First of all I would like to praise you and congratulate you on the fact that you are dealing with things like absorbers and room acoustics! Most audiophile sound lovers have already taken a look at the frequency response of their speakers. But there is often a lack of knowledge that the sound that arrives at our ear is not just direct sound from the loudspeaker. Since we usually consume music in a closed room, the surrounding walls with their reflections contribute greatly to the sound.

If you already know me from other blog articles, you already know that I am not a friend of purely subjective statements. I love my measuring equipment because I can use it to eliminate the fluctuations in my judgment. And it helps me understand why a measure sounds good or bad.

And to add the icing on the cake, I left the speakers and the microphones 100% unchanged. I only removed the absorbers completely from the room. As a result, the differences in the measurement diagrams can actually be pushed 100% onto the absorbers and do not result from a change in the speaker or microphone position. Clever, isn't it ?! 😁 But have a look yourself to get an impression of what was actually before / after in the room:

Difference like day and night

Before I show you the objective measurements, let me tell you as a little motivation to continue reading and trying out that the difference between the empty living room and the 12 absorbers that have now been built is gigantic. And in every way. The frequency response is significantly smoother, no annoying reverberation of individual frequencies, the overall reverberation is significantly shorter. Ultimately, much more of the music arrives at the ear and less is changed or added by the space. If you are looking for pure music enjoyment and you want to hear as many details as possible from your recordings, then absorbers should also be the means of choice for your room.

My absorbers

In another blog article I had already described in detail how I built the absorbers. The main component is rock wool with a density of 50kg per m³. I chose a size of 120 x 60 x 20cm because this is comfortable to carry on my own. For one thing, I already knew that I would certainly like to convert more often - if only to take measurements occasionally. On the other hand, there will certainly be a move at some point. With such small absorber units, the transport is very easy and you can choose a different arrangement, especially at the next location.

The measuring equipment I used

Even though I work with Smaart from Rational Acoustics for most jobs these days, I used AFMG's Easera software for this test. The measurement results are very similar for both, but for Easera I liked the presentation better with the option of drawing several curves at the same time in one diagram. Since we are particularly concerned with the differences between the two situations, this seemed to me to be advantageous. Five of my iSEMcon EMX-7150 were used as measuring microphones, integrated via my 8-channel Dante interface Salzbrenner NIO Xcel 1201.

Improve the frequency response

Let's start with the biggest problem we find in small rooms like living rooms and recording studios. Due to room modes, the straight frequency response in the high-gloss brochure quickly turns into an ascent and descent, especially in the range below 200Hz. Erasures of 10dB and more are not uncommon. Depending on the music, this is not that difficult. With acoustic music with double basses that use all the tones of our scale (in particular, classical music and jazz), it is very annoying when the individual tones sound differently loud.

We come to the first measurement diagram: the frequency response of the left speaker. Incidentally, these are Kali Audio LP-6 (Thomann*) and Genelec 7350 (Thomann*) subwoofers. The phase on the subwoofer has been adapted for the greatest possible addition in the transition range at 90 Hz. This comparison is not about the aim of seeing a perfect frequency response, but primarily about illustrating the difference between the mineral wool absorbers can have on the frequency response.

3 differences are striking:

1. The total level in the empty room is almost 2 dB above the situation with absorbers. The red curve shows almost only the direct sound for medium and high frequencies, while the blue curve also has a lot of reflections and reverberation.

2. While the bumps in the treble range are approximately the same on both curves (and thus my oral promise shows that the measurement was absolutely the same and nothing was changed), the bass range looks as if it had been smoothed out considerably. However, the smoothing does not come from the software, but actually from the absorbers. The ups and downs that we observe due to reflections and room modes in the empty room were leveled by the absorbers in many places. Ultimately, this was exactly the purpose I wanted to achieve with the absorbers. With this diagram I am very happy to observe that there is still a change even at 50Hz! Even with a 20cm absorber depth, an improvement can be achieved in the subwoofer area!

3. Apparently there are not only improvements. In some places small erasures are now visible that were not available before. In the medium and high frequency range we had a reasonably diffuse sound field in the empty room. The erasure from one wall reflection was thus offset by the increase from the other wall reflection. If we now treat two walls (the wall behind the speakers and the rear wall), then a number of reflections are completely eliminated. However, the reflection from the ceiling and floor remains. And their effects are even more noticeable as they are not offset by other reflections. The next big step is the development of a way to attach ceiling absorbers!

Now you will be puzzled by a point in the frequency response: It changes a little at 50Hz, but the cancellation at 58Hz remains unchanged. Yes, absolutely correctly recognized!

This extinction can now have two options: either a room mode between the ceiling and the floor [small excursion into physics: frequency 1st mode = speed of sound / wavelength / 2, thus f = 340m/s / 2.75m / 2 = 61.8 Hz ]. Or it is the ceiling reflection [difference between reflection and direct sound is roughly: 4.50m - 1.50m = 3.00m. First cancellation at f = 340m/s / 3.00m / 2 = 56.6Hz.] Due to the observation that the frequency shifts slightly when the microphone is moved, the cancellation appears to be from the ceiling reflection and not from a mode. Since I put my 12 absorbers only on the speaker wall and on the back wall in this first phase, the ceiling reflection is still untreated. I am therefore confident that, with the same type of absorber, I can also eliminate this problem if I create a way to position absorbers on the ceiling.

Reduction of reverberation

This point may be taken for granted. Of course there is some reverberation in an empty room. And when adding absorbers this is shortened. So far, nothing new. Why is this point so important for our sound quality?

You see time and time again that people paper their rooms with thin foam mats. This has an effect on the reverberation time in certain frequency ranges, but only for high frequencies. Low frequencies are unaffected by 2cm of open foam. And as a result, we get room acoustics that are anything but natural and unfortunately do not give much pleasure in playing music. The highs are completely dampened and the bass and part of the mids continue to cause problems in the frequency response and have a long reverberation time.

So it is high time to check how the 20cm absorber performs in this discipline.

Just in passing: Even for the recording of speech for YouTube videos or the like, I would personally prefer to use 5cm rock wool instead of these lightweight acoustic mats. Because even if we do not need the absorption at 50Hz, we need it in the range 200-2000Hz, and not only at 5kHz.

First a word about the procedure: normally an omnidirectional loudspeaker (usually a Dedokaeder) is used to measure the reverberation time (T30). It emits sound evenly in all directions. In my case, I just used my normal speakers. In absolute terms, the times determined are not entirely correct. Nevertheless, the differences between empty and full space are so great that we can read everything that is important to us for our purposes.

As mentioned, there is always a risk with absorbers that we over-dampen a room in the heights and that we do little in the bass. It made me very happy to see this measurement result. The reduction of the reverberation time from over 0.9s to about 0.6s goes in the right direction, especially since only two walls have been treated so far. With additional absorbers on the ceiling in the area of ​​the first speaker reflection, this will even be shortened a little.

But I'm absolutely thrilled with the uniformity of this reverberation reduction. Easera calculates the reverberation times down to 100Hz. And up to that point the absorber works in any case. It is not apparent that the effect somehow begins to wane at 100Hz. This coincides with the observation on the frequency response, where we could still see a good change even at 50Hz.

Room mode analysis with REW

[Addendum 22.3.2020]
After I have worked into the software REW (Room EQ Wizard), I would like to add the spectogram of these measurements, left without absorber, right with the 12 absorbers in the room.

The long ringing at 22Hz, 50Hz and 58Hz has also remained with the absorbers. There is still a need for optimization with additional absorbers on the ceiling (for the mode at 58Hz) and in the transverse direction (for the 22Hz and 50Hz mode). However, all other irregularities above 60 Hz are noticeably reduced. Ultimately, the weakening of the room modes shown here is the reason for the previously noted smoothing in the frequency response.

My conclusion:

everything done right so far! The 20cm absorber depth seems to fit well. The next step is the construction of the ceiling cloud.

And if you only have good speakers in your living room or recording studio so far, but have not yet bothered about acoustic modules, I can warmly recommend the use of rock wool absorbers from today!

Blog-article: Before and after comparison of broadband absorbers (part 2)

Blog-article: DIY boardband absorbers

Blog-article: How to measure room modes with REW (freeware)

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