Center Speaker Resonances

[Retread]

This is a question about mounting the Ventriloquist center speaker, specifically how rigid, flexible, or damped the mounting needs to be.

Based on Dr. Hsu's comment on the speaker stuffed so full of fiberglass that the ports are inoperative (and that the newer ones have no ports), I'm assuming there are no issues on spacing at the rear of the speaker.

In the dim past I had a job doing shake tests of electronic equipment. I had a huge 10,000 force-pound shaker that could put serious hurt on equipment. Some equipment was fairly flat relative to the other dimensions and had corner mounts. When mounted to the head of the shaker and vibrated, the equipment would hit a resonance, slap against the shaker head, and throw parts all over the facility. From that I learned to appreciate the fact that resonances in mountings could have some fairly serious consequences in what appeared to be rigid bodies.

So, my questions:

1) What if the center speaker is suspended on a completely un-stiff medium, such as a pair of ropes? Is the mass of the box such that there is an inherent resonance in the audio range?

2) What if the center speaker is mounted on fairly rigid objects? I have a 10' ceiling with 4"x12" wooden beams. My center speaker is mounted to a beam centered on my front-projection screen. I drilled four small holes in the speaker top and bottom and bolted on short segments of 1" perforated steel strapping, one inch for the two top straps and about three inches for the bottom straps. These I bent to just the right angles to point downward to my listening area and bolted the whole thing to the beam. Does anyone have any idea of the liklihood of this assemblage resonating in the range of 100 Hz?

3) Has there been any guidance on the use of dampened mounting for the Ventriloquist center speaker when mounted to a rigid surface?

[johnnyfulcrum]

hummm.. I'm not at all an expert - but I wouldn't think you'd have to worry. With Music or HT, wouldn't the frequencies be changing enough that it would be pretty slim that you'd drive a resonance in your mount. maybe with a pure tone of just the right frequency - but then again - a plastic mount probably is not the best structure (unlike a crystal sturcture in a fine glass...) to set up a resonace....

plus the power in the system is (i'd think) no where near "10,000 pound force" - put you mention a physical contact with the head of the shaker - I bet most things would shatter hitting a shaler like that - resonance or no resonace...

just my two cents....

johnnyf

[Retread]

The thing about resonances is that any stimulous stimulates them. One may not perceive the response because of the short duration of the stimulous, but it's there.

When I was doing the "shake, rattle, and roll" testing, we used both sine and white noise waveforms. The shaker was driven by racks of huge vacuum tube servo amplifiers with response down to DC (the techs liked to set it for about a half cycle per second with a throw of about six inches for demos). There were banks of displays showing the amount of energy going into each third-octave. We'd start a white noise test at low level and gradually increase the level to Apollo vibration specs (15-G, if I remember correctly). Any resonances would show up in the displays. If the electronics chassis slapped the shaker head, we were guaranteed to have destruction. However, in a lot of cases the display for a particular resonance would zoom off-scale under white noise. When that happened, we could save the item if we could get to the shut-off switch in time. Mostly we couldn't get to it in time. Zoom-to-destruction would happen in a fraction of a second, and the item would be irretrevable.

My mounts are steel strapping bolted to wooden beams. The steel straps are fairly rigid. I think rigid mounting tends to overwhelm the natural resonance of the mounted object and translate resonanace to that of the mounts and mounting surface. But they can introduce resonances of their own. Idealized mount damping tends to isolate the mounted object from the surface to which it is mounted and let the object assume its free resonance. All of which is why I'm curious about the natural resonance properties of the center speaker as a complete unit. I'm wondering if it may be appropriate to use rubberized shock mounts to isolate the speaker. But if it has a natural resonance in the audio range, that might not be good.

[johnnyfulcrum]

I may need a definition of white nooise - but it it's a bunch of (constant) random frequencies mixed together - then you have a constant driver for resonance. where in music in HT - I'd think you'd have constantly changing frequecies - not one "hanging" around enough to drive the material into resonace.

?

[Retread]

Originally posted by johnnyfulcrum
I may need a definition of white nooise - but it it's a bunch of (constant) random frequencies mixed together - then you have a constant driver for resonance. where in music in HT - I'd think you'd have constantly changing frequecies - not one "hanging" around enough to drive the material into resonace.

?

White noise is random energy with uniform distribution. Pink noise is white noise that is frequency-conditioned to fit with the usual way of measuring audio systems. They both contain all frequencies within the bandwidth of interest.

Music, when looked at with a spectrum analyzer, also pretty much has all frequencies within the bandwidth of interest. If you look at it within 1/24 octave bands, you'll still see a lot of energy in most bands.

All it takes to stimulate a resonance is some energy at its frequency to kick it a bit. That's why you get room resonance effects.

Example: my wife was listening to a Chris Isaak CD and complained of a buzz at a particular point. I shut off the CD, played a sweep signal, and spotted a buzz at 158 Hz. It turned out to be a steel J-hook in a steel bracket on the back of a beam behind my right-front satellite. I'd taken down my screen and had not tightened the nut. The J-hook was dancing in the bracket, making the buzz. I had to run the sweep signal to make it buzz long enough to track it down, but the energy in the Isaak song was exciting it enough to annoy my wife.

[Retread]

I'm doing buzz and rattle tracking on my system. Last night I noticed a buzz coming from my Ventriloquist center speaker. It turned out to be the grille on the center speaker resonating and slapping the face of the speaker. I took it off, and that buzz went away. Probably have to put foam padding behind it.

I also noticed a buzz coming from my VTF-3R. Pushing down on it cut down on the buzz. Remembering that Dr. Hsu said that the sub might rattle if you put the spikes on metal, I substituted rubber furniture coasters for the hard plastic furniture coasters I had under it (vinyl flooring over concrete). Buzz went away.

Then I found I had a buzz from another area. Turned out to be the door on a cabinet resonating.

I'm still tracking.

[Sorny]

Tracking down rattles from things in the room can get frustrating. I just pre-emptively put little felt pads on anything that looked like it might rattle. Sadly, I can't do anything about the entire kitchen rattling at ~28Hz in my house... ;)

Sorny