(1) trail off/away可用于声音，意为“逐渐失去强度；变弱”。

• When he saw who had entered the room, his voice trailed off in fear.
  等看清进入房间的是谁时，他的声音因害怕而变弱了。
• He began by shouting, but his voice trailed off/away into a mere whisper.
  他一开始还大喊大叫的，但他的声音渐渐变成了耳语。

 (2) tail off/away则指“(在数量、质量、力度上)逐渐减少、降低或逐渐消失”。

• Your work has been tailing off this year. We expect you to keep up your former standard.
  今年你的工作质量一直在下降，我们期待你保持以前的水准。
• Membership has tailed off recently, but should improve at the beginning of the new year.
  近来成员减少了，但来年初应会有所改观。
• The path tails off/away into the woods.
  小路消失在树林深处。
• The article begins well, but tails off/away in the final paragraphs.
  这篇文章虎头蛇尾。
• His voice tailed off/away into silence as he lost courage.
  随着勇气的丧气，他的声音也越来越弱，终至沉寂了。

 (3) taper off可作“(使)一头逐渐变细；逐渐减少(弱)；逐渐停止”解, 指在数量、质量、力度上逐渐减少、降低时，可与tail off换用。

• The end of the pipe tapers off so that it can fit into the narrow hole.
  把管子的一端逐渐变细，这样就能把它放进这个窄小的洞口了。
• The organization tapered off in about a year after its founder died.
  创始人去世大概一年后，这个组织就岌岌可危了。
• House building is being tapered off as the demand for renting property is falling.
  随着租房需求的减少，建房数量也在减少。
• The storm is beginning to taper off now.
  暴风雨开始减弱了。