KG 4.2 Speaker Reapir Question

[Murf]

I recently aquired a used set of KG 4.2 speakers. The middle speaker has a 6" tear at the outer edge of the cone of the middle 10" speaker. I am not sure which speaker this is because the KG 4.2 has both an active 10" woofer and a passive 10" radiator.

What is the arrangement of the speakers in the cabinet? How important is it that I repair this speaker (I can't hear any distortion) and what are my options?

[colterphoto1]

6" is a pretty big tear. I would imagine that the upper cone (middle speaker) is the powered woofer. To check, remove it and see if it has a big magnet attached to the back of it. The other cone speaker will not have this, it is the passive one.

Normally, with paper type cone material, a small rip can be repaired with fingernail polish or other hard cement type adhesive. To fix this, you're going to need some material that is very lightweight, strong, and takes an adhesive.

Is there a speaker repair business in your town? Look under PA or musical instument repairs. Or check with Klipsch to see what a replacement woofer would cost.

Michael

[Murf]

Thanks for the advice. I thought about trying to repair it but I hadn't thought of using a lightweight material. Perhaps a piece of silk or mesh might work.

Your help is much appreciated!

[colterphoto1]

You don't want to change the weight of the cone any more than you have to, it changes all the specs of the woofer's performance.

Good Luck

M

[Speedball]

On my 4.2's the bottom one is passive w/o a magnet.

[Murf]

What the heck does a passive speaker do? If I had to guess I would say that since these speakers aren't vented that the passive speaker is there to equalize the pressure as the active woofer moves in and out. But does the passive speaker actually produce any sound?

Murf

[Speedball]

Hello Murf,

I just listened very closely to the woofer and passive woofer and very little if any sound radiated out of the passive. Like you said maybe it is for relieving the pressure or something.

What finish are your 4.2's? mine are Oak with oil finish.

[Murf]

My 4.2s are also the olied oak cabinet. Beautiful finish and great sound.

The passive and active speaker is new to me. I am convinced that the passive and active speaker allow for pressure to equalize between the two speakers. This would allow for the woofer to be played at high volume without blowing the speaker and would probably also buffer the sound of the speaker creating a richer and more natural sound. Of course this could be complete BS!

Thanks for the feedback.

Murf

[T2K]

So what are passive radiators, what do they do, how do they work, and what are the pros and cons of using them?

1. A passive radiator is essentially a weight on a spring. The weight is the combination of the cone material and whatever mass loading might be used - this is the critical part of PR design and must be correct for the part to function properly. The spring is partly the stiffness of the suspension materials (which are usually quite flexible), and mostly the air trapped inside the cabinet. (Later I will mention the passive radiator system as a sub-class of all "vented" systems. They are mathematically identical, the typical vented system using a "port," or hole in the box, with a tube attached to it, to create the "mass and spring" effect - the mass being the air column in the tube)

2. A passive radiator is tuned (by the mass loading) to resonate, like a tuning fork does when its tone is played on an instrument, at a frequency below the active woofers linear response range. Unlike the tuning fork, which is designed to resonate very loudly at one specific frequency, with a response which falls off very rapidly above and below it, a PR will respond to tones on each side of its resonance with a carefully controlled roll off. These are typically 18 dB/octave slopes, which while pretty steep, give the device a useful range of about half an octave.

As I said, this tuning is below the "normal" response range of the active woofer. All woofers, as the frequency gets lower and lower, eventually make less acoustic energy (noise, or music if you are tasteful!). The frequency at which they start to drop in output is usually described by a slightly lower point, called the "-3 dB" point. This is the frequency at which the woofers output is 3 decibels lower than the typical pressure it produces over its usable range. By tuning the PR so it becomes excited at a frequency below this, it contributes to the systems total output by adding about a half octave of bass extension to the speaker. Its tuning is such that at any given point, the combined output of the passive and the woofer adds up to the level produced by the woofer on its own at higher frequencies. Of course, below the PR's resonance, reponse falls off pretty sharply, but "between" that resonance and the woofer normal roll off point, is a small amount of bass repsonse that other wise would not exist.

3. It may seem logical that the passive would operate somewhat like a lever, with its cone moving out at the same time as the woofer cone moves inward, and vice versa. This is not the case, however. In fact, both cones could move in or out together (this is called "in phase"), or any combination of opposite motions ("out of phase" - the most extreme being "180 degrees" out of phase - like the lever effect described above). Ideally, in order to reinforce each others sound, they would be exactly in phase, but in reality, due to the physics of this sort of resonant system, they are out of phase to some degree most of the time.

It is also interesting to note that at the resonant frequency of the PR, its motion is at a maximum (this makes sense of course), and the woofers motion is at a miminum!

4. This is one of the advantages of this type of system - it takes the burden away from the (typically) smaller diameter woofer of producing the lowest tones (which require the most air movement to be equally loud) and shifts it to the larger diameter PR, which often also has a suspension capable of greater linear excursion (motion in and out). Another obvious advantage is that the point at which the bass response starts to drop off is lower, giving a more extended linear bass response to the system. Also, it can allow the use of a smaller diameter woofer, which results in more accurate and better dispersed response at the other bass and low midrange tones.

5. Unfortunately there are some drawbacks as well. One, alluded to above, concerns the "phase response" of the woofer-PR system. Ideally, all the frequencies produced by an audio system (and the chain of recording gear ahead of it) will be in phase. This means that the compression phase of a wave (see "What is Sound") in the recording environment will result in a compression phase in the listening room, at any and all frequencies. It has been found that so long as the overall phase response does not contain any sudden changes or discontinuities, the human ear/brain system will not notice or be bothered by it. However, when the phase response changes rapidly over a small range, it can be noticeable - some people are more sensitive to this than others. The very nature of the resonant system employed results in phase problems of this nature in this narrow range of tones.

Another problem is that due to the fairly sharp tuning of the PR, that is the way its response falls off rapidly above and below its tuned resonance, the frequencies below its tuning will roll off very rapidly - and in addition, the air in the speaker box no longer acts like a spring to control and restore the motion of the PR and especially the woofer. A woofer in a sealed enclosure (see What is Acoustic Suspension) moves less and less at lower frequencies, because it cannot compress the air in the cabinet enough to move in and out very far, and so it will not tend to "tear itself apart," or produce unwanted high levels of distorted signal as it moves out of control.

An acoustic suspension system also tapers off in its response at only 6 dB per octave. The vented, or passive radiator, system rolls off at 18 dB per octave and thus produce very little usable bass below its resonance - and what it does produce can be rather distorted.

For these two reasons, vented systems in general are a poor choice for small speakers that roll off much above 40 Hz - there is too much signal present,in the range they respond to poorly, for them to work well. A larger speaker will work better with this sort of set up because at the frequencies that might be troublesome, there is rarely any musical signal present.

[Murf]

What a great article!

So is it correct that this works basically like a drum with a a beater and resonant head? The bottom head resonates when you strike the top head. The tuning of the resonant head slightly lower than the top head creates a tone that lowers as it disentegrates. Conversely, a resonant head that is tuned slightly higher than the beater head has a tone that goes higher as it didentegrates.

Murf