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Old 29th October 2019
Lives for gear
Jens Eklund's Avatar
Let me first apologize in advance, but I´m taking the opportunity to address the most common rookie mistakes I see people make over and over again. Unfortunately; you managed to cover them all.

1. Low frequency behaviour and how to deal with it:

You have little to no LF treatment. It´s an unfortunate common misconception that all room modes relate to the corners. This is simply not true. The most problematic room modes (lower order axial modes) use the entire walls related to the mode, not only the corners. The first axial mode related to length for example uses the entire front and rear wall, so in your case; the total amount of treated area deeper than 8” seen by this mode is about 11,5%. This is what to expect in terms of performance from a porous absorber only 8” deep compared to a 13” deep absorber:

Advice Wanted: Best Option(s) for Additional Bass-Trapping-8-inches-vs-13-inches.jpg

Threating only 11,5 % of the relevant area with an absorption coefficient of about 0,25 (at the theoretical resonant frequency of your first mode related to length) is doomed to fail.

Same goes for axial modes related to width but with even less percentage area treated (since the room is a bit longer than wide) and when it comes to modes related to height, you have close to no effective treatment.

If relying only on velocity based absorbers (porous only) to tackle the low modal range, they need to be deeper (13-14” or deeper depending on fc of first mode and flow resistivity of the wool used) and you need to cover a lot of the relevant surface area. I consider the entire rear wall to be a minimum. If front wall untreated (in the lowest range); you have then treated 50% of the surface area related to axial modes related to length.

A more effective way of absorbing low frequencies is to use pressure-based absorbers. Not only are they more effective in the low frequency range (for a given depth) but also has the advantage of being reflective above the modal range, so any surface not related to early reflections can be treated with some form of pressure-based absorber. Helmholtz arrays (slotted panel) being the most common type used, easy to predict and most forgiving:

helmholz not very popular?

Also, a note on how to identify your modes:

Need help to tune my control room. (First Measures Included)

2. Early reflection control:

Ceiling acoustic panels

3" Roxul enough for early/first reflections?

3. Diffusers:

Your room is too short (3,67 m) to benefit from diffusers. The purpose of having diffusers on the rear wall is to scatter the energy in the horizontal plane so that this energy can return to the sweet spot via the rear sidewalls. If these surfaces (rear side walls) are treated with broadband absorbers; this energy return will be lost and thus defeat the purpose. The return should ideally arrive at around 20-25 ms after direct sound but down to about 15 ms is ok (at least that’s what I think personally, 12 ms is considered bare minimum). Also, if diffusers are used; they need to offer good performance in the range 300-400 Hz and up (to at least about 3-5 kHz).

Here´s an example of what I would consider minimal treatment in order to reach satisfying results in a small room like yours:

Advice Wanted: Best Option(s) for Additional Bass-Trapping-1.jpg

Dotted vertical and horizontal lines are nodal lines for first and second axial modes related to length and width. We usually try to avoid sitting in the nodal lines related to length, especially the first (centre). Second nodal line might not be critical if room is heavily treated. If possible; also try to avoid placing the speakers in the nodal lines related to width.

If not only porous absorption on rear wall, possibly something like this:

Monitors placement/room acoustic

If one wants to take it one or two steps further, you could add treatment to front wall (to combat the first null due to SBIR related to front wall), doesn’t need to be very deep (8” is ok) assuming the rear wall is properly treated to deal with first and second order axial mode related to length). Possibly using HH arrays close to the floor and ceiling and leaving the middle section (height) broadband to avoid early midrange reflections.

Angled HH arrays on rear sidewalls effectively solves possible flutter echo issues and adds low frequency absorption targeting low order modes related to sidewalls.

Advice Wanted: Best Option(s) for Additional Bass-Trapping-2.jpg

If diffusers are used (represented in green), assuming the room is long enough; rear sidewalls needs to be reflective above the modal range in order to be able to reflect the scattered energy back to the sweet spot.

Advice Wanted: Best Option(s) for Additional Bass-Trapping-3.jpg

As far as the ceiling goes; the same principle as the side wall treatment can be applied.

Again, sorry for being harsh, but the red pill is all I have to offer

/Jens Eklund
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Advice Wanted: Best Option(s) for Additional Bass-Trapping-8-inches-vs-13-inches.jpg   Advice Wanted: Best Option(s) for Additional Bass-Trapping-1.jpg   Advice Wanted: Best Option(s) for Additional Bass-Trapping-2.jpg   Advice Wanted: Best Option(s) for Additional Bass-Trapping-3.jpg