Firstly, the required low frequency load volume established by the electro-acoustic parameter set of the TAD 1601A bass driver established the need for large amounts of air. Further, much of our exhaustive drive-unit evaluation was carried out with the drive-units suspended by cables in free air. This stark reference point allowed us to make meaningful and well founded decisions about which type of enclosure design we should pursue. It was found that all design approaches and variations incurred trade-offs, manifesting various unwanted resonances and colourations. However, with constant referencing back to free-air operation we eventually converged on one specific recipe and trade-off set that would prioritise the specific virtues we were looking for:
With regards to enclosure wall damping, once again carefully controlled experiments (the construction of identical mass/geometry prototype enclosures with and without advanced constrained layer damping techniques) showed us that damping was not without trade-offs. In a general sense, one misconception regarding damping is that it modifies only the amplitude and storage time of resonant energy. However, what is often overlooked is that damping also reduces the “Q” of any resonance, which effectively results in the “bell” shaped energy distribution spectrum of a resonant mode being spread over a wider range of frequencies. Further, there are other musically destructive consequences related to commonly employed damping regimes: a disturbance to the natural balance of upper harmonic formation and decay. Common damping techniques always have a frequency specific characteristic, and inevitably attack upper frequencies much more aggressively than lower ones, effectively turning the enclosure into a high frequency sponge. Enclosure wall resonance and re-radiation do compromise performance, but the standard damping solutions commonly thrown at these problems are often even more musically questionable, disturbing the natural life and vitality of the drive-units, and ultimately, the musical experience.
With regards to enclosure shape, once again careful experiments referencing to drive-unit voice in free air proved beyond all reasonable doubt that enclosure sidewalls that were not parallel to wave propagation gave rise to far more disturbing colorations and signatures than the simple side to side flutter or discrete frequency standing-wave effects often cited as the problem with rectangular enclosures. We conducted careful experiments using experimental variable-angle geometry enclosures, and it was found that trapezoid type enclosures were in fact one of the worst possible design choices when faced with a lesser of two evils type situation. There are those who will find this challenging, having read somewhere that parallel walls are bad. They are those who are yet to empirically test this as we have. Make no mistake, enclosure cavity resonances are a problem, and absolutely do compromise the performance of any box loudspeaker, but the proposition that non-parallel wall enclosures somehow eliminate cavity resonances is flat-out untrue, and we discovered that these types of designs incur even worse sonic penalties compared to the rectangular/parallel approach.
The ostensibly straightforward solution finally arrived at should not
give the impression that more exotic shapes, bracing, and damping
regimes were
avoided due to complexity or cost. This is not so. From the outset, no
limits were placed on design due to any such issues…The
only arbiter guiding design choices was whether a particular
approach subjectively enhanced or
detracted from our desired sonic characteristics. This approach left many
generally accepted loudspeaker design practices discarded along the wayside,
casualties of careful empirical evaluation and an obsessive search for
ultimate musical expressiveness.
Finally, it has been claimed by some that rounded enclosure edges reduce diffraction effects. This is only true when the radius of the rounded edge is large in comparison to the planar surface it is terminating, and/or large in comparison to the wavelengths of the frequencies of interest. From this it is obvious that in the majority of cases where small radius chamfers are used there will be no advantage other than cosmetic, and in our case the desire for absolute minimum, cut-to-the bone baffle dimensions, and the sonic portrayal associated with this, dictated sharp edges.
The dispersion characteristics of any non horn-modified
transducer are frequency dependant, and primarily related to the size of
the wave-launcher at interface with free air. Sound wavelengths decrease
in size with increasing frequency, so the relationship between the size of
the wave-launcher and the lengths of the waves it launches establishes a
frequency dependant dispersion characteristic of decreasing dispersion with
increasing frequency (beaming).
In the case of the TAD TD4001, the moment the waves exit its short
throat, they expand just as they might have, had they been launched
by a direct radiating
dome the same diameter as the TAD TD4001 throat exit. Of course, because
the throat exit has a diameter of 50mm, the gradual transition to beaming
occurs at a lower frequency than would be the case with say, a typical
25mm dome tweeter, but against this, because the TAD
TD4001 is
operating down
to a frequency about an octave lower than the typical crossover point of
a 25mm dome, it has superior dispersion in this octave due to the beaming
characteristic of woofers that would be typically covering this range in
small two-way systems…apples vs. oranges. However, it must be understood
that the crossover voicing was finalized with a specific set-up geometry
of exact toe-in so that both loudspeakers face directly on axis to the
listener position. If intended performance is to be realized, the user
must duplicate
this set-up geometry (please refer to set-up instructions).
Finally, it is worth mentioning that we feel that a circular expansion
of high frequencies is desirable, as opposed to the horizontal
vs. vertical
modifications of some horns, and typical ribbon transducers.
Firstly, acceleration capabilities can be predicted by simple physics calculations based around parameters of both force and mass. In other words, power to weight ratio. In this context, it must be understood that the Alnico motor system of the TAD 1601A is immensely powerful.
Secondly, in order to generate a given sound pressure level at a
given frequency, the excursion response demanded of the TAD 1601A is
an order of magnitude less than that which would be demanded
of a small format woofer
trying to
do the same job...In other words, a small woofer will be required to cover
a greater distance than a large woofer for equal sound pressure…even
an Olympic sprinter will not beat the average Joe if the Olympic sprinter
is required to cover 100m whilst the average Joe is only required to cover
25!
In other words, the small driver’s “speed” is offset by
the high velocity demands placed upon it.
The model 16 system exists not as the result of some marketing plan or business idea. It was developed first and foremost as a self-indulgent labour of love for the personal enjoyment of the members of the deep team, with allowance for further replication. The motivation was simple…we couldn’t find the exact formula we wanted in a loudspeaker. Modern designs just didn’t sound funky, vintage designs just didn’t have the fidelity. We wanted detail, image formation, air, but we also wanted groove, funkiness, and mid-bass punch.
A large part of the model 16’s performance is directly due to the amazing
TAD drive-units. However the essence of these transducers could so easily
have been diluted if not for the radical design features and purity of execution.
The model 16 is a unique niche-device for those who know what they want…We
look forward to sharing our passion with like-minded listeners…
