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This week we continue our discussion on Flutter Echo/Flutter Tones in Home Theater as featured In Home Theater Acoustics Vol. 5 of a five-part series from Home Theater magazine.

The following was written by ASC founder & president Art Noxon, PE Acoustical   ~Enjoy!

Flutter Tone Science

If we stand at the end of a long, narrow room such as a hallway and clap, we will hear the flutter echo as it returns to us each round trip. If the hall is 20 feet in length, the flutter echo returns after every 40 feet of travel. The time for the round trip is controlled by the speed of sound. In this example, the sound of the clap makes a round trip some 1130/40 or 28 times a second, which sounds like the note of 28 Hz, around the lowest note of the piano keyboard. However, if we stand in the middle of the room and clap, we hear a different flutter tone. In this situation, part of the clap sound travels towards each end wall. Being in the middle means that each end wall is only ten feet away. Both sounds return to us after only 20 feet of travel. They pass by and head off towards the opposite wall, only to return to us after another 20 feet of travel. This situation produces a flutter tone of 1130/20 or 57 Hz, a full octave above the basic flutter tone of the hall.

If we were really doing this experiment, we would quickly find that we must stand to the side of the hall so as to let the two end walls have a clear view of each other. If we stand in the center of the hall, the flutter is quickly damped out because of the absorption of our body. In this position, with our back to the side wall, sound travels away from the clap equally in both directions, up the hall and down the hall. When we stand at the midpoint of the hall and clap, the two wave fronts race towards the two end walls, reach them and reflect back to soon pass by the clapper at the same time. These two pulses, having arrived at the same time, are heard as one loud pulse. Positions non reversed, the two pulses race for the opposite far walk, and again repeat the course. For this position, the double-strength pulses are heard every time they make half of a full round trip of the hall.

Another important position to stand at is the end of the hall. We already know the flutter echo occurs at half the rate as when we stood in the middle of the hall. But let’s look at the pulse timing detail. Again, two pulses expand from the clapper’s position, one heads toward the far end wall and the other toward the near end wall. The first reflection, off the near end wall, hits us after an overall travel of only three or four feet. It races by and follows the other pulse down the hall, lagging by six to eight feet. They both hit the far end wall and return towards the clapper’s position. The leading pulse flashes by and on to hit the nearby end wall. By the time it again hits the clapper, the lagging pulse also hits the clapper. This creates the effect of a single-hitting, double-strength pulse. Then the lagging pulse moves past and towards the nearby end wall. It reflects and, after a bit, again passes by while heading for the far end wall. In the meantime, the leading pulse had already long left the scene, heading again for the far end wall and a repeat of the cycle.

What we have here is a triple pulse event whose timing is that of a full round trip in the hall. The three pulses are so close together that they sound as if they were one pulse. This combining effect is well-known in pro and high-end audio. It is called the Haas effect, after the scientist who did a lot of work in this area of hearing. What he found is that when high-frequency reflections, such as those in the hand clap arrive within ten to 15 ms (thousandths of a second), they fuse together and sound as one.

Next, we take a few steps down the hall and repeat the hand clap test, listening for any changes in the sound of the flutter tone. If we moved five feet off the end wall, the two pulses would be 20 feet apart and heard as separate pulses because they arrived outside the sound fusion time period. However, the same sequence of events still occurs. The only difference is the separation of the two distinct and small pulses. In the middle position, double-strength pulse effect still occurs. As we change positions along the length of the hall, we change the timing of the discrete echoes that make up the flutter tone. We also find that as we approach the middle of the hall, the two single echoes get far away from the double pulse and closer to each other. When they are within about six feet of each other, the fusion effects begin and the two pulses start sounding as if they were one and the upper octave flutter tone is heard. Get just a few feet off dead center of the hall and the upper octave disappears and the lower flutter tone begins to reappear.

The timing of the two separated pulses is what accounts for the changing of the character of the flutter tone. As we move closer to either of the end walls, the timing between the two separate pulses gets closer together, sandwiching the double-strength pulse until the end wall is reached and they are essentially all on top of each other. As we move closer to the center of the hall, the timing between the two separate pulses again gets smaller. This time, they do not sandwich and are as far as possible from the double-strength pulse. Finally, at the center, the time between them goes to zero, creating a second, double-strength pulse.

All the pulses contain energy, the same amount of energy. Whenever they return to the clapping position, together they combine into a stronger, double-strength pulse. Even more, when they arrive at the clapper’s position within six feet of each other, they still combine into a single, double-strength pulse. When a clap originates within three feet of an end wall, all of the pulses arrive at effectively the same time and the result is heard as a four-times stronger, low-frequency flutter tone. Then again, if the clapper is within three feet of the middle of the hall, the separated pulses arrive close enough together to combine and double up in strength. Either of these extreme conditions is about as easy to detect.

When the two separated pulses are not close to the doubled-up pulse, the lower flutter tone is quieter, less noticeable to detect and that is good. Also, when the separated pulses are not combined due to a midpoint clap position, the upper octave flutter tone is not heard. That is also good. Clearly, we now know that the most non-stimulating position for flutter tone generation will be more than four feet away from either end wall and a few feet off the center of the room. By experimenting, additional information is developed. Anywhere in the end third of the room seems to strongly stimulate the lower flutter tone. The thirdway point seems to stimulate the third octave, along with the fundamental flutter tone. The middle of the room really generates the second octave flutter tone within a foot or two of the center point.

Using our 20-foot room as an example, the ambience speaker ought to be located ahead of the 1/3 point, but two to three feet off the center. That puts it at about seven to eight feet off either end of the room, probably the rear wall for home theater. As a general rule, the ambience speaker can be placed 38 percent of the room length off the back of the room. This position will ensure that minimal flutter tone coloration is introduced into the room.

This section has been intended to be a baseline guide for the anti-flutter tone positioning of the surround speakers. To this, we next add some enhancement devices to both increase the presence of the ambience signal and to continue to reduce the telltale presence of flutter tones in the home theater setup.

Enjoy the entire artcle