Whenever you're trying to detect something, you'll always need to take into account

• how well your system can detect something. 
• how much information is going to be lost because of external noise.
• what frequencies you're using.

Because all kinds of marine animals communicate in tones with different frequencies, marine biologists can use passive monitoring sonar to listen in on whale (and other animal) songs. Some animals even use sound to see, making the waves we pick up more interesting.

Marine biology isn't the only system to use passive sonar, though. The fishing industry also uses sonar and the Doppler effect to monitor the balance between the supply fish and the breeding fish.

The most common use of sound waves happens with ultrasonic frequencies in industrial and medical applications. These are usually called ultrasounds.

Ultrasound—which is basically a mini-sonar system—sends out acoustic waves at a frequency above what most adults can hear (20 kHz). Kids have better hearing than adults, but for most ultrasound testing you won't hear a peep (source). When you get tested using ultrasound, the machine is actually sending high frequency waves through your body to see what gets absorbed and reflected. All that can get translated into a pretty good image of what's going on inside your body without hurting the tissue.

Ultrasound creates images by sending out waves and listening for their reflections from inside your body. When an ultrasound wave travels inside an object made up of different materials (like, say, bone and muscle), each time it finds a boundary (like between bone and muscle or between muscle and fat), part of the wave is reflected and part of it keeps going. The reflected pieces are detected and used to build an image of the object based on the speed of sound and how far the wave traveled before it was reflected back.

The best part, though, is the fact that ultrasound is non-destructive, meaning that it won't hurt you the way CT and other radiation can. This form of non-destructive testing can even generate three-dimensional ultrasounds. It sends sound waves in multiple angles to get a 3D image of your insides.

3D ultrasound and CT scans both give 3D imaging of objects like the human body, but CT scans use radiation. Yes, like what Marie Curie discovered, and yes, like what killed Marie Curie. Of course a CT scan or two isn't going to up your chances of getting cancer too much, but if we've got something we know won't cause cancer, that sounds like the better option, don't you think?

(Source)