Shopping for a battery train horn and stuck between an 18V pack and a 20V MAX pack? The bigger number looks like it should mean a bigger blast, but the two labels describe the same five-cell battery, and your horn can't hear the difference.
The Short Answer: No, 20V MAX Is Not Louder
A 20V MAX battery does not make a train horn louder than an 18V battery of the same size. Both are lithium-ion packs with five cells wired in series, and under load they deliver the same working voltage. The "20" and the "18" are two different ways of reading the same pack: one at its peak with nothing connected, the other at its typical working level.
You can see it on our own spec sheets. The Extreme Series Train Horn for Milwaukee® 18v Battery and the Extreme Series model built for DeWalt® 20V MAX™ packs list the same four powder-coated aluminum trumpets, the same three volume levels, and the same 500+ short or 200 sustained blasts on a 6Ah pack. The only part that changes is the battery mount.
Where the "20V" Number Comes From
DeWalt states it plainly in the fine print on its own battery pages: maximum initial battery voltage, measured without a workload, is 20 volts, and nominal voltage is 18. That footnote is the whole story in two sentences.
Here is the cell math behind it:
- Nominal voltage: a lithium-ion tool cell is rated around 3.6V. Five in series gives 18V. That is the number Milwaukee® prints on M18™ packs, and Makita® and Bosch® label their 18V lines the same way.
- Maximum voltage: fresh off the charger, each cell can read about 4V with nothing connected. Five of those gives roughly 20V. That is the number printed on a 20V MAX™ pack.
- Under load: once a motor starts pulling current, a freshly charged pack quickly settles back toward its 18V nominal level, whatever the label says.
DeWalt applies the same convention across its lineup. Its 12V MAX packs are listed at 10.8V nominal, and its FlexVolt packs are listed at 20V, 60V, or 120V maximum with nominal figures of 18V, 54V, and 108V. So when a FlexVolt pack runs a horn in 20V mode, it is still an 18V-nominal battery doing the work.
Several power-tool reviewers have put the two labels on a multimeter and called the 20V figure a marketing choice rather than an electrical difference. Same cell count, same chemistry, same working voltage.
What 18V vs 20V Looks Like in Decibels
Suppose a 20V MAX pack really did hold its full 20V the whole time a compressor was running. Would you hear it? Decibels are logarithmic, and OSHA's technical manual gives the rule of thumb: doubling the sound power raises the noise level by only 3 dB.
Going from 18V to 20V is an 11 percent voltage increase, nowhere near a doubling. Run the numbers generously and assume a compressor's output climbs with the square of voltage (roughly 23 percent more power). That still works out to less than 1 dB. Most people would not pick that out on a side-by-side test, and in practice even that small gap disappears because the 20V MAX pack drops to the same 18V under load.
| What changes | Effect on loudness |
|---|---|
| 18V label vs 20V MAX label (same Ah) | None in practice. Same pack voltage under load. |
| Hypothetical full 2V gain held under load | Under 1 dB, theoretical. |
| Doubling the sound power | About 3 dB. |
| Standing twice as far from the horn | About 6 dB quieter at your ears. |
The last row is the one most buyers miss. OSHA notes that sound pressure level from a point source drops about 6 dB every time you double the distance. Where you stand and where the trumpets point matter far more than the number on the battery.
What Actually Makes One Battery Train Horn Louder Than Another
If the battery label doesn't decide loudness, what does? Four things, in order of how much they matter:
- Horn tier. Trumpet count and compressor size set the ceiling. BossHorn's line runs from Dual (130 dB) through Quad (140 dB) up to the Extreme and Boss Series at the top. Moving up a tier changes the sound far more than changing batteries ever will.
- Volume setting. Our Extreme Series spec page lists three levels (full, medium, soft). If a horn seems weak, check the setting before you blame the battery.
- State of charge and temperature. A pack near empty, or one that sat overnight in a freezing truck bed, sags harder under the compressor's load. We break that down in our guide to whether a low battery makes a train horn quieter.
- Distance and aim. Per the 6 dB rule above, trumpets pointed at the crowd from the tailgate will always sound bigger than the same horn pointed at a wall.
If loudness is your main goal, start by choosing the tier, then pick whichever battery platform you already own. Our Extreme Series train horns come in versions for most major battery platforms, and every version uses the same trumpets and compressor.
So How Should You Choose: 18V or 20V MAX?
Choose by the batteries in your garage, not by the number on the sticker. The platform you already own is the right one because it saves you buying a second set of packs and a second charger.
- Already running Milwaukee® M18™ packs? Get the Milwaukee-compatible horn. Any M18™ pack from 2.0Ah to 12.0Ah fits.
- Already running DeWalt® 20V MAX™ packs? Get the DeWalt-compatible horn. Our spec page lists 20V MAX™ packs from 2.0Ah to 12.0Ah. For the full fitment and FlexVolt breakdown, see our DeWalt 20V MAX compatibility guide.
- Own both? Then the difference is pack size, not voltage. Our M18 vs 20V MAX blasts-per-charge comparison shows which platform's largest pack gives you more blasts.
- Own nothing yet? Pick the platform you're most likely to buy tools for later. The horn will sound the same either way.
The one battery spec that does change your experience is amp-hours. A bigger pack doesn't make the horn louder, but it keeps the horn at full strength for more blasts before the voltage starts to fall off.
FAQ
Is a 20V MAX battery more powerful than an 18V battery?
Not in any way a train horn can use. DeWalt's own fine print lists 20V MAX packs at 18V nominal. Matching amp-hour packs from an 18V-labeled platform and a 20V MAX platform store about the same energy and run the compressor at the same working voltage.
Will a 20V MAX-compatible horn hit a higher dB rating than the 18V version?
No. Within a tier, our horns for 18V-labeled and 20V MAX-labeled packs share the same trumpets, volume levels, and rated blasts per charge. The battery mount is the only difference.
Why does my horn sound stronger on a fresh battery?
Right off the charger, a pack starts slightly above nominal and holds up better under load, so the compressor spins a little harder for the first blasts. That is true of 18V and 20V MAX packs alike. It's about charge level, not the label.
Can I run a FlexVolt pack on a 20V MAX-compatible horn?
FlexVolt packs fit 20V MAX tools and run in 20V mode, which DeWalt lists as 18V nominal. You get more capacity and more blasts per charge, not more decibels.