Myth: "My studio must have a floating floor!"

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Myth: "My studio must have a floating floor!"

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Postby Soundman2020 » Sat, 2020-Jan-25, 00:22

A common belief among first-time studio builders is that they have to "float" their floor to stop sound getting in and out... because "everybody knows" that's the right way to do studio floors!

Except that it isn't...

This is a myth! You do not need to float your studio floor (with very, very few exceptions)

The vast, vast majority of studios (even high-level professional studios), do not have floated floors. Some do, yes, but only when very high isolation is needed, or under special circumstances, such as if the studio is built on an upper floor in a commercial building, or very close to a railway line, a major road with heavy traffic, an airport, or another source of loud, low frequency vibrations.

So why do I say that it is a myth? Doesn't a floating floor isolate the studio, so sound can't get in and out? Yes it does, but most studios already have enough isolation for the purpose, and do not need more. And for the few that do need it, it is complicated to do, and expensive. High isolation is expensive.

So let's start with that: How much isolation do you need for your studio? Let's assume, for argument's sake, that you plan to record and mix live rock bands in your studio, and you are concerned that this will annoy the neighbors. A typical rock band can put out a level of around 115 dBC (decibels measured on the "C" weighting scale) inside the tracking room / live room / rehearsal room, and maybe heading for 120 dBC. But let's call it 115, to make the math easy, and also because that is realistic. On the other end of the scale, many noise regulations set a maximum allowed level of something like 40 to 45 dBA (decibels measured on the "A" weighting scale), measured at your property line. That's how loud you are legally permitted to be (regulations are different all around the world, so do not assume that this hypothetical situation applies to you! Check with your local authorities to find out what the level really is for your place).

Now, you might assume that you can just subtract one number from the other, and that's how much isolation you need. So you might think "115-45=70, so I need 70 dB of isolation". Not correct. For two reasons:

1) Distance. Sound level naturally drops off with distance, at a theoretical rate of 6 dB each time you double the distance, but in reality it is a bit less. maybe 4 to 5 dB. Let's call it 5. Thus, if you measured a level of, say, 55 dB outside your studio when you are standing two meters (6 feet) away from the wall, then the level would naturally fall off to just 50 dB at twice that distance, which is 4 m (12 feet), and by the time you get 8 m away (24 feet), the level would be down to 45 dB. So as long as your property line happened to be 8m away from the studio, it would be OK to have levels of around 55 dB outside the walls. Distance is your friend, here.

2) Weighting scales. Note that the measurement inside the room was "115 dBC" and the legal limit was "45 dBA". Those are two different measurement scales! The "C" scale is used for measuring very loud sounds, because "C" is similar to the way the human ear perceives loud sounds, while the "A" weighting scale is used for quiet sounds, once again because the "A" weighting curve closely approximates how our ears and brains respond to soft sounds.

So what's the difference between these two scales?

This is the difference:
SOUNDMAN-recording-studio-acoustic-design-SPL-DB-Weightings--A-and-C--scales.gif
SOUNDMAN-recording-studio-acoustic-design-SPL-DB-Weightings--A-and-C--scales.gif (38.23 KiB) Viewed 480 times
SOUNDMAN-recording-studio-acoustic-design-SPL-DB-Weightings--A-and-C--scales.gif
SOUNDMAN-recording-studio-acoustic-design-SPL-DB-Weightings--A-and-C--scales.gif (38.23 KiB) Viewed 480 times
The green curve shows the sensitivity for "C" weighting, and the red curve is for "A" weighting. You can see that there's a very large difference between them for low frequencies, but for medium and high frequencies they are very similar. In fact, above about 800 Hz they are practically identical. Clearly, when you set a sound level meter to "C" weighting, and use it to measure the sound of typical contemporary music, then switch it to "A" weighting and measure again, it will show a much lower reading on "A" than on "C".... because on "A" it is not taking into account most of the low end: drums, bass, throbbing keyboards, roaring electric guitar, etc. It is just not very sensitive to those sounds, on "C".

This is GOOD for you! Because even though you are producing 115 dBC inside your studio, if you do get a visit from the cops because someone complained about your music, the cops would have to use the "A" scale to measure the level! Not the "C" scale. So their meter, set to "A" would show a level that is considerably lower than your meter set to "C". Depending on the type of music and how far away you are, the difference could be as much as 20 dB, or maybe only 5 or 6 dB, but there will be a difference, and it will be in your favor.

As an aside: you might want to know WHY we even have these two curves? Why not just use one single curve? Because human hearing is not linear, and works differently for loud and soft sounds. Many years ago, way back in 1933, two scientists, Mr. Fletcher and Mr. Munson, did some research on a bunch of people, to find out how sensitive our hearing is to different sounds. They discovered that it is not linear: we are more sensitive to some frequencies, and less sensitive to others. Not surprisingly, our greatest sensitivity is in the frequency range of the human voice, and we are less sensitive outside that range. In other words, we hear best at the frequencies that we use to speak. But they also discovered that this is not a "constant" relationship: it changes with the loudness of the sound. For very loud sounds, we hear all frequencies pretty much the same, but for very quiet sounds our hearing is much less sensitive to low frequencies. They then drew out their results as a series of curves on graph paper, and that become known as the "Fletcher-Munson curves". Later, other scientists did additional studies and refined the curves a bit, so now they are just called the "equal loudness curves".

This is what both sets of curves look like:
fletcher-munson--and--equal-loudness-curves-together.jpg

The blue ones are the original Fletcher Munson curves, and the red ones are the updated Equal Loudness curves. They show how sensitive our hearing is for different frequencies. at different levels. They gave names to these curves, based on the spot where they cross the 1 kHz frequency point. So, the "100 phon curve" shows how your hearing works for very loud sounds, such as a rock band inside the room where you are, playing at about 100 dB, and the "40 phon" curve shows how your hearing works when listening to that same rock band playing the exact same song, but far, far away, where you can barely hear them.

Now for the interesting part: You can see that the "A" weighting curve for sound level meters is sort of an upside-down version of the 40 phone Equal Loudness curve, and the "C" weighting curve is an upside-down version of the 100 phon curve. If you noticed that, and thought they might be related, then you are right! "A" weighting is, in fact, the 40 phon curve! And "C" weighting is the 80 phon curve. (They are flipped upside down because the Equal Loudness curves show LOUDNESS, while the weighting curves show SENSITIVITY, which is the inverse of loudness).

So, that's why all good sound level meters have a switch to select between "A" and "C" scales; so you can validly measure how your own ears perceive quiet sounds ("A") and loud sounds ("C").

But what does all this have to do with deciding if you need a floating floor, or not?

Patience... we'll get there...!

So far, we have figured out that you might have levels of 115 dBC inside your room, and that you would need to get them down to maybe 45 dBA outside, but you cannot just subtract those two levels, because one is "A" and one is "C". Also, there is no way to convert between them! This is not like converting between kilograms and pounds where there is a simple fixed relationship (1 kg is 2.2 pounds, always). Because these two decibel scales are measuring two different things, there is no such simple conversion here. It depends on the levels and the frequencies. There could be a 25 dB difference, or there could be no difference at all, and it wold change second by second for any give song.... So you cannot convert between them!

What to do then, to determine how much isolation you need?

Two possibilities: measure both levels using "A", or measure both levels using "C". Both are valid. Personally, I prefer to measure both using "C", to be more accurate, but using "A" is fine too, since that's the level that the cops would have to use, if they came to check your levels with a meter.

Which now gets us back to the point: If you measure 115 dBC inside your room, then flip the meter over to "A" weighting instead, the meter will show a LOWER level. Maybe 5 dB lower, or maybe 20 dB lower, but it will be lower. Let's assume that the difference averages out to about 10 dB (a reasonable guess for most contemporary situations). So, your level measured inside the room might be 105 dB, and you need to get that down to 45 dB at your property line. So you need a TOTAL of about 60 dB isolation between the inside of your room, and the fence around your house. Assuming your studio will be about 4m (12 feet) from the fence, that gives you a 5dB advantage (because as we saw above, sound levels decrease with distance). So, taking that into account, you only need 55 dB isolation (60-5=55).

And now we can get back to your floating floor, and why you probably don't need it.

Most home studios are built in garages or basements. Most garages and basements have concrete floors. Concrete is very massive (heavy). In fact, each cubic meter of concrete weighs about 2,400 kilograms! (about 5,300 pounds). [In imperial units, each cubic foot of concrete weighs about 150 pounds]. So your concrete slab floor is VERY massive: it weighs many tons. And mass is exactly what you need to stop sound. In fact, there are only 4 physical factors that you can use to stop sound:
  1. ) Mass,
  2. ) Stiffness,
  3. ) Damping,
  4. ) Decoupling
In other words, you can (1) build a barrier so massive that sound waves just cannot move it or get through it, (2) you can build a barrier so rigid and stiff that sound waves cannot make it move or get through it, (3) You can build a barrier so "mushy" and soft and "springy" that sound cannot get through it, and (4) you can build two barriers that are so well separated from each other that sound cannot get from one to the other. That's it. There are no other ways of using creating physical barriers that can stop sound. For best effect, you should try to combine a couple of those in a good isolation wall.

Your concrete slab has two of those in its favor: it is very massive, and it is quite rigid. But there's something else: for garages and basements, the concrete floor is almost always done as "slab on grade". Meaning that the concrete rests on top of the ground (usually with a couple of other layers of materials in between). That provides #3 on the list: damping. To understand this, think what happens if you hit a cymbal with a drum stick: it rings out loud and long. But if you then touch it with one finger, the tone changes: it is now dull, and doesn't ring so long. If you put two fingers on it, it rings even less. Because your finger are "damping" the vibrations: they are absorbing the sound. If you got a whole bunch of people to put their finger on that cymbal, so that it was completely covered with fingers, then it would not ring at all! It would just produce a sort of dry "tunk" sound when you hit it with the drumstick, but it would be very quiet, and very dead.

Your concrete slab rests on Mother Earth, which is like having millions of fingers resting on it from below. In fact, the entire planet is damping your slab! That's hard to beat. The entire planet is there to absorb all of the vibrations and sounds in your slab. It is very well damped.

Thus, a concrete "slab on grade" floor in garage or basement is about the best you could hope for! It has 3 of the 4 factors that make for good isolation: high mass, high rigidity, and high damping. So it is really good at isolating already! (If you were to add the 4th element (decoupling), that would be a floating floor.)

So how well does a typical "slab on grade" isolate? About 70 dB. There's something called the "flanking limit", which basically measures what the maximum amount is that any given material can handle, and most typical slab-on-grade floors in home basements and garages have a flanking limit of about 70 dB. In other words, that's how much isolation they can provide. Thus, if you could build your entire studio the same way, using damped massive concrete walls and ceiling too, then your studio could provide 70 dB of isolation.

But you don't need that much! We already saw that a typical home studio needs maybe 55 dB of isolation, and that is plenty. So your plain old concrete slab floor already provides much better isolation than you need.

Therefore, you do not need a floating floor!

End of story.

Forget the "floating floor" myth.

To recap: The only studios that MIGHT need proper floating floors, are ones that:
1) Need more than 70 dB of isolation
2) Do not have a concrete slab-on-grade floor.
3) There are high levels of vibration in the slab coming from external sources.

That's it! The second situation here might happen, for example, if somebody wanted to build their studio on the seventh floor of an office building. Even though the floor is going to be a concrete slab, which is good, it is not being supported by the ground any more. So it loses two out of the three advantages it had: it no longer has the entire Planet Earth damping it, and it is no longer rigid. Now, it acts more like a drum head: a membrane stretched across a frame. So it no longer has a flanking limit of 70 dB: Maybe 40 or so. Thus, in this situation it would probably be a good idea to install a proper floated floor, to attain the high level of isolation needed.

Forget about floating your floor. You likely do not need it. (and even if you did, it would be VERY expensive and complicated to do...

Just to put all of this in perspective, this is what a properly floated floor looks like:
properly-floated-floor-01--concrete-slab--MED-ENH.jpg
Note the thick reinforced concrete slab, and the thick steel girders and framing. And here's a close-up of that thick steel frame, along with the large steel coil springs:
properly-floated-floor-02--spring-detail--MED-ENH.jpg
. Anything less than that is not going to work.

Myth: BUSTED! :)



- Stuart -




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