Plural of deadener; things or substances that reduce sound, vibration, or intensity.
From 'deaden' (to make less intense) + '-er' (agent noun suffix). 'Deaden' comes from 'dead' + '-en' (verb-forming suffix), with roots in Old English 'deað' meaning death, extended to mean 'lacking life or intensity.'
Deadeners are everywhere in your world—from the foam in your headphones to the insulation in car doors—yet we rarely notice them because their entire job is to make sounds disappear. This is a perfect example of how the most important things often work by being invisible.
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