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There are several passive ways to optimize a room and fix standing wave issues with different kind of absorbers. The absorption performance of all types of absorbers highly depend on how you use them. I will go in detail with some rule of thumbs and helpful information in this guide. An overview about the most used absorber types:

- Porous absorbers
- VPR Absorber
- Slot absorber
- Panel absorber
- Helmholtz-Resonator
- VPR Absorber
- Active Phase related Chassis-Absorbers
- Diffusor that act as acoustic absorber

Porous absorbers

Known materials are open-cell foam like Basotec, Rockwool or one of my favorite material Caruso Iso-Bond. All of these porous types are available in different weights and densities which must be chosen carefully to work most effective in your room. One rough rule of thumb is, the lower the air flow resistivity of the material, the better the effect for low frequencies. But it´s not just that simple. The lower the problem frequency the more material thickness you should consider because the absorber-thickness must be at least 1/4 of the half wave-length to achieve an absorption effect. For example a 100Hz wave has a length of 3,43 meter and you need at least 1/4 of the half wave length to get an absorption effect for 100Hz which results in a min. 40cm thick absorber with a low density of 5000 air flow resistance. In this scenario you will effect an absorption rate of 0.75. The highest absorption with 40cm thick material can be achieved with a more dense material with 3000 air flow resistance which will have an absorption rate of 0.87. The highest absorption for 100Hz could be achieved with 70cm thick absorber material with an air flow density of 1000 rayls/m. This would affect an absorption rate of 1.0. This means at least 1/2 of the half wavelength with the adequate air flow resistance will affect the best result. But there is another trick to reduce the air density and the costs which I will explain after this diagram.

Some sweat point measurements with available absorber panels Caruso ISO-BOND WLG040/20KG and WLG035/40KG. I´ll show alternatives with air-gap if they make an improvement.
Problem Frequency, Absorption rate and the most effective thickness of the individual absorber type:
100Hz Absorption 0.80 | Don Caruso WLG040, most effective absorption with 40cm thickness
100Hz Absorption 0.99 | Don Caruso WLG040, most effective absorption with 10cm thickness + 60cm air gap from wall to absorber
120Hz Absorption 1.00 | Don Caruso WLG040, most effective absorption with 10cm thickness + 50cm air gap from wall to absorber
180Hz Absorption 1.00 | Don Caruso WLG040, most effective absorption with 10cm thickness + 30cm air gap from wall to absorber
200Hz Absorption 0.92 | Don Caruso WLG040, most effective absorption with 30cm thickness
200Hz Absorption 0.94 | Don Caruso WLG040, most effective absorption with 20cm thickness + 10cm air gap from wall to absorber
300Hz Absorption 0.98 | Don Caruso WLG040, most effective absorption with 20cm thickness
600Hz+ Absorption 1.00 | Don Caruso WLG040, most effective absorption with 20cm thickness + 10cm air gap from wall to absorber
900Hz+ Absorption 1.00 | Don Caruso WLG040, most effective absorption with 20cm thickness

500Hz+-100 Absorption 1.00 | Don Caruso WLG035, most effective absorption with 50mm thickness + 10cm air gap from wall to absorber
500Hz+-100 Absorption 1.00 | Don Caruso WLG035, most effective absorption with 50mm thickness + 10cm air gap from wall to absorber

For the ceiling and if high air gaps 10cm-50cm + are possible, the 30-50mm Don Caruso 40KG WLG035 becomes very effective like seen in the following result:
60-90Hz Absorption 0.98 | Don Caruso WLG035, most effective absorption with 50mm thickness + 100cm air gap from ceiling to absorber
100Hz Absorption 0.99 | Don Caruso WLG035, most effective absorption with 50mm thickness + 70cm air gap from ceiling to absorber
140Hz Absorption 0.99 | Don Caruso WLG035, most effective absorption with 50mm thickness + 50cm air gap from ceiling to absorber
300Hz Absorption 1.00 | Don Caruso WLG035, most effective absorption with 50mm thickness + 20cm air gap from ceiling to absorber
500Hz Absorption 1.00 | Don Caruso WLG035, most effective absorption with 50mm thickness + 10cm air gap from ceiling to absorber

Without Air gap the 50mm thick Don Caruso 40KG WLG035 starts to absorb from 1000Hz+ with an absorption rate of 1.00

As you can see in this diagram the target frequencies can be influenced heavily by using less material but increasing the air gap. The 120Hz is a good example. Why is it most effective to have an air gap of 50cm and using only one 10cm thick Don Caruso WLG040 plate? The reason is, the one panel reduced the air resistance and the air gap behind the panel cooperates with the panel in a way the wave is absorbed more efficient as if you would choose to stuff the whole corner with the 5000 air density material. There is one notice to work with an air gap. The air gap produces a bell absorption which influences these target frequencies a lot +- around 100hz in the low frequency range but frequencies above or below are not much absorbed with this way of treatment. For an most effective deep frequency broadband absorption you still should consider using a super chunk instead the air gap style. For example if you have 50cm space you can build a 50cm thick low frequency super chunk in your room with 1000-2000 air flow density wool. You will achieve a broadband absorption from 60hz up to 20K. If you add a 80cm Super chunk the frequencies absorption rate of 0.9 starts from 40hz and goes broadband up to 20K. Porous absorbers should be places at least in the pressure maxims of the room. The benefit if you treat corners is, you can fix frequency issues of each close wall of this corner. So for porous Absorbers, the density, air gap and the position of the absorber must be considered very well.

If you decide for porous absorbers, take care to keep your room acoustic allive. This is done by keeping at least in total 30% high-frequency reflections with a slot wall in front of the absorbers or wrapping 30% of the wall in a thin slotted plastic foil. This way you will still have diffuse high-frequency reflections in your room but the absorption is not effected. Some people who just have low problems and want to keep the room most alive with diffuse high-frequencies can add diffusers or slotted wood plates in front of the absorbers.

In the following Scenario you can see a room with a standard treatment for an acoustic treatment that is effective on deep frequencies and not to dry by keeping more than 30% of the surfaces high frequency reflective. On the front there are three VPR Absorber panels with steel panels calculated for 70hz. The side panels are porous Don Caruso WLG035 10cm thick panels to reduce broadband reflections at 210Hz. The ceiling clouds have a large air gap 30-60cm and the lower dense Don Caruso WLG040 is used to reduce deep and high frequencies at 180Hz-20K. There is no reflective foil added to the ceiling so they absorb everything in the higher range ahead the center room position. The pressure maxim corners are stuffed with 62cm wide WLG040 3-angle cut corner traps. The listening position is set with diffusors and an porous absorber wall in the back.

VPR Absorber

This Absorber technic is invented and well documented by the German Fraunhofer institution. The VPR Absorber is made with a Don Caruso WLG035 100mm thick porous absorber panel and a steel plate. The Absorber thickness, the steel plate size and thickness plays a role and is calculated to have most effect on your target frequency. The function of this absorber is a spring-warmth function. The wave makes the steel panel vibrate and the energy is absorbed by the porous low density material behind the panel. Commercial products can be seen as with the steel plate inside of two Don Caruso WLG035 Iso-Bond panels. The benefit of this absorber is, you require almost no deep space and you can get rid of ultra-low energy below 100Hz by a total dimension of the absorber of 120x62x12cm. Those panels can be mounted easily in a frame and the wall.

Slot-Wall absorbers

Slot absorbers are similar to Helmholtz absorbers. Slot-Size and Panel-Thickness but the Volume of the Body is necessary as well to calculate for the target frequencies. The benefit of these walls are, they look great and have a kind of diffusing effect as well. They are not so easy to calculate but they have an effect. I would say they are not so effective like the porous absorbers but if they are angled, they can also have positive reflection effects to get rid of direct reflections in on the listening position. The bandwidth absorption of this resonator is adjusted by the wool inside the body. The absorption itself is the energy loss due to forth and back air movement between the slots if the slot-wall-resonance was hit by the signal. The calculation base for a slot-wall is as follows:

f = 2160 x sqrt ( r / (( d x C x D ) * ( r + w )))

Where:
f = resonant frequency (Hz)
r = slot width
w = slat width
d = slat thickness
C = mouth correction factor (1.2, normally)
D = depth of the cavity.

Panel absorber

Panel absorbers are boxes with a panel on top. The Box has it´s own resonance point and this can be calculated very exact by choosing the correct panel thickness and calculating the dimensions of the volume. The more wool is inside the Panel absorber the more broadband it works. The Panel must vibrate freely, so the wool must be fixed in a way, it does not get to close against the panel from the inside. If this is done accurately you can feel the panel absorber working on the specific frequency. The absorption is not that high like with porous absorbers or VPR absorbers. Helmholtz-Absorbers are more efficient as well but in some cases it makes sense to build panel absorbers. The Benefit is, they do not need to be much deep. They can be constructed on the whole wall and reflect other frequencies and only effect little energy absorption on the target frequency.

Helmholtz-Resonator

Like the Slot-Walls the Helmholtz-Resonators reduce the sound energy by vibration and air flow through the holes. The Helmholtz Resonator has the most effect if it is built to reduce a target frequency. As more you make it broadband with wool inside the body, as less is the absorption effect on the target frequency. One pro is, it can be integrated and calculated easily in most room environments. One con is, if it is not proper calculated, the frequencies are not reduced effective and it can resonate itself or enlarge the room hall effect.

Active Phase related Chassis-Absorbers

Active Absorbers often can be found in modern home cinemas together with other passive absorbers. Active absorbers are a reversed working Speaker chassis. They eliminate standing waves by inverting the wave 180° at the end of the wall, so it will be absorbed completely at the listening position.

Diffusors that act as acoustic absorber

Found in bigger studios that does not have heavy frequency issues, they tend to use big diffusor walls to prevent standing waves and keep the room acoustic most alive.

One last tip. Absorbers and Diffusors are not necessary in a well-planned studio room but in most cases they are used to improve the sound or to modify the sound in a way the listener is happy with.
For Home-Cinemas or Cinema-Studios that cut films and work a lot with effects, they more tend to need a dry sounding room than a full-service studio that wants to keep alive for the recording room and the control room. In most cases a mix of several well planned absorbers and diffusors does the trick but calculating and having the target in mind can save a lot of time and money. If you have any questions, feel free to contact me.

Don