For a single echo to occur, you must have a reflective surface. When you call out ‘hello’ in a canyon or from a mountain top, the multiple echoes you hear are coming from different mountain sides that are distanced further apart. Sound travels at approximately 1100 feet per second (335 meters per second). So if it takes one second for an echo to come back to you, the mountain is likely 550 feet away (167 meters) as the sound has to get there and the echo has to then come back.

When treating the acoustics in a room, the math is the same, only the distances are, for the most part, much shorter and the echoes much closer together. For instance, a room that is 20’ wide x 50’ long with a 14’ high ceiling will have 3 surfaces that could cause an echo. Calculating the time for the sound to travel from one point reflect off the wall and back, would be calculated like this:

Length of the room or distance = time for sound to travel
Speed of sound (1100ft/sec)

20 ÷ 1100 = 0.018 seconds or 18 milliseconds

50 ÷ 1100 = 0.045 seconds or 45 milliseconds

14 ÷ 1100 = 0.0127 or 12.75 milliseconds

These calculations show the time it would take for the sound to exit the wall mounted speaker and hit the opposing surface. We need to double these if we intent to include the return path. What we do know is that the ear (or the brain) can detect an echo or delay that is more than 7 milliseconds. The longer the delay; the more noticeable the distraction. It gets more interesting when you add in the fact that most rooms are built in a rectangular shape and that opposing parallel surfaces can sustain the echo for a long time. Next time you walk into a large empty space, clap your hands. You will likely hear multiple echoes as the sound is reflecting off all the walls. In smaller rooms, we call these short echoes ‘room flutter’. This is usually the most problematic type of echo that one encounters.

The best way to help tame room flutter is to eliminate parallel surfaces. For an echo to survive, it requires two parallel walls so that the sound can reflect back and forth and back again. By treating opposing walls and eliminating open parallel surfaces, you can generally solve most of the problems in a room. This may sound easy, but it is not always practical: Doors, windows vaulted ceilings, air ventilation, furniture and aesthetics are factors that can limit your ability to install material where it would be most effective. By applying some good old common sense, you can often come up with a work-around that can be very effective.

When treating parallel wall surfaces, you can either treat a complete wall or treat opposing wall sections. By absorbing all of the energy on one wall, the echo will simply die out before it gets started. As mentioned earlier, this is not always practical. You may want to start your panels above the waste so that chair-backs and wondering feet do not damage the panels. Ceiling to floor reflections are often best tamed with carpet and acoustic drop-down or T-bar ceilings. For larger rooms with high ceilings, using baffles is usually the most effective solution.

When installing absorptive panels, you will find that placing the absorptive panels so that they are not directly across from each other will reduce echo more effectively and save you money! Best of all, after you have installed a reasonable number of panels, if you feel the results are not sufficient, you can always add more. Just clap your hands once and listen. If you hear a trailing high frequency echo, you have a flutter problem to solve.