It depends on many more factors and on one's priorities...
What are the advantages and disadvantages of these different size drivers? If a 1.4" driver with a 3" voice coil falls in the middle of the 1" & 2" shouldn't it have the best of both worlds? Do smaller throat drivers image better? Do the higher frequency's break up sooner on 4" voice coils than on 1.75 voice coils. What are some of the best modern drivers for home hifi?
I'm afraid it is not as simple...
Answers to your questions depend on many other factors, beside throat and voice coil diameter.
Among these:
1) presence or lack of an internal conical throat, and - if present - design thereof:
this contributes to determining the frequency beyond which the driver will 'beam' in pretty much all horns/waveguides (except those with a 'pinched' throat that rely on diffraction to widen the beamwith - but these are IMHO best avoided anyway...);
2) compliance of the surround and size of the back-chamber:
both of these influence the resonance frequency and hence the driver's usable lower frequency range (provided that the horn onto which it is mounted is large enough...)
3) diaphragm material and mass:
these will affect two things:
a) the high frequency mass break point, beyond which the driver requires equalization, which can be either electrical - for 'constant directivity' horns/waveguides - or mechanical - as brought about by 'classical' exponential/hypex/tractrix horns, which lead to some 'beaming' in at least one plane;
b) the frequency beyond which the diaphragm 'breaks up' and is no longer pistonic. This frequency is
typically ~12kHz in the 1" exit CDs and ~6kHz in 2" exit CDs... but the exact frequencies vary, and especially in the case of Be-diaphragm CDs can be almost one octave higher.
So, all this considered, there can be no clear-cut answers to your questions.
As an example, a few 'old style' 1" drivers with deep magnets, long and slow-flare internal throats, compliant surrounds and large back-chambers, coupled to conventional exponential horns with a cut-off frequency (Fc) of ~400Hz are perfectly happy when used down to ~800Hz and can mate well with a suitable 15" woofer.
Drivers of this type have become as rare as hen's teeth, however. Examples would be JBL's 2426, Fostex D1400 and TAD TD-2001.
The former two have titanium diaphragms, and this, coupled to the top-octave beaming caused by the long internal throat, means that they are better used as mid-range drivers below ~7-9kHz, with a supertweeter taking over beyond that
[*]
The TAD otoh sports a Be diaphragm, and as a results works quite well in two-way systems from 800Hz all the way up to 20kHz - provided you can live with the same kind of beaming beyond ~9kHz.
Most modern 1" drivers, instead, have 'pancake' magnets, short or non-existant internal throats and 'fast-flare' phase plugs, stiff surrounds and vestigial back-chambers.
These are best coupled to waveguides or high-Fc horns and used as tweeters > 1.5kHz.
They don't beam quite as much in the HF, but they can't mate well with large direct-radiating woofers, and ideally require an 8"-10" midrange cone to ensure a smooth directivity match. Also, while their throat geometry minimizes top-octave beaming, in most cases the uppermost frequencies are still produced by the diaphragm working in 'break-up' mode, and will therefore not sound as 'smooth' as a properly designed super-tweeter.
Similar considerations apply to 1.5"-exit and 2"-exit drivers too...
In the end, there is no single 'perfect' solution. Pick your poison and live with the compromises it entails....
[*] for my
personal choice of compromises, see:
Project | Homebuilt Hi-Fi - A user submitted image showcase of high quality home built hi-fi components.
Marco
