Final answer:
The unseen effect when reflected sound is within the integration interval is the phenomenon of echo, where sounds are perceived as fused together. The time interval for the loudest echoes depends on the distance traveled by the echo and the speed of sound, which is used in the formula time = distance/velocity.
Step-by-step explanation:
When discussing the unseen effect that occurs with reflected sound, we are referring to what is called integration time in acoustics. Integration time is the duration over which reflected sounds are summed together to be perceived as one sound. For speech, an integration interval is typically around 30 milliseconds, and for music, it is around 50 milliseconds. This can lead to the phenomenon of echo, where the reflected sound arrives at the listener's ear within this integration interval and is blended with the direct sound, creating a sense of reverberation or fullness in the listening space.
To answer the question about the time interval for the loudest echoes, based on the provided velocity of sound (330 m/s), we use the formula time = distance / velocity. The distance for the echo to travel would be twice the distance from the sound source to the reflective surface because the sound has to travel to the reflective surface and then back to the listener. If we are considering the time interval between two echoes, we would have to calculate the distances involved for both echoes and then apply the formula to find the respective time intervals.