Six microphones positioned around HomePod allow it to pick up all the sound in a room. When you say “Hey Siri”, advanced signal processing, together with echo and noise cancellation, allows HomePod to hear you without the need to raise your voice — even if you’re across the room with loud music playing. After HomePod recognizes the words “Hey Siri”, what you say is encrypted and sent anonymously to Apple servers without being tied to your Apple ID.
Source: Apple