What b4 does.
When we are born our perceptual systems are capable of discriminating sounds that occur in any language. During the first year, our perception begins to zero in on the particular set of sounds that are contrastive in our native language(s) (L1s). For example, a child whose parents are L1 English speakers will pick up on the fact that /b/ and /p/ are contrastive in English (e.g., “bet” vs. “pet”). As our perception becomes attuned to our L1(s), we become less sensitive to non-native contrasts. When we begin to learn another language (L2) later in life our L1 acts as a filter, altering our perception of L2 sounds. Consequently, we may not detect differences between contrastive L2 sounds, and we may fail to notice the difference between our accented pronunciation of the L2 and the target pronunciation. This is the case with L and R among Japanese speakers. We might also have trouble connecting the sounds to the letters of an alphabetic language.
The proven method for improving the ability to hear the sounds of a second language is HVPT – High Variability Phonemic Training. That is, minimal pairs on steroids spoken by numerous people. In other words, the contrastive pair is pronounced with emphasis and exaggeration. The point is the students have to hear the sounds. If they can’t hear the difference – which they can’t in normal spoken conversation – they can’t learn them. The sounds have to be made salient, audible, then students can learn them.
HVPT is what b4 does.