To compare noise levels in water to noise levels in air, one must subtract 26 dB from the noise level referenced in water. Because of these differences in reference standards, noise levels cited in air do NOT equal underwater levels. A different reference level is used for underwater sound (1µPa 1m).

The reference level used in air (20µPa 1m) was selected to match human hearing sensitivity. In order to be useful, the sound levels need to be referenced to some standard pressure at a standard distance. Early acousticians working with sound in air, realized that human ears perceive differences in sound on a logarithmic scale, so the convention of using a relative logarithmic scale (dB) was adopted. Note on Acoustic Noise Level Units: Hydrophones measure sound pressure, normally expressed in units of micropascals (µPa). The two examples below show sound waves that vary in frequency and amplitude. Small variations in amplitude (short pressure waves) produce weak or quiet sounds, while large variations (tall pressure waves) produce strong or loud sounds. It is related to frequency because the lower the frequency of the wave, the longer the wavelength.Īmplitude describes the height of the sound pressure wave or the loudness of a sound and is often measured using the decibel (dB) scale. Wavelength is the distance between two peaks of a sound wave. The frequency of middle ∼ on a piano is 246 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic. Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. To the human ear, an increase in frequency is perceived as a higher pitched sound, while a decrease in frequency is perceived as a lower pitched sound. The basic components of a sound wave are frequency, wavelength and amplitude.įrequency is the number of pressure waves that pass by a reference point per unit time and is measured in Hertz (Hz) or cycles per second. The compressions and decompressions associated with sound waves are detected as changes in pressure by the structures in our ears and most man-made sound receptors such as a hydrophone, or underwater microphone.

Sound waves radiate in all directions away from the source like ripples on the surface of a pond. When underwater objects vibrate, they create sound-pressure waves that alternately compress and decompress the water molecules as the sound wave travels through the sea. Ocean acoustics is the study of sound and its behavior in the sea. NOAA Pacific Marine Environmental Laboratory Click image for larger view.Ĭlick image to hear a scale of various frequencies (576 K, QuickTime).

In this example of a sound wave, the period of one cycle of this wave is 0.5 seconds, and the frequency of this wave is 2 cycles per second or 2 Hertz (Hz).

The basic components of a sound wave are frequency, wavelength and amplitude.

A high pitch sound has