The sciences involve quantitatively measuring properties of the natural world. When a scientist is making measurements, they must first identify the specific properties they will measure, how those measurements will be made, and what units they will use. For example, if a scientist is measuring the property of water depth, it would not be enough to say the water is 250 deep. That could mean 250 feet, 250 meters, or 250 fathoms. The scientist must provide the units for that value.

The standard for measurement is the International System of Units (SI), commonly known as the metric system. SI units are universally used in science and engineering. While the United Sates recognizes SI units as the international standard, it is voluntary to use metric units in the US. Most people in the United States use US customary units, such as the pound, gallon, foot, and Fahrenheit, on a daily basis. However, US customary units are defined in terms of SI units.

It is also important to understand how to convert units from one unit to another. Most Americans know that 12 inches is equal to 1 foot, but it may not be known that 1 nautical mile is equal to 1.85 kilometers. It is well known that 1 kilometer is equal to 1,000 meters, but not so well known that 1 micrometer is equal to 10^{-6} meters.

There are fixed units such as meters and relative units such as decibels (dB). Relative units are related to specific conditions. For example, decibels in water have a different relative value than decibels in air. Scientists have agreed to use 1 microPascal (μPa) as the reference pressure for underwater sound. In air, however, scientists have agreed to use a higher reference pressure of 20 microPascals.

**Sound intensity given in dB in water is therefore not the same as sound intensity given in dB in air. (Read more)**

Below are some common units used in underwater sound, what they are used to measure, and how they can be converted between each other.

### Common Units

#### Distance

1 meter (m) – a unit of distance; equal to 3.28 feet

1 kilometer (km) = 1,000 meters

1 centimeter (cm) = 0.01 meters (100 cm = 1 m)

1 millimeter (mm) = 0.001 meters (1000 mm = 1 m)

1 micrometer (μm also 1 micron) = 10^{-6} meters

1 nautical mile (nm) = 1.85 km or 1.15 miles

**US Customary units**

1 foot (ft) – a unit of distance; equal to 12 inches

1 inch = 2.54 cm

1 mile = 5,280 feet or 1.6 km

#### Weight/Mass

1 gram (g) – a unit of mass

1 kilogram (kg) = 1,000 grams = 2.2 lbs

1 metric ton = 1,000 kilograms

1 ton – a unit of weight; equal to 2,000 pounds (lbs)

#### Time

1 second (sec) – a unit of time

1 millisecond (msec) = 0.001 seconds (10^{-3} sec) (1000 msec = 1 sec)

1 microsecond (μsec) = 0.000001 seconds (10^{-6} sec) (1,000,000 μsec = 1 sec)

#### Temperature

degrees Celsius (°C) and Fahrenheit (°F) – units of temperature

To convert from Celsius (°C) to Fahrenheit (°F):

(°F) = (°C x 1.8) + 32

To convert from Fahrenheit (°F) to Celsius (°C):

(°C) = (°F – 32) ÷ 1.8

#### Salinity

1 part per thousand (ppt) = 1 gram of salt per liter of solution; a unit of proportion equal to 0.001

Practical Salinity Units (PSU or psu) = the conductivity ratio of a seawater sample to a standard potassium chloride solution. Since ratios have no units, it is not true that 35 PSU exactly equals 35 ppt.

#### Frequency

1 Hertz (Hz) = 1 cycle per second – a unit of frequency

1 kilohertz (kHz) = 1,000 Hertz

#### Pressure

1 pascal (Pa) – a unit of pressure

1 micropascal (μPa) = 10^{-6} Pa – commonly used when measuring sound pressures.

1 atmosphere (atm) = 14.7 lbs/in2 = 101,325 Pa

#### Intensity

decibel (dB) – a measure of the intensity of a sound; 1/10 of a Bel.

Decibels are a relative unit comparing two pressures; therefore, a reference pressure must also be indicated. In underwater acoustics, the reference pressure is 1 microPascal, so the true unit of intensity for underwater sound is dB referenced to 1 microPascal. In air, scientists have agreed to use a higher reference pressure of 20 microPascals. Therefore, the true unit of intensity for sounds in air is dB referenced to 20 microPascal. **Because they use different reference pressures, sound intensity given in dB in water is not the same as sound intensity given in dB in air**.

### Additional Links on DOSITS

- Introduction to Decibels
- Introduction to Signal Levels
- How does sound in air differ from sound in water?

### Additional Resources

- Google Calculator
- Conversion Calculator
- Unit Conversion Tools
- National Institute of Standards and Technology (NIST), Weights and Measures