The following terms are used to understand and define a PA system. A well-designed system should provide the following:

• Intelligibility. The goal is to achieve easy comprehension of the spoken word.
  Stability. The announcements broadcast over the PA system should be free of feedback and spurious pick-up. Feedback—the endless cycling of loudspeaker output back into the microphone input—is the result of improper loudspeaker location and insufficient electronic gain control. Pick-up of unwanted outside signals can be caused by an aging system or poor installation. In the case of poor installation, the audio signal cables act as an antenna to pick up and amplify signals from outside the PA system. This can be resolved by using proper ground-ng and shielding techniques and minimizing cable loops that promote electromagnetic induction of signals into the system. 
• Clarity. Freedom from distortion or noise. Distortion mixed with noise hinders speech intelligibility, especially under low SNR conditions. 
• Linearity. The PA system’s output at the listening position should vary in direct proportion to the sound source. A linear system provides high-quality reproduction (fidelity) of the input sound. A system that does not do this is nonlinear. 
• Naturalness. The PA system should sound balanced and natural. Given that a PA system is primarily a means for broadcasting the spoken word, the range of frequencies important to understanding speech (nominally 200 to 4,000 Hz) will be present without some frequencies being predominant or lacking. 
• Adequate sound level. The amplitude of the sound signal is a measure of loudness and is usually measured in decibels (dB) of sound pressure level (SPL). The PA system should be loud enough to be heard in the area served without being objectionably loud. 
• Uniform sound coverage. In the region served by each loudspeaker zone, the entire area should receive evenly distributed sound levels. Neither hot spots where sound is noticeably higher, nor dead zones where sound is absent are desirable. Ideally, the uniformity is about ± 1 dB. 
• Adequate ratio of direct-to-indirect sound. Direct sound travels from the loudspeaker directly to the listener’s ears. Indirect sound is reflected off one or more surfaces before it reaches the listener. Too much indirect sound interferes with the clear understanding of speech. Echo and reverberation are examples of indirect sound that can compromise intelligibility.
• Adequate SNR. The PA system sound level must be sufficiently above the ambient noise level to achieve intelligibility. Ambient noise sources include HVAC systems, aircraft operations, human activity, concession mechanical equipment, TVs, escalators, and people movers.