Subvocalization, or silent speech, is defined as the internal speech made when reading a word, thus allowing the reader to imagine the sound of the word as it is read (Carver 1990). This is a natural process when reading and helps to reduce cognitive load, and helps the mind to access meanings to enable it to comprehend and remember what is read.
There is no evidence that normal non-observable subvocalizing will negatively effect any reading process (Carver 1990)(McWhorter 2002). At the more powerful rates (Memorizing, learning, and reading for comprehension) subvocalizing is very detectable by the reader. At the less powerful faster rates (skimming and scanning), subvocalization is less detectable. For normal competent readers, subvocalizing to some extent even at scanning rates is normal. However, speed reading advocates generally teach lengthy prescriptive practices to eliminate subvocalising when reading as they claim it "places extra burden on the cognitive resources, thus, slowing the reading down." Normal reading instructors may simply apply remedial teaching to a reader who subvocalizes to the degree that they make visible movements on the lips, jaw, or throat (McWhorter 2002).
It may be impossible to totally eliminate subvocalizing because people learn to read by associating the sight of words with their spoken sounds. Sound associations for words are indelibly imprinted on the nervous systems, even of deaf people, as they will have associated the word with the mechanism for causing the sound. Subvocalizing is an inherent part of reading and understanding a word, and micro-muscle tests suggest that subvocalizing is impossible to eliminate. Attempting to stop subvocalizing is potentially harmful to comprehension, learning and memory. At the more powerful reading rates (100-300wpm), subvocalizing can be used to improve comprehension.
- Carver, R.P-Prof (1990) Reading Rate: A Comprehensive Review of Research and Theory. (1990)
- McWhorter, K. (2002) Efficient and Flexible Reading. Longman
- A technique for eliminating subvocalization (part of a speed reading tutorial) 
- NASA researching on subvocalization  as a new way to interact with computers.
Other ways to improve Reading Better and Bypassing the Subvocalization problem