WOW! This antique technological backwater has been honored with a post
from Dick Pierce. I haven't seen a post by Dick for years in any of the
groups I frequent, but for my money Dick is easily the most knowledgeable
speaker guy I have encountered. I was troubled by the obvious
inaccuracies of "Sanders" post, but I don't know enough about speakers to
post meaningfully on the subject, thanks to Dick for giving us the
straight scoop.


Regards,

John Byrns


In article ,
(Dick Pierce) wrote:

Sanders wrote in message
...
Foam surrounds and ports will never give accurate response. It only
allows for a slower base response to go lower in a smaller box. To
understand that after each note, the speaker must get back to "0" as
fast as possible. Foam surrounds need a sealed box to do this. Good
bass reflex speakers count on a rigid surround and tuned port to
accomplish the same task.

Sorry, but this is a load of nonsense on several points:

1. MOST foam surrounds have a lower mechanical compliance or,
equivalently, higher mechanical stiffness than typical "rubber"
(actually one alloy of polybutdene or another) surrounds.

2. In most resaonble high compliance woofers that one would find
in a vented enclosure, the surround is NOT responsible for
most of the mechanical stiffness anyway, it's the centering
spider.

3. Your analysis that "the speaker must get back to '0' as fast
as possible", while it might seem intuitively correct, is wrong.
Once you are above the mechanical resonance of the loudspeaker,
the driver is MASS controlled, NOT stiffness controlled, as you
assert. As such, it is NOT the ability to return to a given
position, but the need for acceleration. Now, as we hopefully
remember:

F = ma

Where F is force applied by the voice coil, m is mass of the cone
and a is it's acceleration. Notice the complete absence of a
stiffness term: the suspension does not playb a dominant role at
all over most of the operating bandwidth of the speaker.

And it does NOT need to accelerate "as fast as possible," but
only at the rate determined by the incoming signal.

At resonance, the speaker is predomintly resistance (either
mechanical or, for the most part electrical) controlled and ONLY
below resonance is it stiffness controlled and only then does the
suspension's stiffness play a role. And, at that point, it is only
the magnitude of the stiffness that counts.

4. If your assertion were correct, all drivers with high compliance,
low stiffness surrounds would have worse transient response than
stiffer drivers. Yet drivers with high compliance/low stiffness
surrounds, as a general rule, tend to go deeper in the bass, for
the simple fact that their mechanical resonances are lower. With
a lower mechanical resonance, the badnwidth of the system is wider,
and wider band systems, all else being equal, have better transient
response, as defined by the fundamental relation:

dT * dF = 1/2

where dT is the uncertainty in time and dF is the uncertainty in
frequency (e.g. bandwidth). What this fundamentally means is that
wider bandwidth systems (large dF) have better transient capabilities
than narrow band systems (small dT).

5. Optimally tuned reflex systems do NOT require suspensions with high
stiffness, as you claim, they require suspensions of the correct
stiffness, which is dependent upon the entire system design. One
can (and many people successfully have) designed reflex systems
using high-compliance drivers. In, for example, the classical lossless
B4 maximally flat alignment, one of the requirements is that the
driver compliance be about the same as the driver compliance. That
means that if you are designing a system with a large cabinet volume,
as a means of positioning your self high on the efficiency side of
the efficiency/bandwidth/volume relation, you MUST have a high-
compliance driver. Otherwise the driver resonance will be too high,
tuning will be significantly compromised and the system will, in fact
have fairly dismal transient response, contrary to your assertion.

6. Improvements in transient response of reflex systems can be had
by moving in the realm of QB3 alignments, which can only be had
with low-Q, low resonance drivers, which is, despite your assertion,
contrary to the notion of stiff suspensions. All other things being
equal, the stiffer the suspe4nsion, the higher the total Q and the
higher the driver resonance.

7. Please explain what the difference is between a "ported speaker"
and a "bass reflex speaker." (subtle hint: there isn't any)


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