Mark & Mary Ann Weiss wrote:

I realize this is probably an economic limitation, but it
seems to me
that a few "price-no-object" driver manufacturers should
be making
something like this:

I have some Dynaudio Acoustics woofers that are 8" and
have 4" dia
voice coils.

Figure that voice coil diameter if functional (not window
dressing) relates to voice coil power handling capacity.

For their size, they sound incredible, big, deep, loud
and very responsive to transients, not to mention the
power handling
of 300W.

Actually, Dynaudio woofers are the laughing stock of the
industry when they have substandard Xmax, and odd
Thiel-Small parameters.

Woofer power handling specs can be pretty crazy. There are
standards for measuring them, but are they being followed?

Now, I work with a lot of 18" woofers in sound
reinforcement
applications, but the voice coil sizes are rarely larger
than 3" or 4".

One thing you have to look at is the balance between
sensitivity and power handling. A woofer driver can have
sensitivity that ranges from about 83 dB/watt to 103 dB/watt
with some lying outside even this very wide 20 dB range. An
efficient woofer doesn't need a voice coil that can handle
megawatts in order to be suitably loud.

There is an iron law of physics that ties box size, low
frequency extension, and efficiency together. For example, a
big box allows higher efficiency for a given low frequency
extension. A small box means you either have low efficiency
or not much bass extension or both.

SR speakers tend to compromise low frequency extension (less
bass) and box size (larger) for efficiency (more efficient).

Studio monitors tend to compromise efficiency (less
efficient) for low frequency extension (deeper bass) and box
size (smaller).


I know there is a British company called Precision that
makes 21"
woofer with 6" voice coils, but if we consider the scale
of things,
an 18" woofer should do amazing things if it had a 10"
voice coil.

Depending on the context, such a large voice coil could be a
cosmic waste, and even limit the performance of the speaker.

More of some parameter that looks cool does not always give
better performance.

The larger coil reduces the length of unsupported piston
area,
provides enormously more power for accurate transients,
and has more
rigidity against lateral movement.

None of which may be issues in a particular system design.
Enough is enough if you have enough.

I also suspect that you didnt' mean to say that "The larger
coil provides enormously more power for accurate transients"
because the coil is not a source of power, the amplifier is.
Transients are minor issues when it comes to voice coil
heating because they are of short duration. The key
parameter relating to handling extreme transients relates to
strength, and its easier to make a smaller voice coil
stronger.

Plus with a motor this large, the Xmax could be huge.

I don't believe that voice coils are the major issue when it
comes to Xmax. My informants tell me that its easy to make a
voice coil long, but its hard to provide a cone suspension
that is compliant enough to allow large excursions, while
holding the cone in the proper path with enough precision to
avoid mechanical damage while the cone is stroking. A larger
diameter voice coil vastly increases the weight of the
magnet assembly for a given amount of flux density.

Imagine an 18" woofer with a redesigned
skiver/suspension surround and the ability to stroke 8".

There you go - you mentioned the cone suspension as being a
limiting factor for Xmax. Nice that we agree, eh? ;-)

Such a driver would displace the volume of twenty 18"
conventional woofers
and be capable of infrasonic low frequency extention.

In fact the justification for some exotic woofer designs
relates to beating the normal limits to Xmax due to
tranditional cone suspension systems.

In a sound reinforcement situation that is not a permanent
installation, such a driver would be the holy grail--just
hook up a
10kW amplifier to it and rock the house.

There's a trend towards powered speakers for SR. The weight
of the amp and the weight of the speaker are now combined
into a single parameter. It is very difficult to build a
long-stroke woofer that is also highly efficient.

While such a driver might
have a 100lb magnet structure (and if price is no object,
we could
make that a lighter weight neodymium magnet), the benefit
of having
just one, heavy cabinet, over hauling two dozen cabinets
would be
enormous.

The market and the designers are very sensitive to exactly
these issues. The better products on the market are no doubt
the result of careful work to optimize the weight/SPL
performance.

And for the ultimate "Bass Pigs", such as this author :-)
, this
would be the ultimate loudspeaker that could take over the
neighborhood without taking over the livingroom decor.

I've been thinking about this concept occasionally for the
past 6-7
years. It should be a reasonable progression. If Bob
Carver can
invent an 8" subwoofer with a 2.35" stroke, then by golly,
it should
be possible to scale this design up to an 18" driver.

AFAIK Bob Carver didn't invent the drivers his woofers use.
They were developed by a supplier. Bob Carver did have a
patent on subwoofers that he used to IMO extort money from
subwoofer manufacturers. The patent was found to be riddled
with errors and self-contradictions, and was eventually
invalidated in court. This left Carver exposed to damage
claims from the smaller companies that he had been holding
up for legally unjustified cash payments.

I feel that 18" drivers have undersized voice coils.

Thanks for using a word (feel) that denotes emotion, not
logic.

I think I know a thing or two about speakers, but I would
not want to tell speaker designers how to build better
speakers. I know a number of people who analyze and/or
design speakers who heartily agree with my lack of desire to
compete with their demonstrated abilities. ;-)

They could be larger, both extending power handling, Xmax,
upper frequency
usefulness, transient response and efficiency. Such
drivers would
really be useful for extreme low frequency output, as of
course,
Doppler limits the maximum speed the piston can move
without causing
audible pitch shifting.

Well, you can control Doppler by moving the high pass
frequency down.

Are we really at the technological limit with 5" voice
coils on 18" drivers?

All Science is subject to future discoveries and improved
analysis.