Hmm. jag kanske ska klistra in hela meddelandet istället för att klippa bort delar av det (tyckte bara det tog så stor plats då):
The number one misconception about woofers is that moving mass has
something to do with "speed" of response or its high frequency limit.
It does not, at least directly.
What mass effects is efficiency and also the shape of the low frequency
roll off.
A radiator that is small compared to the wavelength it is producing,
experiences an increasing acoustic load with increasing frequency (one
of the few places on sees a frequency dependent resistance with out a
reactance). That radiator in an infinite baffle, driven at a constant
velocity
will produce a +6 dB oct rising response because its radiation
efficiency increases with frequency.
To make a driver like this have "flat response", we must roll off the
voltage response 6 dB per octave, one pole.
This is done by having a much larger amount of mass or weaker motor such
that above some point velocity control is lost and then with increasing
frequency, the velocity falls 6 dB oct. It is this range, where the velocity falls 6 dB /oct that one has flat response.
Electrically, this speaker looks like and acts like an R/C filter, the R
is the coil R+the amplifier source R and the C is the moving mass of the
speaker reflected through the motor, which looks /acts like a capacitor.
Since the moving part is the C, it is the voltage across it (the voltage
/velocity output) which falls 6 dB /oct, which is canceled out by the
changing radiation resistance and now gives flat response. The -90
degree phase shift of the slope is not canceled out as the radiation
resistance is pure resistance. Changing the size of the C (moving mass)
has no effect on the slope angle, just its level and starting point.
99% of the people do not realize that when a point source has flat
response, its mid band acoustic phase lags behind the input signal by
about -90 degrees (once all fixed time delays are accounted for). This
broad band lag is equal to a time delay who's amount increases with
decreasing frequency. This -90 degree operation is how some of the
simpler measuring systems "determine" acoustic phase, it is a Hilbert transform of the amplitude and at low frequencies for a simple piston,
this is a safe assumption.
Consider how a normal "perfect" speaker spreads out a signal in time.
Make an imaginary signal that has equal amplitude content from 100 Hz to
25 Hz, a specific waveshape which has this property.
Take an imaginary perfect flat response speaker who's upper and lower
cutoffs are way past our needed bandwidth.
This mass controlled "flat" response speaker has a -90 degree lag or
delay, at 100 Hz the phase shift is equal to a source 2.83 feet behind
the speaker cone, at 50 Hz, the delay is equal to 5.66 feet, at 25 Hz is
equal to 11.32 feet and so on.
This test signal's wave shape defines the input "time" of each frequency
component.
When reproduced, the highest frequency component at 100 Hz emerges from
the radiator 2.5 ms AFTER the signal arrived at the driver terminals.
At 50 Hz, this component emerges 5 ms AFTER the signal hit the terminals
and at 25 Hz, the signal emerges after 10 ms and so on.
With the driver spreading the signals frequency components out in time, it is simply not possible to retain the same waveshape as the input
signal, lower frequencies arrive progressively later in time than the
original signal.. Any signal reproduced is done so with the spectrum
rearranged in time by the drivers acoustic phase response.
If one had a driver which had a very strong motor or a normal motor but
very low moving mass, one gets an "over damped" response.
This term is from filter design meaning that it is not optimally flat,
excessively damped, rolling off too soon and gradually
Should the slope of the response reach 6 dB per octave, the driver is
operating in the Velocity controlled mode, while the response is not
flat, the acoustic phase DOES track the input signal (zero degrees) and
the different frequency components are not spread out in time.
The waveshape of the input signal is more closely replicated as the
frequency components are in the original "time" although the amplitudes
are off 6 dB/oct. Each 3 dB /oct change in the slope produces a 45
degree change in phase. An over damped response more closely retains the time information where
a flat amplitude response cannot.
A proper LF horn can have flat acoustic response AND roughly zero degree
acoustic phase.
For a person more sensitive to "time errors", they will likely find an
over damped system more realistic.
For a person more sensitive to "amplitude errors" the traditional "flat
response" system will be more satisfying.
For the person lucky enough to have heard a proper lf horn system, you
have heard that one can have "lightning fast" sounding bass and still
make your pant legs flap.
A normal "flat" response point source speaker HAS this kind of delay
built right in and it is unavoidable (currently).
All conventionally driven point source speakers MUST have the phase
shift / delay if they are to have flat frequency response (dictated by the falling velocity, acceleration controlled response needed to offset
the changing radiation resistance with frequency).
Additional reactance's can alter this phase relationship.
For example above midband at the point in the impedance called Rmin, the
electrical series "L" is equal but opposite the reflected moving mass
(capacitive reactance) of the driver thus canceling each out and being
resistive (no phase shift).
Above that frequency, the series Inductance dominates and produces a
roll off with an inductive reactance.
At the bottom end ot the response, for a simple sealed box, there is a
point where the parallel spring or "compliance" of the box and driver
are equal but opposite the moving mass and this point is also resistive
(at box resonance). Below that frequency, the spring constant dominates
(an inductive reactance) and the acoustic phase leads Remember that it is acceleration which produces sound, it is the
amplifier current which produces the force which produces the
acceleration.
While few of you may have a TEF machine or are able to measure real
acoustic phase, most of you can plot the current phase angle with
respect to the voltage drive signal for a woofer. The phase shift curve
of the current vs freq will have the same shape as the acoustic phase
shift (although the degrees are different). If one had a speaker that
was flat midband, one could look at the current phase in that range and
assume the acoustic phase magnitude was about -90 deg .
This acoustic phase IS time, it is essentially ignored in discussions
about how speakers sound, yet it accounts for most of what you guys and
others are talking about.
och
In reality, for a point source (like a woofer who's diameter is small
compared to
the wavelength), it is not the displacement of the radiator but it rate of
acceleration which is proportional to radiated acoustic pressure (while the
pressure inside a sealed IS proportional to displacement).
While an acceleration based response is hard for many to accept, even harder
is
the implication that adding mass to an acceleration controlled radiator DOES
NOT
effect its speed of response or quickness (but does effect its sensitivity).
For the big heavy slow woofer and the quick mid bass speaker, it is its
electrical inductance which governs its "quickness", the larger woofer has
more
inductance and is therefore "slower".
Nå, vad säger ni?
Jag tror han pratar om bashorn vs sluten/basreflex.
well well...
Sen är jag lite tveksam till vissa parametrar... Kan man ha så lågt Qts som 0.14? Har aldrig sett det tidigare:
