typical crossover network

System Design

Crossover

Every loudspeaker design starts with a system concept: the number of drive units, the mounting and layout of the units provided by the enclosure, the enclosure volumes, etc. Then the system acoustic target function can be specified - the acoustic responses of all the sections and their summation to give the overall frequency response of the system.

The Reference 207 is a 5-way system with crossover frequencies at 120, 400, 3000 and 15000Hz. Five sections may seem rather complicated but in reality it is a logical approach. The simplest form for a system of this performance would probably be a 3-way - bass, midrange and tweeter. Because of the increasing need to cater for wide bandwidth signals the inclusion of a dedicated hypertweeter adds one extra section, the other being the dedicated lower midrange unit.

KEF has a strong belief in the importance of the one unit covering the frequency range between 100 and 400Hz: partly because a lot of musical fundamentals are in this region and also because it allows the bass and upper midrange sections to concentrate on what they're primarily designed for. The Reference 105/3 was the first KEF product to have a dedicated lower midrange section: this was continued in the Reference Model 3 and Model 4 and in the Reference 109 where the combination of 15" bass, 10" lower midrange and 6" Uni-Q midrange works particularly well.

Once the drive units and cabinets have been specified the next essential stage is the design of the crossover network. The crossover is essentially the 'brains' of the system - it receives the full range amplifier signal and splits it into the relevant frequency bands for the drive units. Equally important, it 'shapes', or equalises, the response of all the sections so that when combined they produce a smooth, integrated system response. A lot of the final 'balancing' of a speaker is done by fine adjustments to the components in the crossover network.

The KEF 105, released in 1976, was one of the first systems to utilise 4th-order filters exclusively. In that case the filter shapes were 4th-order Linkwitz-Riley - a filter designed to ensure that the two integrating sections are in-phase throughout the critical crossover region. This works well if the crossover frequencies are spread out in frequency but for the relatively narrow bandpass sections of the R207, where the roll-off on one side affects the phase shift on the other,it is the Butterworth filter shape which provides the important in-phase behaviour. The primary advantage of high slope crossovers is that the overlap between adjacent sections is minimised. This means that at a particular frequency the output of the speaker will be coming from just one unit and will not include low level output from the other units. Subsequently the clarity, precision and refinement of the speaker are improved. However, because the crossover between adjacent sections occurs over a narrow frequency range the units must be subjectively well matched to avoid an abrupt change in the sonic character of the speaker.

The type and quality of the capacitors, inductors and resistors used in the crossover network is constantly under review. The KEF Engineers periodically conduct listening tests to check the sonic differences between the various brands and types of components.

With inductors it is always best to use air-cored type for low distortion but where large values and low d.c. resistance is required some kind of core is required. We use iron powder cores - noted for their excellent performance with regard to saturation.

Polypropylene capacitors are now widely used due to their excellent performance, and we have often produced subjective improvement by using two small values in parallel rather than using a single large value. A trained ear can quite easily hear the sonic differences between different brands, so comparative listening is important in selecting the type that best suits the speaker under design. The same applies for resistors, with metal oxide film often preferred to wirewound types. The sound quality of the crossover components is significant but the overall sound of the speaker is still dominated by the amplitude response. A good technical performance for a loudspeaker goes a long way towards making it sound fundamentally ‘right’, and this is achieved by careful and thorough engineering from start to finish - from concept through to production.

The Room Interface

The ability to measure performance under anechoic - or room-independent - conditions is pivotal to the design of an audiophile loudspeaker, as this exposes the ‘raw’ behaviour of the system.

However, it is equally important that the designers have a thorough understanding of the likely effects that real listening environments will have upon the loudspeaker.

For this reason KEF always undertake extensive subjective evaluations in a neutral but ‘real’ listening room and supplement this with unique computer simulations.

KEF’s Engineers have learnt vital lessons from these computer models and have put these findings into practice with the Reference 207 design.

Measured soundfield in a domestic living room (100Hz).

predicted soundfield in the same room at 100Hz
Predicted (FEA) soundfield in the same room at 100Hz.

Listening Tests

Over the years KEF has amassed a considerable amount of data on how the measured acoustic response of a loudspeaker relates to the subjective performance [3], so when we make the first version of a new speaker we know it will sound quite close to what we want. However, it normally takes a few changes before we are completely happy with the sound. The subjective 'balance' of the speaker can be altered by making changes to the relative outputs of the drive units: this is done in the crossover network. Also, we may adjust the acoustic damping inside the cabinet, the type of cables for the internal wiring or the brand of polypropylene capacitors in the crossover network. It is a cycle of listening - component change - measurement - listening. The new speakers are evaluated in a number of different rooms, with different electronics, on AV and music signals, by a panel of expert listeners - and when everyone is happy the speaker is approved.

Boundary Control Device

The Boundary Compensation Device (BCD) is a simple but highly effective method of providing flexibility in location of the Reference 207 within a listening room. When a loudspeaker is placed near a wall the timbral balance is modified by a significant amount of low-frequency gain due to energy reflected from that wall. The BCD allows the listener to compensate for that gain by modifying the crossover to reduce low frequency radiation. New in the R207 is a third setting which boosts bass radiation to compensate for rooms which are subjectively ‘bass light’.

This FEA model of the Sound Pressure Level (dB) in a typical domestic living room shows how dramatic the bass gain can be near a corner (assumes a 'flat response' loudspeaker).