Question

sometimes reflection of sound may be undesirable

Answer 1

Yes , sometimes it is an desirable for example in cinema hall when sound is reflected it may disturb the audience

Answer 2

Have you ever been in an empty hall and tried to speak with your friend? you will notice that you both are not able to speak to each other clearly because you may listen to your echo while your friend is saying something to you. so in that case we can say that reflection of sound is undesirable(except the case if you’re enjoying listening to your echo).