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Re: Doppler effect, spectral side-bands, spikes

Posted by Arbelos (D) on January 1, 2003 at 16:48:32

In Reply to: Re: Doppler effect, spectral side-bands posted by bdiament on January 1, 2003 at 13:24:27:

Reading your comments, i am asking myself if you really read my post and tried to understand it?

*Audibly* this 'balancing on one spike' method has *very* similar effects, though maybe less in degree.
May you consider then that your working hypothesis is not complete yet? It's better not to discard other approaches based on a not complete hypothesis. As explained, both methods indeed do have things in common and have similar effects.
Balancing a speaker on one spike plus felt induces not much more 'seismic' energy than Roller Bearings BTW.

FM or Doppler distortion is IMO inescapable in the world of acoustics. The question is, do we use the right tools to minimze it? And shouldn't we weigh the FM byproducts by audibility?
Your method (which i proposed too several times at different places) obviously seems to reduce these FM products 'audibly' by making use of a masking effect: A louder tone masks a harmonically close tone. This effect comes into play on a very wide scale, because every partial tone will develop very audible side-bands with *higher* frequency resonances, and will stay audible up to higher frequencies.

An example: a 40 Hz tone with eg. 2nd / 3d and fourth harmonics, FM side-bands in ():
Spikes (200 Hz resonance):
40 (160,240) Hz
80 (120,280) Hz
160 (40,360) Hz

Roller Bearings (2 Hz resonance):
40 (38,42) Hz
80 (78,82) Hz
160 (158,162) Hz

now let's guess from that, which will be more audible....? – even if the higher frequency resonance modes would be more suppressed, which i seriously doubt.

To my ears it simply sounds like that: The sound is freed from a subtle but pervasive resonant mess. Timbres are freed, and the resolution noise floor drops to a surprising degree. The distortion is much more masked by the musical signal.

There is in fact (or IMO) only one movement axis where major Doppler problems occur: In the front to back movement. Vertical vibrations are no big issue, because not much is 'decoded', and the ear is much less sensitive to vertical movement than lateral.
Laterally also not much is decoded, slow movements could be audible, though i have not heard a problem.

But structure born feedback comes in the form of bending waves, where the support points will be moved with a slight delay relative to each other. This will result in rocking modes and higher frequency modes translated to the speaker – even with roller bearings... (but not with one spike supported speakers ;-)
The rocking modes will be audible in the front back plane.

IMO the higher *resonance* and flex modes are messing much more than the stiff coupling below. That's where we maybe disagree.

There is *much* more energy residing within the speaker than within the building structure. And within the building structure, there is much more sourced by the speakers pressurizing the walls and floors, energy coming from the speakers and energizing the support resonmances too.

You still put up a good question with the (hypothetical) seismic vibrations.

Spikes reduce the inlet points and make the vibration modes (hopefully) less complex: moving (turning) a hand quickly balancing a stick couples less of that movement to the stick than having the stick flat in the hand.

Spike / speaker interfaces are definitely *not* rigid. They are elastic with a not infinitely low elasticity. This results in Eigenmodes in more audible ranges, compared to the alternate approach: Instead of fighting elasticity accepting it, and induce very high elasticity, *reducing* the resonace point to the minimum: Roller bearings.

One of the ideas of spikes is to couple speakers and other gear to a higher mass, which *hopefully* should be calmer than the speaker itself...

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