For singers in the bass range (E2 - D4).

100 Intervals: ascending and descending min2, maj2, min3, maj3, P4, #4

How to practice:

The starting note will play twice before playing the first and second  note of the interval. During this time, imagine the interval in your  head and really hear it. Once the two notes have, prepare to sing both  the start note and the interval. Do at least 5 minutes a day for good  results.