Cone excursion

[Ken Pachkowsky]

Quick question for you guys.

My old 4350's had a lot of cone excursion when driven hard. These Westlakes produce awsome low end at hi spl's with no excursion visable.

Can someone explain this to me?

Ken

[Guido]

Hi Ken!

At he tuning frequency of the cabinet there is nearly no cone excursion as al the sound is "produced" through the ports.

Below the tuning frequency the cone excursion increase dramatically.

I think these Westlakes are tuned lower than the 4350s and so they have less cone excursion.

Hope I could help a little bit although my english maybe is a little bit circumstantial.:D

[Ken Pachkowsky]

Thanks Guido.

Your explanation is identical to the one I got from Widget this afternoon and makes perfect sence.

Ken

[Mike Caldwell]

Hello
It's always a good idea to use a high pass filter somewhere
in the signal path that is set just below the cabinets tuned
frequency. This prevents over excursion and increases your amps
head room by not wasting power amplifing signal that can't be
reproduced.

Mike Caldwell

[Alex Lancaster]

I agree absolutely with Mike, more so if You are using turntables (LP's), because of feedback.

About the ports "producing" sound, I disagree, they radiate the "back" wave, and increase the SPL, by using energy that would be wasted in a sealed box, but I cannot see them radiating more than the front of the cone; the above is a little simplistic, and, of course there are other considerations.

Alex.

[4313B]

"About the ports "producing" sound"

Vented/ported enclosures are Helmholtz resonators.

Here's a little blurb that might be of interest (One should be able to find plenty of references on the net) - Loudspeakers (Monitors) - Bass Reflex (http://www.music-recording.com/loudspeakers/bass_reflex.html)

The use of high pass filters for protecting vented/ported systems below resonance has been covered in several other threads. JBL has always recommended their use. I use them for high power applications and when I'm at home listening to unfamiliar source material.

[Alex Lancaster]

[QUOTE]Originally posted by Giskard
[B]"About the ports "producing" sound"

Vented/ported enclosures are Helmholtz resonators.

Which are pneumatic springs, driven by the "back" wave, right?

Alex.

[Mike Caldwell]

Hello
As a side note you can use cone excursion and a frequency
generator or a cd with test tones in at least one third octave spacing on it to determine fairly closely.
the tuned frequency of a speaker cabinet.
By watching the the cone movement as you select different
frequencies when you find the frequency the shows the least
cone excursion with low frequency output that will basicly be the tuned frequency of the box. This test really only works in the frequency range from 20hz up to about 90hz or so, where movement of the cone is the most visiable.


Mike Caldwell

[boputnam]

Originally posted by Giskard
Vented/ported enclosures are Helmholtz resonators.

Helmholtz Resonance
A Helmholtz resonator or Helmholtz oscillator is a container of gas (usually air) with an open hole (or neck or port). A volume of air in and near the open hole vibrates because of the 'springiness' of the air inside. A common example is an empty bottle: the air inside vibrates when you blow across the top, as shown in the diagram at left. (It's a fun experiment, because of the surprisingly low and loud sound that results.)

Some small whistles are Helmholtz oscillators. The air in the body of a guitar acts almost like a Helmholtz resonator*. An ocarina is a slightly more complicated example. Loudspeaker enclosures often use the Helmholtz resonance of the enclosure to boost the low frequency response. Here we analyse this oscillation, informally at first. Later, we derive the equation for the frequency of the Helmholtz resonance.

The vibration here is due to the 'springiness' of air: when you compress it, its pressure increases and it tends to expand back to its original volume. Consider a 'lump' of air at the neck of the bottle (shaded in the middle diagrams). The air jet can force this lump of air a little way down the neck, thereby compressing the air inside. That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring (diagram at right). The jet of air from your lips is capable of deflecting alternately into the bottle and outside, and that provides the power to keep the oscillation going.

Now let's get quantitative:
First of all, we'll assume that the wavelength of the sound produced is much longer than the dimensions of the resonator. For a typical bottle, the sound produced has a wavelength of a few metres, so this approximation is pretty good, but it is worth checking whenever you start to describe something as a Helmholtz oscillator. The consequence of this approximation is that we can neglect pressure variations inside the volume of the container: the pressure oscillation will have the same phase everywhere inside the container.

from: University of South Wales, Physics Department - Link (http://www.phys.unsw.edu.au/~jw/Helmholtz.html)

emphasis, added... ;)

[4313B]

"a container of gas (usually air) with an open hole (or neck or port)."

"I have bubbles in my tummy...It's just air. It's not stink. Promise."

(After a hearty meal of barbecued hamburgers, Jessica Simpson searches deep within herself...and finds gas. Unscented, bubbly gas. Funny, I thought the air was all above the neck…)

[boputnam]

Giskard!! Yer being goofy! We all know where THAT ends...

[4313B]

Oh! Ok! I wasn't real sure and was simply trying to be helpful. I'll delete my post then.

[boputnam]

Originally posted by Giskard
"a container of gas (usually air) with an open hole (or neck or port)."

"I have bubbles in my tummy...It's just air. It's not stink. Promise."

(After a hearty meal of barbecued hamburgers, Jessica Simpson searches deep within herself...and finds gas. Unscented, bubbly gas. Funny, I thought the air was all above the neck…)


Oh! Ok! I wasn't real sure and was simply trying to be helpful. I'll delete my post then. Oh NO!! It is TOO funny...

[JeffW]

Originally posted by Giskard
[B(After a hearty meal of barbecued hamburgers, Jessica Simpson searches deep within herself...and finds gas. Unscented, bubbly gas. Funny, I thought the air was all above the neck…) [/B]

Well, Jessica may indeed be filled with bubbly air, but upon closer examination, I find she also has a good amount of cone excursion.