the site. In the photoacous- of all, soot is very effectively absorbing light. Secondly, tic cell a Kapton®foil is present that was coated with it appears that the photoacoustic effect is significantly copper and blackened with carbon soot from a candle increased due to the extraordinary thermal properties flame. The body of the photoacoustic cell was made of of the soot layer, as elucidated by means of a physical plexiglas, to allow the chopped light beam to illuminate model. The porosity and thermal conductivity of the the carbon blackened copper coated Kapton foil. This soot layer are quantified. foil acts as a photoacoustic transducer. The collimated The application of a horn to passively amplify the ra- sunlight is modulated by means of a rotating mechanical diated sound is not new. It is widely applied in musical chopper. Due to optical absorption by the carbon black, instruments. For technical apparatuses, for instance, it a periodically varying heat flux is induced into the air 6 has been applied in a thermoacoustic device. However, above the surface. This causes pressure variations in the to the best of our knowledge, the combined use of solar cell cavity at the chopper frequency and its harmonics. power and an acoustic horn to amplify the sound created The cell is connected to an acoustic horn, which acts as by a photoacoustic device has not been shown before. an impedance matching device between the cavity and The paper is organized as follows. Section II treats the the open air environment, making the sound audible. design of the Heliophone, including a discussion about Details of the photoacoustic cell (Fig. 2) are given be- the acoustical properties of the photo-acoustic cell and low. The foil (width of 9 mm) was placed in the middle horn combination. Using a numerical model, the (pas- of the spherical cell cavity (inner diameter 15 mm, ra- sive) amplification effect of the horn is quantified. Sec- dius Rs=7.5 mm). As the width of the foil is smaller tion III discusses the experimental results. Section IV than the diameter of the cell cavity, the lower and up- summarizes the conclusions of this work. per half of the cell cavity can communicate with each other. Inside the photo-acoustic cell the sound pres- II. THE DESIGN OF THE HELIOPHONE. sure level was uniformly distributed up to a frequency of about fmax = c/λ, with c being the speed of sound In the proposed configuration of the Heliophone (Fig. and with λ equal to 2 times the typical dimension, giv- 1, 2), sunlight is focused by means of a compound ing fmax ≈10kHz.
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