Originally Posted by Milt99
Dr. Toole, I'm sure you have already answered this question in this sprawling thread at least once but there is no nested searching option I know of.
What are your views on digital room correction?
I know that various Harman subsidiarys like Lexicon & JBL Synthesis use certain forms of it.
However I do remember a white paper from some years back by Sean Olive.
I do not remember if you were a contributor or not, but the summary seemed to me that at that time, room correction,
at least among the software evaluated was considered pretty much superflous.
AFAIR, 3 systems were tested, Anthem ARC, Audyessy.
Can't remember the 3rd but the only other one I recall from that time was Trinnov.
The following is a stock answer to your FAQ which one day will be added to the companion website to my book: www.routledge.com/cw/toole
. It is open access, no need to buy the book.
The complete answer is in many pages in my book, but to reduce the argument to its elements there are four facts to remember.
First, an omni mic and analyzer are not equivalent to two ears and a brain. Rooms curves show evidence of phenomena that are not problems for binaural hearing humans. Equalizing these phenomena can degrade good loudspeakers.
Second, from comprehensive anechoic data on a loudspeaker (the spinorama for example) one can predict with good accuracy the steady state room curve in a typically reflective room. However, the reverse is not true. The anechoic spinorama can identify good or bad loudspeakers - this capability is seriously compromised if in-room measurements are all that is possible.
Third, a loudspeaker that is not flat on-axis but which has fairly constant directivity as a function of frequency can benefit from in-room equalization, but in order to know that one needs comprehensive anechoic data. If one had such data, the optimum equalization (above the transition frequency) would be based on the anechoic data, not a room curve. Also, if one had the anechoic data, one should not have purchased the loudspeaker to begin with.
Fourth, about 30% of our overall opinion of sound quality is attributable to bass performance - not only of the loudspeaker, but of the individual room acoustics through which it is communicated to listeners. EQ is one tool to improve the situation, but it can only work for one listener unless room modes/standing waves have been attenuated by proper use of multiple subs. This strategy is discussed in tiresome detail in Chapter 8. Steady-state room curves are the metric for this part of the frequency range and it should be smooth. Tilt, if any, is a matter of taste for the program being played. The "circle of confusion" is especially active in this part of the frequency range so a bass tone control is useful, especially if one enjoys older recordings.
Harman has spinorama data on its loudspeakers and therefore can anticipate the shape of the expected room curve above the transition frequency – somewhere around 400-500 Hz in small rooms. In these cases it is possible to interpret curves measured in a customer’s listening room. Over this frequency range equalization of small fluctuations should be avoided as they are most likely evidence of non-minimum-phase acoustical interference – not correctable by equalization, but capable of degrading the sound from well-designed loudspeakers. Only broadband “tone control” operations should be used. Addressing bass frequency issues is different, requiring much higher resolution.
Room EQ/calibration schemes are widely used these days, even in inexpensive receivers. Here I will quote from one of my papers: Toole, F. E. (2015). “The Measurement and Calibration of Sound Reproducing Systems”, J. Audio Eng. Soc., vol. 63, pp.512-541. This is an open-access paper available to non-members at www.aes.org http://www.aes.org/e-lib/browse.cfm?elib=17839
There I say at the end of the first paragraph: “The implication is that by making in-situ measurements and manipulating the input signal so that the room curve matches a predetermined target shape, imperfections in (unspecified) loudspeakers and (unspecified) rooms are measured and repaired. It is an enticing marketing story.” Most manufacturers now feel the need to incorporate some form of room EQ in their products, but from my perspective the challenge is to prevent them from degrading good loudspeakers. In the absence of comprehensive and accurate anechoic data on the loudspeakers any such equalization operation is a gamble.
Reading the instruction manuals for some of the more popular EQ algorithms one finds instructions suggesting that if, after equalization, the customer is not pleased with the sound, one should open a user-friendly window allowing one to modify the target curve. To what? Based on what? If it is how it sounds playing commercial music then one is including the circle of confusion into the EQ – it is not any longer a “calibration” but a tone control exercise that, once set, is fixed for all future programs. Dumb! Tone controls are definitely useful, but they need to be accessible for change according to program needs.