RT not being an issue in small rooms is an incorrect inference. High RT in treble frequencies, results in overly bright rooms, while at lower frequencies, bass transients and separation are obfuscated. Getting acoustical energy, especially below transition frequency, to dissipate in small rooms needs astute design and significant commitment. If anything, high RT is a big issue in small room acoustics. A significant percentage of maladies misattributed to speakers and electronics is a direct consequence of high RT listening environment.
I didn't have time to watch the CIRMMT video. I did re-read Ch. 13 edition 2.
There two big things I noticed, all analysis is based on ideal (perfectly rectangular) rooms and use of
Waterfall Plots to visualize modal ringing.
The inverse correlation of frequency resolution and time resolution in Waterfall Plots is mentioned. I read that as a caveat emptor for the nature of visualizing a plot based on frequency, amplitude and time. I suspect, it is a consequence of the Fourier Transform math. There is no mention of the inverse correlation of resolution generating incorrect filters or resulting in incorrect application of DSP filters. Consequently, I chalk that one to inexperience or overzealous EQ by the user.
I use the Waterfall Plots below transition frequency, toggling frequency resolution with time resolution. Well below transition, I prefer time resolution. Closer to transition, things get murky. Of course, well above transition, only frequency resolution matters, and life is easy completely ignoring time.
Starting with mathematical modeling of an ideal room and then correlating it to observations made in an ideal physical room make perfect sense in the pursuit of science and understanding. This approach is not feasible in the real world. At minimum, the ideal room models completely fail (exempt height, since most of us have a roof exactly parallel to floor across the whole room).
I believe our approaches overlap in that we see EQ as a tool of last resort. I will send days playing with room setup and speaker/subwoofer locations, only adjusting distance and level settings, to get the best possible coupling of source to room. If I notice a persistent peak or dip, I'll break out the room mode analysis spreadsheet to help correlate observations to idealized mathematical expectations. This helps understand which battles are not winnable on placement alone and which to focus on. Only
then will I try EQ to control the worst peaks or dips.
I hear you on the topic of people considering EQ their path to audio nirvana or obsessing on speakers and electronics (or cables
) while turning a blind eye to the room or good speaker placement. To them I say, get Bose-d
.
Grab your book!
Why RT is not needed in small rooms:
p.62 Sound Reproduction 2nd Edition Section 4.3.4 "In the acoustical transition from a large performance space to a 'small' kind of room, it seems the the significant factors are reduced ceiling height (relative to length and width), significant areas of absorption on one or more boundary surfaces, and proportionally large absorbing and scattering objects distributed throughout the floor area."
p.63 "These [small rooms] are not Sabine spaces, and it is not appropriate to employ calculations and measurements that rely on assumptions of diffusivity. Schultz [1983] states, 'The amount of sound-absorbing material in the room cannot be accurately determined by measurement, either with the decay-rate (reverberation-time) method or the steady-state (reference sound source) method... One cannot trust the predictions of the Diffuse Field Theory for a non-Sabine room."
p.63 Section 4.3.5 "A measurement of reverberation time in a domestic-sized room yields a number. When the number is large, the room sounds live, and when the number is small, the room sounds dead... The numbers measured are small compared to those in performance spaces, and so the question arises if the late-reflected sound field in a listening room is capable of altering what is heard in the reproduction of music."
p.64 [after explaining that late reflected sounds are diminished in small rooms] "Nevertheless, excessive reflected sound is undesirable, and an RT measurement can tell us that we are in the ballpark, but for that matter, so can our ears or an 'acoustically aware' visual inspection."
Why RT measurements in small rooms are difficult:
p.63 "Reverberation time is a property of the room alone, and a correct measurement of it should employ an omnidirectional sound source capable of 'illuminating' all of the room boundaries. The reason for this is that it is assumed that the boundaries consist of areas or reflection and absorption and the central volume of the room is empty. The several formulae by which we estimate RT confirm this, and the values of absorption coefficient for the materials are 'random incidence' values, meaning there is an assumption of some considerable diffusivity in the sound field. Some practitioners incorrectly use conventional sound-reproduction loudspeakers as sources. The directivity of these is such that the resulting reflection patterns and decays are not properties of the room, but of the room and loudspeaker combination - a very different thing."
Why Waterfalls are NOT being recommended (theres only 4 waterfalls in the 50 page chapter):
p.240 Section 13.4.1 "Natural Acoustical Equalization Versus Electronic Equalization"
"Before moving on, some things should be said about waterfall diagrams:
-They are highly decorative
-They contain a lot of information
-That information is compromised in both time domain and frequency domain axes, and the compromise can be manipulated to favor one or the other, but not both. In other words, one can have high resolution in the frequency domain and sacrifice resolution in the time domain, or the reverse. All of this is most relevant at low frequencies."
p.241 Figure 13.20 Shows 3 waterfalls for a room with a predicted second order length mode. (e) has no visual indication of the mode, with the other two having varying results, whereas (b) the steady-state high resolution measurement shows that predicted mode loud and clear.
p.243 Figure 13.21 "A comparison of steady-state frequency responses.." ... that is extending from p.242 talking about 'positional' equalization vs. electronic equalization. The waterfalls look near identical despite the differing equalization, whereas the steady-state vary hugely.
p.244 Figure 13.22 Is quite similar in it's comparison of steady-state vs. waterfall as above, but it also uses a non-rectangular room.
p.246 Figure 13.23 Is again compared in the same way, after p.245 "However, because we know that low-frequency room resonances generally behave in a minimum-phase manner, we know that if there are no prominent peaks protruding above the average spectrum level, there will not be prominent ringing in the time domain. It it this indirect, inferential knowledge that permits us to confidently use frequency responses as a primary source of information about room behavior at low frequencies."
'Mathematical modeling of an ideal room':
p203 Those are the 'Back of the Envelope' calculations I have been recommending. Perhaps, you will note that the way I have discussed this page on the forum, is to simply encourage people to take the physical measurement of two parallel boundaries and calculate the modal frequency so that can be understood in acoustic measurements. That has nothing to do with an ideal room, and he also verified that the 'ideal' room is very much a myth. How this is 'not feasible in the real world' really comes down to your willingness to understand how much control you do have, and where to apply it. But to have presumption that you can look at a frequency response measurement (without phase, as people here have been doing) and 'know' what to do is absurd. Why would you want to guess, when you
can know?
