An apparatus for controlling ultrasonic power delivery to a surgical area, comprising: a device configured to encounter vacuum pressure at the surgical area; a vacuum sensor configured to monitor vacuum pressure encountered by said device; and a computer configured to compute an ultrasonic pulse profile (403, 404, 405) for delivery to a needle configured to vibrate based on the ultrasonic pulse profile received, wherein the computer is configured to compute and apply at least one pulse to the needle, each pulse having a pulse shape formed based on actual sensed aspiration vacuum pressure; characterised in that: the ultrasonic pulse shape comprises a predetermined ultrasonic pulse shape portion and an altered ultrasonic pulse shape portion; the altered ultrasonic pulse shape portion is either an increased energy portion comprising an increase in energy proportional to an increase in sensed aspiration vacuum pressure or a decreased energy portion comprising a decrease in energy proportional to a decrease in sensed aspiration vacuum pressure; wherein the altered ultrasonic pulse shape portion is added to the predetermined ultrasonic pulse shape portion in order to add an amplitude spike to the predetermined ultrasonic pulse shape portion, having a duration smaller than the pulse width of the predetermined ultrasonic pulse shape portion, to vary the ultrasonic power delivered to the needle directly proportional to the sensed aspiration vacuum pressure. a device configured to encounter vacuum pressure at the surgical area; a vacuum sensor configured to monitor vacuum pressure encountered by said device; and a computer configured to compute an ultrasonic pulse profile (403, 404, 405) for delivery to a needle configured to vibrate based on the ultrasonic pulse profile received, wherein the computer is configured to compute and apply at least one pulse to the needle, each pulse having a pulse shape formed based on actual sensed aspiration vacuum pressure; characterised in that: the ultrasonic pulse shape comprises a predetermined ultrasonic pulse shape portion and an altered ultrasonic pulse shape portion; the altered ultrasonic pulse shape portion is either an increased energy portion comprising an increase in energy proportional to an increase in sensed aspiration vacuum pressure or a decreased energy portion comprising a decrease in energy proportional to a decrease in sensed aspiration vacuum pressure; wherein the altered ultrasonic pulse shape portion is added to the predetermined ultrasonic pulse shape portion in order to add an amplitude spike to the predetermined ultrasonic pulse shape portion, having a duration smaller than the pulse width of the predetermined ultrasonic pulse shape portion, to vary the ultrasonic power delivered to the needle directly proportional to the sensed aspiration vacuum pressure. the ultrasonic pulse shape comprises a predetermined ultrasonic pulse shape portion and an altered ultrasonic pulse shape portion; the altered ultrasonic pulse shape portion is either an increased energy portion comprising an increase in energy proportional to an increase in sensed aspiration vacuum pressure or a decreased energy portion comprising a decrease in energy proportional to a decrease in sensed aspiration vacuum pressure; wherein the altered ultrasonic pulse shape portion is added to the predetermined ultrasonic pulse shape portion in order to add an amplitude spike to the predetermined ultrasonic pulse shape portion, having a duration smaller than the pulse width of the predetermined ultrasonic pulse shape portion, to vary the ultrasonic power delivered to the needle directly proportional to the sensed aspiration vacuum pressure.