The way good dipoles work comes down to understanding what the ear/brain does with time delays due to reflections. Simplifying things, very short delays, such as 0-2msec, are used to assign the direction of the sound and this time interval is crucial in forming a stereo image (cabinet edge diffraction, etc falls into this range). Longer delays, up to 50msec, are grouped to form an ambient impression. Delays longer than 50msec become discrete echoes. Sound travels about 14 inches in 1msec.
Provided we don't overdo room reflections (like Bose does horribly) and control them, we can use these room reflections to enhance ambience and the depth of the stereo image, as a good properly placed dipole does. These room reflections can also cause destructive cancellations at some frequencies as in the case of floor and ceiling bounce and in the low bass. Floor bounce is typically in the 2 to 3msec range (a crucial time delay) and is one of the things which gives away the fact that we are listening to speakers. The ceiling bounce is more benign, since it arrives later, around 4-8msec (high ceilings help here). Ceiling bounce becomes important when we have suspended ceilings (which should be avoided). Good line source dipoles have a desirable dispersion pattern. The dispersion pattern is such that in plan all the energy goes to the front and back in a figure of eight pattern, with no side dispersion, which minimizes reflections from side walls. In the vertical plane all of the energy is confined to being a parallel projection out from the line source dipole, thus minimizing floor and ceiling bounce. The problems of low bass will be dealt with below. The setting up of a good dipole in the room is more complex than a monopole. The room has to be large and the dipole away from the back wall. However, the dipole has some good characteristics lacking in monopoles that can be used to great advantage.
The Carver line source dipole consists of a dipolar ribbon on an open baffle working from 200Hz up. Below 200Hz is covered by line source dipolar woofers on an open baffle. The ribbon is made from 0.00035" thick aluminium bonded to DuPont Kapton film 0.0005" thick by a subcontractor to Carver who specializes in adhesive technologies. Four quarter-inch aluminium foil strips with an extra space between each pair of aluminium foil strips are used (over the extra space is put a row of magnets). As the 4" wide aluminium foil bonded to Kapton film comes off 100 metre spools, it is run through a crinkling machine comprising special rollers that squeeze a leather texture into the material. The crinkling is done to suppress membrane vibration modes. Carver then uses a patented technique to uniformly tension the film on a particle board rectangular frame. The ribbon is then clamped and electrically bridged at both ends and suspended in a strong linear magnetic field over its entire length provided by thirty linear feet of 6" bar magnets glued into a metal frame. The 60" Carver ribbons consist of two half-inch ribbons with a folded "voice coil" of 20' long suspended between three double rows of magnets. The ribbon speaks through the two vertical half-inch ribbons, there being a row of magnets centrally located between them (which you may pick out on the photographs). The cavity in which the ribbon speaks from also leads to a geometrical cavity resonance which is taken care of by the crossover. The ribbons have a constant resistive impedance of 4.6ohms.
The aluminium foil bonded to Kapton film after crinkling.
The 200Hz crossover point for the ribbon is recommended because there is a membrane resonance slightly below 200Hz due to the tension in the film. There is also a larger geometrical resonance at 140Hz due to the 60" length of the ribbon and these resonances show up in a review of the Genesis II.5 speakers in Jan 1995 Stereophile which uses this same ribbon for good reason. An active Clearview crossover was used in this implementation to overcome the falling ribbon response near crossover due to quarter wave cancellation on the ribbon open baffle (there is more on this later), null the cavity resonance due to the width of the cavity the ribbons sit in and overcome the ribbons fall off in the extreme treble. A third order (18db/octave) crossover rate on both sides is used to the woofers at 200Hz. An active crossover gives flexibility at the expense of the need for two stereo amplifiers.
The following information is needed to understand what Carver has done with the bass panels which operate below the crossover frequency of 200Hz. Herewith follows a lesson by Dick Pierce from a posting of his on the newsgroup rec.audio.tech, 14 Nov 1995:
""Q" is one of those dimensionless numbers that causes no small amount of consternation amongst those who don't understand what it means, as well as among those that THINK they do! However, the principle behind Q, when used in the context of loudspeakers, is VERY simple. It is simply the ratio between energy storing and energy dissipative mechanisms at resonance. In electrical terms, it is the ratio of the reactance to the resistance.
A high Q indicates that for the amount of energy stored in a resonant system, the mechanisms that dissipate that energy are small. So a high-Q system will tend to have a resonance that decays slowly, because the amount of resistance available to dissipate the energy is small compared to the amount of energy stored. A low-Q system will tend to dampen the resonant motion quickly, because the energy is dissipated quickly and removed from the resonant system.
There are primarily 2 energy dissipating mechanisms available in a loudspeaker driver: mechanical and electrical (there is another, acoustical, but it is VERY small when compared to the other mechanisms). The mechanical dissipative mechanisms are primarily the frictional losses in the driver's suspension, and, to a lesser extent, acoustic absorption. There are, essentially, two electrical mechanisms for energy dissipation: the DC resistance to the voice coil and the output resistance of the amplifier. In almost all cases, the DC resistance of the voice coil completely dominates.
These two mechanisms, mechanical and electrical, determine, respectively, the mechanical Q (Qms) and the electrical Q (Qes) of the loudspeaker driver. Their parallel combination determines the total Q (Qts) of the loudspeaker driver." Amen.
When we mount a loudspeaker driver onto a baffle system we also have to take into account the Q of the baffle system to arrive at the total system Q. To work out the total Q of the driver and baffle system you simply multiply the baffle system Q with the total Q of the loudspeaker driver. Closed boxes store energy that interacts with the loudspeaker driver in complex ways, especially in vented enclosures. Boxes themselves also have resonances. Normally a high-Q closed box is combined with low-Q loudspeaker driver to give a desirable total system Q. But when we mount a loudspeaker driver on an open baffle this situation is reversed. An open baffle stores no energy and has a low-Q of 0.2 and Carver chose to use a high-Q woofer with a total Q of 3+ to arrive at a desirable total system Q.
Carver's high-Q woofer was also chosen for another good reason to do with mounting a woofer on an open baffle. As we decrease in frequency or increase in wavelength, the system initially behaves as an infinite baffle. When the wavelengths are long enough to be a quarter of the baffle dimensions, the waves begin to cancel each other around the edges of the dipole baffle. The wave travels out to the edge (1/4) and back to the opposite side of the vibrating speaker cone, where it is exactly out of phase and cancels out. Quarter wave cancellation on an open baffle is a first order phenomenon - the roll-off occurs at 6dB per octave. When we reach the free-air resonance point of the high-Q woofer we add to this the second-order sub-resonance fall-off of the high-Q woofer to end up with a third-order or 18db per octave fall-off below the free-air resonance point of the high-Q woofer (this makes a good rumble filter in the Carver case).
In loudspeaker literature we can look at the family of curves for the frequency response of a loudspeaker driver on an infinite baffle as we decrease the frequency. Starting with the rolled-off curve when Q=0.5 (critically damped), then the Butterworth graph with Q=0.71 (maximally flat), then a little ripple at Q=1, then clearly a bumped-up graph at Q=1.4. When the Q is higher than any you can usually find in loudspeaker driver catalogues you start to get boosting above the resonance point of the loudspeaker driver, and a sufficiently high-Q will result in a slope of about 6dB per octave above the free-air resonance point of the loudspeaker driver. This increase of 6dB per octave of the high-Q loudspeaker driver can be used to counteract the 6dB per octave quarter wave cancellation to give a flat frequency response right down to the free-air resonant frequency of the loudspeaker driver. This is a much more elegant solution to the problem of quarter wave cancellation on an open baffle to that used by Celestion, etc of using a conventional low-Q woofer with electronic equalization since it does not involve additional amplifier power and the necessity of electronic equalization equipment. High-Q woofers are relatively easy to design/make. Both ways of overcoming quarter wave cancellation on a baffle entail the use of long throw woofers for the safe operation of the woofer.
In the Carver case a large sloped open baffle is used with at least three 12" high-Q woofers mounted on it in a line source dipolar array. The sloping of the baffle spreads out the frequency at which quarter wave cancellation occurs on the baffle and makes it look not so big. The large open baffle has a low spouse acceptance factor and needs to be in a large room away from the back wall. The reason for using multiple woofers is that it takes four times or 6dB more amplitude on an open baffle at resonance than an equivalent sealed box to achieve the same level of sound output. With the Carver 12" (30.5cm) woofers Carver uses a special technique to roll the foam surrounds under pressure to get the proper low resistance in them. Small magnets with a light stiff paper cone and modified spiders and the most flexible annulus yet devised are used to raise the Q of the Carver woofer. The Carver woofers have a DC resistance of 16 ohms so that the overall resistance of them when multiple woofers are connected in parallel on one open baffle is still reasonable. The Carver woofers have a maximum linear excursion of 3cm (1.18") and maximum mechanical travel of 5cm (2"). The Carver woofers have a free-air resonant frequency (fs) of 22Hz and when mounted on the above open baffle are nominally flat down to the fs frequency of 22Hz with an f3 frequency of 17Hz. At the free-air resonant frequency of 22Hz the woofers will flap around, but I have yet to see this happen on any music that I have played on this system, low frequency pipe organ notes and all.
One major problem that was referred to earlier is room interactions with dipolar bass. The article in Nov 1991 Hi-Fi World and its follow up article in Dec 1995 have a lot to say on this. The following enigmatic sentence from the Nov 1991 Hi-Fi World article sums this up. "Whilst room alignment is difficult and we have no verified empirical rules for it, when a dipole has been room aligned successfully it will give more even sounding bass than a monopole." The flatter in-room response of bass dipoles is achieved by toeing them out suitably (rather than in) and thereby using cancellation effects to suppress room-resonance modes. However, the optimum bass configuration conflicts with the optimum imaging configuration, which requires toe-in. Thus, if Celestion is right, the only practical means of optimising in-room bass and imaging in planar speakers is by the use of separate baffles for woofers and the mid-range/tweeters. In any event, it would be helpful if Celestion's computer program or similar to aid in arriving at the optimum position for a dipolar bass panel in a room was more widely available, rather than being limited to empirical means for finding this.
David Graebener, here pictured, is one of the founders of Speakerlab, a
Seattle-based loudspeaker design and manufacturing firm. This picture
was taken from a Speaker Builder (2/92) article about the building of a 20ft
ribbon for Boeing which has a marked resemblance to the Carver ribbons.
Here is the cover from this issue of Speaker Builder.
View larger picture (30K) plus other photographs
This resemblance is not surprising because David Graebener and Jim Croft from Carver developed the Carver ribbons and woofers.
To find out what the Carvers sound like after all this you will have to read on on other WWW pages pointed to by the WWW page above this one.
For suggestions, comments contact
Last modified 29 October 2001.