SUMMER MUSINGS: "Final" ceiling absorption 😉. AI-assisted summary of Floyd Toole's thoughts on early reflections treatment & discussion on this.

Hey guys and gals, it's July and thus time to enjoy some summer jams, BBQ, and family time ahead. And of course getting into the "Summer Musings" season here on the blog!

Let's "shoot the breeze" and talk about a couple of things in this post related to discussions over the last while with room treatments and acoustics you might want to consider for your own space.

First, as you can see in the picture above, despite the self-proclaimed statement that I was "done", I just had one more little addition to add. I cleaned up and optimized my seating area a little and realized that the rear ceiling panel wasn't exactly overhead. So I added an extra 2' x 4' x 0.8" PET panel immediately overhead above the main listening position. [I also spent some time optimizing the height channels and DSP recalibration along the way.]

The reason I did this is because my ceiling is of a typical low 8' height. If I sit with ear height around 3.5', that means the distance to the ceiling is only 4.5' above the ears which makes reflections coming from up there potentially quite prominent.

These would be steeper angle reflections, typically of second and third-order that have bounced off other surfaces already like the ottoman I often place in front of the sofa or the rear wall. These could be conceptualized as part of the "reverberant build-up" rather than the specular early reflections we're treating when placing absorption panels at the first reflection points. These would also be later arriving waves (likely ~15-60ms window) on the ETC graph.

Other reflections that an overhead panel could help break up may be the vertical flutter from floor-to-ceiling which for an 8' (2.44m) height would be c/2d = 343/(2×2.44) = ~70Hz.

To be clear, this additional panel isn't going to change measurements like the RT60 or the ETC a huge amount, we're just looking at incremental but measurable changes. So for comparison, and for my own reference one day looking back on where I am today, here are how my measurements look in July 2026 with my room set-up as pictured above with the ottoman moved out of the room for "reference" listening.

Dirac Live Bass Control DSP is active (15-point spatially averaged +/- adjacent seat, +/- 6" up/down-forward/back) which is how I normally listen.

I'm using a variant of the Dolby Atmos Music Target Curve as previously described. For these measurements, let me focus on not just the combined Left+Right curve but also the individual Left and Right channels. I targeted 80dB SPL for each of the left and right channel amplitude; so combined L+R is about 6dB louder.

Reverberation time well in the low 300ms range. Nice and flat across midrange and upper frequencies.

"Music Clarity" C80 index sits primarily between +15-20dB, 1kHz midrange at around +19dB for the left and right channels. This indicates a "direct", "articulate" sound without completely erasing room reflections or becoming too "dry" which is often described as what happens if pushed well over +20dB.

Next let's have a look at the Energy-Time Curve for each channel across the first 40ms covering early reflections - timeframe of the Haas/precedence effect in psychoacoustic research. With the ceiling absorption in place, the 2.7-2.8ms broadband ceiling bounce energy I was seeing previously is now well suppressed. However, nothing's perfect and it looks like there are a few more early left channel reflections than right. The room isn't perfectly symmetrical so this is not a surprise. All early reflection peaks are below -11dB which is good to see.

And if we look further out in time across 200ms, adding a little smoothing (0.2ms), we can compare the two channels further:

Left and right channels basically the same with steady decay as per the arrow. Higher order reflections after the first 20ms clearly of lower level.

And here's the 50dB range color spectrogram of Left+Right channels together with peak 92dB SPL allowing for strong sub-bass and bass, cursor at 300ms for landmarking. Looks well controlled down to 80Hz at least and I have no concerns down to 20Hz with visible decay by a very reasonable ~800ms. Bass sounds tight with my dual subs (Paradigm SUB1 and SVS SB-2000 Pro) - more annoying is the occasional rattling of things on my shelves when the music/soundtrack hits hard below 30Hz!

One more graph!

A peek at the time-domain step response across 10ms from the individual left and right channels (more accurate to say "channels" than "speakers", I think) and also both channels playing together, captured at the listener sweet spot about 9-10' away from front speakers. It's a look at the multiway speaker and subwoofer integration achieved with DSP.

I would love to see a step response measurement like this in a treated room at the listening sweet spot with something like the current "high end" darling US$800k Wilson Audio Autobiography speakers optimized with all the mechanical sled gears dialed in (by Daryl Wilson himself, maybe?) and supposedly "time aligned" without DSP. 😏

Overall, I can't complain, and certainly happy (for now). 🙂

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Moving on to our next topic for this post, recently I saw a thread at the Steve Hoffman Forums where there were some discussions about what Floyd Toole thought about early reflections:

"I thought Toole said that early reflections actually enhance the listening experience."

I think it's times like these where we can use AI to do a little research and summarization for us - here's what Anthropic's Claude says about this topic (with some light editing on my part):

Floyd Toole's position on early reflections is one of the more contrarian and research-backed stances in audio acoustics — it directly challenges the conventional treatment wisdom so it's worth understanding carefully.

Toole's Core Position
Toole argues, based on decades of controlled listening research at NRC Canada and Harman International, that early reflections are not inherently harmful and may actually be beneficial for music listening — particularly lateral reflections. This puts him at odds with the standard recording studio treatment philosophy that dominates most acoustic treatment advice.

His argument rests on several pillars:

The Precedence Effect and Fusion Zone
Toole acknowledges the Haas/precedence effect but interprets its consequences differently from the treatment-first camp. Reflections arriving within roughly 25–30ms are fused with the direct sound by the auditory system — they are not heard as separate echoes but as an enrichment of the direct sound. The question then becomes not "are reflections present" but "what do they contribute perceptually."

His research found that listeners in blind tests consistently preferred rooms with some controlled early reflections over heavily damped rooms, particularly for music. The preference was especially strong for lateral reflections which contribute to spaciousness and envelopment — two of the most valued attributes in concert hall acoustics.

Lateral vs Vertical Reflections
Toole makes an important distinction that most treatment guides collapse:

Lateral reflections (side walls) — Toole considers these potentially beneficial. They increase the interaural cross-correlation (IACC) difference between the two ears, which the auditory system interprets as spaciousness and envelopment. This is the same mechanism that makes concert halls sound immersive. Absorbing all lateral reflections produces a "dry," "in-your-head" sound that many listeners find fatiguing and unnatural over long sessions.

Vertical reflections (floor and ceiling) — these are less beneficial because they arrive at both ears simultaneously, contributing little to the sense of spaciousness while still adding tonal coloration via comb filtering. Toole is more sympathetic to treating these, particularly the floor reflection.

The Reflection-Free Zone vs. Live-End Dead-End
The conventional LEDE (Live End Dead End) studio design — absorb everything at the front, diffuse at the rear — was developed for mixing engineers who need to make critical tonal judgments. Toole argues this is the wrong paradigm for hi-fi listening rooms because:

·    Mixing engineers need to hear exactly what is on the recording without room coloration

·    Hi-fi listeners are trying to reproduce a convincing musical experience, where some room contribution is natural and even desirable

·    The two goals are different and require different acoustic solutions

Loudspeaker Directivity as the Key Variable
One of Toole's most important contributions is emphasizing that the speaker's directivity pattern determines how much early reflection energy reaches the listener, and that a well-designed speaker with controlled, consistent directivity produces reflections that are spectrally similar to the direct sound. Such reflections are relatively benign because they don't significantly alter the perceived tonal balance.

A speaker with irregular directivity — where off-axis response has peaks and dips at different frequencies than on-axis — produces reflections that are spectrally different from the direct sound, causing genuine tonal colorations. His argument is therefore: fix the speaker first, then worry about the room. A well-engineered speaker in a moderately reflective room often sounds better than a poor speaker in a heavily treated room.

What Toole Actually Recommends
His practical recommendations for hi-fi listening rooms are more nuanced than either "treat everything" or "treat nothing":

·    Avoid flutter echo and long discrete echoes — these are always harmful

·    Control the floor reflection — he accepts this as genuinely problematic due to its vertical arrival angle and early timing

·    Don't over-absorb lateral reflections — a moderate amount of side wall reflection contributes positively to spaciousness

·    RT60 in the range of 300–500ms for music listening in typical rooms — he considers heavily treated rooms with RT60 below 200ms to sound unnatural and unpleasant for music

·    Diffusion is preferable to absorption at side walls and rear wall for hi-fi applications

·    Speaker placement and toe-in are more important variables than treatment for most rooms

The Honest Synthesis
Toole's research is real and rigorous, but it has limits worth acknowledging:

It was conducted with specific loudspeakers — the Harman research speakers with very well-controlled directivity. Results may not transfer directly to speakers with irregular off-axis behavior.

Preference ≠ accuracy — Toole's tests measure listener preference, which tends to favor spaciousness and envelopment. Listeners who prioritize imaging precision and detail retrieval over spaciousness may prefer a more treated room.

Room size matters — his research covered a range of room sizes, but small rooms produce early reflections at shorter delays than larger rooms, where the fusion zone argument is stronger.

The practical takeaway is that Toole gives you permission to not over-treat, especially side walls, and to prioritize speaker quality and placement over aggressive absorption. But his research doesn't invalidate the ceiling and floor treatments — those are the reflections he is most sympathetic to controlling.

Fascinating stuff with quite a bit there worth thinking about. As the summary suggests, Toole's position can be controversial but we need to analyze this in a nuanced fashion. As I underlined in the "Preference ≠ accuracy" section, there is the issue of subjective listener preference: are you one who enjoys "spaciousness", or would you prefer precise imaging? Not that this is mutually exclusive of course!

As often seen in the audiophile world, depending on who and where we ask, no doubt all kinds of opinions can be solicited - including folks who prefer scratchy vinyl and vintage tube amps with high distortions as their "preferred" sound even if such opinions are not high-fidelity transparency many others would value. It's also important to remember that typical "untrained" listeners (including some audiophiles who may have never heard a neutral reference sound before) might prefer adjustments like colored "smiley face" EQ. We have to be mindful of this whenever we talk about research involving listeners.

Importance of experienced/trained listeners, see Toole's presentation.

In terms of the actual research suggesting that lateral reflections can be beneficial then, I see that Toole talks about it in his book Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers, Rooms and Headphones (4th Ed) on page 119 (bold mine):

"The tests were done many years ago, but at that time stereo was in its prime, and the selected listeners were very experienced and knowledgeable. The questionnaire was intended to elicit as much information as possible from the listeners and much was revealed. After 40 years, the results still generate animated discussions, including flak, in audio forums."

He's referring to studies from 1985 and 1986 using a vinyl source and some Rega Model 3, KEF 105.2, and the electrostatic Quad ESL 63 (dipole with weak lateral radiation) speakers of that era. Some more quotes from the text:

"The notion that early lateral reflections in the listening room would be audible problems was not reinforced. The reflections in the arrangement in Figure 4.6 arrived at 4.1 ms and 10.1 ms, within the range thought to be problematic; however, as shown in Figure 4.7, due to loudspeaker directivity, the reflections were attenuated by roughly 10 dB when they reached the listener. These were flat, plaster-on-block side walls—perfect specular reflectors. Yet what they contributed to the listening experience could be interpreted as being more positive than negative. 

....

In the overall description of listening perspective, for the music recorded with a natural perspective (choral, chamber and jazz), the modal listener response was “you are there” for the Rega and the KEF and “close, but still looking on” for the Quad. According to the definitions of those phrases . . . the Rega and the KEF gave listeners some impression of being enveloped by the ambient sound of the recording environment, with the Quad tending to separate them from the “performance” (Toole, 1986, p. 342). This all sounds very much like the influence of lateral reflections, apparent source width (ASW), early spatial impression and the associated variations in interaural cross correlation (IACC) which correlates with a sense of envelopment. For listeners who appreciate such illusions, wide dispersion loudspeakers and some amount of room reflections appear to be desirable."

Interesting thoughts and analyses but also there seems to be speculation in what he said. This to me suggests that lateral reflections are not disastrous but not necessarily desirable either depending on the speaker quality. Here are the reference papers:

Toole, F. E. (1985). “Subjective Measurements of Loudspeaker Sound Quality and Listener Preferences”, J. Audio Eng. Soc., vol. 33. pp. 2–31.

Toole, F. E. (1986). “Loudspeaker Measurements and Their Relationship to Listener Preferences”, J. Audio Eng. Soc., vol. 34, pt. 1, pp. 227–235, pt. 2, pp. 323–348. [PDF]

Clearly these are very old studies and I wonder how replicable the results have been. I could have missed discussions in the book describing research beyond these early National Research Council (NRC) works from the 1980's. I wonder, now in the 2020's, if we were to repeat the experiment(s) with current speakers, hi-res digital recordings done with modern production qualities, recruited trained studio and audiophile listeners, would the results be the same?

Remember that in science there are no popes, no high priests, no god-given prophets regardless of how much we respect the researcher (or writer, or reviewer, or designer) - so be free to decide for ourselves so long as we remain rational and can explain our beliefs. For me, I'm more interested in achieving precise imaging through room and speaker symmetry, hearing more of the direct sound, and reduction of reflections which inherently are of lower fidelity. If high-fidelity "transparency" is my goal, then I would argue that this applies not just to playback hardware, but also room characteristics. This is especially important when I'm listening to high quality and minimally-mic'ed recordings that already capture the room ambiance in the signal and do not want my small room to assert itself onto what is there. For example, if I'm listening to Harry Belafonte at Carnegie Hall (1959), do I really want to hear the reflections from my small room mixed in with that recording? Would I still be listening to Harry through my hi-fi system in a way that is somewhat faithful to the sound of Carnegie Hall encoded in that recording? 🤔

The same can be said for many "audiophile-quality" recordings like The Ray Brown Trio Live At The Loa (1988), the audiophile classic Jazz At The Pawnshop (1995), binaural captures like Sara Bareilles' Brave Enough (2013), many classical works like the Mercury Living Presence series (50's to 60's), or even the commonly-played-at-audio-shows Nils Lofgren's Acoustic Live (1997).

[Taken to an extreme, if one's goal is to maximize spaciousness, one could look at getting some Bose 901's with their rear-facing drivers used to create room-filling "immersive soundstage"! 😏 While I love immersive sound, this ain't my idea of hi-fi surround sound. I have not heard Toole comment about these Bose speakers, we know they have poor tonal response, imprecise imaging (negative Directivity Index!), room dependence, are not able to reproduce a recording engineer's intent - all of which I believe go against NRC/Harman principles.

For 2-channel playback, also look into crosstalk cancellation/ambiophonics for IMO a better, more expansive sound.]

So to prioritize imaging, I prefer applying absorption to the first reflection points to reduce those early reflections and allow the room to seem bigger than it is. I see nothing wrong with making our dedicated hi-fi listening spaces behave like studio control rooms where engineers and producers hear as much into the mix as possible; I also don't understand how that's necessarily fatiguing (mentioned in the AI summary) unless the music itself is fatiguing or our rooms become uncomfortably anechoic (highly unlikely unless we go nuts with absorbers across all the walls, ceiling and floor!).

[For completeness, of course we can also use diffusion (like my LP rack with variable "well depth") a few feet behind the listening position to enhance ambiance. 

In larger rooms (say 3,000–4,000 ft³ and above), side wall diffusers can be used with good effect, perhaps even preferable. Diffusers however require physical distance from where one is listening in order to effectively scatter and separate the wavefronts from each other. A rough rule of thumb is you need about 1' of distance from the panel per 1" of panel well depth - here's a table to consider when using standard QRD designs:

This is why in practice, other than the rear and front walls if you're sitting a few feet away, it's likely much more efficacious to use absorption treatments in small rooms. Other designs like fractal, curved, non-periodic designs could allow closer effective distance.]

I think Ethan Winer's position about "Early Reflections Are Not Beneficial" without any special consideration for lateral vs. ceiling vs. floor aligns with my sentiments as a well-written rebuttal to the Toole position. So long as we remain rational (vs. magical thinking) and can defend our positions, it's all good.

[An important technical point Winer makes is that if one's speakers are excellent and have controlled off-axis response, comb filtering would be even worse with such good speakers from the room reflections! So, paradoxically, the better one's speakers, the more important it would be to deal with significant reflections. I don't see how anyone can counter this argument.]

With that, I think this is enough audiophile geekery. 😁

Happy July 1st Canada Day to fellow Canucks, eh? And wishing all the American friends a wonderful Fourth of July for their 250th year of independence.

Hope you're all enjoying the music.

Let's end with Radiohead's "The National Anthem" from Kid A (2000):

Addendum: July 4, 2026

I just wanted to add to the discussions of what it would look/sound like in my room if I removed the side absorption panels given the asymmetry as one can see here:

Notice that my right wall is significantly dominated by a wood and glass cabinet where I put some of my headphones, CDs, BluRays, etc. It was this asymmetry that got me to buy side absorption panels back in 2015.

So here are the left and right front channel ETCs with the IKEA Mittzon absorption panels removed:

As you can see, the side wall reflections around 3.1ms now are the strongest peaks on the ETC (compare to the picture I showed earlier above) with clear asymmetry between the left and right sides.

I was a little surprised that it was the left peak that had the higher energy reflection despite this being just the flat drywall side. Thinking about this, I suppose the fact that the right cabinet being textured and of different materials might have a role to play in broadening out the ETC energy over time (like a weak diffuser?).

Obviously without the side panels, RT60 goes up a little (around 350ms), C80 down a little (averaging +16dB or so). What is most audible in my room is the reduced "tightness" in the stereo imaging. For example, voices are not as well anchored where they should whether dead center or at various positions (for example, the singers in "One Day More" on Les Misérables). This is more unpleasant to me than whatever gains from a sense of "spaciousness" might be claimed by not using side panels.

My take is that if the room construction and speakers are laid out symmetrically, and the side reflections are something like -10dB as per Toole's research from the '80s (compared to my reading of -7.6dB in my room off the left wall), then I suppose maybe the side reflections might not be of concern, if not even preferred by some as suggested by Toole.

Otherwise, I think for most audiophile listening rooms I've seen, I suspect some side absorption to balance out the right/left first reflection points would be beneficial. Especially for those would-be subjective-only "influencers" who serve the Industry with asymmetrical rooms (like this guy 😉).