What does this mean? Read on as ASC founder, president and TubeTrap inventor, Art Noxon, PE discusses speaker positioning for maximum sound – as featured in a five-part series by Art in Home Theater Magazine
Ambience speakers are readily becoming standard in home theater
As we survey audio systems for home theater, a trend appears. We consistently find one or two subwoofers, two or three stage speakers, and two ambience speakers. In the last two sections, we studied the subwoofer as it fits into and plays the listening room. Here we will study ambience speakers, the kind that are becoming a standard for home theater.
The Dolby surround signal is a mono signal usually fed to two speakers located towards the back of the room. This signal is unique in audio because it is rolled off at 100 Hz. This doesn’t mean that there should be no deep bass in the ambience effect. It does mean that the deep bass is generally understood to have no directionality. Our head is too small, our ears too close together, and our hearing too insensitive to be able to tell which direction low frequency sounds are coming from. Remember how no one worries where the subwoofer is placed, except for visual or room mode control? That’s because we can’t tell where the bass is really coming from. The way we “know” where bass comes from is by focusing on where the upper partials of the bass sound are coming from. The Dolby surround signal contains the upper partials of the ambience bass, so we think the ambience bass is coming from the ambience speakers. But really, it’s the subs and main speakers that get the signal and do the generating of the ambience deep bass.
There are two basic kinds of ambience speakers these days
…although more may pop up as time goes on. The first, most basic type are simply small, book shelf type speakers on speaker stands or mounted on the wall. The ambience signal can be beamed either: directly at you or away from.you/bouncing around the room a bit before it hits you. If the speaker is aimed directly at you, you will hear it and know where it is. Our hearing is very sensitive to sounds beaming directly into one ear. After all, what do we do when we can barely hear some sound? We turn our head to the side, so one of our ears can hear the sound more directly. For the ambience speaker setup, the orientation of the speaker is a matter of personal choice and the experiments should be made. Many people prefer not to hear the ambience signal directly and their ambience speakers are turned somewhat towards the wall and face either forwards or backwards.
The second type of ambience speaker is called a surround speaker and is recommended by the THX people. In this system, the choice about how we hear the ambience signal has been made for us. This speaker is mounted high on the side wall and set up to not beam any sound directly at the listener. These speakers are specified to be primarily dipole type speakers. This means that they play backwards and forwards equally strong, but not at all to their side, which is, of course, where the listener is located.
The dipole speaker familiar to us in hi-fi is usually a thin sheet of material that is forced back and forth by either magnetic or electric fields. The forward wave is exactly out of phase from the backwards wave. When the sheet moves forward, a positive pressure wave is sent forward while a negative pressure wave is sent backwards. Not so for most surround sound speakers. This type is often comprised of two dynamic speakers wired out of phase and playing back to back. There still is a positive wave sent out in one direction, while a negative wave is sent in the other, being equal in strength but opposite in phase. There are numerous dipole speakers and the goal here is not to propose or evaluate which might be better than the other, if such would even be possible. The goal here is to explore the effect on the sound of these speakers that is imposed by the room in which they are located.
The dipole type surround speaker is a strange kind of speaker to the world of audio and it will, without a doubt, undergo a number of transformations as it evolves into its mature form. To begin with, it is not a full range speaker because the surround channel is rolled off at 100 Hz. For the most part, these speakers have been a small speaker cabinet with two speaker baffle boards, one set to face forward and the other to face backwards. Usually, we see each panel forward and the other facing backwards. Usually, we see each panel mounted with a single driver. Two-way speakers are also used, sometimes with the tweeter offset from the main driver, other times with coaxial drivers.
…of this style of speaker is to “play forwards and backwards” so as to illuminate first the room and not first the listener. This directional effect only works for a limited frequency range of the speaker. Small-sized drivers are directional for upper, mid, and high frequency ranges, but become omni-directional for the lower ranges. This directionality effect occurs at a predictable frequency based on the size of the drivers, as well as the cabinet in which they are mounted.
A good demonstration on the directionality of a speaker can be achieved by setting a small loudspeaker outside of the house on a table that is placed in the middle of the open yard. Then, while keeping some fixed distance away, walk all the way around the speaker while it is playing some tune with which you are familiar. You will hear the full range of sounds of your speaker when you are in front of the speaker, but as you move to the sides, and especially when behind the speaker, the highs drop off substantially, but not the lows. Male vocals, for example, sound pretty much the same no matter where you are, but sibilance, the “tsss” sounds, dramatically drop off behind the box.
If you get an identical second speaker, wire them up in phase and place them back to back. You’ll hear bass range everywhere and the sibilance will be heard in two beams, One forward and the other opposite. Listening directly off to the side of the speaker pair, you’ll hear the midrange and bass. Now reverse the phase of the two speakers and listen. All of the bass drops out, yet the two mid/high back to back beams remain. To the side, there is a strange drop in all sound. So it is with the dipole speaker. The dipole effect is limited to the upper ranges of the speaker because the bass shorts out, acoustically speaking. At some low frequency, the dipole speaker simply sloshes air back and forth around the edges of the speaker and makes no more sound. This is nc different than listening to a bare speaker and then mounting it onto a piece of plywood. We increase the distance between the front of the driver and the back and, in doing so, give the speaker more range in the bottom end.
Because the surround dipole speakers are fairly small, they short out at fairly high frequency, around 400 Hz. And, so, there must be another system in place to generate sounds below this natural dipole cutoff. There are a number of ways to accomplish this. The most straightforward way is to use a single lower frequency driver reversed, large-sized, and directional midrange drivers. Offset or coaxial tweeters will accompany these large midrange drivers to get full high frequency range. The main thing to keep in mind during the evolution of this style speaker is that the orientation of the low frequency drivers is irrelevant as to the directionality of the lower registers. Omni is omni and it doesn’t matter which direction the midbass speaker(s) points.
There seems to be only a couple of rules to follow
…when placing the surround dipole speakers. Mainly, they have to be placed high on the side walls, directly to either side of the listener position. They can be positioned in front or behind the listener somewhat, but must be angled so that the side of the speaker points to the listener. Above all, never place them in bookshelves no matter how convenient it may seem. The honky, tonal resonances this setup produces will be almost unbearable, not to mention that the walls of the bookshelf will catch the ambience signal before it gets to the room. These surround speakers are to fire along the side wall towards the front and back walls. Next, there are three factors to be considered in the placement of ambience speakers — resonance, self-canceling, and flutter.
Whenever a speaker is placed in a room, it needs to be positioned so as to minimally stimulate room induced coloration effects. This is especially true for ambience speakers because their effects are in direct competition with the room’s natural ambience for the listener’s attention. If the ambience speakers are located improperly, they will strongly stimulate the local room effects and their capability of generating the desired audio track ambience will be reduced by the sound masking effects of the room’s acoustics.
We know the ambience speaker is to be located high on the side wall by the listener. Beyond that, we seem to be left to our own resources. The lower frequency play of the speaker can be used to determine the most neutral vertical location on the side wall. The high frequency characteristics of the speaker can be used to determine the most neutral front-to-back position for the speaker. In the following sections, we go over the details that determine the most neutral position for the ambience speaker.
Continue reading Art’s article here to learn how to position those speakers for a great listening experience.