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In Reply to: RE: Measurements vs. Reviewers posted by John Atkinson on August 20, 2015 at 04:16:10
I do agree with JA regarding nearfield measurements with one exception , regarding vented enclosures , i do find port output as measured regarding velocity and scaling never really matches anechoic measurements, GP measurements do a better job if weather allows ....
What say you John..
Edits: 08/20/15Follow Ups:
> i do find port output as measured regarding velocity and scaling never
> really matches anechoic measurements, GP measurements do a better job
> if weather allows...What say you John?Scaling the measured port and woofer responses in the ratio of the square
roots of the radiating diameters gives excellent results when there is one
woofer and one port. With multiple radiators, you need to do more of what
Floyd Toole calls "hand waving" to get a measured result that accurately
represents the speaker's low-frequency output.Ground-plane measurements can be very accurate at frequencies where the
half-wavelength is much greater than the height of the radiator above the
ground when the speaker is laid on its side. But you do need good weather
and a fairly large flat paved space.Andrew Jones tells me that the best way to assess the speaker's
low-frequency behavior is to drill a hole on the side of the enclosure
the exact diameter of the microphone capsule and insert the capsule
inside the speaker. But I don't think such destructive testing is feasible
in the context of a magazine review.
John Atkinson
Editor, Stereophile
Edits: 08/23/15
The ported design inevitably fails even without the chuffing because it is inherently a high Q design. The port offers little resistance to the flow of air at its resonant frequency and its harmonics, high resistance to the flow of air midway between them. The resonant frequency is the low frequency cutoff because response falls off at 24 db per octave below that.
Acoustic suspension designs are far superior. They like all woofer/enclosure combinations obey Newton's second law of motion. Damping is controlled by the stuffing which forces the driver to lose energy by having to push and pull air between the fibers creating aerodynamic loss. Q (or b, the velocity related coefficient) can bet set for any value. the spring constant K is a function of the Ideal gas law P1*V1=P2*V2 and of course Force F=Pressure*Area. Acoustic suspension speakers have additional advantages in that the cone is usually heavier to get Fs low and therefore stronger and the restoring force is applied by pressure that is uniform over the entire surface of the cone. This eliminates shearing forces that tend to break up the cone both radially and circumferentially. This is why Acoustic Suspension speakers can theoretically be designed to any low frequency response in a small enclosure. They also fall off at only 12 db per octave which means that they can be equalized until their power handling capacity whether mechanically or thermally is reached.
By contrast the ported speaker has both a b (damping coefficient) and K (restoring spring force) that are highly frequency dependent. They are also much more prone to cone breakup. This is why the measurement of harmonic distortion versus output SPL is so critical. Without it you don't know if what you are measuring is true undistorted bass or doubling.
You should take more time to spin the science and while at it explain how the added Mass (spring) makes them superior ....
Newton's second law of motion is a second order linear differential equation;
F(t)= m*d2x/dtE2 + b*dx/dt +k*x
The problem for ported speakers is that in their case b and k are functions of frequency, in an acoustic suspension speaker they are not.
The increased mass of the cone means that it will be stronger for a given material and therefore be less likely to break up. Cone breakup is a major cause of harmonic distortion. The cone literally flexes by twisting because the mechanical forces on it are not applied uniformly.
This is not a spin, this is college physics for scientists and engineers 101. You can not only find this equation in every such text but the formula for calculating the system resonant frequency and the frequency response as a function of Q.
The chuffing in a ported speaker is due to air turbulence at the entrance and exit of the port. Flaring the port rather than having it end abruptly helps some.
While both topologies offer their plus and minus, Vented speakers can have the port tuning as low as need be, it will require the correct drivers and of course relatively( low FB) large cabinets, there is absolutely no advantage in going sealed over vented , not to mention the additional output afforded by the vent, distortion will be no higher at the same SPL ..
Regards
Edits: 09/09/15
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