Room Acoustic Thread - PART 5

Discussion in 'Studio' started by Sinus Well, Sep 23, 2019.

  1. Sinus Well

    Sinus Well Audiosexual

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    PART 5: CHOOSE THE RIGHT MODULE FOR THE JOB - ABSORBERS

    Most of the sound absorbers used in practice can be assigned to two absorber types or combinations of both.
    The more common absorbers are the porous sound absorbers. These are mineral and organic fibers, foam, textile curtains, etc.
    They are relatively easy to build, but have the disadvantage that they require a lot of space.

    The second type of absorber are resonators, especially in the form of membrane resonators, perforated membrane resonators
    and Helmholtz resonators. It requires slightly more planning in the production than in porous absorbers.
    In addition, a vibrometer for modal analysis is needed.



    Thread Content:
    1. Porous Sound Absorber
    2. Membrane Resonator
    3. Helmholtz Resonator
    4. Diaphragmatic Absorber
    5. Which absorber for which purpose?



    1. POROUS SOUND ABSORBER:

    The absorption of sound by porous materials is mainly based on the conversion of sound energy into heat energy. It is caused by the friction of air particles moving in the pores at a certain speed.
    This is related to the existence of pores that must be open and so deep and dense that sound energy can enter the material and this friction process can take place.

    [​IMG]

    In addition to the parameters porosity, structure factor and sound entrance angle, the flow resistivity has a significant influence
    on the sound absorption. Weight or mass is not a primary criteria in the selection of an absorber material, as it is only the result
    of porosity, structure and density. This means that the degree of sound absorption, with the exception of the entrance angle
    and wall distance, depends only on the thickness and the flow resistivity of the material.

    For our purposes, the flow resistivity should be between 3000 and 16000 Pa*s/m².
    MKS Rayls/m are also used instead of Pa*s/m².
    These 2 values are interchangeable for our purposes and can be converted 1-to-1:

    • 1000 MKS Rayls/m =1000 Pa*s/m² = 1 kPa*s/m²

    For bass absorption, a low flow resistivity (3-5 kPa*s/m²) is needed
    so that the long sound waves can go deep into the material.
    For broadband mid/high frequency absorption, however, a higher flow resistivity (6-16 kPa*s/m²) is recommended.
    By increasing the distance from the absorber to the wall, even lower frequencies can be absorbed by impairing the linearity.

    In Part 4, we determined the frequency to be absorbed and the entrance angle of an early reflection. With these two values, we can now find out which absorber is suitable for us.
    For this we use Whealy's Online Calculator: Porous Absorber Calculator



    Start with the following values:
    • Set Air gap to 0mm
    • Set Octave subdivisions to a Third
    • Set Smooth curve
    • Set Angle of incidence on the angle calculated by you
    • Set Start graph at near to the fundamental frequency you want to absorb
    • Then you can play with the values Absorber thickness and Absorber flow resistivity.
    • The fine tuning can then be done with the Air gap.

    The optimum degree of absorption is achieved if the air gap corresponds to half of the absorber thickness.

    [​IMG]



    A small overview of suitable products:

    BASF

    Basotect: 7-20 kPa*s/m²

    CARUSO ISO-BOND
    WLG 035: ≥ 10 kPa*s/m²
    WLG 040: ≥ 5 kPa*s/m²
    WLG 045: ≥ 3 kPa*s/m²


    ISOVER
    Akustik TP1: ≥ 5 kPa*s/m²
    Integra ZKF 1-032: ≥ 5 kPa*s/m²
    Integra ZKF 1-035: ≥ 5 kPa*s/m²
    Integra ZKF 1-040: ≥ 5 kPa*s/m²
    SSP 1/SSP 2: ≥ 11 kPa*s/m²

    Heraklith Heralan

    DP-3: > 5 kPa*s/m²
    DP-4: ≥ 7 kPa*s/m²
    DP-5: > 7 kPa*s/m²
    DP-7: > 7 kPa*s/m²

    Molton
    100% Cotton, 300g/m²: ≥ 4 kPa*s/m

    Owens Corning
    PF 3350: 3,4 kPa*s/m²
    OC 701: 14 kPa*s/m²
    OC 703: 27 kPa*s/m²


    Rockwool
    Sonorock: ≥ 6 kPa*s/m²
    Sonorock Akustik: ≥ 6 kPa*s/m²
    Sonorock Plus: ≥ 6 kPa*s/m²
    Termarock 30: > 7 kPa*s/m²
    Termarock 40: > 10 kPa*s/m²
    Termarock 50: > 16 kPa*s/m²



    In order to find out how high the flow resistivity of an insulating material is, it is usually sufficient to look at the data sheets.
    Although the resistivity is rarely mentioned here, but the data sheet gives information about the nominal density (kg/m³).
    That's all we need to calculate the flow resistivity:



    [​IMG] [​IMG]

    Source disclosure: Building Acoustics

    Some interesting scientific papers on the use of activated carbon to absorb bass frequencies:

    Please note:
    In many cases it is necessary for glass and mineral wool to apply a trickle protection between
    the insulating layer and the perforated cover.
    This is particularly necessary for blankets to prevent trickling small fiber particles.
    This purpose is served by thin sound-permeable materials such as fleeces or stinging nettles,
    whose specific flow resistivity must be smaller than that of the insulating material.

    When working with these materials protective clothing should also be worn to prevent irritation of the skin and respiratory system !!!





    2. MEMBRANE RESONATOR:

    Membrane resonators have the maximum sound absorption at their resonant frequency. This is the resonance frequency of the mass-spring system, in which the membrane acts as a mass and the cavity as a spring. The membrane is characterized
    by its surface density. U
    sually rigid steel sheets or aluminum sheets are used for the membrane.

    There are both: hollow & half closed designs with flexible membrane (Membrane) and insulation filled & closed designs with inflexible membrane (Diaphragmatic).
    Membrane resonators consist of a hollow chamber framework which is covered on one or both sides with thin perforated membranes made of aluminum (round or slot-shaped openings) and overlying cover membranes made of aluminum or sheet steel.
    [​IMG]

    [​IMG]

    Like other resonance absorbers, they can be tuned to specific frequencies, but they can also be used in conjunction with porous absorbers, such as with applied foams, for broadband applications.

    Membrane resonators can be calculated with the Excel Spreadsheet Porous Absorber Calculator V1.59.
    It does the math for you, but remember: Without testing your designs with a vibrometer, you have no guarantee of success.

    A similar principle is followed by the popular Limp Mass Absorbers, which use MLV as their mass.




    3. HELMHOLTZ RESONATOR:

    Helmholtz resonators are resonance absorbers for low frequencies. They consists of a resonator neck, which acts as
    a mass (mass of moving air)
    and a resonator volume, which is due to the trapped air, the spring of this spring-mass system.
    In simple terms, a helmholtz resonator is the same as an oversized empty beer bottle.
    The field of application of these modules is where specific frequencies have to be absorbed in narrowband.
    Building Helmholtz resonators in DIY projects requires the highest level of accuracy and is therefore not recommended for beginners.

    The figure below contains approximate data on the equivalent sound absorption surfaces that can be achieved with Helmholtz resonators at the resonance frequency.
    The values are displayed as a function of the resonator volume. They apply to the possible fact that the resonators are located in wall or ceiling surfaces.
    When arranged in a space edge, they are twice, when arranged in a corner four times as large. This is caused by the higher sound pressure level due to the pressure accumulation.

    [​IMG]


    A simple calculation is possible with the following tool: Helmholtz Absorber Calculator by acoustic-modelling.



    4. DIAPHRAGMATIC ABSORBER:

    A diaphragmatic absorber is a sound absorber that focuses on the sound absorption of longer wavelengths at lower frequencies. While the absorption performance of the membrane resonator is achieved primarily by the density and resonant frequency of the membrane and the chamber may be semi-open, a diaphragmatic absorber achieves its performance primarily by the trapped air behind the membrane and the insulating material therein, which determines the rate of absorption and Absorbs resonances of the body.

    [​IMG]

    A diaphragmatic absorber consists of three parts. The front wall, which slows down the longer wavelength by moving in sympathy with the sound pressure applied to it.
    The heavy box itself, which forces the front wall to move, because the box itself remains rigid. And the internal insulating filler material that absorbs resonances in the box and thus contributes to the overall performance of the device. The density and depth of the case add to the degree of absorption you can achieve with a diaphragmatic absorber, and the internal case filling contributes to the rate of absorption that occurs from that level.

    While the purchase or self-construction of a diaphragmatic absorber is one of the most costly of the solutions offered, it still offers the highest performance in broadband absorption of low frequencies.

    [​IMG]

    I have tried a few concepts, from building concepts on GS to building concepts on other forum pages. Some were good, others were not. I do not want to advertise, but in the end I bought licensed building plans at Acoustic Fields and built some self-customized BDA's. The results are outstanding. These have multiple membranes and the cabinet is made up of several layers of MDF, which are isolated from each other by thin layers of MLV. As an insulating material I have chosen active carbon pallets.

    There are some nasty voices about the ACDA series (especially on GS) claiming that this absorber has bad, barely existing absorption levels and would be nothing but hi-fi voodoo. These products are characterized by the fact that they particularly absorb the frequency range of 30-50Hz and hardly affect the rest of the frequency range, which is IMHO absolutely impressive for this size. ACDA-12 Testing Results




    WHICH ABSORBER FOR WHICH PURPOSE?

    Yes, that's the big question.
    Let's start with the pro & contras. You have 4 options:
    1. Porous absorber
    2. Membrane resonator
    3. Helmholtz resonator
    4. Diaphragmatic absorber

    Porous absorbers can be used in many ways. They are cheap but have one drawback: If you want to absorb bass then you need a lot of space. If you have a large room and can do without 1/4 or 1/3 of the room, good! Then maybe this is the right choice for you!

    The materials for a membrane resonator are also relatively cheap. But: You need the appropriate measurement technology to determine the exact resonance frequency of the material. Heavy plates are used to absorb low frequencies. This should also be considered.

    Helmholtz resonators are good for narrow-band absorption. They are difficult to calculate and should not be the first choice when it comes to treating the acoustics of a room. They are particularly helpful when certain narrow frequency ranges can not be controlled exclusively by other technologies.

    Diaphragmatic absorbers are expensive but relatively easy to build. They require comparatively little space and can be tuned to specific frequency ranges. Depending on their design, they can be very heavy. My buildings weigh about 90kg each.


    So what should you do?

    From my experience, I would start with diaphragmatic absorbers and/or membrane resonators. First, I would treat the critical areas on the front wall.
    These are usually the room corners and room edges. There I would stack the diaphragmatic absorbers.
    Air cavities behind the absorbers I would fill with insulation wool. Then I would treat the critical areas of the rear wall.
    Only then I would take care of the bass treatment in the rest of the room, if measurements show that something needs to be done here.

    After the bass range is reasonably under control I would treat the early reflection points on walls and ceiling. Depending on the problem, this can be solved either with porous acoustic panels (matched to the problematic frequency range) or with diffusers. The floor can be laid out with a thick carpet to minimize reflections. However, care must be taken to ensure that high frequencies are not absorbed disproportionately.
    Further reflection points may, if necessary, either be treated in the same way or be redirected by reflecting concave angle plates to another absorber or diffuser matching the frequency range of the reflection.
    Please note that after each change, room acoustics must be measured in order to determine success (or failure).

    More information can be found here:
     
    Last edited: Jan 9, 2024
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