Home theatre speakers
Part 1: What is wrong with conventional design?
Conventional home theatre speakers
The hifi industry has been very slow to adapt its thinking about how loudspeakers should be designed for home theatre. The most minimal of changes have been made to make conventional more suitable for home theatre. These changes are mostly superficial. The centre channel is often made to be placed horizontally on top of an entertainment cabinet and the surrounds have been made smaller for wall mounting. In essence, this represents the least amount of change possible in order to market these as suitable for home theatre. In many ways they fall short.
In a nutshell, conventional design has failed to re-think home theatre, starting with a design brief and performance goals. It's not a case of form follows function, but rather creating a familiar form factor the market will easily accept.
What should we expect in a home theatre speaker?
A serious home theatre speaker should meet these targets:
- ability to reach THX reference levels (105 dB peak SPL in the listening position) with clarity
- high sensitivity to achieve this in average rooms with affordable AV receivers
- high power handling to achieve reference levels in larger rooms with higher powered amps
- low dynamic compression of transients
- constant directivity
- a large effective listening area extending to all seats
- ability to create a centre image
- surround field maintained in side seats without collapsing into the nearer surround
- freedom from listener fatigue at high SPL levels
- neutral and accurate uncoloured sound
How conventional designs fail
Conventional designs fail to meet even the most basic requirements. Typically their maximum output level requires added power amps to overcome their low sensitivity and they typically use fragile drive units which are unsuitable to deliver a dynamic experience. They typically sound strained and compressed when turned up and the result is a diminished home theatre experience. Many of them perform quite well for music listening in the sweet spot at moderate levels, but fail to deliver a dynamic experience shared in every seat.
Centre channel dilemmas
A centre channel is the standard solution to fix the problem of maintaining a centre image in side seats. It does tend to work fairly well, however, in most home theatre systems a centre channel creates more problems.
The most common problem is that they are oriented horizontally. This is acceptable in a design that is intended to be placed this way, with a tweeter mounted vertically over a midrange driver and then flanked by woofers either side. This arrangement is the exception to the rule, due to the extra complexity and expense. A properly optimised 3 way passive crossover is expensive in parts and these designs tend to be taller than most would prefer, to get a midrange driver that is not too small. Very small mids are low in sensitivity which is a problem in this application. In most cases, a central tweeter is flanked by two midbass drivers. The horizontal dispersion is a problem in all seating locations except the middle sweet spot that does not require a centre channel at all! As such, this is a very poor solution.
It would be preferable to have main speakers that can reduce the reliance on a centre channel. In many systems, this can be achieved with the right design. This not only saves cost but avoids the awkward problem of combining an effective centre channel with a TV and entertainment unit.
In dedicated rooms this situation can be avoided and a centre channel that is an exact match of the front speakers can be used.
Surround speaker problems
Surround speakers are intended to immerse the listener in a sound field that extends beyond the boundaries of the room, creating the impression of being in a much bigger space. This works easily in the sweet spot, but in many systems the options for placing surrounds are less than ideal and often side seats are close enough to the surrounds that the sense of space collapses into the nearer speaker. The surround field fails.
The conventional solution is dipoles and bipoles which double up on drivers or add small wideband drivers firing to front and rear. To achieve a reasonably low cost and compact speaker like this is usually a serious compromise. Cheap small drivers are used to keep the cost down, yet many of these speakers are also quite expensive for the level of performance. Typically the box is twice as big as necessary for a given output level. Since these need to be compact to sell, they are quite restricted in output typically.
In part 2, we discuss a simple yet effective solution to these short falls.