Sunday 7 July 2024

Seating variations, multi-seat correction & multichannel DRC. The audio "sweet spot".

A look at the rear of my room and the sofa seating - surround speakers, LP rack for a little bit of diffusion room treatment. For best practice, it's important to not sit too close to the back wall. I have 3' between the couch and the LP rack, and the rack is about 17" deep before that back wall. 

Before putting the UMIK-1 microphone away after using Audiolense XO as discussed last time, I thought it would be interesting to explore in greater detail what happens with frequency response and time-domain performance as we sit off the "sweet spot" where the measurements were collected and correction calculated for.

In the image above, I've put a meter stick on my listening couch to identify the seating positions 1 and 2-feet to the right and left of that sweet spot so I can take detailed measurements of what happens to the sound coming from the 5 main speakers at those positions with and without DSP turned on.

Here's what the 5-channel frequency response looks like at the sweet spot and up to 2-feet on either side of that with the DSP off, and then with the DSP on (click on image to zoom):


Clearly at the sweet spot, we have achieved the goal of tightening frequency response and I would say what within 1-feet on either side, the frequency response is still not bad and an improvement over the non-DSP result at the sweet spot. No need to think that we need to put our heads in a vise in order to maintain the benefits of DSP correction! Plus or minus a foot is IMO not of concern.

While the high frequencies continue to follow the intended target curve (the "Default House Curve" traced out last time), out at 2-feet on either side, we see an accentuation in a 30-40Hz sub-bass bump with the DSP turned on. It's not too bad compared to non-DSP 2-feet to the right, but clearly more pronounced 2-feet to the left closer to that side wall.

Let's now look at what happens to the time-domain step response of the 5 main speakers in a 5.1 layout. Notice that I've normalized each graph to the center channel anchored at 0ms. We can see how far ahead or behind the initial impulse arrives compared to that center channel:


As we move the microphone over seated position to the right and left, we can see the relative closeness in time of waveform arrival. As we move right, we can see the right front and rear speakers get closer. Also, as we get closer to the speaker, the amplitude also increases; the exact amount will vary and could be complex depending on room dimension and absorptive/reflective surfaces near that seating position.

On the whole, although moving off from the sweet spot will "de-focus" the time domain for signal arrival between the speakers, the overall shape of the step response still retains the intended minimum-phase morphology with strong initial impulse and reasonably smooth decay (indicative of time-alignment of the drivers in each speaker).

Multi-Seat Correction:

Regarding that frequency response and the negative effects when sitting away from the sweet spot, suppose we want to improve the quality for those sitting out at 2-feet, what can we do?

Well, in Audiolense, we can run multi-seat correction. We do this by capturing a number of measurements so that the software can "average" out the correction needed. This is done in the Correction Procedure Designer window:

Use the "Correction → Correction Procedure Designer... " window to set Multi-Seat correction. First, you need to measure at the locations you want to correct for (see Multi seat correction tab grey arrow) with those .measurement files in the default Measurement directory. Drag the measured locations you want to use to the right window - notice I'll correct for "--Sweet Spot", 1' Left+Right, and 2' Left+Right (blue arrow). Then you can click on "Show multi seat impact" to preview the effect and make sure to select "Sweet Spot Measurement" (green arrow) to make sure the program knows which is the sweet spot it should prioritize for (notice the 3.00dB preference). 

Now have Audiolense create the new filter and save out the file for the DSP in Roon, JRiver, etc. For confirmation, let's have a look at the frequency response at the sweet spot, and 2-feet on either side with DSP off, single-seat filter measured at the sweet-spot, and multi-seat measurement correction that tries to find a solution across my couch:

Note the 1/12-octave smoothing used in these graphs. Typically in magazines, we see 1/6-octave or even 1/3-octave smoothing which will obscure details and appear better. One could make a case that with human hearing, 1/6-octave smoothing would be adequate. 

As you can see, the tightest frequency response is when we have a single-seat correction measured at the sweet spot. Audiolense is doing everything it can to correct that measurement, but the price of trying to fix the 40Hz null resulted in accentuation of surrounding frequencies 2' right and left of that sweet spot.

Notice that with multi-seat DSP, we reach a compromise. Compared to the No DSP measurement, that 40Hz null has been ameliorated somewhat at the sweet spot but not as much as with the single-seat correction. The benefit is then less of that 40Hz accentuation at 2' on either side of the sweet spot. 

While not shown, there are no concerns with the step responses whether single-seat or multi-seat correction applied.

To some extent, multi-seat correction finds a compromise compared to the best solution at the sweet spot. Only you can decide on whether and how much multi-seat correction you need or want. If for the most part you'll be listening to music alone in the room, then a single-seat correction would be just fine. If music listening is often more of a social event, then sure, capture sweeps at the other commonly-used seating positions. Your sweet spot quality will still be prioritized despite consideration to the other spots. ๐Ÿค—




Summary:

It's obvious that for any physical sound system with discrete speakers, there will be a listening position where the frequency response would be smoothest, where equal inter-channel balance can be achieved, and best time-accurate performance. This is our "sweet spot". 

Whether we're in our smaller sound rooms at home, or in a large modern cinema, let's face it, there is inevitably an ideal sweet spot not too close to any individual speaker or too close to our walls.

For audiophiles, it's not typically difficult to identify where best to sit given one's speaker layout or to optimize sound for that position. Obviously in very challenging multi-use rooms, we might have to make compromises. However, it has been a kind of "Holy Grail" in home/movie theaters to achieve relatively uniform performance, especially the bass quality, across multiple seats.

Modern room correction systems like Dirac Live Bass Control are able to calibrate the subwoofers separately to further smooth out bass response. I see back in 2023, the guys at Audioholics were talking about Dirac Live Active Room Treatment (ART) as the next step (see here as well). Basically, ART allows not just subs but any speakers with frequency response <150Hz to participate in bass correction. Check out the Trinnov Waveforming "bass steering" tech as well with ideas that will require quite a bit of investment such as speaker-array absorption of sound waves. It'll be interesting to follow these developments over time, so keep an eye out on this stuff and the price of admission which should diminish even if for now very expensive and targeted to hard-core enthusiasts!

To reiterate, I think as audiophiles we must try to appreciate the importance of progress in sound reproduction first by understanding our systems' performance within the rooms we use. High-fidelity reproduction is not about hanging on to old traditions like tube gear, vinyl playback, or the silly cottage industry of unimportant snake-oil products. Think about whether your system is capable of coherent 20Hz-20kHz reproduction in your room when you're enjoying the music - this is at the heart of truly "high-end" performance, and it can be defined objectively in many ways, incorporating the science of acoustics, and improved with modern techniques like DSP.

Audiophilia, if defined as "the love of audio hardware", but dissociated from in-room performance, would be at best an incomplete pursuit and of questionable value, IMO.

These days, lots of people write reviews, have opinions, and make YouTube videos of speakers and sound systems. I would argue that even if a person only does subjective reviews, all hi-fi reviewers need to be transparent with the audience about what their room looks like and can demonstrate that they have an idea of the room frequency response as a sign of basic competence. The sound "system" is not just a set of components, but must always include the room when describing sound quality!

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Here are a couple pictures from the Rolling Stones Hackney Diamonds Tour the other evening in Vancouver from up in the stands:



With Mick Jagger and Keith Richards both 80 years old now, Ronnie Wood currently 77, it would not be surprising if this is their last tour this side of the pond even if not the last studio album. The boys were impressively agile for octogenarians going pretty hard for more than 2 hours with the show ending after 11:00PM; relevant considering the issue with aging politicians in the news lately. ๐Ÿคจ


Finally, just a quick recommendation to have a listen to U2's Achtung Baby (1991, DR12 in multichannel) in multichannel/Atmos if you have a chance. After the bluesy americana of Rattle and Hum (1988), I remember fondly the massive departure the boys took into the dreamy, distorted soundscape of Achtung. This kind of synthetic production lends itself well to being creatively reimagined in a multichannel remix. Whether one likes U2 or not, this was still a fantastic example of artistic renewal for an already very popular band that inevitably resulted in their continued relevance. I don't think many artists have been able to successfully negotiate such transformations in their career. Also a wonderful contrast to the nasty grunge sounds of the early '90s on the radio. ๐Ÿ˜

Alrighty folks, off to enjoy the warm summer weather. Until next time, hope you're enjoying the music!

Postscript: (2024-July-08)

I thought I'd keep playing with the set-up of the subwoofers to remove that 40Hz dip in my sweet spot. Setting phase on both subs to 0° without even repositioning or volume setting, and going with a multi-seat correction at the sweet-spot and 1' left and right, I get this:


Good enough! Sweet spot performance good, especially of the front left/right/center speakers, nothing too horrifying even at the 2' left/right sitting positions; more bass 2' right, less 2' left. ๐Ÿ˜

Yeah, maybe there's something I can do about the dip in the left surround channel around 130Hz at the sweet spot. One can tweak ad nauseum but there comes a time to be satisfied and just go listen to some nice music...

4 comments:

  1. Hello, Arch. I really enjoyed this article although, as a Manhattan resident living where an extra room for AV can easily add $100,000 to the cost of an apartment, it wasn't very relevant to my situation. But your discussion of optimizing the "sweet spot" did make me think about videos on the psychophysical phenomenon of time-intensity trading available on YouTube from the people at Audioholics.

    Going back to at least 4 years ago
    (for example, https://www.youtube.com/watch?v=fMwWYIESOk4 ), Gene Dellasala and his speaker specialists, Matt Poes and Jim Larsen have been describing how setting up relatively narrowly directional speakers so that their axes cross in front of the seating position can provide a wider sweet spot in which not only is the system's tonal balance optimal, but the phantom center image is perceived to be in the correct, center of the sound stage from a wider seating area.

    I also think it's interesting that the Ohm Walsh, quasi-omnidirectional speakers (https://ohmspeaker.com/) use directional super tweeters that, when set up according to the instructions, cross axes in front of the seating position. This company sells direct and doesn't advertise in the audio press, so they don't get reviewed often, but the few reviews available on YouTube mention the almost uncanny way the phantom center image seem to remain in the sound state. (I have no connection to Ohm, except that I plan to travel to their show room in Connecticut this fall for a demo, before I purchase my next pair of speakers.)
    Well, anyway, thanks for another interesting article.

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    Replies
    1. I meant, "the phantom image seems to remain in the center of the sound stage"...sorry for the typo, and I hope this now makes sense...

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    2. Hey there Mike,
      Thanks for the note. Yeah, I'm sure space is at a premium and the price high assuming one even finds a good place to buy/rent these days out in Manhattan and other major cities!

      Nice video at Audioholics on expanding the sweet spot by "extreme" toeing with time intensity trading as a possibility with narrow dispersion speakers. A great trick which back in the day when I was using some Tannoy bookshelves really worked well! I remember trying it on my current Paradigm Signature S8's when I bought them about a decade back but that set-up didn't stick for long; looked a little odd in the multichannel system but it might be interesting experimenting for both the front and rear pairs...

      They also had a good discussion in that video about 2-channel stereo and the importance of lateral reflections for improving spatial cues whereas multichannel systems can provide the cues in the surround and height channels so a room with more absorption and lower RT60 could/would work better.

      Hey let us know how the Ohm demo goes this fall, and nature of the phantom channel that system projects!

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  2. hello Arch.,
    as usual clear and very interesting post. most audiophile are hardware addicted, don't matter understanding systems' performance within their man's cave.
    do you think they care about listening positions on the sides of the sweet spot? they don't need to share feelings, just show kinda-of-obsessions with expensive toys.
    I don't have a room-correnction system yet, I will repeat REW measurements to improve the listening experience even with friends sitting next to me, with a cold beer, talking about life and music.

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