Evaluation of piezoelectric speaker drivers for loudspeaker and array speaker design

Abstract

Abstract
This thesis is an evaluation of piezoelectric speaker drivers for loudspeaker design. It goes through the electrical and sound generating properties of the speaker drivers and general theory of piezoelectric speaker drivers. A prototype single speaker driver loudspeaker has been realized and evaluated by measuring the frequency response and sound pressure level (SPL), total harmonic distortion (THD), directivity and power consumption. Further two types of array speakers (linear and square) has been realized and evaluated for the same properties as the single speaker driver loudspeaker. The different ways of connecting the speaker drivers together, parallel or series connection, has been compared in regards to SPL, power consumption and THD. The speaker drivers generates approximately 60 dB SPL at one meter distance away from the speaker between 300 Hz to 20 kHz. Parallel connections proved to increase the overall measured SPL (over 70 dB SPL at one meter for four speaker drivers), however the amplifier was placed under more stress due to it needing to supply more power. Series connections proved to be less problematic for the amplifier, however there was no increase in SPL compared to a single speaker driver. The THD measurements showed that series connections resulted in less THD compared to parallel connection and single speaker drivers. The directivity measurements showed that the square array became more directional in both the vertical and horizontal planes compared to the individual speaker driver. The line array became even more directional compare to the square array, but only for one plane and the other plane showed a similar directivity pattern as the individual speaker driver. They perform similarly to electromagnetic speaker drivers with comparable dimensions in area of speaker diaphragm and could be used in applications where thickness/size is limited.

Popular Abstract

Dancing crystals make music
Speakers as you know can play music, the traditional speaker uses a magnet and electromagnetic forces created by electrical currents to make sound. Another way to make sound is with piezoelectric speakers. These do not have a magnet which allows them to be made smaller than the traditional speakers. Piezoelectricity is not something new, it was discovered by the Curie brothers in 1880. But what did they discover? When you push and mechanically deform some crystals they will generate an electrical voltage at the surface of the crystal. It was also confirmed that if an electrical voltage was applied to the crystal it would deform as if someone had pushed on it, even if no one did. These types of crystals can therefore start to shake or vibrate if an alternating current is applied to them.
We Humans can hear frequencies between 20 Hz to 20 kHz. If an electrical signal of frequencies within this range is fed to one of these piezoelectric crystals they will start to vibrate with a frequency that we can hear. As they vibrate the vibrations are transferred to the air that surrounds the crystals and sound waves are created. These can therefore be used as speakers. They have been used as speaker devices for some time, they are very good at creating high frequency sound and have been used as tweeters and buzzers. Tweeters are used as a complement of regular speakers to increase performance in the higher frequency range to increase the overall sound experience. Buzzers are used to create single frequency sounds, for example there is a good chance that the “beep” sound at the supermarket register is made with buzzers. The biggest setback of this kind of speaker is that they lack in the lower frequency range. As frequency goes down they become less and less effective. In recent years a large effort in trying to create good full-frequency-range piezoelectric speakers have been made. There are a few examples of these already on the market but most of them are for in-ear applications such as earphones and hearing aids. During this work the possibility of full-frequency-range piezoelectric loudspeakers have been researched. There are a handful of available speakers available for consumers but they might become interesting in the future, their potential lies in their small size and could open up possibilities of speakers systems that can not be made today. From this work it was found that these types of speakers are as of today more suited for smaller sound systems, desktop speakers, laptop speakers or loudspeakers in mobile
phones. They do appear to be “on-par” with the traditional electromagnetic speaker in terms of loudness and energy efficiency. They also behave differently to the traditional speaker in an electrical sense which means that they need to be handled differently to electromagnetic speakers. The frequency range is good, but not great, they are more suitable for speech rather than music.

• author: Brandel, Felix
• supervisor: Johan Nilsson
• organization: Department of Biomedical Engineering
• alternative title: Utvärdering av piezoelektriska högtalare för högtalare och array högtalardesign
• course: BMEM05 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Speaker, Piezoelectric, Array speaker
• language: English
• additional info: 2024-21
• id: 9172889
• date added to LUP: 2024-09-09 12:47:13
• date last changed: 2024-09-09 12:47:13