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A Study of Vibration Isolation for Floor Standing Loudspeakers

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Sound is vibration. It can be the desired vibration of a musical instrument passing to the air and then to the listener’s ears. It can be the wanted vibration of a loudspeaker transducer. It can be the unwanted vibration of a loudspeaker cabinet passing to the floor or supporting surface, which then vibrates in sympathy, producing interfering and unwanted additional signal paths. These phenomena can be measured and assessed including the potential psychoacoustic impact of the additional signal paths. In this study, the author measured the sound and vibration produced by a floor-standing loudspeaker: Vibration induced into surfaces and sound produced in the air, with the loudspeaker mounted on either damped isolators or on solid wood blocks between the speaker cabinet and the floor.
Analysis of the data reveals:

  • Vibration isolation under speakers produces measurable differences compared to wood blocks: more than 15 ms in reverberation time in some frequency ranges; low level artifacts in the waterfall; up to a dB in distortion in some ranges; up to a dB in frequency response.
  • The type of shell construction affects isolation performance — specifically solid barriers vs. limp mass.
  • In general, isolation produces an improvement in performance and potential audible benefits, minding the shell construction of the room.

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Scott Dorsey
Scott Dorsey

Comment posted May 16, 2021 @ 15:36:34 UTC (Comment permalink)

In the audiophile community there has been a lot of unsupported arguing going on about the effectiveness of decoupling loudspeakers from the building structure (to prevent conducted vibrations from vibrating the building and causing secondary radiation from the structure), versus tightly coupling loudspeakers to the building structure (in order to use the heavy and rigid structure as a sink for vibrations).

This paper begins to answer those questions, and has some really good data that is pertinent to a very common construction method.  Mr. Katz does a great job of quantifying the degree to which loudspeaker vibrations are conducted into the resilient building structure and how effective isolation techniques really are.

My suspicion is that a more massy construction, such as a room located on the first floor over top of a slab foundation, would make tighter mechanical coupling more beneficial because the floor would be more effective as a stable sink of vibrations.  It would be very interesting to see additional measurements of different structures!

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Robert Katz

Comment posted November 27, 2021 @ 14:56:43 UTC (Comment permalink)

Hi, Scott. What I've found over time is that the resilient and resonant floor of the second floor of the house is a big contributor to the anomalies measured in my paper. You can walk on the floor and it vibrates a little. 

All my measurements point to the probability that if the floor were rigid, the results would show an even more remarkable improvement with the isolatotrs versus the wood blocks. I am firmly in the camp that isolators improve the reproduction over rigidly mounting the loudspeaker to the floor. That goes for thngs like "tiptoes" as well, which couple and decouple at different frequencies. And any engineer can predict that would be a bad thing. Rigid is rigid. Semi-rigid is very bad.
I have other data from my superb listening room on the first floor, which is directly on the cement slab. While waiting for my isolators to be specially constructed (to deal with 300 pound loudspeakers) I had the speakers mounted on wood blocks to get them to the height I wanted. I took measurements. I have waterfalls from that time period. Then, after switching to isolators, I took new measurments and I have waterfalls taken from that period. 
The test microphone is within a millimeter or two of the identical distance for each measurment in the first floor room. The loudspeaker height and position is also within a few mmillimeters. I'm particularly careful. 
If AES permits links, here is a link to an online article I wrote, comparing the two waterfall measurements. The measurement on isolators is distinctly better:

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