WHY OMNISPHERE?

A small technical paper

Omnidirectional sound reproduction by means of a pulsating sphere has always been the theoretically ideal design concept. Unfortunately, the concept can never be physically realized for wide frequency range sound because the sphere would have to be, simultaneously both large and small - small at high frequencies to represent a point source at the short wavelengths involved, to prevent lobing, and large at low frequencies to enable enough air to be moved to develop adequate acoustic output.

A number of interesting attempts have been made in the pursuit of this ideal by developing pulsating spheres of limited bandwidth, none of which have met with commercial success. The latest was in 1989 when MB QUART of West Germany developed a football shaped radiator, 50cm long with a response down to 800Hz, operating on a magnetostriction principle.

Other approaches to achieve an omnidirectional or quasi-omnidirectional type of sound have been more successful and with more manufacturers marketing product, such speakers are becoming increasingly in vogue for the upper-mid and high end markets.

The advantages of Omni sound are being increasingly recognized in terms of naturalness of reproduction and freedom from "sweet spot" listening. Conventional front firing speakers cannot recreate the sound fields of acoustic instruments that propagate sound omni-directionaly. Piano's, violins, guitars, drums and cymbals are not front firing and hence cannot be reproduced naturally throughout a listening area by front firing speakers.

A major advantage of the Omni over conventional front firing speakers is that the Omni's sound output, at the listening position, is much less subject to coloration by the addition of reflected sound from room boundaries and furnishings. This is because the Omni has the same response on and off axis, whereas the rectangular boxed front firing speaker degrades off axis and suffers from extremely poor, bass heavy, response off the sides and back of the enclosure.

Off axis sound is inevitably reflected back to the listener in some part and in the Omni case adds only room reverberation - but in the front firing case adds coloration as well due to the poor quality of the reflected off-axis sound.

It should be noted that, since the Omni speaker provides more off-axis energy that the front firing type, they will sound best in a reasonably non-reverberant space.

However desirable, true Omni sound reproduction is very difficult to achieve and until the development of new patented technology that led to the market introduction of the OMNISPHERE, there was only one such speaker being produced in any volume, the OHM WALSH F model. The original model was a true omni-radial but was reportedly deficient in high end extension and had very low efficiency. Newer versions have improved efficiency, but have been redesigned for reduced rear radiation and the addition of directional tweeters so are no longer omnidirectional over the full frequency range.

Other manufacturers have opted to produce what I choose to call quasi-omnidectional speakers. BOSE, with their direct/reflecting systems and DBX with their Soundfield model, achieve this through use of multiple drivers facing in different directions to generate a spacious room filling sound.

The Magnaplanar and Electrostatic speakers can also be classed as quasi-omni because they radiate in a figure 8 bi-directional out-of-phase dispersion pattern

It can be seen from analysis that none of the forgoing approaches to omnidirectionality meet it's basic requirement of flat power response, which requires that the sound output energy at every musical frequency be distributed evenly in all directions. This precludes the use of conventionaly mounted multiple drivers, except at low frequencies where their size and spacing represent a point source by virtue of being small compared to the wavelength of sound being reproduced.

A good analogy in representing the nature of sound waves emanating from a speaker is the example of wave or ripples developed when a stone or stick is plunged into a body of water. When there is only one impulse, the ripples start from that point and expand smoothly in ever expanding circles. This corresponds to the sound waves in air, generated by a single point source of sound.

If a grouping of several stones or sticks are plunged into the water, each one will generate it's own expanding set of waves. These then interfere with one another, causing cancellation and augmentation of wave motion. The same thing happens in the sound output of a speaker when there are multiple sources that are spaced more than a fraction of a wavelength apart. The distortion of the resulting sound field results in what is commonly known as lobing or picket fence response to relatively continuous musical tones and the smearing of and/or production of multiple images of single musical transients.

The first requirement in meeting the criteria of flat power response, is that the sound sources must be physically small with respect to the wavelength of sound they produce.

Secondly the drivers, which are inherently directional, must be so packaged that they meet the omnidirectional dispersion and frequency response requirements over a full 360 degrees horizontaly and 45-60 degrees vertically.

Meeting these requirements at high frequencies imposes a severe limitation on the maximum ceneter to center to center spacing of tweeters, when more than one is used, if lobing and generation of multiple first arrival transients is to be avoided. For example, when tweeter dome centers are spaced apart by a differential in distance to the listener equivalent to a 1/4th wavelength at the listening position, a -3db null results and when the differential reaches 1/2 wavelength, the null can be 20db or more. Transient smearing and generation of extra transients can result from a spacing difference of as little as 0.27 inch, representing a 20 microsecond difference in arrival time.

Vertically mounted tweeters exhibit the same problem but to a much lesser degree because the variations in height of listening position is minimal compared to the variation in horizontal position.

Table 1 illustrates the impossibility of achieving null free, non-lobing response and accurate transient response for multiple tweet-ers mounted on adjacent sides of a square cabinet or around a cylinder or a sphere, a la BOSE or DBX. This deficiency also applies to side by side mounting of tweeters on the same surface, when the listener os off the center listening axis (sweet spot) and iS thereby closer to one tweeter than the other.

Table 1.

Frequency 1/4 wave 1/2 wave
Hz inches inches
20,000 .17 .34
10,000 .34 .68
5,000 .68 1.36
3,000 1.30 2.27
2,000 1.70 3.40

Table 2.

Frequency 1/4 wave 1/2 wave
Hz in ft. in ft.
30 9.4 18.9
50 5.7 11.3
70 4.0 8.1
100 2.8 3.7
200 1.4 2.8
400 .71 1.4
600 .47 .94

Turning to low frequency considerations, consider the case of the out-of-phase bi-polar speaker using a large radiating surface area or multiple drivers, radiating from both front and back sides. Examples are: the Magnaplanar and Electrostatic types and, at low frequencies, the Carver "Amazing" loudspeaker.

The out-of-phase figure 8 radiation pattern produced by this form of construction results in some cancellation of sound output from the speaker sides, with a maximum null occuring at 90 degrees, having maximum effect at low frequencies.

The OMNISPHERE exhibits none of the limitations or defects of the quasi--omni speakers and produces a true omni-radial response through use of new technology, now patented.

Output above the 2kHz crossover frequency is provided by only one tweeter which acts as a true point source, providing perfect omni-radial dispersion by driving the space between two opposed spherical surfaces. The tweeter produces only one first arrival of transient musical material and does not exhibit any lobing or picket fence response on continuous tones.

Below crossover, at the proportionately longer wavelengths of low and mid-band frequencies, the four 6" bass/midrange drivers act as the equivalent of an omnidirectional single point source due to their unique close mounting on a 45 degree pyramid below a reflecting sphere. The effect of the somewhat large size of the array at their highest output frequencies is mitigated by the sphere which acts as an acoustic mirror (virtual radiator) smoothing the response in the crossover region and blending it with the overlapping tweeter output.

Time alignment is perfect because the tweeter is located physically behind the the bass/midrange drivers for any listening position -something that no other speaker manufacturer can claim.

The behavior of the OMNISPHERE at low frequencies differs from and is superior to to not only the in-phase bi-radial but conventional speakers as well. Low frequency output crosses over from the bass midrange drivers to the floor loaded ports at about 55Hz. With the ports at only 2" above the floor, the distance is too short for any nulls or reinforcements to take place within the port frequency range.

The smoothness of response of the OMNISPHERE in the upper bass and lower midrange will, of course, be affected by the position of the speaker relative to room reflecting surfaces and by the degree of room damping, as would any speaker. However, the advantage of the OMNISPHERE over both front firing and quasi-omni speakers is that sound propagation in this frequency range is uniform in all directions which decreases the severity of individual resonant response anomalies due to room characteristics by averaging out a larger number of lower intensity reflections. Casters provide an easy means of moving the speaker about in the room to find the most desireable location.