Bowers and Wilkins Nautilus

Speakers start at £300 from Bowers and Wilkins, and rise to around £40,000 a pair for these - the firm's top-of-the-range Nautilus for which it received a Queen's Award for Innovation.

These are huge speakers, 1.2m tall, so there is no need for reflex ports to boost bass response. Instead there is a tube attached to the rear of each driver to remove rearward energy. The tube is filled with absorbent material and shaped to taper the sound to zero at the far end. "It is like a horn in reverse, " R&D head Dr Gary Geaves told Electronics Weekly. "No energy by end of tube means no reflections."

The characteristic snail shape comes from the need to squeeze the bass speaker tube, which is long because of the wavelengths it has to handle, into a manageable space.

Nautilus tubes are used for the tweeters and mid-range speakers of many of the firm's products.

Exterior smoothness is to prevent sound diffraction from sharp corners which otherwise blurs the sound field.

Active cross-overs mean four amplifiers per speaker are required, plus a pre-amp for each side.

Making Loudspeakers

Chipboard and plastic at the low end, and MDF (medium-density fibreboard) in the middle. What do high-end speaker makers make their cabinets out of?

"MDF is pretty good material, but you can find better," Dr Gary Geaves, head of research at speaker maker Bowers and Wilkins, told Electronics Weekly. "We use wood in our very high-end speakers, built up in layers to follow curves with very high precision. Wood is quite stiff and has quite good damping."

The cabinets are made by a 90 year-old Danish wood working firm, formerly a furniture maker, that B&W bought in 2002. "The had to get used to building up wood to higher tolerances for loudspeakers," said Geaves.

Other materials used include speaker housings moulded in a synthetic, mineral-filled resin.

"They get expensive because you use a use a lot of exotic materials and R&D," said Geaves.

Artificial diamond is used for some tweeter domes.

"It is CVD [chemical vapour deposition] polycrystalline diamond which has mechanical properties identical to single crystal diamond," said Geaves. "The Young's modulus to density ratio is very very high."

Made by industrial diamond firm Element Six, the diamond is deposited over a former, then the former is removed leaving a free-standing diamond dome. "The diamond dome is produced in relatively high volumes," added Geaves.

Any material is considered for loudspeaker parts, but there are limits.

"We spend a lot of time on materials, pushing resonance up as far as possible.

There is potential for composites and honeycombs," he said. "You can make all kinds of fancy materials. Concrete is great material, but you have to be able to sell it."

Computer aided design is important in enclosure and component design.

"We do an awful lot of modelling on all sorts of aspects, although rarely complete systems," said Geaves. "Finite element analysis is good for mechanical components, and we do a lot on acoustic fields, on diffraction, nautilus tubes, and the design electromagnetic circuits."

As for the actual sound transducers, there are many possible types including electrostatic and ionic designs. "The bang for buck is high in the conventional moving coil loudspeaker, which is very inefficient," said Geaves. "Horns are much more efficient, but suffer from sound colouration."

What is important in cabinets is not to have sharp corners. "Sharp edges act as diffraction sources which colour and blur the sound. Shapes with nice smooth edges reduce diffraction," said Geaves.

Trading sensitivity for bass

Dr Gary Geaves, head of R&D at B&W loudspeakers, explains how sensitivity (vertical axis) can be traded for bass response allowing small speakers to produce good bass providing sufficient drive power is available.

Big loudspeakers can inherently produce lower bass notes than little ones, so the trend to space saving loudspeakers is increasingly challenging manufactures.

"Our core business is audiophile who has room in their life for big speakers. As loudspeaker designers, we fundamentally don't like things getting too small as we come up against the law of physics," Dr Gary Geaves, head of R&D at B&W told Electronics Weekly. "You can't beat the laws of physics, but you can push up against them as far as possible."

The big problem with small speakers is bass response - or lack of it.

For a given enclosure, said Geaves, sensitivity can be traded for bass response (see whiteboard photo).

This fall in sensitivity means more power is required to drive the speaker for a given sound level, but if the power is available, small speakers can be made with acceptable bass response.

One way to extend bass response is to have a hole in the speaker enclosure - a resonant port that allows energy from the rear of the speaker to add to that coming from the front of the speaker. To get the phase right, a tube of the correct length extends inside the enclosure from the port.

"A port is a bung of air bouncing on spring provided by the air in the rest of the cabinet," said Geaves.

If the resonant frequency is just below the enclosures natural roll-off frequency, bass response can be extended - providing the resonance is well controlled and doesn't add a huge hump to the response curve.

The firm recently introduced a loudspeaker docking station for iPods called Zeppelin.

"Ports offer many advantages," said Geaves. "With Zeppelin we wouldn't have entertained not putting a port in it."

Getting the port to behave properly is far from simple. "A port is an engineering challenge," said Geaves. "You have to get an air bung appropriate to the stiffness of the air inside cabinet, and you may have to wind the tube up in the enclosure to fit."

If the port is pushed too hard, the air in it becomes turbulent and makes its own sounds. "If you do too much, you can provoke turbulent noise - even if the port is straight - particularly if the tube is too narrow," said Geaves, however, "plusses generally outweigh negatives - especially if size is important".

An alternative to a port is an auxiliary bass radiator (ABR), which is essentially a passive loudspeaker - with no magnets or coils - open to the cabinet at the rear and the world at the front. Its mass and suspension stiffness can be carefully controlled to get the right effect.

"An ABR is a cone bouncing on a column of air," said Geaves. "Ports and ABRs are two ways to the same thing."

The University of Salford has an explanation of loudspeaker basics: see www.acoustics.salford.ac.uk/acoustics_info/loudspeakers/