Acoustically Designed Cabinets

In order to understand better why the Vivid Audio philosophy of using enclosures with rounded contours leads to an improved performance when compared to those with sharp corners we have to look at how sound behaves in the two cases. First let's look at the way the rounded enclosure helps with the high frequencies.

Acoustically Designed Cabinets – Mid and High Frequencies

Vivid Audio Acoustically Designed Cabinets Sound energy is carried through air by means of pressure waves. The way in which these waves interact with their surroundings is not always intuitive and one of the most difficult ideas to grasp is why a wave radiating from a loudspeaker is reflected by a sharp cabinet edge. We're all familiar enough with sound bouncing off a hard surface, that's the echo you hear if you shout at a smooth wall. Imagine putting two flat boards either side of the tweeter; the sound bounces off the walls like ripples in a pond.

But there's no hard surface at the edge of the cabinet, just free space, so what's going on? As the sound moves outwards from the tweeter it forms hemispherical waves with one edge running along the flat cabinet front, sometimes referred to as half-space, until it reaches the cabinet edge and the full-space beyond. It turns out that the sudden change from being bounded on one side to being completely free is almost as big a shock as running into a hard surface and a sort of negative echo radiates from the edges .

Minimizing reflection Re-radiation from sharp discontinuities, known as diffraction, is also something that concerns designers of sneaky military aircraft. They aim to create as little radar reflection as possible to avoid detection. The B1 bomber has a radar signature just one per cent of that of its less discreet predecessor the B-52.

Much of this benefit has been achieved by keeping the surface as smooth as possible. So how does this re-radiation affect the performance of the loudspeaker? Some of the sound clearly takes the direct path to the listener's ears and some of the sound bounces off the corners and heads back towards the listener's ear where it mixes with that coming directly from the driver. Sometimes the waves add and sometimes they cancel depending on the wavelength and the position of the listener. This ʻinterferenceʼ causes the irregularities in the response shown by Olsen.

Cabinet with sharp edges   Curved cabinet without edges

In the case of the curved cabinet there is no sharp edge and hence no interference. Clever crossover design and other tricks can lessen the effects of interference directly in front of the speaker but when you place the speaker in a normal room with reflective walls you hear the main sound followed by a sort of average of all the irregular off-axis output so the subjective result is still coloured by the sharp corners.

Replacing the flat baffle and sharp edge with a single smooth curve means there is no longer a single point at which the sound space changes from half space to full space. In turn this means that there is no interference and a smooth off axis response.

Tweeter on top

Another frequently encountered shortcoming of many speaker systems is a jump in the dispersion between the different drivers. A typical cone mid-range driver concentrates the sound into an ever tighter beam as the frequency approaches the point at which it crosses over to the tweeter(2-3kHz). However a small tweeter mounted on a flat baffle, or worse still in free space, has a very wide dispersion at the lower end of its range. So if the response is flat through crossover when heard on-axis then there will be an excess of high frequencies off-axis. This excess reaches the listener after reflecting from the walls of the room and the subjective effect is of an over-bright balance.

Acoustically Designed Cabinets – Low Frequencies

At the bass end of the spectrum it's no longer the wave-like nature of sound which is a concern but the sheer movement of the air. Vivid Audio loudspeakers use the vented style of enclosure to improve low frequency distortion performance. This means introducing a carefully optimised duct which connects the inside of the box to the outside. Sound output from the rear of the bass unit causes the air in this port to move in and out and for a certain band of low frequencies the majority of the system output is from this port. Benefits include reduced cone excursion and hence distortion but only if the air in the port moves smoothly.

All too often vented loudspeaker systems have simple lengths of tube with no attention paid to the manner in which these are terminated. As the air moves back and forth through such tubes a distinct 'chuffing' sound may be heard that becomes rapidly more significant as the drive level is increased. The cause of this undesirable distortion is turbulence and it represents a transformation of the desired airflow into useless eddies. Turbulence occurs whenever there is an abrupt change in the direction of a flowing fluid. You can see it as you swish your hand through water or in the wake of a boat and you can hear it when you hiss through clenched teeth or in the roar of a jet engine. In every case the energy in the smooth stream is being converted into rapidly spinning vortices where it dissipates into waste heat.

A quick look at any design where fluid movement is an issue reveals a simple solution – streamlining. Be it plane or a fish, sharp steps are nowhere to be seen. And so the same can be applied to loudspeaker ports.

Vivid Audio loudspeakers feature bass ports with gentle flares at both the outside and inside ends and deliver a bass performance that far surpasses any simple tube.

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