Hearing loss rarely affects all frequencies equally, so hearing is usually distorted. Low frequency loss impairs the ability to hear vowels. Categorisations of hearing impairment relate to intensity or amplitude. Hearing impairment can be measured according to decibel loss determining categories of hearing impairment. Regarding severe and profound hearing loss, a distinction, important for future communication, is made between prelingual and post-lingual loss. A deaf child born into a deaf family which uses sign language tends to develop that language at a similar rate to that at which a hearing child develops language. Spencer and Marschark refer to a 'fruitless argument' about the degree to which the primary goal should be, 'acquisition of the spoken vernacular' or the 'acquisition of a fully functional language system regardless of the sensory modality in which it is received or expressed'. Two broad perspectives are recognised: those emphasising the auditory-oral and those emphasising the visual manual.