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Forte cabinet vibration and/or audible resonance


sputnik
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I have just joined recently but I have been viewing this forum for three years or so. The amount knowledge and experience here is truly impressive. I hate to ask a stupid question of such an elite group but as I was once told there are no stupid questions just stupid people who ask questions.

Is it noise or is it music? Maybe I just have the volume too high, but at one or maybe two lower frequencies I hear what sounds like resonance from my Fortes - not bad but the speaker cabinets also vibrate and the grill cloth frame buzzes (my horn diaphrams are fine). The cabinets are solid as are the bass and radiator cones. As best as I can describe it, the sound goes from "bum-boom-bom" to a "bwa" sound that you can feel as well as hear things rattling around the house. It doen't sound like a normal progression - it's sort of like a certain low note gets a little goosing. It's especially noticeable when playing certain Enya music with long sustained deep electronic bass. The effect is not specific to any location relative to the speakers, it's pretty much all over. It's very rare, but I don't know if it should happen at all.

I'm wondering if the bass and midrange might be producing the same frequency at the crossover point or is the effect caused by my amps or pre amp (old NAD 2100s and 1600)? I'm curious to learn anything. Thanks.

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I have Fortes as my main speakers and have never noticed anything like that. Are the speakers touching anything? Are they sitting on anything? Do you have another pre and amp to switch in and test with? I think I would start with a switch of amps and see if you can get it to happen. If you can't get to happen again, it is in the amps, if it follows, it is in the speakers. I wish I could be more help.

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The effect isn't at the lowest extreme. There is a great rumbling seismicity down there. This happens a bit higher up and at higher (not quite ear splitting) volumes. I'm not sure if it near the crossover point. I've got a couple of old amps and receivers I could try when my wife is out of the house. Thanks.

I just had another thought. Could the bass driver and passive cone go "out of phase" at a specific frequency at higher volumes?

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I have the same problem. I bought my Fortes used and one of them has a vibration when playing lower frequencies at a moderately high volume. I'd like to know the solution to this also. I don't think it's a problem with Fortes in general, though, since mine I bought in pretty crappy condition (cosmetically).

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I think the principle is that people feel stupid to ask some questions. It far more tragic to have a question and not ask it than to expose oneself to anticipated mockery. But we don't do that. Really. Ask away. Besides, it gives us an excuse to run off at the mouth.

My thought is that the program material contains infrasonics, i.e. very deep bass notes.

In the earlier days of hi-fi, this sort of thing arose even though there were no infrasonics recorded on LPs. You couldn't modulate the groove enough to create them. Nonetheless, record warp, motor rumble, and off-center spindle holes could produce very low frequencies which were effectively detected by the stylus and pick up. Then they were passed to the amp and speakers.

Filters were suggested. We don't see that suggestion these days.

The recording world has changed and it is not uncommon to have strong 30 Hz or lower in a CD. Note the Telarc CD's which have a warning suggesting that speaker destruction is possible at high levels. Maybe Enya should have the same.

Part of the problem in those days was that bass reflex (vented) speaker bass diaphragms were prone to actually try to follow the infrasonics. Sealed box designs were not prone to this. None the less, the Forte with a passive radiator shares many aspects of the vented design. It may be prone to the same issue.

The bottom line is that old problems have returned but the old lessons have been forgotten.

I'd suggest that you take off the front grill to see how much the woofer is moving at the critical moments in the music. Also check the excursion of the back passive radiator. Either one or both may be driven to their limits. This gross excursion should be very visible.

It would be interesting if you report what you find. I might well be wrong.

It could also be that the grill or some other loose component is rattling. It could even been a cabinet door or window frame in the room. A walk around might allow you to identify the source.

Gil

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Thanks Gil, I'll experiment with that tomorrow - the wife is asleep now. I think I have identified most of the room effects - fireplace, shutters, baseboard heat, stuff on shelves etc. The speakers are pristine. The magnetic buttons on the grills do buzz along with this vibration effect. If you put your hand on top of the speaker it is a more pronounced vibration than normal - like resonance. The effect is from the sound producing elements of the speakers and it occurs in both of them. I would not call this a real problem - more of a sonic curiosity. Thanks again for the attention.

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You could always remove the grills when listening to the speakers. You could also try stands and spikes. If the speakers aren't sitting flush on the floor they could be producing some nasty vibrations. Plus Fortes raised a bit off the ground really open up.

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The original Forte' to my knowledge had rubber grill inserts and not the magnetic ones, are these Forte's or Forte' II's? A pair of '87 Forte's that I had recently had the rubber insert type grills. I don't recall what type were on my '88's.

I think the noise that you are noticing is coming from the passive radiator over-excurting during demanding passages at higher volumes because I noticed the same thing on my '88 Forte's on some songs at higher volume levels.

Play the passage and watch the radiator when the noise occurrs and see if it is moving erratically. When you are really jamming the radiator seems to not move evenly since there is no pole piece to guide it.

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Well, l I came home over my lunch hour(s) and tried a few things. I have a test cd that, among other things, produces ascending tones begining at 2 Hz and ending at 22 kHz. The NADs "normal" inputs filter out signals below 10 Hz and above 40 kHz. The woofer cones and radiator were oscillating slowly (sewing machine speed) in the very low range, well below 32 Hz. No sound but I didn't know they did that - thanks for pointing it out Gil. Nothing obscene though like they were going to shake out or something.

Next, I sat next to the speakers and barely touched the active and passive cones with each hand (not quite exactly like Mr. Spock doing the Vulcan mind meld with the Horta). As the tones got higher, both active and pasive cones vibrated faster with decreasing travel (is that excursion?) till sound became audible. They seemed to react in a similar fashion until that one frequency (where the slight vibration happens) when the passive cone stopped vibrating. Just the opposite of what I was thinking - not over excursion but none or hardly any. The passive cone vibration resumed only faintly with higher frequencies til the mid range took over. This did not happen at a more mature (lower) volume.

I notice this on LPs as well as cds so the source isn't the cause. Where I notice this most easily is an 1986 Enya cd - its just titled Enya. The cover is a black and white photo of Enya in a little black dress kneeling beside two mean looking dogs - to me, it looks like she is about to say sic'em. The effect occurs at 20 seconds and 50 seconds into track 9 "Epona" and also at 1:40 in track 12 "Boadacia". The super low stuff on track 14 "Dan Y Dwr" sounds full and potent. If anyone has this cd and Fortes, I would like to know what you hear.

At higher volumes and at a specific frequency could the passive radiator just not correspond with the active cone and just sort lose sympathy with the pressure changes in the cabinet and cause the cabinet to "hum"? Or, is it that infrasonic effect? The whole thing goes away at less juvenile volumes so it's not really a problem - I still love these speakers in this set up in this room.

My speakers are Forte IIs. On a side note, the little magnetic buttons on the grills can spin and move and that sort causes a tamborine effect at high volumes with vibration. Scotch tape over the magnets fixed that.

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----------------

On 2/23/2005 11:08:41 AM Frzninvt wrote:

The original Forte' to my knowledge had rubber grill inserts and not the magnetic ones, are these Forte's or Forte' II's? A pair of '87 Forte's that I had recently had the rubber insert type grills. I don't recall what type were on my '88's.

My 1 series grill frames are made of plastic not rubber. They have knobs that push into sockets in the face of the speaker. One of the frames is slightly bent out and the top and bottom, this makes it a little harder to get it to fit correctly.

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