A Product of Intensive Research & Development
Beneath the beautiful outer design of KARRI acoustic loudspeakers is the product of intensive research and development in transducer behaviour, crossover phase and frequency accuracy, and room interaction. The following provides some explanation of how just some of this research and development has been incorporated into loudspeaker designs that are incredibly natural and lifelike.
KARRI acoustic speakers use dipole mid-range and tweeter configurations. Despite being a more complex arrangement to design, they offer three distinct advantages, all of which contribute to unrivalled performance in providing an acoustically convincing recreation. Before discussing these advantages, let’s examine the polar radiation patterns of various speaker topologies:
This design uses front and rear firing drivers, with both sets in-phase. That is, both front and rear drivers fire in and out at the same time. This design produces strong sound fields to the front and rear of the speaker, as well as to the sides. While some main speakers have been designed with this configuration, this design is more suited to uses in which a diffuse pattern is preferred, hence the frequent application in surround speakers.
The ideal omnidirectional speaker would behave as a point source, producing a ‘ball’ of sound that is perfectly spherical. In practice, this can be difficult to implement at higher frequencies and can also introduce undesirable scattered room reflections.
In most applications, with dipole woofers, very large horns, and line arrays being a few exceptions, lower frequencies become inherently omnidirectional, as ratio of wavelength to baffle dimensions increases. As the frequency rises, this ratio becomes smaller and the sound radiation transitions to directional, with this transition area known as the baffle step. This must be carefully considered within the speaker design, and is taken into account is the crossover frequencies implemented between the omnidirectional bass section and dipole mid-range and treble section in KARRI acoustic bass reflex / dipole hybrid models.
In dipole (or open baffle) configuration, the radiated sound field to the back of the speaker is out of phase with the front of the speaker. This produces a unique acoustic short circuit to the sides of the speaker, to which there is a radiation ‘null’. As will be seen further below, this can be put to very good use in controlling reflected sound within the room.
For a recreated audio recreation to be perceived as convincing, the design must take into account more than the frequencies and acoustic power emanating from the drivers. The way people perceive sound is highly attuned, and so must the designer be in the considerations of the loudspeaker and its interaction with the room.
Naturally Balanced Ratio
A question that naturally arises now is this: if reflected signals can detract from the recorded spatial and timing cues, should I treat the wall behind the speakers to eliminate these too? The answer is: No. If you’ve ever spent time with speakers in an anechoic chamber you’ll know that the sound is anything but natural! Our auditory system and perceptual processes interpret multiple spatial and localisation cues such as time, phase, frequency, and amplitude differences. An averaged natural ratio of direct versus reflected sound is 70/30. Dipole speakers provide a very good in-room approximation of this ratio. The combined effect is a placement of the instruments that is precise in imaging and convincing in depth and soundstage.
No Box Colouration
Despite considerable variation in design and price of ‘box’ speakers, these speakers typically share one unfortunate trait: they sound like sound reverberating from a box. While this can, to an admirable extent, be reduced through exotic structural design and materials, the inherent costs of doing so lend very high price tags to such designs in the pursuit of a quality of which the open baffle, or dipole, speaker is naturally endowed.
KARRI acoustic hybrid dipole/bass reflex speakers take the best of both worlds, while avoiding the deficiencies. This is achieved by utilising dipole arrangements for the critical midrange and treble frequencies, while retaining efficient bass reflex loading for the bass. These bass reflex units are carefully designed and constructed to ensure that the radiated sound is that from the drivers and reflex port, and not the enclosure. By employing aspects such as sides incorporating curved sides formed by multiple layer wood laminations bonded for permanent tension, cabinet resonance is both minimised and pushed to a frequency far above those covered by that section of the speaker. Our bass reflex enclosures also utilise internal bracing and glass wool damping.
The precedence effect is the name given to a perceptual phenomenon whereby a reflected sound arriving at the pinna within a very short time interval (1 to 5 milliseconds) of the direct sound, is perceived as part of the same primary auditory event. Effectively, the sound is perceptually smeared and the direct and reflected sounds become one and the same.
It is easy to see that when sound is being radiated from a loudspeaker as well as reflected from the side walls and wall behind the speaker, this smearing is substantial and masks the original reflections of the recorded event, as well as critical, small time interval acoustic cues of instruments. This is exacerbated by side radiation from the loudspeaker being reflected of the centre of the wall behind the speakers or a display screen, smearing the stereophonic cues.
Dipole speakers, with their acoustic null to the sides, excite far fewer and less intense side-wall and centre reflections, preserving the critical recorded timing and spatial cues. The rear radiation of dipole speakers, on the other hand, travels a longer path from the driver to the wall behind the speaker and out to the listener. It is for this reason that it is preferable to place a dipole speaker so that the rear driver element is at least 85 cm (5 milliseconds return) from the wall behind it, with 1 metre (6 milliseconds) or more being ideal. For example, the KARRI acoustic Nullaki model places the rear of the dipole elements 55 cm forward from the rear of the speaker, so an additional 45 cm between the speaker and the wall provides the desired 6 milliseconds reflected time distance.
“Dipole just does something that I’ve not heard from monopole or traditional speaker designs to date.”
– Marc Rushton, StereoNET