The Devil And The Auto RTA: Don’t Forget To Use Your Ears

Of course, we’re often working with limited amounts of equipment – if all you have is a pair of loudspeakers on stands, your options are limited. Still, turning the mains a few degrees in one direction or another (or using a block of wood to angle them down, thereby avoiding the ceiling) can still make a big difference!

The point here is that before making any measurement of any kind, look at your system, and listen to it, to make sure you’re not fighting unnecessary battles with the wrong tools.

I’ve been hired by more than one company to solve some problem or other, only to leave my measurement equipment in its case the entire time because what was really needed was someone with knowledge of how loudspeakers interact with each other and a room.

Let’s return to the subject at hand: why an RTA won’t give you a useful measurement, and why an auto RTA will only turn that bad measurement into bad system equalization decisions.

The first problem with the majority of RTAs is limited resolution. Commonly available 1/3-octave units are far too inaccurate to be of any use in the first place, like trying to drive a car while looking through Venetian blinds.

The “standard” resolution seems like enough because most of us have 31-band equalizers on our mains and monitors, but useful analysis requires at least 1/12-octave measurement, and preferably 1/24- or 1/48-octave!

This may seem like overkill, but the reality is that sound is much finer resolution than most of our equalization or measurement tools, and the finer detail one can see the better one’s decisions will be.

Here’s an example. I took two measurements of a sound system, one at low 1/3-octave resolution, and one at 1/48 octave resolution. Figure 1 shows some trends, but doesn’t really help me see anything.

Figure 1: Measured 1 meter 1/3 octave response of two overlapping EONA ADRaudio U 61 HH loudspeakers indoors, no equalization. Note the pronounced low frequency hump due to loudspeaker placement in one corner of the room. It is next to impossible to tell whether the uneven response above 500 Hz can be adjusted with equalization.

In Figure 2, it becomes immediately obvious that I have a serious comb-filtering problem that’s affecting my system’s high-frequency response and making it sound tinny.

Figure 2: Measured 1 meter 1/48 octave response of the same loudspeaker system. Comb filtering peaks and valleys of as much as 24 dB are now apparent in the higher frequencies due to the overlap in pattern between the two closely situated full range loudspeakers, as well as room modes and placement of the measurement microphone. While the low-frequency buildup centered on 250 Hz can probably be solved with equalization, it is now apparent that the problems in the higher frequencies are due to improper loudspeaker placement and attempting to equalize them would only change the loudspeakers’ tonality for the worse.

Had I tried to change system equalization based on what we see in Figure 1, I would have adjusted the overall tonality (which may help) but solved none of the real problems.

Figure 2 shows me that the problem I have can’t be solved with equalization anyway, but I may be able to solve it by re-thinking my loudspeaker placement.

The problem in this contrived case happens to be that I have more than one loudspeaker reproducing the same signal covering the same listening position – the loudspeakers
interfere with each other and cause comb filtering.

The solution is either to replace the two loudspeakers with one loudspeaker that’s louder but offers the same coverage, or to splay the two loudspeakers further apart so their coverage patterns are no longer overlapping.