India, Aug. 10 -- Each Kanwar yatra season, a wave arrives before the truck does. On the eleventh floor of a Noida or Gurugram high-rise, it slips through the double glazing that stops everything else. The window frame starts to rattle. The chest shifts before the ears catch up. In a Delhi colony two streets back from an arterial road, it arrives all the same - the setback that muffles traffic noise does nothing for the bass. Doors flex on their hinges.

Put a sound meter next to you and it will likely read within legal limits. That is because the trucks are loud in a way that the meters miss.

Bass - the low-frequency energy a subwoofer wall throws out - carries further than higher-frequency sound. It gets through more obstacles. It shak...