Method and apparatus for pre-loading a piezoelectric transducer for downhole acoustic communication
A downhole acoustic transmitter has a piezoelectric transducer, an enclosure in which the piezoelectric transducer is housed, a transducer preload means which applies a selected compressive force against the transducer such that a mechanical preload is applied to the transducer, and an acoustic tuning element which has a first end coupled to the transducer preload means or the transducer, and an open second end. The acoustic tuning element is not coupled to anything but the transducer preload means or transducer, so the transducer preload means effectively has a second open end and thus can maintain the same preload compressive force on the transducer even when the transmitter is subjected to tension and compressive forces during operation.
1. A method for acoustic transmission from a downhole location, comprising:
(a) applying a compressive preload force in an axial direction against a piezoelectric transducer by a transducer preload means such that the piezoelectric transducer is compressed, wherein an assembly comprising the piezoelectric transducer and the transducer preload means is attached to a first end coupling of an enclosure of a downhole acoustic transmitter;
(b) tuning the acoustic impedance of the piezoelectric transducer by attaching a first end of an acoustic tuning element having a selected acoustic impedance to the transducer preload means or to the piezoelectric transducer, and wherein a second end of the acoustic tuning element is spaced from a second end coupling of the enclosure such that an axial gap is defined there between; and
(c) applying a voltage to the piezoelectric transducer to generate an acoustic transmission.
2. The method as claimed in claimed in claim 1 wherein tuning the acoustic impedance of the piezoelectric transducer comprises selecting an acoustic impedance of the acoustic tuning element that when combined with an acoustic impedance of the piezoelectric transducer, matches the acoustic impedance of an inner face of the first end coupling contacting the piezoelectric transducer.