The way a "heavy" fog muffles the sound of the London traffic

London is usually a right old racket, but a proper thick fog turns the city’s volume knob down to zero. It’s not just in your head; the air itself is literally eating the noise of the traffic.

Think of those tiny water droplets as a billion microscopic speed bumps. Sound waves usually zip through clear air like a winger on a counter-attack, but in a fog, they’re constantly crashing into liquid particles.

These droplets soak up the sound energy and scatter it in every direction. By the time the roar of a double-decker bus reaches your ears, it’s been tackled into a muffled whisper.

Hold on, if sound is bouncing off everything, shouldn't it get louder?

Nah, it’s the opposite of a megaphone. When sound waves hit those droplets, they don't get a boost; they get lost in the sauce. It’s like trying to sprint through a crowded Tube station at rush hour—you’re bumping into everyone and losing steam fast.

Instead of a straight line to your ears, the sound becomes a disoriented mess. The energy actually gets converted into a tiny bit of heat, though not enough to boil your tea, unfortunately.

By the time the noise finally finds you, it’s exhausted. It loses that sharp "edge," which is why everything sounds like it’s wrapped in cotton wool rather than being amplified.

Does this mean the fog is just a giant filter for the bass?

Spot on. It’s essentially a giant, natural low-pass filter. High-pitched noises have short wavelengths that are roughly the same size as those tiny water droplets.

Because they’re similar in size, they constantly smack into the droplets and lose their energy. It’s like trying to dribble a football through a crowded pub—you’re bound to lose the ball.

But low rumbles have massive wavelengths that just roll right over the droplets without even noticing them. That’s why a distant bus sounds like a deep thud rather than a screech.

So a siren's high-pitched scream is basically toast in a thick fog?

Bang on. You’ll hear the low-end 'thrum' of the engine long before that 'nee-naw' reaches you. The siren’s sharp edge gets shredded by the droplets, leaving it sounding weak and confused.

It’s exactly why ships use foghorns that sound like a giant burping into a tuba. That deep, floor-shaking rumble is physically so large it can ignore the fog and travel for miles.

In a proper pea-souper, emergency services are fighting a losing battle. The sound is there, but it's stripped of its piercing power.

Just how 'massive' is one of these low-frequency waves then?

We’re talking absolute units. A deep foghorn blast has a wavelength that stretches past a double-decker bus with room for a black cab at the end.

While a high-pitched whistle is only a few centimeters long—tiny enough to get tripped up by mist—the foghorn’s wave is a 17-meter giant.

To a wave that big, a water droplet is like a pebble on the motorway. It doesn't even register; the sound just rolls over it without losing rhythm.