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Our builder has asked whether we have any objection to his laying sound insulation on top of the suspended ceiling he's going to install, rather than attaching it to the actual ceiling above. He advises that doing otherwise will increase room echo, a very important consideration for what will be classrooms without any soft furnishings etc.

The builder is reputable and we have no reason to doubt him but this seems to me perhaps the echo issue is a lie. I realise this would also make difficult access to the space above the drop ceiling.

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The level of echo is based on the material of the suspended ceiling itself, not what's behind it. The sound insulation between the floor and ceiling will only reduce the sound travelling between the classroom and the level above. To cut down on the echo within the room, focus on sound absorbing material on the ceiling and walls of the classroom. Soft materials like fabric and carpet are best for this job.

Edit: To better answer your question, the only reason I can think of for laying the insulation is that you have no attachment points where the insulation would have less of an effect. Also, many builders put insulation between the joists, and the joists themselves transmit and possibly reflect some sound. But your concern about limiting access above the ceiling is valid. Also, many workers are unlikely to return the insulation to its proper location after moving it to do some work above the ceiling. Some light fixtures may generate excessive heat if insulation is laid directly on top. And finally, the added weight may not be good for the suspended ceiling, possibly causing the tiles to warp or other kinds of failure. For the ease of maintenance alone, I'd go for the insulation attached to the floor above.

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Thanks for your reply. I must say though that this conflicts with what I've read elsewhere, that unless a suspended ceiling absorbs all sound then it will hit the actual ceiling and reflect. While our drop ceiling tiles have acoustic properties they by no means insulate the space above to any great extent. –  mark Jun 14 '12 at 12:08
    
I should avoid words like "only" since there can be some echo of sound off the floor above, but I don't believe it will be the majority of your problem, especially the high pitched sounds from children. Focus on the solid unobstructed surfaces in the room for the best benefits, e.g. solid walls and hard floors. –  BMitch Jun 14 '12 at 12:51
    
Thanks again. For sure these are considerations but really my concern at this moment is this issue with having the sound insulation laid on top of the drop ceiling, rather than attached to the ceiling a short distance above. I suspect this is the easy path for the builder and that the echo as justification is a falsehood. –  mark Jun 14 '12 at 13:03
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One last thought, since I don't have direct experience with this situation (our sound insulation is between multiple units in condos), I'd ask the builder if he can show you some places he's worked on where it was done each way. –  BMitch Jun 14 '12 at 13:34
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I think I realise why he wants to do this. He's calculated the ceiling insulation without taking into account the ventilation ducts which are now fitted. To wrap all of these ducts will be a considerably larger quantity of insulation, whereas the drop ceiling is completely flat. I think I am going to allow him to do this, with conditions. –  mark Jun 15 '12 at 8:11

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