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Large Format Studio Monitor History:

The term point-source is often used to describe the optimum sound source. The advantage is that the sound from a point source comes from one location.  Sound starting from the same place and time and emitting together from the source in phase,  results in a coherent sound wave.

There have been several large format studio monitors on the market over the last 40 years that have been designed to try and achieve the point-source advantage.  This has been attempted by physically aligning the driver's voice coil or acoustic centers in either the vertical or horizontal plane.  Aligning the axis of two drivers in both the horizontal and vertical planes with both drivers mounted on a baffle for a two-way system is not achievable considering obvious mechanical reasons.  Measurements show that aligning the driver's vertical axis's results in vast improvements in sound quality for the horizontal plane. Yet even these designs still lack the coaxial driver advantage where both the horizontal and vertical axis is aligned.

Although coaxial drivers are aligned in both the vertically and horizontally axis, they are not typically aligned in the Z axis for various mechanical reasons depending on the HF driver configuration. To align the HF and LF drivers in the *JBL/UREI 813 and 809 studio monitors,  Ed Long applied his **Time-Align passive crossover techniques to achieve a time-coherent or true point-source performance.  Coaxial driver systems like the ***Altec 604,  JBL/UREI 813 and 809 monitors, although coaxial driver based systems, depended on passive crossover circuitry which was limited to power input and contributed undesirable harmonic distortion and phase anomalies at high power levels.

Today there are several large monitors on the market that use digital or analog controllers to control crossover and time delay functions, however most of these loudspeaker systems are using dual woofer configurations with a horn in between the two drivers and they are vertically aligned.  Once again these systems have good horizontal off-axis frequency response but exhibit vertical off-axis frequency response cancellations as discussed earlier due to the large spacing between the woofers.

Finally, there is a horn that doesn't sound like a megaphone. Early on in the history of audio it became obvious to the best minds in the field, that to gain high SPL levels and maintain extremely low distortion levels you needed two things:

1.  High velocity of the air molecules at a low pressure (free space) giving us high SPL.

2.  Reduced diaphragm motion which at a given frequency lowers the velocity of the diaphragm resulting in lower distortion.

What the designer needed was an acoustical impedance matching device between the high pressure, low velocity diaphragm and the low pressure, high velocity of a high SPL free space. This device is called a horn. This gave rise to horns being used widely in large format studio monitors many years ago.  To this day, horns are still in wide use for sound reinforcement where their unique ability to provide unmatched efficiency and directionality makes their use mandatory.  Unfortunately horns are not as simple as one would hope and they typically do not provide a flat, smooth response.  In other words, they sound like megaphones. Horns are actually band-pass devices.  They are limited in their low frequency ability by their length and size of mouth.  Their high frequencies are limited by the throat geometry (for pattern control) and by the physical distances internal to the compression driver itself.  For example, the distance from any part of the diaphragm to the throat must not exceed 1/4 wavelength before significant cancellation of the signal occurs.  At these two extremes, efficiency is lost and between these frequencies the horn excels increasingly at producing distortion energy at higher SPL levels, creating a hump shaped frequency response curve.  This gives us that characteristic horn "honk".  To make matters worse there are a host of other resonances caused by many different geometric features, which are constraints of the practical design causing many dips and bumps in the response.  All of these issues are simply the result of the horn's system transfer function.

The transfer function is a mathematical representation of what you get out vs. what you put in.  It's described mathematically by the singularities of the transfer function, which we call poles and zeros, and whose locations cause the bumps and dips.  Until now it has been virtually impossible to cancel these poles and zeros using passive components or even active analog electronics.  Since this is a mathematical problem, a more detailed and accurate means of calculating the opposite of these poles and zeros is needed.  The advent of faster DSP provides us with a solution.  By computing complex algorithms without sample delay, the new DSPs are capable of not only eliminating these undesirable poles and zeros but are also capable of correcting for any variations in their locations caused during the manufacturing process. In other words, finally, a horn that doesn't sound likes a megaphone.

Poor radiated pattern control can also lead to problems in your mix.  The excessively wide and uncontrolled radiated patterns that are exhibited by modern studio monitors cause reflections off surrounding objects, most commonly, video monitors and the mixing console itself.  Similar to the vertically mounted drivers, this energy also arrives at your ears at a different time and phase which can cause cancellations or peaks in your frequency response, misleading you to EQ the mix incorrectly. 
This is why we at Equator spent so much time designing for the proper control of the radiated power from our monitors.

*JBL/UREI are trademarks of Harman/JBL. **Time-Aligned is a trademark of Ed Long Associates. ***Altec 604 is a trademark of Altec Lansing Inc.