Earlier I hinted about other tutorials on Ethan's site which runs counter to published science. This one took a while to prepare as I wanted to run some personal tests to demonstrate the same point in addition to quoting the literature. If you go to Ethan's site, you see a decidedly cold reception to electronic correction/EQ. The back up for that is in sentences like this: ". Modal ringing - an extended decay at some, but not all, bass frequencies - is just as damaging as a skewed low frequency response, and EQ cannot reduce ringing. Only bass traps can reduce ringing."
It goes to say others disagree but that he has not seen it work over a "large area." Not sure how that is related to it not correcting ringing.
As a way of background, when you play your speakers in your room, so called "room modes" or resonances serve to change the frequency response of the speaker. So much so that you can measure your speaker in an anechoic chamber all you want. The response in the room will be determined largely by the room at frequencies below transition (200-400 Hz). When we measure the frequency response in this region, we see pretty wild swings, as much as 30 db. Every 10 db perceptually causes us to think something is twice as loud. So 30 db is pretty bad. None of this is in disagreement. What is in disagreement is what happens in "time domain." Time domain is what it says. It shows you what happens to a signal one sample after the other. Room resonances manifest themselves as "ringing" in that if you excite the room with a pulse, the room keeps signing the tune even after the note is gone. Time domain resonances can be shown to be audible below transition frequencies. A base note that lasts longer than it is supposed to will sound different than one that is not. Ethan in his statement says that electronic EQ is half a solution if it only works in frequency domain and can't impact time domain ringing. Naturally he positions acoustic products as the full solution even though he acknowledges in his article that as frequencies go down, they can get pretty unwieldy.
Back to the point of contention, Ethan backs his point of view with a test of Audyssey auto EQ. He shows before and after "waterfall' displays which show the combined frequency response and time, indicating no change in time domain. Note however that his thesis summary is not limited to what Audyssey does but the entire domain of electronic EQ. This is the graph he shows for the sub which is the scenario I tested also:
So I set out to test this phenomenon in a controlled environment. I put a sub in a corner, turned off the crossover and used Room EQ Wizard to run a sweep. I generated a waterfall display with carefully set parameters to make the output visible *and* correct. There are a lot of pitfalls here. If you want to read about that in general, and this test specifically, click on the first link in this search: https://www.google.com/search?q=site%3Awhatsbestforum.com+Acoustic+Measurements%3A+Understanding+Time+and+Frequency&rlz=1C1SNNT_enUS374US375&oq=site%3Awhatsbestforum.com+Acoustic+Measurements%3A+Understanding+Time+and+Frequency&sugexp=chrome,mod=0&sourceid=chrome&ie=UTF-8
This is what we get:
The back wave shows our frequency response which is not very pretty with peaks and valleys as one would expect. Front to back shows how long the frequencies are lasting. This display is rather compromised and can look different with different settings in REW. Again, the link I provided earlier describes this in detail. For the purposes of this exercise though, we can live with it.
Eyeballing for a peak, I chose 53 Hz as a frequency to go after to pull down. I also made a wild guess on how wide to make my parametric EQ. All the testing by the way was done on a JBL SDEC-4500 Synthesis EQ which gives me plenty of these filters with very fine grain control of frequency and “Q.” This is what we get now if I overlay this EQed correction in green over the earlier brown measurement of no EQ:
The blue line going through the graph from front to back is set at 53 Hz. We see right away that the frequency domain correction at 53 Hz has been accompanied by respective change in time domain in reduction of ringing. The faded brown shows what used to be there which is now missing in green.
In the article I show addition of filters one at a time but for brevity, let’s jump forward and look at what happens with four filters in play:
Like clockwork each change in frequency domain, bringing a peak down has resulted in the time domain (back to front) improvements. The blue line for example is for 110 Hz and we see that it has done its job.
I don’t think it gets any clearer that what Ethan says in his online article does not hold water. Ringing is changed for the better. It really had no choice though
. The science says it should. Room resonances with respect to peaks in a room are what we call “minimum phase” phenomenon. Such a system has a phase response which tracks its amplitude (Hilbert transform). In that sense, if you fix the amplitude with a filter with reverse response to the resonance, the correction applies to the phase just the same and yu get correction in both frequency and time domains. The literature tells us the same things. Here is Dr. Toole:” Using equalization to reduce the bump also attenuates the ringing so both problems are solved simultaneously.”
Dr. Toole shows the following measurements:
He does the work with both coarse and fine grain EQ and in both cases we see a reduction in time domain ringing that accompanies the flatter frequency response on the left.
Peter. Craven and Michael Gerzon in their AES DSP conferences titled, “PRACTICAL ADAPTIVE ROOM AND LOUDSPEAKER EQUALISER FOR HI-FI USE” come to the same conclusion and back it with measurements as I have done:
Per Rubak and Lars Johansen in their AES paper draw on the above work and opine the same way:” As pointed out by Craven & Gerzon , room equalizers based on Digital Signal Processors are able to reduce the reverberation time considerably, even if we only use minimum-phase equalizers. Our preliminary test results are in agreement with this important potential for DSP based equalizers. Therefore we have put focus on objective test methods concerning the improvement of the room acoustics using equalizers.”
So you have my test data plus three authoritative sources all contradicting Ethan’s statement. Listening tests to my quick optimizations confirmed the same. I played guitar strings on the sub alone and it was rewarding to hear every single pluck with EQ, which was smeared without before. So subjective evaluation follows objective analysis.
Here is one of Ethan competitors arriving at the same results as I have in a real room/project: http://www.acousticfrontiers.com/Room-Correction-Services.html"Below: waterfall chart showing before / after results from application of room correction at 25Hz and 50Hz by Acoustic Frontiers. Notice that both the peaks in the frequency response and ringing in the time domain have been eliminated. These measurements were from a 9 seat dedicated home theater that we calibrated - see the full case study."
Nyal has provided the above data to Ethan on this forum yet we still read what is written on his site.
Where did Ethan go wrong? If you look at the Audyssey correction, you see that little of what it did matched what I have done here. Its “after” EQ still has pretty ragged peaks so not much correction was applied. Some peaks were taken down others raised. The angle of presentation also makes it hard to see what the back frequencies are doing as there is a dominant one in front. So the fact that correction didn't come about in time domain is natural: it didn't work in frequency domain either. So this is not a good example to use to make a point, nor is it reasonable to have it represent all classes of EQ systems as Ethan has done. He goes on to lament about this class of products and manufacturers:” Unfortunately, the popular audio press gushes uncritically over products like these, printing press releases as fact and never actually testing the validity of manufacturer claims. The appeal of a small electronic device that claims to replace large and visually imposing acoustic panels is undeniable. But wishful thinking does not make it so! Even the vendors themselves offer no real proof that their products work as claimed. “
While he is somewhat justified in saying that companies like Audyssey don’t provide much visibility into how well their system works, others such as Harman have done so in publishing both papers and listening tests results. Here is test data for a number of EQ systems I reference in my article, http://www.madronadigital.com/Library/BassOptimization.html
In this test, two EQ systems did worse than no EQ. The Audyssey on the right and Anthem next to it. The yellow is Lyngdorf followed by the highest scoring results on the left by Harman. The green is optimized for multiple seats and blue for a single. All three results outperform doing no EQ (orange). You can read more about this test here: "The Subjective and Objective Evaluation of Room Correction Products," Sean E. Olive, John Jackson, Allan Devantier, David Hunt, Harman International, 127th Convention, Audio Eng. Soc. Preprint 7960. http://seanolive.blogspot.com/2009/11/subjective-and-objective-evaluation-of.html
The excellent multi-seat results from Harman (which is deployed in “ARCOS” for the JBL Synthesis EQ I used) shows that we can achieve excellent performance across a wide area unlike what Ethan implied. Indeed EQ done right is a powerful technique. Combine that with multiple subs and SFM (explained in my article) and you may not need any low frequency acoustic products.
Hopefully you now see why I am so unhappy with the tutorials on Ethan’s site. No matter which way you dice it, they steer people away from science and always toward one thing: buy more acoustic products. Every mistake I find results in sales of more acoustic products. I get that he is in that business. Nothing wrong with promoting his products in the best light possible. My issue is him providing these articles as often as he does as guiding people new to this field on how acoustics works. I am great fan of him when it comes to simplifying concepts as he does but we can’t lose track of what the science really says. And in these major points it simply does not say what he says. Many of these other techniques that he either ignores on his site or gives backhanded compliment are quite essential and powerful in optimizing the sound in our rooms. You can't ignore them. They are great problem solvers as a subwoofer is much easier hidden in the room as compared to 17 bass traps!
As you see, it takes a lot of work to write these posts. I do them because I have passion around getting the right info out there. I am so frustrated that I wasted so much time on mistaken notions as read online and Ethan's articles. It is time that we direct people to the right data and not cover up the hole in the purse as was said here.