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This is an area where a lot of mistakes are made, and it's never good news when a designer / acoustician / structural engineer comes into a studio because there is a noise transmission issue and the conclusion is... "We have to tear it down".
This is something we see every year. Often in DIY spaces - but sometimes in professional spaces that were somehow modified along the years and where the conditions for the floating floor to function have been altered.
You cannot float properly on:
- Any type of foam - with 2 exceptions
- Any sort of rubber
- Hockey pucks
- Tennis balls
- Your grand ma.
Why: because none of these behave as a proper spring system, with a known behavior that can be calculated. Some like Rockwool or foam once loaded will stiffen and the tiny bit of elasticity you had the first days will disappear really fast and transmission of the LF and LMF will be back with a vengeance. All these are good for is attenuating footstep noises, if you're lucky.
What you need to achieve proper floating is:
1 - A Mass-Spring system that can be properly calculated - so a system you have a clear control over.
2 - Products that are designed for this exact task (I will name a few) and come with a clear guide to their behavior under load - which is often the result of intense lab testing and quite a few real-life tests.
3- Seek the lowest possible natural frequency for your system. Never exceed 10Hz within a studio design.
What's the natural frequency of a spring system:
It can be seen as a single degree of freedom mechanical behavior. It's basically the spring's natural oscillating frequency, and is dependent on mass and stiffness. Having this information is absolutely essential, and should always be communicated by the designer every time there is a floating floor calculation.
with f(0)= Natural Frequency in Hz
M= Mass in KG or daN (about equivalent)
The rest of the calculation for a floor is a bit more complex, but if some want then I will go through it at a later stage.
This natural frequency information is very important because it will determine both the areas where:
- transmissibility is >1, so when the floating floor system will actually resonate and transmit more to the structure.
- transmissibility is <1, so the area where the floating floor system is efficiently decoupling.
The natural frequency being the resonant frequency of the system, this is the area where transmissibility is >1. Therefore this must be as low as possible. The transition to the efficient decoupling area is gradual and varies with the type of spring system used.
As a rule of thumb, a floating floor should have at least a 90% efficiency (decoupling value) at 20Hz to make sure it is efficient over the usual musical bandwidth of 20Hz-20kHz.
Which means you should always keep your f(0) under 10Hz.