PART 4: ACOUSTIC ROOM MEASUREMENTS


So in Part 2 we determined our listening position and in Part 3 we figured out where to put our acoustic modules.
Now let's look at which frequencies need to be absorbed to keep the frequency response of the room under control.
Before we take care of the reflections of high and mid frequencies, we have to manage the bass response.



Bass Frequency Measurement:

In REW please create a current measurement (both Speakers + Sub) of your listening position and generate a waterfall diagram.

Then click on "Freq. Axis". Here you can choose between linear or logarithmic representation of the frequency response.
We choose the logarithmic representation!
Please click on the button "Limits".
In the opening window we limit the visible frequency range. In the dialog box "Left" we enter 20Hz and in the box "Right" 300Hz.

"Top" and "Bottom" determines the displayed dynamic range. We are interested in a dynamic range of 60dB (RT60).
In the box "Top" you enter the highest displayed peak in the range between 20-300Hz.
In the box "Bottom" you enter the level, which is 60dB below the peak.
For example, if the highest peak at 120 Hz is 88 dB (you can round up +-5 to 90 dB), then you enter 30dB for "Bottom".


To be able to evaluate everything properly, we now have to determine the display axes and the time range.
To do that, please click on the button "Controls" and transfer the following values:

Slice: 30
X: -10
Y: 48
Z: 150
Overlay: No overlay
Overlay Transparent
Transparency: 75%

With "Time Range" and "Window" you determine the reverberation time that you would like to achieve in your room.

The reverberation time (RT60) indicates how long it takes for a certain frequency to drop 60 dB from the peak value.
Our goal is therefore to bring all frequencies as evenly as possible to (resp below) the selected RT60.
The average standard for studios here is 300 ms. This is especially true where recording and mixing happens in the same room.
If this is the case, just enter 300 ms for "Time Range" and "Window".

These settings help us to identify bass problems more easily.

Now compare the ringing frequencies in the waterfall measurement with the results of Sengpiel's Room mode calculator to determine on which room axis the modes are.
Using the low-frequency pressure sketch, you can then determine at which point in the room the mode has to be absorbed.
From experience, I would advise to first treat the front wall and then the back wall.
We also take a look at the SPL frequency measurement. Our goal is to smooth the mountains and valleys and to achieve a linear frequency image at our listening position.

The frequency to be absorbed or the frequency range to be absorbed indicates which type of absorber is best suited here.
More about this in Part 5. After each treatment with a module, a new measurement is needed to evaluate the success.

Please note: You should always treat the bass in the room first. There is no point in treating the high and mid frequencies first.
If the bass doesn't work, your mix will not work. It's that simple.



Early Reflections / High- and Mid Frequency Management:

After the room modes are treated, we take care of the early reflections.

To make sure early reflections don't negatively impact our mix position, there are several concepts:

• We can attach absorbers that absorb the reflection.
• We can direct the sound to another location in the room by attaching a reflective surface that changes the angle.
• We can attach a diffusor to distribute the sound chaotically and to make the sound distribution in the room homogeneous.

Especially in smaller rooms, it is advisable to attach diffusers to the Early Reflection Points of the side walls or the room ceiling.
In larger rooms, the live end-dead end concept is also popular, with diffusers attached to the front or back wall, while absorbing on the opposite side.
In small rooms, this is usually not possible because these areas are needed for bass management.
In our example room, it is also not possible to install diffusers on the side walls as they should be at least 2 meters away
from our listening position so that we are not affected by diffusion artefacts. So we use absortion.

Determining the fundamental frequency that needs to be absorbed is quite simple.
For this you need the sketches of the reflection points, which you made in Part 3.
Now measure the length (in meters) of A and B.

• A (meter) + B (meter) = X

X is the wavelength to be absorbed.
To calculate the frequency from X, use the wavelength calculator from Sengpiel.
The correct converter is located at the bottom of the website.

Then we still need the reflection angle ɑ. Use your Triangle Ruler and the sketch.
Make note of the frequency and the reflection angle: You need it to choose/build a suitable absorber.

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PART 5 ->

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Last edited: Sep 25, 2019