Home Soundproof Garage - Garage Conversion Stage 5 Soundproof a Room Stage5 Part 3
Soundproof a Room Stage5 Part 3

Flutter Echo

As something of a technical aside, this stage of the construction is very useful for demonstrating (and then fixing) a very annoying problem in acoustics. This problem is known as Flutter Echo, and if you are not aware of what conditions cause this problem, then its appearance and solution will be a mystery to you.

Take a look at the following picture showing the insulation applied to the inner walls. Notice where there is no insulation. None has yet been applied to the ceiling and obviously, none will be applied to the floor.

flutter echo parallel paralell soundproof room surfaces insulation reflect deflect absorb30mm Rockwool slabs applied to inner walls

So we now have a room with 4 highly absorbent walls, but a reflectiv e floor and ceiling. When you reach this stage in the construction, it is very easy to demonstrate what a flutter echo sounds like. Simply empty the room of all your building equipment, then stand in the centre of the room and clap your hands.

What do you hear? Its a sort of rapid "boing-oing-oing-oing-oing" sound.

The next question is this; what causes the sound to bounce around in this strange way?

flutter echo parallel paralell soundproof room surfaces insulation reflect deflect absorbInsulation applied to walls and ceiling

The reason is quite simple. Remember the analogy of a "sound-balloon" I gave here? Sound is expanding away from the sound source (your hands) in all directions. However, as the sound hits the boundaries of the room, a portion of the energy is absorbed, and the rest is reflected. What we have done is make the walls of the room highly absorbent, but left the floor and ceiling reflective. This means that any sound moving in the horizontal plane will be absorbed very rapidly, but the sound moving vertically will continue to reflect long after the horizontal sound has been absorbed. So a flutter echo is merely that part of the sound field that is travelling (in our case) in the vertical plane - the absorbency on the walls has removed the horizontal components of the sound field. As you can hear, a flutter echo will become very annoying, very quickly.

A flutter echo can occur in any direction, between any pair of hard, parallel surfaces; we just happen to have created a vertical one between the floor and ceiling. Whenever you have absorption unequally distributed in a room, and one pair of opposing surfaces are significantly more reflective than the other surfaces in the room, you will find a flutter echo has been created.

How can a flutter echo be fixed? Simply find the pair of offending surfaces, and if possible, apply absorbency to one them. This will kill the flutter echo (if this is not possible, then avoid placing a sound source between these two surfaces).

In our situation, we are going to apply absorbency to the ceiling anyway, so this has only been an exercise in demonstrating the phenomenon.

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