ISO 24504:2014 specifies methods to determine an appropriate sound pressure level range for spoken announcements in environments where ambient noise is less than 80 dB. The specified methods follow the concepts of ISO/IEC Guide 71 and includes consideration of older persons with decreased hearing ability to determine sound pressure levels of spoken announcements. The spoken speech levels specified are for products and public-address systems.
ISO 24504:2014 is applicable when a loudspeaker producing a spoken announcement is located a short distance from the user in an environment where the sound pressure level with a standard frequency weighting A of ambient noise does not exceed 80 dB. It is applicable to spoken announcements that are audible to persons with normal hearing for their age when presented by a target product under quiet and anechoic conditions, and for both recorded voice and synthetic speech announcements.
It is not applicable to products providing private information such as automated teller machines in public spaces, nor to spoken announcements heard through headphones or earphones, or with the ear close to the speech sound source such as ear speakers. It does not specify the sound pressure levels of spoken announcements for systems with automatic sound pressure level control to compensate for fluctuating ambient noise levels, nor those presented in emergency situations such as signals for fire alarms, gas leakage and crime prevention (covered in ISO 7240‑16 and ISO 7240‑19), or in automobiles (covered in ISO 15006). It considers only the audibility of speech and not the process of speech understanding.