The acoustic bell curve is a concept that helps you to plan a budget when considering acoustic treatment. Logic dictates that the more panels you put up, the more energy you will absorb. At some point above 50% of surface coverage, the reflections have been reduced so much the room sounds "dead" and unnatural. At that point your on the way to achieving a pseudo-anechoic chamber.

The bell curve shows this effect. Put one 24" x 48" panel in a large room and nothing will happen. Add a few more panels, still nothing. Add more and all of a sudden, at around 10% of surface coverage, the curve gets very steep and the room response begins to change. Add more and the room begins to sound good for speech and music. Add still more and you've got a recording studio, then a voice-over booth. Finally, if you carry the curve off the chart you eventually get to the anechoic chamber.

So how many acoustic panels should I install? The answer is generally limited to your available budget and what the room is used for. The more material you install, the more control you will have over the sound and the more money you will have spent. The hard part is finding a point on the bell curve that will have the desired effect on the acoustics while remaining within your budget. Generally, 25% surface coverage will make a "normal" sounding room while 35%-50% is desirable for recording studios and voice-over booths. Anything less than 15% surface coverage is usually due to budget limitations.

Coverage:	10%	15%	20% to 25%	30% to 35%	40% to 50%
Effect:	Little or no reduction. Hard to understand someone a feet away.	Bright sounding room (many reflections). Good for performing acoustic music. Not so good for speech communication and amplified live music.	Normal room sound. Excellent for music listening and public speaking.	A "quiet room". Good for recording music and speech.	Very pronounced effect. Desirable for specialized recording and acoustic testing.

For a quick estimate of how much acoustic treatment a given room will need calculate the square footage of the floor area. In a room that is 16' x 20' the floor would be 320 sq. feet. If we calculate the area of all the room surfaces (12' ceiling, floor and four walls) we get 1504 sq. feet. Twenty five percent of that is 376 sq. feet which is relatively close to the square footage of the floor.

This amount of acoustic treatment (about 25% of total surface area) will generally produce a significant difference in a room and is an excellent figure to start with. For a gym, it would dramatically improve the intelligibility of speech but might also be beyond what the facility can afford.

The next step is to add up any absorbing materials already in the room. For a residential room that may include carpeting, heavy drapes and sofas. Those room fixtures will help reduce the amount of acoustic treatment you need to reach the 25% figure. In the gymnasium there is no room fixtures like carpet that absorb sound so reaching the 25% mark will cost more.

This is how the bell curve is used to calculate what would adequately suit your needs and help you plan budget for acoustic treatment.