[ejbragg]

Frequency Regions Defined

There are basically 5 Regions of Frequencies defined in acoustic engineering (actually there maybe more now, but for starters, I will base my methods of attack on the following frequency "regions":

I. Lowest Frequencies (Stiffness Controlled, assisted by material bending capabilities)
II. Low Frequencies (Resonance Controlled, assisted by vibration/ resonance resistance)
III. Mid Range Frequencies (Mass Controlled, assisted by impedance of compression waves)
IV. Higher Frequencies (Coincidence Controlled, assisted by materials which impede vibration breakthrough)
V. Highest Frequencies (Extended Mass Law, assisted by impedance of compression waves)

In this thread we will discuss the regions in detail. Another thread will be used to discuss the general overview approach for handling each region. Additional threads will be created to discuss the specifics of treatment for each region.

 

[ejbragg]

Region V

This first discussion is very geeky - getting into some physics without necessarily getting into the math. It will likely be boring for most, but if you really want to understand what happens to sound in air and other materials, this is the place we have to start. :nerd:

I will actually begin with the highest frequencies first, because they tend to be the easiest to sort out. The highest frequencies are those that are mostly outside the range of hearing, but transition to within comprehension. Generally speaking, it's about 10 KHz and up. Note I say "generally speaking" because there is really no immobile frequency threshold. Sound properties tend to change according to temperature, air pressure, humidity, and so forth. There are, in fact some weird things that can occur to all sound waves in air that has different properties, such as when the sun goes down and the ground is warm and the upper air is getting cold. Such discussions are outside the focus of this thread, and probably belong in the live sound category.

Region V frequencies are the highest sound frequencies we recognize as human listeners. If you set a signal source in a room which emanates a Region V frequency, the amount of signal loss in air is higher than that of any other region. Air molecules are like rubber bouncing balls, striking each other when there's a disturbance. The highest frequencies cause such chaos that the overall disturbances often cancel each other out before the wave spreads very far. This causes a higher loss of these waves in air. (Note however, that such waves traversing through a stiffer material has much less loss.)

Because of this property, the transmission loss (in air) will be 10 dB higher per doubling of frequency. For example, let's say your extremely efficient high frequency speaker is creating a 10 KHz signal, and your measuring device reads 30 dBa at 10 feet distance from the speaker. If you raise the frequency to 20 KHz, using the same input signal power, the SPL at the same listening position will now read approximately 20 dBa: The air will be absorbing 10 dB [more] of sound pressure before it reaches you! (And the air was already absorbing a lot of power at 10 KHz for the same reason.)

Another property of this frequency region is that the primary method of travel through other material types is via compression waves. If a stiff panel, such as glass, is close enough to the source to be excited vibrationally, the waves can actually travel through the glass, and the only loss will be contributed to the air / glass boundary loss; then on the opposite side ... the glass / air boundary loss. [Such losses are not guaranteed to be inaudible.] The appropriate method for stopping these frequencies is to either increase the material density (which increases the boundary loss) or decrease the material stiffness (which tends to absorb the compression wave).

 

[ejbragg]

Region IV

Region IV frequencies are strange frequencies, indeed, providing some of the strangest properties of all regions. These frequencies are often in the range of 5 - 11 KHz (there is always some bleedover between categories). These frequencies have less loss in air than Region V, but still lose a lot more power than lower frequencies, usually somewhere between 6 - 10 dB per octave doubling, with a general loss of about the same SPL loss per doubling of distance. (Note that Region V SPL loss per distance doubling is harder to predict).

These waves cause problems in a weird way:

When such a wave front strikes a surface, and its wavelength is greater than the thickness of the material, its tendency is to travel along the surface of this new material. This problem is also a product of the angle of incidence of the collision: the greater the angle of incidence (closer to parallel), the greater the transmission of the wave front along the surface.

When such a wave is traveling along a wall and it comes to a sharp edge, such as a corner for a doorway, the sound escapes back into the air and causes what is known as "edge effect". This phenomenon makes the doorway (or any other sharp transition in the surface) radiate sound. Because sound travels along hard surfaces much faster than in air, such radiation reduces clarity in a listening environment.

The primary problem associated with Region IV frequencies is that of vibration break through. A possible method for controlling such frequencies is to keep the hard surfaces in a room as perpendicular as possible to the line of audio travel. However, this can actually be counterproductive, as such walls can merely cause unwanted reflections back toward the source, as well as the listener position. Nevertheless, merely splaying walls in a room, although assisting in preventing room modes at mid frequencies, allows upper-mid frequencies a surface to travel! Perhaps a usable method for treatment of Region IV frequencies is an absorbent termination in the walls.

 

[ejbragg]

Region III

Region III Frequencies are generally around the range of 600 Hz to 6 KHz. There are certain similarities between Region III and Region V ranges, mostly due to the fact that they both act primarily as compression waves in materials. Like Region V, Region III frequencies can be controlled by high mass material; however, stiffness may not be as detrimental.

There are some differences: Region III are very predictable in air. With every doubling of distance from the source, the SPL decreases by about 6 dB. Furthermore, every doubling of frequency (every increase by 1 octave) is also accompanied by a 6 dB increased loss.

Although high frequencies act as rays and low frequencies act as waves (see other categories in acoustics for more information), mid frequencies tend to act as a sort of hybrid. Although a limp wall can reflect upper frequencies generally well, such is not the case with mid frequencies. Because the wavelengths are longer, they tend to penetrate walls more easily, so higher mass is the answer for stopping these compression waves, and the lower the frequency, the higher the mass.

There are 2 limiting frequency boundaries associated Region III: a "resonant frequency" and a "critical frequency". The resonant frequency is the lowest frequency at which mere mass can stop the transmission. Below this point, the audio begins to vibrate or pass through the material (or surrounding materials). Any wave that begins to propagate through an object despite its mass is transitioning into a different region, either I or II (depending upon the material's properties). The critical frequency is the highest frequency at which a material can control a wave front before it begins to escape via surface propagation. Note that different materials tend to fail to block sound at differing frequencies, so each material has a different "Region blueprint", if you will.

Generally speaking, a well-behaved range of frequencies occurs when the material used to block it falls within the range:
(2 * Fr) < controlled region < (Fc / 2),
where Fr = Resonant Frequency,
Fc = Critical Frequency

In other words, you must stay well within the range to be effective, because the transition can be very gradual.

 

[ejbragg]

Region II

Region II Frequencies are low frequencies (approximately 50 Hz - 700 Hz), and they come with some real challenges in the area of control. In fact, Region II is often more difficult to control than Region I. Because the wavelengths associated with these frequencies are great, these waves tend to propagate right through most barriers. Furthermore, such audio, if it leaks into the structure of a building, can spread throughout its entirety, being heard in every room, as well as outside the building. This is due primarily to resonance.

Region II Frequencies have a tendency to penetrate the surface of walls rather easily. One of the characteristics of these long wavelengths is the ability to create high pressure on one side of the wall, and low pressure on the opposite side simultaneously. At the next instant, the pressures switch sides. The aftermath is much like a wall that is being shaken at high speed by many hands in perfect synchronicity. These waves are absorbed by one side of the wall and cause the opposite side of the wall to radiate the sound like a giant speaker. Furthermore, as the structure is influenced by the vibration, other walls (and ceilings and floors, and sometimes furniture) in the building become sound radiators, as well.

Although a concrete wall might be appropriate for Region III, such rigidity is not very effective for Region II. Although high mass helps increase the boundary loss at the air / wall transition, these frequencies also require dampening to absorb vibration.

There is another problem. The wavelengths are so long that a massive wall deep enough to overcome the wavelengths is not feasible. For example, to stop a 100 Hz wave penetration by sheer density and thickness, one would have to build a non-rigid, dense wall 5 ft 8 in. thick! ((1130 ft/s) / 100 Hz) / 2) A realistic approach must include some form of absorption of these frequencies, such as plenum cavities and bass traps (see other audio treatment categories for more information on bass traps).

The treatment at these frequencies requires 3 methods of attack:

1. High mass material
2. Dampening
3. Plenum barriers

The plenum barrier is in a sense, a double wall with an air cavity within. The idea is to trap the low frequency on its way out. More info on this later....

 

[ejbragg]

Region I

Region I Frequencies often range from 0 - 60 Hz. These fall into the extreme low audio spectrum, become difficult to reproduce and hear, and pose some interesting characteristics. Like Region II, these frequencies tend to require a lot of mass. However, they do not tend to traverse through entire structures in the same manner, unless these structures are extremely rigid. Furthermore, making this last statement a bit of an irony, it is rigidity, rather than flaccidity that actually controls this region. In other words, a solid concrete wall, erected solidly on a massive concrete foundation goes a long way toward controlling these frequencies.

Again, one still needs to approach the design with the long wavelengths in mind - deep wall cavities are a great way to further reduce noise travel. However, structure-borne vibration is less likely, and furthermore, such frequencies are often difficult to hear, anyway. Not only so, but most usable audio, except for sound effects for motion picture, is above this range.

There is one more strange personality of Region I frequencies. As the frequency doubles, the SPL DECREASES by 6 dB. In other words, if using the method explained in Region V... say you have a 60 Hz, 30 dBa signal at 10 ft distance from the source. If you drop the frequency to 30 Hz, the signal at the listening position will drop to 24 dBa, provided your speaker is highly efficient even at these frequencies. In other words, the loss of these frequencies is not strictly related to hearing deficiencies in humans. There is actually a real loss associated with these frequencies in air and other materials.

 

[ejbragg]

Very Important Note About Regions

I provided example frequencies for the different regions. But please note that these are only approximations. The REAL test for which region you are treating is actually defined by the material itself. In other words, 600 Hz may be classed a Region II frequency for drywall. However, it might be classed as a Region III frequency in concrete block. It depends strictly upon the materials and how they REACT. The reaction of materials to certain frequencies are actually the defining principles for the regions described herein. Hope that makes sense! onder: