Room Acoustic Thread - PART 3

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

  1. Sinus Well

    Sinus Well Audiosexual

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    PART 3: HOW TO DEAL WITH ROOM REFLECTIONS


    Before we make further acoustic measurements, we first want to determine which room modes may be present in your room and what points of reflection we need to take care of.
    Every room is different, so one-fit-all solutions make absolutely no sense. We need to find out what problems are in your room before we can take care of a solution.
    For this we need paper, pen, calculator, triangle ruler and a compass to draw circles.


    Determine your Room modes:

    Here are the basics of the speed of sound and wavelengths:
    • The speed of sound is 343m per second at a temperature of 20°C.
    • A 100 Hz sound wave is exactly 3.43 meters long.
    • A division of the Hz value by a factor of 2 multiplies the wavelength by a factor of 2.
    • 50Hz = 6.86 meters
    • 200Hz = 1,715 meters
    As I said in the introduction, room modes only refer to the frequency range below 300 Hz.
    To find the possible modes of your room and it's harmonics, you need to measure length, width and height accurately.

    Please enter your room dimensions on Sengpiels Room mode calculator.
    Write down the results for axial, tangential and oblique modes separately. These will be needed later for reference.



    Determine High frequency & Mid frequency Reflection Points:

    To find out where we need to absorb or diffuse reflections, we first need to create a detailed floor plan of our room.
    Don't worry, you don't have to complete a degree in architecture. :winker:

    All you need is a double meter ruler (or laser measuring meter) and a free 3D CAD software, such as pConPlanner.

    You already know the height, length and width of your room.
    Now measure the exact dimensions and positions of windows, doors, dormers and other angles in your room.
    Also important are the dimensions and height of your tabletop, the height and position of your speakers,
    and the exact position and height of your head at your listening position (plus an extra radius of 30cm to 50cm around your head).
    With these values you can easily create a 3D sketch of your room in pConPlanner.

    As an example, I briefly made a sketch of a fictional room, as many will find it at home.
    (I've purposely included some problematic issues that we will discuss later)

    We need a sketch with top view and a sketch with side view. Both sketches should be printed four times on the largest possible sheet of paper (at least DIN A4).


    [​IMG]
    [​IMG]

    • The blue dot represents the listening position.
    • The red dots indicate the center point of our speakers.
    • The green dot is the center of our subwoofer.


    Take 1 paper from each sketch:

    We use a thin pencil to draw straight lines from the red dot in all directions into the room. These lines symbolize high-frequency and medium-frequency sound waves.
    When these lines hit a wall (or window / door) they are reflected.

    The following applies: The angle of incidence is equal to the angle of reflection.
    Please work as accurate as possible!
    Each of these lines is reflected 3 times.

    Some of these lines will cross the blue point (or its radius).
    At this line, we mark a small red X in the spot where it first hits a wall after leaving the speaker and reflects to our listening position.
    With a blue X, we mark the point where the line hits a wall, but then has to be reflected by further walls before it reaches our listening position.

    The more lines we draw, the more detailed our reflection analysis becomes:
    There are areas on the walls where many blue or red X's are side by side.
    Now let's take two more sketches (1 of each) and draw red and blue boxes around the areas where the Xs are grouped on the other sketches.
    Name each box, eg with "SW" for side wall or "BW" for back wall, "C" for ceiling and "F" for floor... Single (ungrouped) Xs can be transferred to the same location as an X again.


    In the end, the sketch with the top view should look like this:

    [​IMG]

    • The areas marked in red show the reflection points at which the sound is reflected back directly to the listening position.
    • The areas marked in blue, on the other hand, show the spots where the sound is reflected back to our listening position only by detours.

    Now put these 4 sketches aside and keep them well. We will need them later.




    Determine Low Frequency Pressure Areas:

    Take another paper from each sketch:

    Now our compass is used.
    We position the tip of the compass on the green dot and draw circles around the green dot.
    We do this until the entire room is filled with circles. The finer you draw the web, the more detailed the analysis will be in the end.
    Now mark the 4 points on the walls, which lie on an axial line with the subwoofer. Also mark the corners of the room and the two points that correspond exactly to half of the room length.

    [​IMG]


    Now again draw circles from all the marked Xs !!!
    When you're done, check the walls for areas where many lines overlap.
    Mark the center of these areas and start drawing circles around them again.

    Now you should have a pretty good idea of where in your room the reflection points and pressure zones of the bass range are:
    Where most lines overlap !
    Not only the areas on the walls are interesting for us, but also the areas in the middle of the room.
    Translate these areas to a clean sketch and repeat the procedure on the sketch with the side view of the room.


    Please note that such an analysis will never reflect 100% of the reality !!
    There are a lot of parameters that influences the radiation and pressure behavior.
    This includes, for example, the design of the subwoofer, the thickness and resistance of the wall materials, etc.
    Nevertheless, such an analysis is well suited to visualize possible problem areas in the room,
    so that you're be able to measure and treat them with purpose.

    So, if you want, you can now place your measuring microphone in the appropriate areas and measure the frequency response
    to find out which resonant frequency is highlighted in that area. It is not a must, but it can be very educational and enlightening, especially for beginners.





     
    Last edited: Sep 25, 2019
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  3. reliefsan

    reliefsan Audiosexual

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    Quality!

    Thank You from all of AS!
     
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  4. Sinus Well

    Sinus Well Audiosexual

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  5. Choosename

    Choosename Producer

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    How in all directions? like a star *?

    I am interested in a visual representation of how to find the reflection points, not in words, more in visual enviroment if it is possible. I lost in words.
     
  6. Sinus Well

    Sinus Well Audiosexual

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    Yes, like a star. The lines that return to the listening position after a single reflection are the most important for the treatment, those that return after two reflections are the second most important, and so on.

    This is the same as if you were using the mirror trick. For the first reflection points, this is easy to do with a mirror and a helping person. For waves that only reach the listening position after several reflections, it is difficult to do with mirrors. Many mirrors and many people to help. It is easier to draw stars. You just have to draw them very precisely. It's only an approximation, but it works ;)
     
  7. Choosename

    Choosename Producer

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    Thanks, I have found this tool https://amcoustics.com/tools/amray
    co.png
    Still it depends on the number of arms you create on the star, and without mesurement and direction of the speakers, are not all this just a theory? Because the speaker gets more power on the front of magnets, and least on sides and rear.
     
  8. Sinus Well

    Sinus Well Audiosexual

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    Thanks, I will put the link in the article.

    You are correct, the energy radiated by the speaker is different off-axis than on-axis. Furthermore, different materials and material thicknesses, mass, etc, have different properties in terms of reflection and absorption. Therefore, not all walls are the same, and not all doors are the same, etc.

    However, this is of little interest at this point. This method is used to determine 1. the position of the first, second and third reflection points and 2. their approximate fundamental frequency (see PART 4). It does not replace measurements, but complements them. When dealing with reflections (drawing stars), this method will give you a fairly accurate idea of exactly where your modules (and which ones) should be positioned. A measurement alone will not tell you where in the room to fix the problem that shows up on your listening position.

    Drawing circles, on the other hand, will give you a rough idea of where problematic areas in the room might be in terms of pressure zones and room modes (<300Hz). However, the latter must be verified by measurements, as the results of this method can deviate greatly from reality.

    Even in case no measurements are possible (for whatever reasons), lots of improvements in room acoustics can already be achieved just by drawing stars, analyzing the drawing and treating it with appropriate acoustic modules.
     
    Last edited: Jan 9, 2024
  9. Choosename

    Choosename Producer

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    Cool, I will create a room soon or later, and i am at first contact with this theme of room acoustics. I hate DIY!
    Very usefull thread!! Congrats. And thanks for support.
    I will update.:chilling:
     
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