Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Not so....

Posted by pburant (A) on April 12, 2007 at 16:41:46

In Reply to: Re: "always thought that compliant material under a floorstanding speaker was a no-no" posted by HumanMedia on April 12, 2007 at 16:22:14:

This is a very simple analysis: compare the mass of the moving portion of a driver to the mass of the cabinet. There is more than an order of magnitude difference.

This is why for some implementations people can get away with hanging speakers.

Now, if you're going to isolate speakers (generally by using compliant materials, but there are other methods), you need to be wise about how you achieve the isolation. You don't want an isolation system that has a resonant frequency in the audible range - the resonant frequency of the system needs to be well below 20Hz - preferably less than 5Hz.

I can see how people might get confused when considering the 'laying flat on the carpet' vs. spiked example. When you lay speakers on a carpet, unspiked, the speakers are neither well coupled nor well isolated from the floor. This creates opportunities for things to go wrong. If you instead spike the speakers to the floor (thru the carpet), you've now effectively coupled the speakers to the floor. Depending ont he floor type, room layout, etc, this will generally increase the apparent bass output at certain frequencies, and there is also a good chance the speaker was voiced to sound best when set up this way.

On the other hand, if you effectively decouple the speakers from the floor (placing the speakers on roller bearings and the roller bearings atop a constrained-layer platform is one such way), you are preventing a lot of the cabinet's vibration from mechanically coupling to the room structure. Particularly with old, wooden floors, this can be a great advantge since the mechanical coupling of the speaker to the floor can have a few very bad effects. For one, mechanical coupling is much more energy efficient than acoustic coupling, so the chances of sending the floor or larger portions of the room structure into resonance are much greater, and this can naturally muddy your bass response (sometimes a contributor to 'one-note bass'). Second, since sound travels much more quickly thru solids than gases, this also allows bass waves to reach the listener via the structure more quickly than thru the air, and thus can have a smearing effect on the sound.

In short, one cannot say for all systems whether coupling speakers to the floor is the best approach or if isolating them instead is better, but both approaches have merit and the entire set of variables needs to be considered for proper analysis. Of course, trial and error usually leads you to the best result most quickly, but my overall point is that you can't say with a blanket statement that decoupling speakers from a floor is bad.

-Pete

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