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Case: Fixing room acoustics for a F205 system

A local customer has a F205 + T6018 system in a small, dedicated room. He already had some acoustic treatment in place. Now he wanted to improve the sound. Being local, he can easily take a trip over to our place and compare to our Room2 - this means he has a reference, so he can hear how sound quality improves with better acoustics.

But surely, can he achieve a similar performance. That seems almost impossible.

Yes, it can be done, and it is not very difficult or expensive. There is no magic involved, just pure engineering and a little construction work.

Stay tuned, I will show what he did to improve the room, and we have measurements to show the quite large difference in performance.

Measurements show the improvement after implementing proper acoustic treatment. And those differences are huge - which also correlates with how the sound is perceived - a significant and obvious difference, that has huge impact on sound quality.

When we analyze measurements for room acoustics, it is mainly what happens in time that is of interest. The spectrogram is a picture of how the sound start and decay, across the frequency range. We compare before and after fix - pre-fix left:

Spectrogram pre-fix and after

Obviously, all settings for scaling and range and signal processing must be equal, to get comparable graphs. Here are the settings I use for spectrograms:

We see a a much more frequency-linear decay, and especially in the 100Hz-1KHz range, there is a significant improvement. Differences in the bass region is mostly due to different calibration target.

The GD often gives some useful information, and here we see a huge improvement in the important 100Hz-1KHz range:


Decay gives much of the same information as the spectrogram, but presented in a different way. We see a much more frequency-linear decay, cleaner and more tidy behavior in lower midrange:

All decay graphs use these settings:

Don't put too much weight on the first arrival here (the very first, top, line), as the measurements are made L+R and the mic is not exactly centered, and then seaeting or other objects close to the mic measurement position will affect the early/direct sound. Look at the smoothness and resonances further out in time.

This may look a bit complicated. First, measurements must be properly done, then there is difficult analysis required to make any sense out of it.

In reality, it is not that difficult. To see what is necessary to fix with acoustic treatment, it is not necessary to do any measurements, a description and pictures of the room is sufficient. Measurements can be used to validate, but it is not really necessary. Final adjustments will always be done by listening, so measurements here will be more like something that is interesting to look at afterwards.

Measurements are necessary to calibrate the system in the bass range, where efficient acoustic treatment is not practical to do in most rooms.

Measurements can also be used for objective analysis of you existing room, to see if there is potential for improvement. Many enthusiasts have never hear a properly treated room, so there is no reference. Measurements can then show how a room compares to rooms that work reasonably good, and thus tell whether there is room for improvement.

Typical problems are insufficient absorption in the upper bass - low midrange, and in rooms with lots of absorption, it can often sound very dead and dull. Both these problems are solved with proper acoustic treatment.

One last look at the measurements before we look at the practical implementation.

REW now has a RT60 Decay graph window, where you can see the decay profile. This is very useful as it is easy to see initial early decay drop and profile of decay, across the frequency range.

NOTE: These pictures show one situation in each, with 250Hz left and 2KHz right.


After acoustic fix:


Here we see some improvement in early deacy, and a significant improvement in overall decay time, and decay is slightly more even across the frequency range.

Now to the most interesting part - how the room was fixed. Bjarte has modelled his room in Sketchup, and provided excellent images of the room with the acoustic treatment.

The room:

Front wall view:

Similar to my Room2 (https://www.kvalsvoll.com/blog/forum/topic/rom2/), absorption mainly 200mm thick, covering whole wall surfaces and then slats and diffusors to adjust reflection level at higher frequencies.

This gives very efficient decay control down to 100Hz, which can be seen in the measurements. The bass range is not fixed even with the 400mm absorption on the back wall, we see there is a reflection at 80Hz that was not affected. Bass is fixed with eq on the bass-system, and now Bjarte has installed 2x T6 small subwoofers at the back, I may explain why in another thread.

Keep in mind that the pre-fix in the measurements here is not a bare room, there was already some acoustic treatment in place. I have visited this room two times, and got to hear the before and somewhere in the process before it was completed. There was a huge improvement in clarity and spatial holography, very acceptable sound, and in a different league compared to the typical hifi-showroom.

Acoustic treatment that actually works is not magic, it is not expensive compared to what many put into speakers and amplifiers, and all rooms can be fixed.

Some of this you can buy - such as diffusors, the largest treatments are built on-location in the room. Do it yourself, or hire a carpenter. There is no reason not to do this in your own room, resulting in a sound quality difference so huge you really have no idea before you experience it yourself.

The F205 with its controlled radiation makes it much easier to fix acoustics.

It is not necessary to kill all reflections from 1. reflection points, and it is not required to have a completely left-right symmetrical room.

The solution can then be focused on getting enough absorption in the 100Hz-100Hz range, placed on surfaces spread around the room. Reflection level is adjusted by adding reflective or diffusive surfaces.

Some rooms may have restrictions in what can be practically done. In a typical living-room, it will not be an option to treat all surfaces, but reasonable results are still possible to achieve. The ceiling will be possible to treat with 20cm thick modules in most rooms. Even doing a complete wall can be possible - the end result can be made to look just like any other ordinary wall, you will not see the treatment.