When discussing flutter tones in Hifi & home theater one of the first things the novice acoustician does upon entering a room is to deliver a sharp clap of the hands. This is followed by a grave shake of the head and comments about how bad the room sounds. Next comes a proposition to fix the room and the fee. The unsuspecting client then administers a sharp hand clap, nods the head in agreement, and gives the guru a retainer. The only problem here is that these people are busy buying and selling modifications to the sound of their own hand clap. We don’t listen to a speaker while holding it in our hands, yet we can be tempted to consider acoustics based on the sound of our own hand clap.
Home theater audio systems have an ambience channel. It usually delivers a bandwidth-limited (no bass), mono signal to a pair of speakers that have been mounted high on the wall and to the side of the listener. If you stand on a chair and clap your hands in the location of the ambience speaker, you will hear a very funny and undesirable sound effect. Is this really something we can hear? If so, do we want to listen to this sound effect or provide it to our clients? If not, there might be something we can do about flutter echo colorations.
The Acoustic Clap Test
On a practical basis, the only time that a self-administered and self-audited hand clap is directly relevant to anything in audio is when the recording engineer is setting up mikes in a studio. Only in this special circumstance does the desired audio signal leave from and return to the same place. Listening to one’s own hand clap duplicates this round trip, acoustic process and thereby is a relevant test. If someone ever wants to know how a loudspeaker sounds to the listener, a different technique must be followed, one that mimics the actual speaker/listener acoustical path.
A hand clap contains only high frequencies. For a loudspeaker, the high frequencies are directional, forward of the speaker box. To properly administer a hand clap that mimics the high-frequency beaming pattern of a loudspeaker, the hands must meet at waist height while the clapper is facing the same direction that the speaker does. The body of the clapper blocks the expansion of the clap sound backwards. The listener is no longer in the clapper position, the listener is now seated in the listening position. This time, the hand clap is cast forward from the speaker position and is heard by the real listener. It is how the listener hears the speaker that counts and not so much how the speaker sounds to itself, at least in hi-fi playback settings.
In order to properly evaluate the consequence on the listener of the strange sound we heard when standing on the chair and clapping our hands overhead and near the mounting position of the ambience speaker, we must repeat the test while a listener is seated in the listener’s chair. True enough, in this case, the zing we hear when we clap is also heard by the listener. And so, is the sound we hear, good, bad, or inconsequential? Certainly, this sound effect is distracting and that alone is enough to warrant its eradication. On the other hand, we want to retain an overhead liveliness so as to promote the ambience signal. We can’t sacrifice the lively quality of the overhead space in the room, yet we must try to get rid of its distracting effect known as flutter echo.
6 Moons Hifi Room
Flutter Echo/Flutter Tones
Before we try to solve our problems, let’s spend some time learning about it. When we administer a hand clap test while located between a pair of uncluttered and parallel walls, we hear a flutter echo. It has a “zing” sound. The flutter echo actually does sound like a tone. The frequency of the tone depends upon the timing of the flutter. A flutter echo is how we hear what really is a rapid sequence of noise pulses. When we clap our hands in the outdoors, we simply hear the single, sharp pulse of noise we call the clap sound. If we clap our hands while standing some distance away, yet facing a wall or building, we will hear a single rapport of the clap, its echo. Then, if we relocate and stand between a pair of more nearby and parallel walls, that single pulse reflects back and forth rapidly between the parallel walls and we hear what we call a flutter echo.
If the walls are far apart, some 60 feet or more, we actually hear the flutter sequence of the echo reflections. But if the walls are closer together, the distinct detail of the staccato seems to disappear, but only to be replaced by a new sound, one of tonal quality. If the walls are far apart, say 60 feet, we hear the slap back at a rate of 1130/60 or 17 times a second and it sounds like the tap-tap-tap of a true flutter echo. However, if the walls are closer, say 20 feet apart, we will hear that slap back pulse of sound at a rate of 1130/20 or 57 times per second. When we, the human listeners, hear a click or noise pulsed at 57 times a second, our ears/brains are tricked into perceiving a buzz-like tone of 57 Hz. And so, the flutter echo we hear when the walls are farther apart becomes a zing-sounding flutter tone when the walls are closer together.
In hi-fi, home theater, and even most recording studios, the parallel wall surfaces are within the range of 15 to 30 feet apart. That means we don’t hear flutter echoes but do hear the flutter tones. Flutter tones are sounds that have a low-frequency character, but they are not to be confused with room modes which also are low frequency in nature. The control of the low-frequency flutter tones, as we will soon see, is accomplished with high-frequency type diffusion or absorption. Of course, control of the low frequency of room modes is accomplished only by means of larger-sized bass traps, usually best located in the corners.
The low-frequency flutter tone is a pseudotone – a trick on our hearing system played by the rapid staccato of high-frequency noise pulses. Sometimes a careful listener can become confused as to how a seemingly low-frequency sound can be eliminated by the introduction of a paper-thin reflector or fabric, especially when common sense leads us to expect that only those large-sized bass traps should have been needed. In order to eliminate the detection of a flutter echo pseudotone, we need only to break up the flutter echo process. It takes very little scattering or absorption of high-frequency sounds to break up the flutter echo sequence, and thereby el.eliminate the accompanying impression of the low-frequency sounds of the flutter tone.
Audio parlor tricks, such as making bass reverberation disappear with nothing more than a carefully placed scrap of paper, are accomplished with the magician’s classic technique, a distraction of words and slight of hand. Only this time, we say that to create the illusion, the hand must be moving faster than the ear. Actually, the clue to the trick will be found in the presentation. The guru claps the hands and says to listen to the low-toned overhang. If you spectral analyze the energy content of a hand clap, you will find no energy below 400 Hz, yet the hand clap generates the perception of typically a 50 Hz sound. It’s a great trick. Practice it and amaze your friends with your superpowers. You could even start up your own business, selling little tinfoil “bass traps” and you’ll probably even get away with it, for a while.