LUNARE TWEETER ARRAY WITH LARGE SWEETSPOT

There are a couple of unique and as far as we know totally original points to our design. One of the design goals for the LUNARE SERIES was a very large dynamic range. An array of drivers is currently the most viable solution for obtaining the high SPLs necessary for a large dynamic range but, the recognized problem with an array of drivers is a very small imaging sweet spot. Our design of the LUNARE has addressed this problem in a unique way. By working with the relative level of each of the tweeters we have been able to achieve a speaker that uses an array of drivers and therefore, has a large dynamic range, while also having a large sweet spot with correct imaging both vertically and horizontally over a large soundstage. In the LUNARE, the center tweeter is fed signal at full level while the other tweeters are successively attenuated as they go out from the center. The reasoning behind this is that because the center tweeter in our array is greater than twice as loud (+6 dB SPL) as any other of the drivers, the ears will psychoacoustically focus on the center tweeter for imaging information, while the surrounding tweeters relieve some of the burden for a given SPL.

An additional benefit due to this arrangement is the fact that the output of the center tweeter is down in level 11.29 dB *(9.11 dB) from an equivalent single tweeter system producing the same SPL. This translates into all of the tweeters remaining in their linear range of operation with minimized compression for much larger SPLs as well as an additional 11.29 dB *(9.11 dB) of headroom over an equivalent single tweeter system. This is further emphasized in the fact that the Dynaudio Esotar tweeter is recognized as one of the most dynamic tweeters available with practically no compression except at output levels high enough to cause the onset of failure. In an effort to minimize any componentry in the signal path the tweeter array has been designed such that the relative attenuation of each of the drivers has been accomplished by series / parallel combinations of the seven tweeters. This arrangement results in a minimum impedance of 19.8 ohms *(9.18 ohms) and an efficiency of 91 dB at 2.84V/1m for the tweeter array. *(LUNARE 3 and SURFACE MISSION 1)

DIFFRACTION EFFECTS MINIMIZED AND INAUDIBLE

A second recognized problem with driver arrays has to do with diffraction effects. All drivers, except those mounted in infinite baffles, experience diffraction of the sound waves around the edges of the baffle that they are mounted on. The diffraction of the sound waves at the edges causes reflections which result in fluctuations in the response of the loudspeaker. The fluctuations are due to the fact that when the sound waves from the source are combined with the reflected sound waves from the edge there is some phase difference between the two because of the length difference in the two paths. The frequency at which the path length difference is equal to one half of the wavelength, corresponding to a 180-degree phase shift, will experience a severe reduction in the response at the chosen listening position. Since the resulting path length difference between the two waves depends on the listening position the fluctuations in the response will vary slightly with listening position as well. Depending on the amount of phase shift at each frequency there will be increases and decreases of the final response throughout the frequency spectrum as compared to the source response.

One common method of dealing with this problem is to position the tweeter horizontally off center of the baffle so that the distance from the tweeter to each edge of the baffle is different causing the fluctuations in the response due to each edge reflection to occur at different frequencies. This is beneficial because it minimizes any audibility of the fluctuations by spreading the effect over the frequency band. A single tweeter system with the tweeter positioned in the horizontal center of the baffle will have the maximum amount of attenuation over a narrow range of frequencies due to the fact that the spacing from the source to each edge of the baffle is the same which causes the amount of attenuation to be additive. In loudspeakers with an array of drivers this problem is further magnified by the fact that the horizontal spacing from the source to the respective edges of the baffle is the same for each of the drivers. In the LUNARE SERIES we have dealt with this problem in a very simple, elegant, and yet unique way. The tweeters in our crescent diffractive array vary in distances from the edges of the baffle due to the crescent shape of the array, which has the beneficial effect of minimizing any attenuation due to edge reflections by spreading the residual effects throughout the frequency band making them all but inaudible. We are only able to do this without compromising the imaging capability of the speaker because of our use of attenuation in the array as discussed earlier.

INCREASED OMNIDIRECTIONAL OFF-AXIS RESPONSE

Dispersion is another effect that has been dealt with in our design. The polar or what is commonly referred to as the off axis response of a source is determined by the “ka” of the source where k is equal to 2p / wavelength and a is equal to the effective radius of the source which includes to a certain extent the baffle the source is mounted on. The smaller the value of “ka” the more omni directional is the response of the source and the larger “ka” becomes the more lobing occurs in the response making the source more directional. There are two ways to achieve a small value of “ka” which equates to good off axis response, first by considering lower frequencies which inherently decrease k since the wavelength is inversely proportional to the frequency. The second method is to reduce a the effective radius of the source. The radius a can be reduced directly by simply using a smaller diameter driver. For instance, by going from 1.5 inch diameter tweeter to a 1 inch diameter tweeter. The problem with this is that the diameter of the source also affects other competing parameters, the most important being the low frequency cut-off. Therefore, the lowest frequency of operation required from the driver will determine a minimum diameter of the source. The baffle a driver is mounted on also affects the off-axis response. A baffle wider than the driver will begin to decrease the off-axis response in a certain sense making the effective radius of the driver appear larger. It should be emphatically stated that these two are not exactly equal only similar in their results. In other words, a 1 inch diameter source mounted in the center of a 10 inch baffle does not have the same off-axis response as a 10 inch diameter source. However, it is definitely true that a 1 inch diameter source mounted in a 1 inch wide baffle will have a better off-axis response than the same driver mounted in a 10 inch wide baffle.

If a driver is mounted horizontally off center of its baffle then the polar response will be correspondingly unsymmetrical, the off axis response on the narrow side of the tweeter will be more omnidirectional and thus better than the response on the wider side. In the LUNARE speakers we have placed the center tweeter as close to the inside edge of the baffle as possible. This causes the best off axis response to occur in between the stereo pair of speakers where most of the imaging information is realized.

FULL MIDRANGE WITH NO CROSSOVER DEGRADATION

The midrange drivers in the LUNARE SERIES are unique in the fact that they are driven full range with no crossover whatsoever. This is done for two reasons. First, it is a recognized fact by most audiophiles that although passive components such as capacitors, inductors, and resistors are necessary in the crossover of loudspeakers they can cause degradation of the sound. Secondly, most midrange drivers are crossed over in the 1 – 3 KHz range, which encompasses some of the upper ranges of vocal as well as piano. In the LUNARE SERIES we have circumvented both of these problems in the implementation of our midrange drivers. The natural high frequency roll-off of the midrange driver occurs with a –3 dB point at a frequency of 5.5 KHz and this is the frequency at which the tweeter is crossed over to mate with the midrange drivers. This goes against standard loudspeaker design practices and would be undoable with normal lower quality drivers.

When typical drivers are driven to the upper limits of their frequency range the cone goes into what are called breakup modes. These are radial and circumferential structural resonances of the loudspeaker cone. When these modes are excited they produce spurious and audibly objectionable output from the driver at the frequencies of the modes. There are an infinite number of these modes spaced throughout the frequency spectrum. The small number of modes that occur within the pass band region of the driver are the only ones with enough energy content to make them audible. Normal crossover designs deal with this problem by using passive devices to attenuate the output of the driver beginning at a frequency lower than the first mode of the cone. This deals with the breakup modes of the cone but it puts the crossover frequency between the midrange and tweeter in a very critical frequency range. We are able to use midrange drivers without any crossover because these problems have been dealt with mechanically in the design of the driver. Our midrange drivers have a cone diameter of 6 inches and a voice coil diameter of 3 inches. By having such a large diameter voice coil the cone is driven from a point radialy one half of the distance from the center to the edge of the cone. This allows the driver to remain in its piston mode of operation and thus out of any breakup modes through a much higher frequency as opposed to other drivers, which are normally driven at the apex of their cone by a small voice coil on the order of 1 inch in diameter.

All of these elements sum up to create a performance of immediacy, delicate detail and powerful transients like no other loudspeaker systems in the world.