External sound involves any type of exterior influences, including external parts and electrical/electromagnetic interference. Interference is defined as any unwanted indicators arriving as either voltage or existing, at any of the amplifier’s terminals or induced in its affiliated circuitry. It may seem as spikes, actions, sine waves, or random sounds. https://low-noise-amplifier89000.blogproducer.com/44644125/low-noise-amplifier-no-further-a-mystery