What happens to a beginner’s perceptual system when one first starts to learn a foreign language? In the context of learning a foreign language, a person with normal hearing in his/her mother tongue will behave as though he/she were hard of hearing (Lian, 1980). Acoustically, each language sound carries all frequencies from about 50 Hz to about 16,000 Hz (albeit at various intensities). Theoretically, at any rate, each sound can be heard in many different ways. The ear seems to have a 'choice' as to what to hear in practice depending on the way the ear has been trained. L2 students tend to make 'choices' in the target language based on what they are familiar with in their mother tongue. Trubetzkoy (1939) refers to this as the mother tongue 'sieve'. For instance, a vowel is physically made up of a complex set of many frequencies produced simultaneously. When a vowel is heard, everything it contains is heard. However, when listening to a sound, it is not necessary to catch all the elements in order to recognize it as recognition only requires some of the sound spectrum. Each sound has a particular 'optimal' frequency (i.e. the frequency band, or combination of frequency bands) at which a native-speaker best recognizes and perceives the sound in question according to their mother tongue. Students who experience difficulty with a particular foreign language sound are considered as not having recognized (i.e. perceive) its optimum. Thus, they are unlikely to be able to reproduce the sound correctly.
If, for example, the sound [i] in French is recorded and listened to successively through octave filters: