Ultrasound (US) can be a very effective method of heating deep tissues. Heat from the US promotes relaxation and improves circulation. Pain relief is often a result of both.
Standard US is applied with an applicator called a soundhead. The soundhead is moved slowly in small circles. Each circle overlaps the last one. A five-minute treatment is about right for a patch of skin two to three times the size of the soundhead. This gives the tissue underneath time to heat up.
Treating the entire low back area in five minutes is not effective. Be sure and ask your therapist about the treatment. Report your lack of progress, but give it some time. It may take three to five sessions to feel a difference.