Tebaldi

 With the new loudspeakers mod. Tebaldi, Opera`s intention is to recreate within the
home environment,  the emotion and pleasure of listening to music:  to provide a stable and focused musical soundstage in three dimensions with correct timbre, dynamics and detail, and to do this with no sense of listening fatigue.
To achieve this goal, every single technical aspect of the design was subjected to intense  musical listening tests at Opera.
 The new Tebaldi loudspeaker uses techniques and components which reflect the state-of-the-art in electroacoustic design, including controlled-diffraction cabinets, drive units and crossovers. Every type of distorsion has been analysed, quantified and reduced. The sonic result is tonally correct, convincing, involving and most of all, enjoyable to listen to. The Tebaldi loudspeaker is elegant and beautifully finished, and will discreetly enhance any room.

The project
The aim of the Tebaldi is to recreate a precise soundstage which is stable and correct in its proportions and in the dimensions and weight of each instrument or voice. The choice of multiple small diameter drive units offers a number of advantages of a mechanical, electrical and thermic nature, and, if employed correctly, can greatly improve the acoustic quality of the loudspeaker.
Cross-Linked-Dipole

The acoustic field in a room is the superposition of direct and reflected sound. Direct and reflected sound are supposed to be un-coherent (so they sum up as power). Under suitable hypothesis things works. The direct sound hold the primary characteristics of the loudspeaker system which are related to frequency and time response (one-dimensional quality). Reflected sound is responsible for what we call the "ambience effect" i.e. all those effects related to the loudspeaker dispersions and room acoustic behaviours (three-dimensional quality). What you actually hear is the superposition of the two sound fields at the listening point (your ear-drums). If the direct sound produced by a loudspeaker is good (and the listening hall is good) you may try to enhance subjective sound quality working on the reflected sound field  i.e. on vertical and horizontal dispersion pattern .
Our ear understands many thing but if you repeat the same thing more times, within a short time, il will understands better. In fact the sound continue to exist “inside” the hear for a while, if a reflection, bringing the same message, arrives during this short time, the ear still understands the same thing but with more details. This does not affect the direction the sound appear to come from.
Tebaldi model is a 120 litres, over 90 Kg., floor standing loudspeaker. You may regard it as two-way system with a passive sub-woofer in the same cabinet. On the Tebaldi' s front panel there are two mid-woofers and thefront tweeter which are responsible for the direct sound (and a good part of the reflected sound as well). Two 8" sub-woofers are placed on one side of the cabinet. On the rear side of the Tebaldi cabinet there are 4 tweeters facing the rear wall, they are high-pass filtered starting from 2 kHz. These drivers feed ONLY the reflected sound field.
You may use rear drivers to compensate the losses in the "power response". This is true for the tweeter you find on the rear panel of Diva, Divina and Callas SP : this rear tweeter radiates above 4000 Hz, in this range the front and the rear tweeters radiate on less then 180 degree angle ,  their radiation patterns are not or little overlapped, and consequently their interference is null or negligible.
In Tebaldi model the rear drivers radiate starting from 2000 Hz and they do interfere with the front drivers (front and rear radiation patterns ARE widely overlapped): the main axis frequency response become a sort of  "comb filter" with picks and notches. In anechoic environment the “comb filter effect” became more evident increasing the distance of the measuring point. In a reflecting environment the “comb effect” is more difficult to detect but it still exist (more evident in a windowed impulse response than in steady state conditions).
To avoid the “comb filter effect”  the rear sound should not  reach the main listening axis. You can separate the front and rear sounds with a large baffle, but this solution has heavy consequences. The solution is the dipole source. Tebaldi model  uses a (double) dipole with the axis perpendicular to the main listening axis of the loudspeaker system (see fig. below). Because of the interference between the sources of the dipole, sound radiation is null on the main listening axis while it takes its maxima toward floor and ceiling.  The “rear sound“ reaches the listener only after reflections on rear walls and is part of the "reflected sound field". This is due to the physical behaviours of dipoles.

What is a dipole:

A dipole is an acoustical device made with couple of identical sources, with opposite phase, separated by a fixed distance.  For all the points that are equidistant from the two sources the sound produced by the dipole is null (simply because 1+(-1) = 0).

Two dome tweeters as a dipole
 

Let's go in some place, out from the “no radiation axis”, far enough from the dipole. At low frequency, where the wave length is longer than the distance between the two dipole sources, the pressure difference generated by the two sources is too small to be heard. At low frequency the dipole acts as an high-pass filtered source. As the frequency increase radiation increase too. When the wave length is two times the distance from the two sources, the vertical radiation pattern became something like an eight (see fig. below). At higher frequencies radiation pattern shows more maxima and minima but there will always be a plane (equidistant from both sources) where radiation is null. Anyway above 4000 Hz drivers radiate over narrower angles and interference with front driver became negligible.  The action of the dipole is important from 2 to 4 Khz where front anf rear radiation are overlapped.

The vertical radiation pattern for a dipole at 3 different frequencies: red = 1000 Hz, black = 1500 Hz e violet = 1871 Hz. See the narrow “no radiation area”.

A simple dipole, in this case, is not enough: the “no radiation area” is too narrow. To enlarge the "no radiation area" I use a double dipole (a dipole of dipoles) with four drivers linked with opportune phase relationship. 

The vertical radiation pattern for the Tebaldi' s double dipole: the "no radiation area" is now about 30 degree wide.

At the end we have a rear radiating system that feeds the reflected sound field without interfering with the direct sound. Note that we do not need to consider phase relation between direct and reflected sound: they are not coherent so they sum as power and not as amplitude. All this will work if the drivers are “identical” and the radiation impedence symmetrical (or the cabinet is symmetrical).
This is the first time that a dipole (or a double dipole) is used in this way so we call this system Cross-Linked-Dipole to distinguish it from other dipole systems.
The most important thing is that the result is clearly audible, the reflected sound field is enhanced by reflections which occur near the sources (like in the Classical Greek Theatre): the whole reproduction seem to be more detailed.
The effect of the rear  system is maximum with classical music, the more instruments the better, the more “confusion” the more details. The effect is at his minimum with mono recordings. This is why you are allowed to exclude the rear drivers. Anyway you need a good amplifier or the dynamical inter-modulation distortion will mask all details (but this is always true).

Controlled-dispersion Cabinets
The cabinet is made in a mix of MDF and plywood, with teardrop-section curved sides, veneered and finely lacquered in the best Italian tradition. The front baffle, only 17 centimeters wide, is finished in leather. This, along with the benefit to appearance, also serves to reduce diffraction.
The internal volume is divided into four sailed enclosures: two are for the sub-woofer with their passive radiator, one is for the two mid-woofers and the fourth is for part of the cross-over network and the external connections.
Internally there are a number of interlocked braces which serve to increase the rigidity across the cabinet.
The mid-woofers’ enclosure in completely filled with acoustic absorbing materials which cancel standing waves and masking reflections also making the sound radiation isothermal.

Tebaldi Specifications
System:    Floorstanding column reflex with two passive radiators
Drive units:     2 x 8-inches aluminium cone bass drivers
2 x 6-inches magnesium cone mid-bass drivers;
                        1 x 1-inch forward radiating treble unit
                        4 x 1-inch rear-radiating treble units ( CLD )
Number of ways:   3-way + rear tweeter
Frequency response:  20-30000 Hz
Cross-over: low pass:  6 dB octave for woofers ( semi-parallel )
                   High pass:   12 dB/octave for the mid-woofers
         low pass:  6 dB octave for mid-woofers ( semi-parallel )
                   High pass:   12 dB/octave for the tweeter
Crossover point woofer / mid 160 / 200 Hz selectable
point mid / tweeter 2000 Hz approx
Power handling:   300 watt RMS
Recommended amplifier:  20 to 300 Watt RMS
Sensitivity:    90 dB/1watt/1 meter
Nominal Impedance:  4 ohm (min. 3.2 ohm)
Room positioning:   at least 20 cm from rear room boundary
                         at least 30cm from side room boundary
Dimension:    1460 x 700 x 300 mm  ( h x w x d )
                57,5” x 27,5” x 11,8” inch   ( h x w x d ) 
Weight:    90 Kg (198 lbs ) each – net weight
    120 Kg ( 264 lbs ) each – shipping weight