# Perspectives in Control for Multiple Frequency Regions: Part II



## ejbragg (Dec 13, 2009)

*Part II: Soundproof Construction*

If you've read through Frequency Regions: Theories of Attack, one of the questions a person might have asked is, _"How can a person control ALL those sound personalities?!"_ The answer is in hybrid construction!

In all acoustically-minded construction or treatment, the low frequencies take precedence. This is because they are by far the most difficult to handle. Furthermore, by controlling these frequency types, you will inevitably control the others, either completely, or to a large degree. The reverse is not true: treating for mid or upper frequencies usually does very little, if anything toward treatment of low frequencies.

*Section 1: Transmission Loss*

The first step is to IGNORE the interior acoustics of a room and concentrate on what might be ENTERING the room from outside its walls, and/or protecting other rooms from what might be COMING OUT of the particular room under design. The term _"transmission loss"_ is the term used to define how much _loss_ a material offers during the attempt of a sound to travel through it. In other words, it defines the sound blocking ability, or "soundproofing" quality. Absorption of a wall might help some, but if not applied smartly, absorption will do very little to block sound from one room to the next; after all it is much easier to reflect audio energy than to absorb it.

As earlier discussed, a barrier which blocks one sound might still pass another. In fact, there is no method yet known to man to completely block all sound except for a perfect vaccuum. Perfect vaccuums on earth are very expensive and impractical, so we must devise other methods for transmission loss.

Starting with low frequencies, we must deal with Region I and II Frequency characteristics. The treatment for Region I is to use massive (heavy or dense) material that is extremely stiff, which block vibrations from passing through. Concrete, brick, glass, and rock are examples of heavy materials that are rigid. However, Region II vibrations are generally too fast to block, and they tend to resonate most modern building materials. Treatment in this region requires massive material that is flimsy, thereby absorbing the vibration. We must somehow build a wall that uses both principles.

Furthermore, there is the problem of having wavelengths so long that solid walls are no longer practical.

*Section 2: STC Ratings*

In acoustics, there is a term call "Sound Transmission Class" (STC). The STC of a material is a general transmission loss measurement for a given range of frequencies. Most textbooks offer this info from 125 Hz to 8 KHz - hardly a full spectrum of info, but it gives us an idea. The STC is defined in units of decibels, and the higher the number, the higher the transmission loss. As an example of this measurement in a practical application, Let's look at the low frequency losses of 4 basic materials:

*Frequency, Hz............... 125.......250......500*
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3/4 inch plywood...............24.........22.........27
1/4 inch glass...................17.........23.........25
5/8 inch drywall................23.........28.........33
4 inch solid concrete..........29.........35.........37

As one can see, the lower the frequency, the less the ability to attenuate the sound. The poured concrete wall clearly shows the best losses at the lowest frequencies. If a person were to double the thickness of this wall to 8 inches, however, the STC rises (theoretically) by a small amount. The result:

8 inch solid concrete..........35.........41.........43

Doubling the thickness only gains an additional 6 decibels of attenuation. Although this is in reality a decent amount of loss, the ear discerns sound on a logarithmic scale, and so the actual audible loss is only about 25% (a subjective figure). Isolation in a competitive studio is upwards of 50+ STC (across all frequencies, averaged). An STC of around 58 is often considered excellent. To attempt to achieve this figure in strictly a concrete wall is not only impractical; it is impossible... Let's argue that you have the funds and room to pour a 128" thick concrete wall surrounding your favorite room (that's 10 ft, 8 inches). There will still be certain frequencies that penetrate simply because the material chosen is a poor barrier against Region II frequencies. There WILL be penetration in the form of vibration.

*Section 3: IIC Ratings*

There is another class of measurements called "Impact Insulation Class" (IIC). Because frequencies can spread through vibration, and because most buildings have rooms that are adjacent to each other via ceilings and floors, this is another category for materials which is a measurement of their ability to absorb impacts which cause vibration. Because I have used walls in my example, I have not referred to this rating, but it would be wise to be aware of its importance.

*Section 4: Facing Multiple Challenges*

So what works against vibrational problems? Panels that are designed to absorb vibration should be included. If a wall facing a high level sound front is soft and pliable, it will absorb at least some of the energy. If a semi-rigid wall, such as drywall is installed, and if it is allowed to move freely, it is possible to arrange it such that this frequency is absorbed. But one might say, "Such a wall is thin and contains not enough mass to prevent the long wavelengths from passing through." The answer is to use BOTH types of walls together, and using an easy trick, we can:


Absorb the Region II vibrations between the walls, using the vibrating panel inner (sheetrock) wall.
Block the Region I vibrations by the outer (concrete) wall
Block the Region I and II long wavelengths by spacing the inner and outer walls apart.

*Section 5: The Plenum*

An interesting fact exists for spaces between walls... certain wavelengths are trapped. In particular, 1/4 wavelengths get trapped in small spaces. Such a space is called a plenum and the principle is used in car mufflers. It is proven that an airtight box with one entry and one exit can resonate at certain frequencies. These frequencies can be absorbed using appropriate materials. It is the basis for bass traps and modal room analysis. But not only 1/4 wavelength, but 3/4 wavelengths are also trapped.... as well as 5/4, 7/4... all odd ordered quarter wavelengths are trapped in such a space. This means that 1 - 1/4 wavelengths, if they fit in the space will be blocked as well, and so on.

If, for example, you build a double wall with a 1 foot airspace within, you will have a trap for (the lowest frequency of) 282.5 Hz. -> ((1130 ft/s) / 1 ft) * (1/4 wavelength). Theoretically speaking, these particular frequencies would be completely annihilated because the wave simply cannot escape. In the real world, there will, of course, be some leakage. On the downside, 1/2 and full wavelengths will pass completely through this wall with practically no attenuation, except for the boundary losses. There is an impedance matching and mismatching using this concept. This is getting difficult to ponder. More bad news before the good...

A 2 ft airspace will give you protection against 141.25 Hz at the lowest frequency. We can already see that we're headed down that road to super-thick walls again. Using this principle, we can separate these walls by about 7 feet and get some protection at 40 Hz. If we can live with a slightly lower standard, say 60 Hz, we can bring the walls together again to about 4 ft. 9 in. But what about the frequency leaks between? The answer for this is also the answer to how we use the space to absorb the vibrational problems specific to Region II: insulation.

Using insulation in the space will do two things: Absorb the vibration of the inner wall, as well as act as a damper for the plenum. Here's how a damper works:

Applying a small amount of absorbent material inside a plenum will decrease the resonance of the wall space. A little absorbent will slightly decrease it, more absorbent will greatly decrease it. By dampening the space, you actually detract from the ability of the wall to totally block those odd 1/4 wave frequencies; they begin to leak through. On the other hand, the bandwidth begins to increase. More frequencies begin to be absorbed. The more insulation, the more the frequencies between the quarter wave modes are absorbed. With enough insulation, you will "smooth out" the peaks and valleys in the soundproofing qualities of the cavity and voila! You have a very decent barrier. The added effect of increasing insulation means you have actually added more mass to the wall, which again, helps block more sound. Although more mass in the walls actually RAISES the frequency of resonance, the low frequency roll-off rounds out and generally absorbs those frequencies even below your target frequency. A good rule of thumb is about 30% frequency roll-off below your target.

*Section 6: Insulation Density*

A note about insulation. The density of insulation has some interesting characteristics. Lower density insulation tends to be more effective than higher density insulation against temperature change. The opposite is true for audio absorption. The greater the density of the insulation, the greater the absorbent properties - _this is a very general statement that is not always true_. This does not mean you can get away with forcing 10 inch insulation batts into 6 inch wall cavities. Such practice causes pressure on both sides of the wall, literally coupling sound waves right through the walls, as well as preventing the inner flexible wall (of our hybrid design) from freely vibrating as it should. The appropriate answer is to acquire denser insulation, such as mineral wool (rock wool) insulation. This type of insulation is manufactured for extreme heat applications, such as smelting plants and such, and it is relatively inexpensive, if you find the right distributor. *Note that there have been numerous claims where simple, glass fiber insulation has proven to be more effective in absorbing low frequencies than the denser mineral wool types of fiber.*

*Summary*

By nature of sound in the physical world, we have just successfully blocked Regions III, IV, and V from penetrating. The downside is that it is almost impossible to estimate the STC of hybrid construction, and there is little published work on the matter that is open to the general public. But using this idea, people in various studios have indeed proven that such tactics allow excellent STC's even in the 50's, with much less wall depth. In fact, a 1 ft deep wall can be made a RATHER hefty sound barrier, if well designed and built. But there is one last, most important point in soundproofing walls. No leaks.

Any barrier with even a small hole is virtually worthless. This statement can easily be taken lightly, but consider the following:

If you were standing 20 ft from an orchestra, playing full blast, you might be getting a sound pressure level of say, 105 dB (pretty loud, actually). If an infinite brick wall, through the floor, ceiling, and both directions suddenly appeared, separating you from this symphonic mayhem, if it was a good wall, you may be hearing the muffled piece at say, 65 dB - the level of normal speaking. Now drill ONE hole in that wall the size of a quarter. According to your ear, you have just released 85% of the sound back into your vicinity. If you stand in front of the hole, at ear level, you're back to about 101 dB. It is not wise to ignore the importance of airtight surfaces. The space between the walls should be assembled such that a water hose placed in the wall would fill it up with no leaks. (It is certainly not recommend that anyone tests their walls this way, but ... you get the picture!)

This hybrid principle has just been applied to walls in this example. It should also be applied to ceilings and floors, as well. There is a plethora of information about how to build these structures specifically, which is outside the focus of this thread. But there are some excellent reading references in the General Tech 101 Forum for more information.


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