Erik wrote: Let's focus on learning
about choices speaker designers make and what that may mean to the
end user.
Duke replies: I can't speak for any
other designers, but for me speaker design starts with figuring out
what the priorities are, and then hopefully finding a way to
accomplish them (or come as close as is practical) within whatever
constraints are imposed. So the system concept comes first, and then
the design specifics follow. Thus I see tweeters (and every other
part of a speaker) as just part of a system, and no more than a means
to an end, which in high-end home audio is usually something like “a
sufficiently convincing perception of listening to live music”.
Erik: First, for any given type of dispersion, speakers need to
roll off more or less evenly. You don’t want to be 15 degrees off
axis and only hear the mid-range. Ideally the speaker’s dispersion
is even across as much of the response as possible, but usually this
can only be done starting in the upper bass.
Duke: Yes! The radiation pattern plays a far greater role in
what we perceive than most people appreciate. Most of the sound we
hear in most home audio systems starts out being dispersed off-axis.
Erik: Next, the wider the dispersion, the more early reflections
you may encounter, which can severely affect the frequency response
and imaging.
Duke: Agreed. In general early reflections are detrimental, but
later reflections can be quite beneficial.
Erik: The very large diaphragms of ESL speakers (Martin
Logan/InnerSound, etc.) have fabulous clarity thanks to this effect.
They can sound like you have headphones on even with very little room
treatment.
Duke: Dipolar electrostats typically have a narrow radiation
pattern that minimizes early reflections, and then have a
spectrally-correct backwave that reflects off the wall behind the
speakers. If the panels are far enough out into the room, that
backwave energy is sufficiently late-arriving to be quite beneficial.
Erik: Drivers with different dispersion patterns _may_ also have
different rate of decay. Consider a hybrid ESL + cone woofer. The
woofer radiates omni-directionally and the wavefront looses energy
the fastest, while the ESL panel is a plane wave, with narrow
dispersion and looses energy more slowly.
Duke: This would be an example of an extreme radiation pattern
discrepancy (narrow-pattern line-source panel combined with
quasi-omni point-source woofer). Unfortunately it is not possible to
equalize such a system so that the first-arrival sound and the
reverberant sound have the same spectral balance (something that
would be psychoacoustically desirable – I can explain why if anyone
is interested). The best we can hope for is a juggling of tradeoffs
that works at our chosen listening distance. In general, the lower
the crossover frequency between woofer and panel, the better, as long
as we don't overly compromise something else.
Erik: Two of the most important measurements for me are
Cumulative Spectral Decay and compression. The first measures energy
storage, or "blur" that a tweeter adds to the sound because
it won’t stop fast enough. The second measures how a tweeter’s
response changes at different volumes.
Duke: We want to minimize anything that's a source of audible
coloration, and in the tweeter region, what's happening in the time
domain matters a great deal.
You mentioned compression – great job of paying attention to
something that matters, but doesn't show up on a spec sheet! Thermal
compression effects not only suck the dynamic life out of the sound,
they are often responsible for a speaker system's tonal balance
changing with volume level, which can happen if the various drivers
have differing thermal compression characteristics (which they
usually do).
Erik: Usually when I hear about issues integrating woofers with
very light tweeters it's a frequency response issue, and integration
with the room issue.
Duke: Note that the “frequency response” we hear is NOT the
one in the published curve – that is only the on-axis response,
which may not even be our first-arrival sound if we're sitting
off-axis. And then the reverberant energy – remember that's
usually most of the sound that reaches our ears - may well (and
usually does) have a significantly different spectral balance. At
the risk of over-simplifying, the “frequency response” that we
perceive is a weighted average of the two. So what comes across as a
room integration issue may well have its roots in a radiation pattern
discontinuity in the crossover region. Thus the room gets blamed for
colorations that can be traced back to problems in the radiation
pattern, which in turn are (arguably) loudspeaker design
deficiencies.
A few additional thoughts:
Imo one incorrect approach to speaker design would be this: Take
“the best woofer”and “the best midrange” and “the best
tweeter” and combine them using “the best crossover” and put
them into “the best enclosure”. This can easily result in an
overly expensive speaker whose different parts do not play well
together, resulting in a lack of coherence that becomes distracting
over time. This would be an example of putting the design specifics
first.
Imo one correct approach to speaker design would be this: Choose
all of the components based on how well they will work together with
each other, with the room, and even with the amplifier towards
achieving an intelligently-selected perceptual goal. This would be
an example of putting the system concept first.
So my opinion on tweeters is, they are just one part of a system,
and imo it's not cost-effective to trade off good system synergy for
the sake of using “the best tweeter”. To give an example, I
don't think anyone has accused Andrew Jones of using the best
tweeters, but those of us who compete with him must admit that he is
a master of system synergy.
Given my particular, perhaps somewhat unorthodox set of
priorities, tweeters whose radiation patterns change significantly
with frequency are not the ideal tools for what I'm trying to do,
though they can be made to work reasonably well with careful juggling
of tradeoffs. My preference is generally for tweeters that use a
particular type of waveguide or horn: Constant-directivity with low
diffraction, with the intention of crossing over where the woofer's
radiation pattern approximately matches the horn's. System concept
first, design specifics second.
