Why speech is harder to ignore than a hum
A steady hum is repetitive, so the brain can learn that it carries little new information and spend less attention on it. Conversation is different: syllables, pauses, emphasis, and voices keep changing. Even when you are not trying to listen, your attention can briefly orient toward those changes to check whether they matter—particularly when a voice sounds close by.
That shift need not become conscious listening. A fragment of a sentence can enter awareness, then pull attention away from the words on the page before you have decided to follow the conversation.
The interruption happens inside the reading process
Silent reading is not always silent in the mind. Many readers form a faint inner version of the words as they go, a process often called subvocalization. It helps hold a sentence together long enough to connect its beginning with its ending and understand the meaning.
That temporary holding space is part of working memory: the limited mental workspace used to keep and manipulate information in the moment. Speech nearby competes for some of the same verbal resources. A phrase from the next table may linger just long enough to disrupt the sentence you were building, so you have to reread a line or recover where the argument was going.
Why the effect varies
The disruption depends on the task and the sound. Dense or unfamiliar writing demands more concentration than an easy passage, leaving less room for a competing voice. Speech that is especially noticeable—or that you can understand—may be more distracting than a uniform background noise, though its effect varies from person to person and situation to situation.
The practical consequence is small but cumulative: a conversation does not have to address you to make reading slower. It only has to keep changing enough for your attention to sample it while your mind is trying to keep a sentence intact.
