All finback whales in the Northern Hemisphere produce sounds with generally similar features. Most sounds are frequency-modulated (FM) down-swept infrasonic pulses from 16 to 40 Hz, lasting about 1 to 2 seconds. Various combinations of pulses often occur in deliberate, patterned sequences usually lasting 7-15 minutes, and these sequences are repeated in bouts lasting up to many days.
The illustration below is a sound spectrogram of a 9.5-minute recording made by Navy ocean-bottom surveillance microphones in the western North Atlantic. This example illustrates both the regularity and variability in the timing pattern of fin pulses, here 17-40 seconds between calls.
To hear the recording, click on the spectrogram. The sound you will hear is speeded up ten times faster than normal speed, which also raises the pitch of the whale sounds by a factor of ten (slightly more than three octaves). Notice that each pulse sounds as though it is a double pulse, or as though it has an echo. That is because the sound actually arrives twice at the underwater microphone, having travelled somewhat diferent paths through the water. This phenomenon is called "multipath".
The illustration below is a sound spectrogram of a 2.5-minute recording made by a Pop-up hydrophone in the Gulf of California in March 1999. The time between calls in this example varies from 4 to 11 seconds. These pulses also sound as though they may be "multipath" double arrivals. But they are not. These are actually pulses made in a paired cadence. To hear the recording, click on the spectrogram. As in the figure above, the sound you will hear is speeded up ten times faster than normal speed. Please note that the time scale on this spectrogram differs from the one above.
Actually, many fin whale vocalizations at normal speed will not "sound like" ANYTHING at all, because the frequency of their calls often falls below the hearing of human beings. Click on the spectrogram below to hear these two calls, the first two calls in Figure 2, in real time. Yes, there will truly be a sound file playing, but you may not be able to detect it with your ears. Be aware that even if you do hear something, it is likely the distortion introduced by your loudspeaker.