The standard way of quantifying a noise level is with a decibel or dB(A) measurement. The lower the dB(A) level the quieter the system. With a number of innovative features that keep noise disruption to a minimum, mini-split systems are ideal for environments such as classrooms, places of worship, healthcare facilities, theaters, and libraries.
Low Decibel Operation
The indoor and outdoor units have been designed to operate at extremely quiet levels. Indoor units run from 19 dB(A) at low speeds to 49 dB(A) at high speeds. Considering that a quiet library has a noise level of 40 dB(A) and the average home's noise level is 50 dB(A), the ductless mini split indoor unit is generally quieter than its surrounding environment.
Outdoor units operate at levels as low as 58 dB(A) - the level of a common office environment, restaurant conversation or background music. The loudest units peak at 65 dB(A) which is quieter than the sound of a vacuum cleaner.
Not only do these sound levels provide you with the quiet you need, but it also gives you more flexibility for placement of the units. When you install a ductless mini split system, you won't have to worry about your neighbors complaining about the noise. The quiet nature of its operations allows for minimal distractions of your day to day activities.
Reference
http://www.mitsubishipro.com/en/professional/commercial-applications/building-applications/quiet-spaces
http://mini-splits.ca/total-comfort/
http://d-airconditioning.co/mini-split.html