ACOUSTIC OPTIMIZATION OF A ROOM

    DIY

    A not unknown problem for anyone involved in sound: the acoustics of the room are insufficient or even beyond the threshold of pain. This is due to so-called "standing waves", echoes and resonances. Each room uses shape, length, width, and height to create individual "room modes" whose weakening or total elimination should be the beginning of any acoustic optimization.

    After I had finished my period of life with almost daily stay in great studios, it became necessary to improve the music room in my apartment significantly.

    Most (like me) have more or less no clue at the beginning, how, where and what to start with. In addition, the willingness to spend a lot of money on the acoustic optimization could be relatively low.

    When searching the Internet for help, you will find a lot of advertisements and offers for acoustic panels, but learn nothing about the further processing with it (e.g. where to place them), which is necessary - not least because of the otherwise inevitable resulting optical destruction: most offered cheaper panels are made of foam or polystyrene (such as styrofoam). So much for the look.


    But the church is supposed to stay in the village. It's not about the construction of a recording studio, but about the acoustic taming of a music room - gladly good sounding and "beautiful" as well.

    I try to sum up my explorations and experiences with the acoustic improvement in the following.

    Ideally, the "acoustics" of a studio, starting from a shell, are captured by measurements and calculations. Following the results, absorbers (for "deeper tones") and diffusers (for "higher tones") are provided at the calculated points and discreetly integrated into individually designed modules in an overall design of the room. The interior walls of the studio are as far as possible decoupled from the actual walls, ceilings and floors of the house. Such a thing is "priceless" and trying it for yourself should start with the shell as well. But that's another job.

  • First: calculate room eigenmodes

    First, there is some theory, but it can be mastered. The blessed Internet offers some room-mode calculators - I have used the Hunecke room eigenmode calculator, which can be seen on the right in the picture. It is only suitable for rectangular spaces, but also very easy to understand.

    After entering the length, width and height of the room, the lowest 20 spatial frequencies are calculated and - that's why I chose it - visualized differently in a graph depending on the frequency. I then played the frequencies with a sine wave generator through my loudspeakers (in the already furnished apartment), walked along the walls and was surprised how exactly the graph shows where the frequencies "rock".

    I can not recommend enough to start with this procedure, because it immediately sets up a kind of basic awareness of how, and especially where the disturbing sound reflections must be slowed down. It makes sense to start at the lowest frequencies, because they cause the greatest destruction and create the basis for further problems in higher ranges (frequency doubling).

    Absorbers are needed to absorb or capture low frequencies. In order not to endanger the living environment or the living air, I deliberately renounce the use of mineral wool of any kind.




    Helmholtz Resonators (plate vibrators)

    For the lowest frequencies, the greatest effort is required, even in terms of space. I have two so-called "Helmholtz resonators". There are two 100x80x60cm boxes, the size of chests of drawers. I came to the used pair of these resonators years ago and they handled the worst problem, some deep standing waves, surprisingly good. This problem (deep standing waves) occurs as soon as a subwoofer is used in the room. Subwoofers require a special treatment in the room, because they deliver very long-wave frequencies. The bass absorbers also require a lot of space (quasi the double basses among absorbers).

    My Helmholtz resonators are designed by mb-akustik and contrued for broadband. Today they are built a little smaller and belong to the "type B". They reach from about 30 Hz! The actual corner frequencies are of course higher and fall up and down in a certain slope (just like an EQ). Although they seem bulky and large, these boxes are an effective solution to "room droning" as they cause, in my perception, more throughout the room through their "counter vibration" than thick foam/mineral wool absorbers in which the deep Frequencies simply "swallowed" on the walls. Nevertheless, some additional porous absorbers will be helpful and are necessary.

    Another advantage of the Helmholtz resonators is the significantly longer durability due to the MDF design. Foam has at some point (after about 5-10 years) largely released its plasticizers and then begins to trickle, or, worse, assesses mold by penetrating moisture inside. Since even with foam absorbers most of the money must be spent for the lowest frequencies, the durability is thus a double important factor! (Cost control)

    The Helmholtz resonators are by far the most expensive part of the acoustic room optimization and I had some inhibitions to buy such expensive "boxes" (new about € 850.- per piece) - I only payed €350 .- for the used pair. These inhibitions, or the doubts on which they were based, however, were blown away when I had (positioned) the resonators in the room!
    They are big boxes with a big effect.

    Some Helmholtz resonators are tuned to selected frequencies and then construed very narrow band. At mb-akustik, these narrow-band resonators belong to "Type A". When buying used so be sure:
    Narrow-band "Type A" resonators are actually only to use in the room for which they were built! Only buy used, if you take over the room!

    All foam panels, absorbers or diffusers, I would buy in no case needed, already because of the integrated evaporations of the previous owners!




    Porous Absorber

    As the most appealing solution between appearance, effect and price, I chose Basotect pyramid foam. My experience with DIY mineral wool absorbers was that the indoor climate (the room air) suffers too much underneath and therefore I absolutely want to avoid mineral wool in the apartment! The Basotect pyramids are made of 7cm thick melamine foam. On a 5cm base sit 2cm high pyramids.

    I therefore built wooden frames for the 100x100cm absorbers and chose to string a thin fabric (the cheapest at IKEA), stretched out the outside and pulled it together and fixed with a bow at the back to be washed or changed for color change. Of course, it could also be stapled with a tacker, but the refresh or maintenance significantly complicate and the material would wear.

    The sellers offer "acoustic glue" for these absorbers, which I consider sub-optimal. If the absorber is glued, it is unnecessarily stiffened and reduced in effect. That's why it "stands" in the front of the 12cm thick frame, leaving 5cm of air between the absorber and the wall. It does not only look more subtle/better than glued pyramids on the wall - it also sounds much better acoustically! (right picture)

    The sound absorption coefficient of the pyramid foam is> 0.9 from about 600 Hz and 4 absorbers in the package cost € 160.-. This achieves a much less painful price range.

    I use these "standard absorbers" only on the walls - they could of course also be used on a ceiling, however, the (delicate and fragile) material is too soft to hang straight and, if it is not to be glued, from below additionally stabilized (perhaps by a diagonal cross of nylon cord). But for the ceiling I have - after some unsatisfactory attempts with different absorbers - found another, much better solution.



    The successor of my pyramid absorbers, now under the house brand: the t.akustik Melamine Pyramid 70mm Grey


    Diffuser

    Especially at my studio table I wasn't satisfied with the sound for a long time and had therefore experimented with different absorber suspensions above this place. However, the changes were rather negative, which took time to comprehend. In addition, mids and heights were superimposed, which sounds like a distortion and made the spatial localization of the sounds from the speakers almost impossible. Especially in smaller rooms (with short reverberation times under one second) it comes quickly to over-damping with absorbers, so now nothing led past a diffuser. Due to the high prices and the sometimes considerable weight that I do not want to have over my head, I had long denied the diffusers.

    As a possible solution, which is easy and comes at a very favorable price, arises the 60x60cm WDS-7 Diffuser, which come in a package of 8 pieces and with € 89.- (complete!) Are incredibly cheap. All I had to do was overcome my basic rejection of EPS foam (a kind of styrofoam), which took some time. Also the "optics" demanded something aesthetic accommodation. The effect convinced me immediately.

    For diffusers, a distinction is made between 1D and 2D diffusers, where 1D radiates the frequencies in the horizontal stereo base and 2D scatters in all spatial directions. Roughly speaking, the 1D is used more on the side walls and 2D rather on the ceiling, front and back wall. However, the smaller the room, the more often 2D diffusers can be more advantageous also on the sides. Important for diffusers is that they are not too close to the ear, so that the scattering gets the required space. For this reason, I came to the small depth (7cm with max 5cm deepening) of the WDS-7, because I am 2m high and the ceiling 2.5m. Most "better" (more expensive) diffusers have a depth of 12-18cm. The still slight absorption of the diffusers (even after painting) is helpfull too!

    Although the WDS-7 diffusers belong in the 1D category, they can achieve a kind of 2D effect by an opposite arrangement. Again: these diffusers are extremely favorable! More expensive EPS diffusers cost already over € 85.- per panel! I can not say how much better they are - but I will order another package of these diffusers for some experiments on the walls.

    With the EPS diffusers, however, more details are to be taken into account during the treatment:

    Because of the softness of EPS foam, I think gluing makes sense. Not punctiform, but flat, to reduce natural vibrations. EPS foam should not be "pressed" - it creates pressure points immediately, which then remain permanent! The panels need not be pressed when gluing because of their low weight. When using e.g. with the Thomann Acoustic Adhesive, it is enough to "hold" them for a few seconds on the wall or the ceiling with very slight pressure. For 2-3 minutes, a subsequent position correction is also possible. The glue seems very soft to the plaster and easy to be removable again - no comparison with mounting glues!

    If left untreated, these diffusers will absorb (too) strongly. To reduce this, I painted the panels with dispersion (except the back) prior to gluing. The hardening of the paint noticeably improves the effect and also reduces the polystyrene look. You need a relatively large amount of paint (the area to be painted on a 60x60cm panel is almost 2sqm and the porous foam takes up a lot of color). The paint also serves as a seal of the EPS foam.

    I used Thomann's acoustic glue to glue it. A cartridge was exactly enough for the 8 panels ("in molded garlands"). If the panels are to be glued flat according to instructions (spread the adhesive with a putty knife), a second cartridge is required. With glue and paint I paid only about € 110.- for the ceiling construction with 8 panels. The light, but somewhat stupid work (the painting of the many mini-surfaces!), I coped with drying breaks, within a day easily.


    Gobos

    Sometimes mobile acoustic partitions are extremely helpful - especially with microphone recordings in the room. As part of my absorber experiments, I had attached two EQ Acoustics Spectrum 2 L5 Tiles (1150 x 575 x 50 mm) to the ceiling, which had not satisfied me. I had noticed that they absorb much more in close vicinity than the pyramid foam - but I clearly prefer the pyramids in the effect on the wall for the room. So I converted it without further ado and am now the proud owner of two gobos.

    To keep them free in the room, I mounted them on Showlite FLS-10 PAR light floor stands. To fix the absorber, I have screwed two metal angles (as "U") with the feet, so that the absorber can be easily inserted and removed again. The front angle is (for aesthetic reasons) only 6cm high, the rear angle is 20cm (so it's more of a "J"). Since the back of the absorber can be used as Velcro, I have attached a little Velcro on the inside of the long angle for additional fixation. When lifting the gobo, of course, the foot stays on the gobo. Very convenient. Such a pair of gobos are really a good thing to have and they make recordings in the room much easier!

    When both gobos are side by side at an angle, they are very good at keeping the room's sound reflections off the mic. The low height of 1.25m also suits me very well, as they do not obstruct the eye contact even when sitting.



    Electronic Calibration of the Listening Area

    After the general room acoustics the listening place (at the mixing desk or at the DAW [Computer]) has to undergo a special treatment: here the optimum should come as close as possible. That may have already become clear through the positioning of the diffusers on the ceiling - they are just above that space. For a real "fine adjustment" I therefore chose an electronic calibration, since it also integrates the used speakers in the process. This requires a software and a measuring microphone. Sonarworks Reference is a very good "package", even for headphones, which can be aquired also with an included measuring microphone for relatively little money. Good measuring microphones are very expensive and I only need them to measure my space, so extremely rare. The supplied measuring microphone is mediocre, but has its own serial number. Each of these microphones was measured before sale, documenting the individual deviations. After entering the serial number on the website, you get a protocol, which is then used by the software to balance the values of the microphone. Great idea! It works.


    The calibration is done as follows: The software "Reference 4" provides different positions for the microphone, which, GUI-supported, are very easy to find. The software reflects the position of the microphone through a point and indicates the position to be taken by a circle. So you can see if the microphone is in the right place (= in a circle). If the position is correct, a signal sequence is triggered via the loudspeakers. In just a few steps, the entire calibration process is completed and an EQ curve is created, which is switched between the DAW and the amplifier. It compensates for the acoustic imbalance at the listening site - also via channel volume adjustments - or, with EQ only, in the headphones.

    In my room the studio table isn't placed in the symmetrical center of the room. This makes one speaker much closer to the wall than the other. Despite the acoustic optimization, there are (of course) more reflections on the nearer side of the wall, and thus a slightly higher volume, which makes the stereo image a bit 'crooked'. These are the issues that Sonarworks Reference 4 resolves well. This increases the chance that own mixes on other systems sound good too! I think, it is the most favorable solution for electronic calibration. Very helpful!



    A Résumé and a Guess

    There is no really cheap solution for the basses. Basses are a serious thing - the base. Do not spill, paddle! The original price of my two resonators makes up half the cost of all the acoustic elements used (at the moment). So far, I have not tried any bass traps or other porous absorbers for the low basses - just by the required amount (without resonators) I estimate an even higher price, as the resonators - including overtime and greater optical impact/devastation. Although, a few extra bass traps to support the resonators, are in the field of my imagination.

    With the standard absorbers and the EPS diffusers, I have supplied myself from the lowest "price level with frequency specifications" and at the same time I'm amazed and delighted with the good effect. However, this effect is achieved only because the space was previously analyzed (eigenmodes), and because the elements were placed correctly and not processed in the state in which they arrived (color, stringed frames, arrangement). It is advisable to read a little into the operation of diffusers, since no instructions for use will come along.

    The acoustic improvements are clear and I'm very happy about that! I will experiment even more with diffusers. There are no perceptible changes in the room air due to the elements used. The changed acoustics noticeably improve the whole "room climate"! Good acoustics also create well-being.

    All supposedly much better alternatives to the absorbers and diffusers I choose mean at least an eightfold increase in costs. I do not do something expensive like this at will. I would choose to make the step to the expert order before and then have a tuned system, which would certainly be expandable in different stages (especially since the company later also is available of all the data to the room). My choice fell on the company mb-akustik.

    As a lonely unsuspecting customer in the broad market, many false purchases are the best choice. Lots of foams with too little effect - good to to be recognized because of missing frequency information - I would therefore consider only products that are provided with technical (frequency) information. Even with the diffusers it is difficult. Many models offered - e.g. on eBay - are without protocols and some QRDs are built with so thin woods, that I fear they will be self-resonant at higher volume.

    Therefore, I prefer to trust the "responsibility" of the larger music stores that offer these elements under their own brands. Compared with the more expensive products (including other manufacturers) of music houses websites, I suspect that the surcharges will be paid mainly for the more complex appearance and more expensive material (wood). Functionally, I suspect only minor differences. So if you are ready for something DIY, the cheaper range can be quite useful!

    The "professional" look of the souped-up foam corners I do not like anyway. I am and remain an avowed anti-professional!



Contact

Jochen Bohnes

27793 Wildeshausen.

Germany

Phone: +49 151 614 910 38

eMail



Used Equipment! Eurorack, ARP-Case, Gong-Stand...