Perfect Tenses(Simple, Continuous)
Sarah (to promise) to be kindly. *
Tom and Tim (to wave) their hands for 20 minutes before we (to notice) them. *
Ben (to bend) his head before he (to say) "hello". *
Ann (to murmur) something for an hour but I can't understand anything. *
The friends (to pull) the sledges for ten minutes before they (to be) tired. *
Look at Sue! She (to repair) her house! *
Daddy (to light) a candle before we (to find) a lantern. *
Kate (to regret) about mistakes which she (to make) in her test. *
My close friend (to knock) at my door for twenty minutes before I (to hear) the sound. *
I think that Mike (to finish) his work. *
Answers
Explanation:
Reflection of sound waves off of surfaces can lead to one of two phenomena - an echo or a reverberation. A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. Why the magical 17 meters? The effect of a particular sound wave upon the brain endures for more than a tiny fraction of a second; the human brain keeps a sound in memory for up to 0.1 seconds. If a reflected sound wave reaches the ear within 0.1 seconds of the initial sound, then it seems to the person that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations - the prolonging of a sound. Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 s for a sound to travel the length of a 17 meter room and back, thus causing a reverberation (recall from Lesson 2, t = d/v = (34 m)/(340 m/s) = 0.1 s). This is why reverberations are common in rooms with dimensions of approximately 17 meters or less. Perhaps you have observed reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound.