Digital technology will drive loudspeaker design, according to Sussex-based hi-fi speaker firm Bowers and Wilkins - and micromachined semiconductors may be the future.
"We are in an interesting time, going from an old paradigm to a new one," B&W Research head Dr Gary Geaves told Electronics Weekly. "Digital MP3 is part of it, digital networks is another."
Digital amplification is another of Geaves factors. Class-D amplifiers dissipate far less power than traditional analogue amplifiers, and so are easier to build into loudspeaker enclosures to make active speakers.
"We have monitored active loudspeakers. We have been sceptical, but when size becomes an issue you have to think active," said Geaves. "The big thing is Class-D which is becoming good quality now, although Class-D is not yet as good as high-end analogue."
Amplification within the speaker enclosure requires mains power, and once mains power is available other things become possible.
"There is a massive customer need for computers to link with rest of world," said Geaves. "Over next five years expect to see and explosion of productivity."
B&W's background is in high-end passive speakers, typified by its 800 series.
Zeppelin iPod dock
Now the firm is starting to think more in terms of active speakers, particularly for small speakers where sensitivity has to be compromised to get reasonable bass response - recently introducing an active stereo speaker with an iPod dock called Zeppelin.
"We have always done sub-woofers that have been powered," said Geaves. "Zeppelin is a classic case of small, and plugs into MP3 players so it needs to have amplification - a case of maxing amplifier power."
Wireless Liberty
Increasingly, music is stored or streamed from computers. Once mains power is available at the speaker, it is becomes possible to remove the wires connecting source and speaker, and to build in digital signal processing.
"Wireless got a bad name in our industry as early systems were poor quality," said Geaves.
Problems included poor synchronisation between stereo speakers which causes the sound image to move around, and susceptibility to interference. Things have improved. "Now we have seen a number of companies that have something like a wireless wire," said Geaves.
In the Autumn, the company is introducing a wireless 5.1 speaker system dubbed Liberty.
Stereo shortcomings
With DSP in a speaker, or in banks of speakers, the inherent shortcomings of stereo could be overcome.
"To expect to reproduce the sound of a band with amplifiers distributed across the stage, or an orchestra, over loudspeakers is somewhat of a tall order," said Geaves. "The fact you get a loudspeaker to sound anything like an orchestra, and carry the emotion, is amazing."
With DSP and networked sound sources, the future of hi-fi could be recreating something close to the original 3D sound field throughout the whole of a room.
"The future could be trying to reproduce what the sound engineer heard when recording the music. Even 7.1 only creates an illusion of sound field over an area - it is not really a sound field," said Geaves. "There are all sorts of research based approaches. What you need is lots more speakers coating the walls of the room - lots and lots of small loudspeakers that don't cost anything, each with local processing."
To achieve a reasonable cost with the array approach "the electronics, DSP and speaker are a block", said Geaves. "It would be interested to see what mileage these is in building it in silicon with MEMS."
Even if this wave field synthesis became available, "all sorts of revolutions would have to happen, the whole industry is set up for stereo and 5.1," said Geaves.
Digital compression
Back in the current world, Geaves has something to say about existing digital compression. "Much above 192kbit/s variable bit rate can be very good. From compromised MP3 you can go to 320kbit/s," he said. "We are almost getting to the state where storage capacities are going insane and you will be able to store music uncompressed."
For digital playback, said Geaves, care has to be taken right through the system: "There are a lot of places in the signal channel to go wrong, but it's good when you get it right."
With people increasingly happy to listen on headphones in noisy environments, or cheap computer speakers, is there room for hi-fi companies?
"There will always be a certain percentage who will rebel against the spirit of age and want the real thing," said Geaves.
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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. |
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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. |
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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/ |