Stand on a pier when a container ship sounds off and you feel it in your chest. Stand next to a locomotive horn and you feel it in your teeth. Both are loud enough to hurt, but they are built for different jobs, and the numbers explain why one rumbles and the other screams.
The Short Answer
At the source, a large ship's whistle and a locomotive horn land in the same territory, roughly 140 to 150 dB. The ship wins on range, not raw output. It is rated to be heard 2 nautical miles away, and it gets there with a fundamental pitch of 70 to 200 Hz, a note so low that air barely absorbs it. A train horn is tuned one to two octaves higher so it cuts through a car cabin at a grade crossing, which is a different problem.
| Horn | Rated level | Where it is measured | Fundamental pitch | Design range |
|---|---|---|---|---|
| Ship whistle, vessel 200 m (656 ft) or longer | 143 dB | 1 meter, 1/3-octave band | 70 to 200 Hz | 2 nautical miles (about 2.3 statute miles) |
| Ship whistle, vessel 75 to 200 m | 138 dB | 1 meter | 130 to 350 Hz | 1.5 nautical miles |
| Locomotive horn (federal rule) | 96 to 110 dB(A) | 100 feet ahead of the locomotive | Chord, roughly 311 to 622 Hz on a five-chime horn | Grade-crossing warning |
| BossHorn Extreme Series battery train horn | 150 dB | At the trumpets | Train-chord midrange | Portable signaling, remote to 2,000 ft |
Two cautions before reading that table as a scoreboard: the ship figure is an unweighted third-octave level while the locomotive figure is A-weighted, and A-weighting knocks about 13 dB off a 160 Hz tone. The ship is also measured at 1 meter, the locomotive at 100 feet.
How Loud a Ship Horn Is, by Law
The International Regulations for Preventing Collisions at Sea, adopted into U.S. law as 33 CFR Part 86 (Annex III), spell out exactly how loud and how low a vessel's whistle must be. The rules scale with the length of the ship.
- 200 meters or longer: 143 dB at 1 meter, fundamental between 70 and 200 Hz, audible 2 nautical miles.
- 75 to 200 meters: 138 dB, fundamental between 130 and 350 Hz, audible 1.5 nautical miles.
- 20 to 75 meters: 130 dB, fundamental between 250 and 700 Hz, audible 1 nautical mile.
- Under 20 meters (65 ft): 120 dB in the 180 to 450 Hz band, audible half a nautical mile.
The same regulation shapes the sound. A directional whistle must stay within 4 dB of its rated level across a 90-degree arc off the bow and be mounted as high as practicable. There is even a crew-protection clause: the level at the ship's own listening posts may not exceed 110 dB(A). That "very loud in this area" sign next to a funnel exists because the rule assumes the whistle is dangerous at close range.
Hardware example: Kahlenberg's KM-165, built for ships over 75 meters, publishes a 165 Hz fundamental and 143 dB in the third-octave band. The horn is about 34 by 18 by 18 inches, weighs 37 pounds, and drinks 106 standard cubic feet of air per minute at 120 psi. That is a compressor room, not a tank under a truck bed.
Older steam whistles went lower still. The three whistles on the original RMS Queen Mary were tuned to a 55 Hz fundamental and, according to Cunard historians, could be heard more than 10 miles across the Atlantic.
How Loud a Train Horn Is
The federal rule for locomotives, 49 CFR 229.129, requires 96 to 110 dB(A) measured 100 feet in front of the locomotive, averaged over at least six 10-second blasts. Work that back toward the horn at 6 dB per doubling of distance and a maximum-legal locomotive horn is producing roughly 140 dB(A) a few feet from the bells. We ran that math in our piece on whether a train horn gun is as loud as a real locomotive.
Independent testing agrees. HornBlasters' 2014 third-party test of the Nathan AirChime K5LA, the five-chime horn on many modern locomotives, recorded 149.4 dB at close range. The K5LA sounds five notes at once: 311, 370, 415, 494, and 622 Hz. That chord is why a train horn sounds like a train instead of a foghorn.
Portable battery horns are rated at the trumpets, the same way the K5LA was tested. BossHorn's Dual models run about 130 dB, Quad models about 140 dB, and the Extreme and Boss Series reach 150 dB and beyond. The Extreme Series Train Horn for Milwaukee® 18v Battery is the 150 dB version: larger metal trumpets tuned for a deeper tone than the standard models, a high-output compressor, and a wireless remote good for 160 feet out of the box or 2,000 feet with the long-range upgrade. It runs on any M18™ pack you already own and weighs about 4 pounds. That is roughly the same source level as a ship's whistle, at one-ninth the weight and with no air line.
So which is louder? At 1 meter the ship's 143 dB and the locomotive's 140 dB(A) are within a few decibels, and once you A-weight the ship's 165 Hz tone, the locomotive and a 150 dB portable horn come out ahead on the meter. The ship is still heard from farther away. Here is why that is not a contradiction.
Why They Sound So Different: Pitch and Wavelength
Pitch is the entire story. At the standard 1,125 ft/s speed of sound, a 165 Hz ship whistle has a wavelength of about 6.8 feet, and the Queen Mary's 55 Hz note stretched about 20 feet. A train horn's 311 Hz low chime is about 3.6 feet long, and its 622 Hz top chime is under 2 feet. A stock car horn sits around 400 to 500 Hz, which is why a train horn reads as "deep" to drivers even though it is high compared to anything on the water.
Trumpet length sets pitch. A 165 Hz fundamental needs a bell on the order of a yard long, which is what the KM-165's 34-inch housing is doing. Train horn chimes are shorter because the notes are higher, and a portable battery horn keeps its trumpets in the train range because a train chord is the sound its owners want. The Extreme Series gets its deeper voice from longer, larger trumpets than the standard quad, not from imitating a freighter.
Strictly speaking, a "foghorn" is the stationary signal at a lighthouse; the navigation rules call what a ship carries a "whistle." Both rest on the same idea: a very low note, pushed hard, aimed forward.
Why the Low Note Carries Farther
Air is not transparent to sound. It absorbs acoustic energy through molecular relaxation and viscosity, and the loss climbs steeply with frequency. Published absorption tables at 68 °F put the loss at about 5.6 dB per kilometer at 1,000 Hz, 12 to 16 dB per kilometer at 2,000 Hz, and 63 to 105 dB per kilometer at 4,000 Hz depending on humidity. Drop into the 70 to 200 Hz band and the curve keeps falling toward a tiny fraction of the 1,000 Hz figure. Over the 2 nautical miles (about 3.7 km) a large ship's whistle must cover, a low fundamental loses almost nothing to the air itself.
Long wavelengths also bend around obstacles more easily, which matters when the obstacle is a wave crest, a breakwater, or another hull. Spreading loss, the 6 dB per doubling rule, hits every frequency equally. Absorption and diffraction do not, and that is why the maritime rules push big ships into the bass register.
There is a trade-off, and it explains the train horn's choice. Human hearing is far less sensitive at 100 Hz than at 500 Hz, which is what A-weighting encodes. A ship's low note carries a long way but sounds less loud per decibel of pressure. A locomotive is not trying to reach a ship 2 miles off. It is trying to be heard inside a closed car with the radio on, and midrange notes near 300 to 600 Hz are the efficient way to do that. Each horn is tuned for the ear it needs to reach. Our decibel comparison chart puts both horns next to jackhammers, sirens, and jet engines if you want the wider picture.
Can a Battery Train Horn Stand In for a Boat Horn?
For recreational boats, yes, by a wide margin on paper. Annex III asks a vessel under 20 meters to produce 120 dB at 1 meter in the 180 to 450 Hz band and be audible half a nautical mile. A 150 dB portable horn sits 30 dB above that, with its energy in a band the rule counts. It also dodges the failure modes of small-boat horns: canned air runs out and hardwired 12V horns corrode. A tool battery lives in a dry bag and swaps in seconds.
What a battery horn will not do is turn a bass boat into a freighter. It cannot produce the 70 to 200 Hz fundamental assigned to ships over 200 meters, and it should not: a small vessel is supposed to sound higher than a big one so mariners can judge what is coming in fog. For the fine print, see our guides on train horns for boats and USCG sound-signal requirements.
FAQ
Is a ship horn louder than a train horn?
At the source they are close: 143 dB at 1 meter for a large ship's whistle versus roughly 140 dB(A) for a maximum-legal locomotive horn at the same distance. A 150 dB portable train horn matches or exceeds either on a meter. The ship is heard farther away because of its low pitch, not because it is dramatically louder.
Why does a ship horn sound so much deeper?
Because the law requires it. Vessels 200 meters or longer must use a fundamental between 70 and 200 Hz, and those frequencies lose almost no energy to air absorption over miles of water. A five-chime locomotive horn plays a chord from about 311 to 622 Hz, one to two octaves higher.
Can I use a battery train horn as my boat's sound signal?
On a boat under 20 meters, a 150 dB portable horn clears the 120 dB Annex III level with room to spare and produces sound in the 180 to 450 Hz band the rule counts. Keep it within reach of the helm and carry a charged spare pack.