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Mid & High Frequency Reflections - Identification and Treatments :

Inaccuracies in mid and high frequencies are caused by comb filtering. The direct speaker signal path reaches the ears at the listening position first, and then very shortly afterwards reflections from hard / untreated surfaces in the room also reach the ears. It’s the combining of the 2 signal (direct and reflected) that causes comb filtering, which in turn creates inaccurate frequency response and ruined imaging.

The most problematic reflections are those that arrive within around 15 to 20ms of the direct signal reaching the ears (ie reflections that arrive very soon after the direct path signal reaches the ears).

Another factor is the strength (relative volume) of the reflected signals. Loud reflections cause more problems than quiet reflections. Loud reflections are classed as being any which are within 15 to 20db in level compared to the direct speaker to ear path signal. Any reflections above -15db will cause issues in the frequency response and imaging, whereas reflections below -20db don’t usually require attention (although for studio control rooms reflections below -25db to -30db may be desirable).

So this gives us a set of ‘standards’ to aim for for mid and high frequencies in terms of individual reflections arriving back at the listening position, which is : No reflections within the first 20ms should be louder than -20db. These standards are well established in broadcast studios via the European Broadcast Union (EBU Tech 3276 Document.. Download here. Section 2 is of relevance), and in critical listening spaces such as recording studio control rooms, film mixing studios (Dolby Standards for mixing rooms), and hi-end hi-fi listening rooms.

The method used to achieve reflections lower than 20db within the first 20ms is via room testing, and specifically use of the Energy Time Curve plot. Below as an example untreated room ETC taken at the listening position :

Note several loud reflections in the ‘untreated room’ ETC above - up to around -12db.

In the ETC for the same room after treatment the following results are achieved :

All reflections are below -20db, apart from a couple of very early reflections (these are from the desk surface, and are easily dealt with using thin squares of absorbent correctly placed).

The improvement in the Energy Time Curve provided a flatter mid and high frequency response, as well as vastly improved stereo imaging.

Identification of Mid / High reflections is best done using the Energy Time Curve plot during room testing, as the exact area that each individual reflection is coming from can be easily identified, which in turn helps to create a tailored / targeted room treatment plan. However, if main reflection points (left & right walls, rear wall, and ceiling) are covered properly (as per example plan below) then most of these loud reflections will be dealt with.

The only other area to be aware of is the wall / ceiling corner on the back wall, and the wall / ceiling corner in the front corner (ie behind speakers). The front wall / ceiling junction can often be dealt with by correct placement and / or angling of ceiling clouds, whereas the back wall / ceiling junction may require custom / narrow corner units (or BF-075 or BF-125 type absorbers angled across the wall / ceiling corner).

Please note that rear wall (ie.. behind listening spot) vertical corners will also create loud Mid / High frequency reflections, so the room treatment for mids and highs and bass control needs to be considered as a whole. The addition of broadband corner bass traps in rear corners will control bass, but also ensure that no loud / destructive reflections are going to be headed back to the listening position.

Also.. BF-125, BF-175 and BF-180 type bass traps used in reflection points will also help to improve bass response, so corner devices help to improve both bass and mids and highs, and wall / ceiling mounted traps placed in reflection points improve mid / high response, and help to control bass.

This gives a complete room treatment plan :