ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA Y SISTEMAS DE TELECOMUNICACIÓN PROYECTO FIN DE GRADO TÍTULO: HIGH-FIDELITY PIEZOELECTRIC LOUDSPEAKER AUTOR: JAVIER FERNÁNDEZ MARTÍNEZ ESPECIALIDAD: SONIDO E IMAGEN TUTOR: DR. KEITH R. HOLLAND UNIVERSIDAD: UNIVERSITY OF SOUTHAMPTON CENTRO: INSTITUTE OF SOUND AND VIBRATION RESEARCH (ISVR) PAÍS: REINO UNIDO Fecha de lectura: 16 de Junio de 2014 Calificación: El Coordinador de Movilidad, _________________________________________________________________ Faculty of Engineering and the Environment Institute of Sound and Vibration Research High-fidelity piezoelectric loudspeaker Javier Fernández Martínez Supervised by Dr. Keith R. Holland May 2014 This report is submitted in partial fulfilment of the requirements of the Degree of Acoustical Engineering, Faculty of Engineering and the Environment, University of Southampton. 2 Abstract This project reports on a literature review about piezoelectric loudspeakers and on an experimental research about how to improve some features of a particular horned piezoelectric tweeter. The work involves an investigation of the performance and principle of operation of piezoelectric loudspeakers to understand how the sound is generated and what its main parameters are. Also, previous research papers about how to improve this type of speakers are reported. The knowledge gained was used to reconsider and re-purpose a particular piezoelectric transducer. After characterising the original state of the device with acoustical and electroacoustical measurements, some improvements were implemented. Moreover, interesting conclusions were reached based on the results of the tests that were carried out. A structural study with a scanning laser was then completed. These sections demonstrated the need for providing the speaker with a rear suspension that guides the vibration of the membrane. Finally, an inverse filter was designed in order to get a flat output response. After simulating the results with Matlab, validating experiments were run in the anechoic chamber with great success. 3 4 Resumen Este proyecto consta de un estudio detallado sobre piezoelectricidad y altavoces piezoeléctricos, así como de una parte experimental consistente en mejorar algunas características de un altavoz piezoeléctrico particular: un tweeter de bocina. El estudio profundiza en cuáles son los principios de funcionamiento y los principales parámetros de este tipo de altavoces. Con el conocimiento adquirido a partir de trabajos de previos sobre el tema e investigación bibliográfica se ha llevado a cabo la parte experimental. Esta parte ha requerido de una serie de medidas acústicas y electroacústicas para, primero, caracterizar el altavoz en su estado original y para posteriormente buscar y validar posibles mejoras, principalmente en la respuesta en frecuencia. Además, se ha realizado un estudio estructural del diafragma a partir de medidas tomadas con un vibrómetro laser Doppler. De estos tres procesos se concluyó que el altavoz bajo estudio tiene un problema en el soporte del cristal piezoeléctrico y se demostró la necesidad de equipar el dispositivo de una suspensión trasera que controle el movimiento del diafragma. Finalmente, se ha diseñado e implementado en Matlab un filtro inverso, con el objetivo de conseguir una respuesta plana a la salida del altavoz. Su funcionamiento fue validado en la cámara anecoica satisfactoriamente. 5 6 Acknowledgements I would like to be grateful to everyone involved in making this project possible. First of all, the biggest “thank you” is for my parents and my sister Laura. You can‟t imagine how grateful I am for your support provided all through my degree, both in Spain and in the UK. This would have never come true without you. I am very happy to have you beside me in any decision that I made. You are a great pillar in my life. I would also like to thank my supervisor Dr. Keith Holland, who gave me the opportunity to develop this project and always kept his door open to my enquiries. A special gratitude to Jana for making easier the adaptation to Southampton and to Marcos for being always disposed to help me with the project. Also to my Erasmus friends and housemates; you have been of great help and encouragement throughout this year; I really appreciate having met you and became such good friends. Thank you, to the rest of my friends and flatmates in León, Madrid and England. You know who you are; there is no need to name anyone. I wish I could transfer you all the energy used in doing this project, aunt. Javi. 7 8 Contents 1. Introduction ................................................................................................... 13 2. Piezoelectric loudspeaker ............................................................................. 14 2.1. Piezoelectricity........................................................................................ 14 2.2. Piezoelectric driver ................................................................................. 15 2.3. Parameters ............................................................................................. 17 Frequency response ..................................................................................... 17 Directivity ...................................................................................................... 18 Resonance frequency ................................................................................... 18 Electric admittance ....................................................................................... 20 3. Researched information ............................................................................... 21 Based on acoustic diaphragms .................................................................... 21 Based on silicone buffer layer ...................................................................... 22 4. The loudspeaker ........................................................................................... 23 5. Acoustic and electroacoustic measurements ............................................... 26 5.1. Acoustic characterisation ........................................................................ 26 5.1.1. Layout .............................................................................................. 26 5.1.2. Results ............................................................................................. 27 5.2. Electroacoustic characterisation ............................................................. 40 5.2.1. Layout .............................................................................................. 40 5.2.2. Results ............................................................................................. 40 6. Laser measurements .................................................................................... 42 6.1. Layout ..................................................................................................... 42 6.2. Results ................................................................................................... 43 7. Inverse filter .................................................................................................. 45 7.1. Theory .................................................................................................... 45 9 7.2. Implementation .......................................................................................46 7.3. Results ...................................................................................................47 8. Conclusions ..................................................................................................49 8.1. Acoustic measurements .........................................................................49 8.2. Electroacoustic measurements ...............................................................53 8.3. Laser measurements ..............................................................................54 8.4. Inverse filter ............................................................................................58 9. References ...................................................................................................59 APPENDICES .....................................................................................................61 APENDIX A1: Inverse filter Matlab script .........................................................61 Total number of words: 6019 10 List of figures Fig. 2.1: Piezoelectric crystal. General idea of working Fig. 2.2: Structure of piezoelectric crystal. Reproduced from [3] Fig. 2.3: Oscillation of the piezoelectric crystal when voltage is applied. Reproduced from [3] Fig. 2.4: Electric configurations of piezoelectric crystal. Reproduced from [http://www.fuji- piezo.com/Bimorph.htm] Fig. 2.5: Piezoelectric diaphragm and modes of vibration of the crystal. Reproduced from [5] Fig. 2.6: Relation between supporting method and frequency of resonance. Reproduced from [3] Fig. 2.7: Cavity of piezoelectric loudspeaker. Reproduced form [3] Fig. 2.8: Electric equivalent circuit of piezoelectric loudspeaker. Reproduced from [10] Fig. 3.1: Schematic view of a piezoelectric microspeaker. Reproduced from [13] Fig. 4.1: Piezoelectric tweeter. Front view Fig. 4.2: Piezoelectric tweeter. Back view Fig. 4.3: Detailed front view of piezoelectric diaphragm Fig. 4.4: Detailed back view of piezoelectric diaphragm Fig. 5.1: Brüel & Kjær 4189-L-001 microphone Fig. 5.2: View of the speaker in the infinite baffle and the microphone at 1 meter distance Fig. 5.3: