Data collected by pop-ups are processed by computers and displayed as a graphic called a spectrograph. The spectrograms above display actual data sets collected within the Stellwagen Bank National Marine Sanctuary. They illustrate sounds of varying frequencies detected over a 24-hour period. These spectrograms indicate that there
is continual loud noise in Stellwagen Bank, at the same frequency range used by baleen whales.

Listen. Can you detect anything? There are invisible waves traveling in the air around you. These waves transfer energy from one place to another. Waves can be distinguished by several factors, including intensity (loudness), frequency (pitch), speed, and wavelength.

Frequency, which we perceive as the pitch of a sound, is a measure of how many waves pass a certain point in a given period of time. Often, this wave characteristic is measured in a unit called a Hertz, or Hz for short. One Hertz equals one complete cycle per second. An average human ear can detect waves of a frequency between 20 and 20,000 Hz. Our brain interprets this range of vibration as sound. Vibrations outside of this range have a frequency too low or too high for us to detect. For example, blue and fin whale calls are infrasonic, or below the range of human hearing.

Another important characteristic of sound is intensity, or how much energy is contained in the sound. We perceive intensity as the loudness of the sound. A spectrogram shows changes in frequency over time. Color variations indicate the intensity of the sound.