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Speaker supports, FM modulation & browsing audio science books...

Posted by Arbelos (D) on January 8, 2003 at 15:17:54

In Reply to: Re: Doppler effect, spectral side-bands, spikes posted by bdiament on January 2, 2003 at 07:07:01:

i found that i missed one aspect of FM modulation: The 'spread' around reproduced partial tones depends on the velocity of the vibration of the speakers. As far as i found the formulas, velocity increases with frequency, with amplitude being constant.
But AFAIK velocity is proportional to SPL, and FM modulation should therefore be proportional to SPL..

I really am convinced that there is no absence of vibration in audio systems, there are simply different ways of shifting (or unifying) the resonance frequencies.

Spikes or other feet try to 'hold' speakers (or other gear), which rigidly links the speakers to their specific support resonances, activating these, specially when these resonances lay within the reproduced spectrum of the speakers. This is clearly the case with every medium hard compliant or 'stiff' support like spikes. (I'd say 50 to 300 Hz). But much less with the VLF resonance of roller bearings.
This results in the above mentioned FM-modulation side-bands, their 'spread' however reduced by the relation of (vibration velocity ) / (speed of sound).
Optimistically (for spikes) assuming equal (actio/reactio) excursions of the different support methods, and hence supposing same levels of FM distortion, the very low frequency resonance keeps this 'spread' extremely narrow, so that it will be much more efficiently be masked by each partial tone of the speakers input signal. I hope you can follow this explanation.

My feeling is that probably in effect the amplitude of vibrations with the roller bearings is even lower than with spikes. But at least it is not an order of magnitude higher, so the advantage of narrower FM side-bands will be kept.

At first the surprising point studying my AES etc. material was that the most sensitive zone of the ear is in the ca. 10 Hz range, ie. exactly in that VLF range...!
But then i realised, that resulting FM side-bands of the eg. 3 Hz wobbling supports would only be close to that sensitivity zone, if the speakers would move with the velocity of the speed of sound... quite improbable.

The tipsy 'balancing on one spike' support does *indeed* react quite similar to roller bearings. Where the speakers stay in vertical axis with roller bearings, when you push the speakers in the middle, they simply tilt with the one spike support. The difference as explained above (low vibration amplitudes IMO) will be quite small, though the felt pads provide an amount of damping which has its pros and cons. Both supports simply *yield*, returning with a very low frequency.
As to wall flexing modes (feedback due to room pressurizing by the speakers), ironically these would be probably even better suppressed by the one spike support, due to only one 'stiff' vertical vibration input path, the speaker balancing rel. calm atop.
The roller bearings are rel. stiffly coupled to bending waves from the support (vertical movement out of phase for the three or four feet).
IMO the roller bearing sonically improves upon the single spike support to a certain, though not vast degree.

Single spike support sounds similar in effect, as you precisely described: Like a very efficient reduction of subtle feedback in the room, an increase in focus and transparency. No doubt a bout that.

If seismic energy would be at work, playing the speakers louder would improve clarity and reduce the assumed negative influence of subterranean vibrations relative to the speaker vibrations, but IMO it does not. Though improving transparency at low levels **too** is a key feature of such a yielding support.

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