The delicate brush strokes across the surface of a cymbal can only be recaptured by a tweeter that’s truly linear in operation. And that means one that uses a dome that stays rigid and operates like a piston throughout its operating range. Most Bowers & Wilkins speakers feature alloy dome tweeters. Our newest tweeters – as found in the 800 Series- feature a ‘crowned’ voice-coil bobbin and a silver-plated pole piece, extending the bandwidth well over an octave above the limits of human hearing, and rendering a separate ‘supertweeter’ quite unnecessary. It is commonly believed that the best materials for speaker diaphragms, be they cones or domes, are those that exhibit high stiffness.
The principle is that the diaphragm then behaves as a perfect piston and does not suffer the time-smearing problems associated with diaphragm break-up. Like many things in life, this simplistic approach has a good deal of truth, but is by no means a universal panacea. No material has infinite stiffness and there will eventually be a frequency at which the diaphragm will cease to behave as a perfect piston. Because very stiff materials also tend to have low internal damping, the break-up, when it occurs, can be very severe. The resulting resonances have what is called a high Q. The term Q has two meanings in acoustics. It can refer to the directivity of a speaker – the higher the Q the narrower the spread of sound – and you will often see this meaning used in public address speaker specifications. In this case, however, Q refers to the sharpness of the resonance – the higher it is, the more the resonance is highly tuned around a single frequency, putting a large peak in the frequency response.
As with a bell, a high Q resonance will ring on long after the applied signal has stopped. This is not good and the designer should make sure that the driver's response in the region of these resonances is well attenuated by the crossover. In practice this means that the nominal cut-off frequency of the crossover filter should be set at least 1½ and preferably 2 octaves below the lowest resonance frequency. Another potential problem with stiff diaphragms concerns directivity – how much the off-axis response differs from the on-axis response. The broadness of sound dispersion depends on the ratio of the wavelength of sound to the diameter of the diaphragm. The higher the frequency, the shorter the wavelength, and the narrower the beam of sound becomes. Excessive variations in the spread of sound with frequency will lead to listeners sitting away from the central 'hot spot' position hearing a different sound balance and a change in the character of the various instruments. It will also impair the sound image. In severe cases, the position of an instrument may appear to change with frequency.
So how does the designer avoid these problems? Making the diaphragm smaller will both raise the lowest resonance frequency and broaden the dispersion. Unfortunately, small diaphragms have to move further than large ones to produce a given level of sound and so tend to produce higher levels of harmonic and intermodulation distortion. The solution therefore is to use more drivers, so each covers a fairly narrow bandwidth and output levels can be kept high, dispersion more even and distortion low. How many drivers? To do the job properly over the full audio band really requires a minimum of four and it is no coincidence that our Nautilus™ speaker, in which all the diaphragms are aluminium, is 4-way.
For other speakers in our range, the use of aluminium is generally restricted to tweeters and bass drivers. A stiff material is essential if one wishes to make a moving coil tweeter that extends well into the ultrasonic region. In the bass, a stiff material is better able to withstand deformation from high pressures inside the cabinet and impulsive forces from the voice coil, thereby giving the best dynamic response. In the midrange, when one driver is used to cover a wide bandwidth, a more flexible material with a specifically controlled break-up behaviour, such as Kevlar® remains a better option. With combined bass/midrange drivers, preference must be given to the requirements of the midrange, where the ear is at its most sensitive.
Find out what Bowers & Wilkins customers and audio enthusiasts are talking about on our blogs, and read in-depth articles in the Sound Lab.
Production Notes: ‘Saving the Tapes’April 26,2015
Will Gregory in conversationApril 21,2015