Ship Noise vs Other Underwater Noise Sources

This guide explains how to compare shipping noise with other underwater sound sources, such as marine life, seismic surveys, and natural events. You’ll learn to measure or estimate source levels, account for distance and frequency, and present a fair comparison. No specialized equipment is required—just a basic understanding of decibels and access to published noise data.

What You’ll Need

• Published source levels (in dB re 1 µPa @ 1 m) for the noise sources you want to compare. These are available from scientific papers, environmental impact assessments, and databases like the Discovery of Sound in the Sea.
• A calculator or spreadsheet to convert decibels and apply transmission loss formulas.
• Optional: a hydrophone and recording system if you want to measure real‑world noise levels in the field.

Step-by-Step

1. Define the comparison reference. Decide on a common reference distance and frequency band. Most underwater acoustics use a reference distance of 1 meter from the source, and a frequency band such as 10 Hz–10 kHz for shipping, or 100 Hz–1 kHz for many marine mammals.
2. Gather source levels. For each source, find its source level (SL) in dB re 1 µPa @ 1 m. Common approximate SLs (broadband) are:
   • Large commercial ship (at 10 knots): ~180 dB
   • Small boat (5–10 m): ~150–160 dB
   • Blue whale vocalization: ~190 dB
   • Seismic airgun array: ~230 dB
   • Lightning strike (underwater): ~260 dB (rarely measured)
3. Adjust for distance. Use the passive sonar equation: Received Level (RL) = SL – Transmission Loss (TL). For deep water, spherical spreading gives TL = 20 log10(r), where r is distance in meters. For shallow water, cylindrical spreading gives TL = 10 log10(r).
4. Apply the formula. For each source, compute the received level at your chosen distance. For example, at 1 km (1000 m) in deep water, TL = 20 log10(1000) = 60 dB. So a ship with SL=180 dB gives RL = 180 – 60 = 120 dB re 1 µPa.
5. Account for frequency weighting. Many comparisons use a frequency weighting (like the audiogram of a marine mammal) to reflect what an animal actually hears. If you have one-third‑octave band levels, integrate them or apply a weighting curve.
6. Create a comparison table. List each source, its SL, the TL at your chosen distance, and the resulting RL. This makes the comparison clear and easy to present.
7. Visualize the results. Plot the received levels on a bar chart or a map with color contours if you have spatial data. This helps communicate the relative impact.
8. Interpret the results. Remember that a 3 dB increase is a doubling of acoustic energy, and a 10 dB increase is perceived as roughly twice as loud. Compare the levels to known hearing thresholds or behavioral response criteria.

Common Mistakes

• Mixing reference units. Always use dB re 1 µPa for underwater sound. Using dB re 20 µPa (air standard) will give a misleading comparison.
• Ignoring frequency content. Broadband source levels can be misleading because some sources emit most energy at low frequencies (ships) while others are higher‑frequency (dolphins). Compare in the same frequency bands.
• Using the wrong spreading model. Shallow water does not always follow cylindrical spreading; it can be more complex. Check the environment before choosing the TL model.
• Forgetting absorption. At high frequencies (>10 kHz) and long distances, absorption becomes significant. For low‑frequency shipping noise, it is often negligible, but for sonar or dolphin clicks, it matters.
• Not stating the distance. A source level is only meaningful when the reference distance is given. Always specify “@ 1 m” or whatever reference you use.

Tips

• Use published source levels from peer‑reviewed papers when possible. The NOAA and BOEM websites have useful compilations.
• For a quick field estimate, you can use a hydrophone with a calibration file to measure received levels, then back‑calculate to source level if you know the distance.
• When comparing multiple sources, plot them on a log‑frequency axis to see the spectral overlap.
• Remember that shipping noise is often continuous, while other sources (like airguns or whale calls) are intermittent. The cumulative exposure may matter more than the peak level.

Troubleshooting

• My calculated received levels seem too high or too low. Double‑check the source level units and the reference distance. Also verify the transmission loss formula: at 1 km, spherical spreading gives 60 dB, not 30 dB (which is for 10 log10).
• I can’t find source levels for a specific vessel. Use an estimate based on similar size and speed. A 100 m cargo ship at 15 knots is around 185 dB, while a 20 m fishing boat might be 160 dB.
• The comparison looks different depending on distance. That’s expected. At close range, a whale call may dominate, but at 10 km, a ship may be louder. Choose a distance relevant to your question.
• My field measurements don’t match the model. Real environments have bottom reflections, surface scattering, and ambient noise that can raise or lower levels. Use a more complex model (e.g., Bellhop) for accurate predictions.

Frequently Asked Questions

How loud is a typical ship compared to a whale?

A large ship at 10 knots has a source level of about 180 dB re 1 µPa @ 1 m, while a blue whale call can reach 190 dB. However, ships are continuous, whereas whale calls are intermittent.

What is the loudest underwater noise source?

Seismic airgun arrays can reach about 230 dB re 1 µPa @ 1 m, and underwater explosions can exceed 260 dB. These are among the most intense man‑made sources.

How does distance affect the comparison?

Because transmission loss is logarithmic, the difference between sources shrinks as distance increases. At 1 km, a 180 dB ship is 60 dB down (120 dB), while a 190 dB whale is 70 dB down (120 dB)—they become equal.

Do I need a hydrophone to compare noise?

No, you can use published source levels and the sonar equation. A hydrophone is only needed if you want to measure real‑world levels in a specific location.

What frequency band should I use?

Use the band that is most relevant to the receiver. For marine mammals, use the audiogram of the species (e.g., 10 Hz–100 Hz for blue whales, 1 kHz–100 kHz for dolphins). For general comparison, 10 Hz–10 kHz covers most shipping and biological sounds.

Is shipping noise the biggest contributor to ocean noise?

In many coastal and open‑ocean areas, shipping is a major continuous contributor, especially in the low‑frequency range (10–500 Hz). However, seismic surveys and sonar can be more intense locally, and natural sources like wind and rain dominate at higher frequencies.

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