Ultrasound uses sound waves to measure bone mineral density (BMD) at the heel, shin, or finger. The process is quite simple. For example, to measure BMD at the heel, you will be asked to place your bare foot in a device (known as a sonometer) that emits high-frequency sound waves. A computer determines the bone density by calculating how fast the sound waves pass through your heel. The machine can provide an estimate of your bone density in less than a minute.