Using prime numbers and room dims to place speakers

ferrofluid
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Using prime numbers and room dims to place speakers Dec 13, 2022 20:16:42 GMT

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Post by ferrofluid on Dec 13, 2022 20:16:42 GMT

For anyone who goes back to see the the only previous discussion started by me, please know my gear has changed since then and the room has been vastly improved by remodeling away two acoustically deleterious features. Now I am dealing with a very symmetrical rectangular room and can place the speakers and chair where they sound best. No furniture or traffic-flow concerns exist. My small, low gear table is about 24” to the left of my seat. Ceilings are 8’10.5”

I’ve been experimenting with a Prime Number system. Deployment of proper room acoustics will happen in the coming months, but for now, I am focusing on speaker and seat placement.

Wondering if anyone is familiar with the Prime Number system:

Divide the L and W of the room by 3, 5, 7, or 11. Place the speakers there and listen. Seat is dialed in by ear as follows. For a given speaker set up, place the seat farther back than you suspect will be ideal. I imagine those who like their ears 20-30% farther away than the spread of the tweeters may start farther back than those who prefer EQ triangles. Just start way back, then move the seat forward incrementally while listening to the quality of the center image. Again subjective. What do you like in a soundstage or tonally?

Almost forgot to say that after choosing the prime spots you prefer, one can experiment by moving the speakers in fractions of an inch in all directions to further improve SQ. Theoretically, anyway. I’ve not done so, yet.

My room is 122” wide. I have tried Prime 5, 7, 11. Placing tweeters 24.4”, 17.4” and 11” from the side walls. By far, prime 7 (17.4) sounds the best.

Room Length is 224. Have tried Prime 3 (74.67” into the room), Prime 5 (44.8” into the room).

By far the best sounding to me is Prime 3 for L, and Prime 7 for width. Tweeters are 87.25 apart and by subjective listening, I settled on a seat position that puts my ears around 89” from the tweeters, so basically an EQ triangle. Before placing the speakers in these prime L, W spots, EQ triangles had not been my preference, so I really am trying to do this by trusting my ears.

I deploy zero toe-in. Bath towels on FR point. My speakers are down-ported 2.5way. The speaker design is known to allow or even thrive much closer to boundaries than is common. My previous speakers required sharp toe-in, for example.

The guy who taught me the prime system is borderline incredulous that Prime 3, L is is beating prime 5 or 7. But he does support me going with what is working in my room. Despite being 74.57 into the room, the LF “hit” is just incredible. And the SS is hard to believe: depth, width, separation of sounds, sounds freed from the speakers as their source, layering, etc.

Finally, curious if anyone has thoughts on something I noticed after completing the process: The room is very very close to being divided equally by my ears and the tweeters. The tweeters and my ears are each about 74” +/- a fraction of an inch from the front and back wall, respectively. And my ears are that same distance to the PLANE of the tweeters if one drew a line from tweeter to tweeter.

Thank you very much,

Last Edit: Dec 13, 2022 20:21:36 GMT by ferrofluid

Hexspa
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Using prime numbers and room dims to place speakers Dec 15, 2022 9:05:09 GMT

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Post by Hexspa on Dec 15, 2022 9:05:09 GMT

Sounds interesting. The way I understand positioning is to find a axial non-null spots and those end up being 20, 32, and 45% the dimensions. Famously, Wes Lachot recommends 38% but I never found that precise spot to be right; probably for a number of reasons. These numbers are based off the harmonic series of the modes - what you will see in practice. 1/3 is 33% and 1/5 is 20% and 9/20 is 45% which seems pretty prime to my high-school math level of training. These are pretty much what you want for the room. Exact spots depend in part on the absorptive properties of the boundaries. More absorptive surfaces make the room seem bigger. There are many calculators for divining room modes so whatever math which helps you 'run between the raindrops' will work. Besides modal issues, you still have boundary interference issues - particularly at the 1/4 wavelength. That's less about prime numbers and more about whether you're using bass management (crossed over subwoofer) or not. You also need to contend with critical distance and how 'live' (colored by the room) do you want your listening experience.

Last Edit: Dec 15, 2022 9:06:42 GMT by Hexspa

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ferrofluid
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Posts: 32

Using prime numbers and room dims to place speakers Dec 26, 2022 7:34:07 GMT Hexspa likes this

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Post by ferrofluid on Dec 26, 2022 7:34:07 GMT

Thanks for the reply, Hexspa. Do you know if there is a setting for me to be notified when someone posts to this thread?

You are correct to link the prime numbers approach to the harmonics. There person said it was based on a much more involved calculation and that using prime numbers was distilled version. In your examples, is there no 1/7 or 14% harmonic? Just curious.

Two developments since my initial post:

1. Eight days ago I added a Kef KC62 sub. In the first 48 hours I couldn’t hear a difference when flipping the phase switch and sub placement was limited by a short RCA. But with a long RCA now and about 200 hours of break-in, this evening I heard a clear difference between the phase settings. 0 degrees is way better than 180 in the current position. I have no idea if that could flip (no pun intended) with the sub in a different location. I will ask my dealer.

2. Even before the sub arrived, I revisited the prime 5 (length of room) after removing some remaining acoustic stuff resting on the floor near the front wall. With that removed, the prime 5 spots sounds extremely good. So I left my speakers there, moved the seat forward in increments and added the sub.

I am interested in revisiting the prime 3 spot again to see if I prefer it or prime 5, now that the sub is in play.

My Lyngdorf integrated amp has 6 built in subwoofer XO’s to choose from. I’m using butterworth 2nd order and have played with XO at 38, 44, 50. I high pass the mains to see nothing below the XO point and the Low pass the sub to see nothing above it. So no overlap.

Now that the phase is set correctly, I have turned the sub volume way down and the perception of LF depth stays just as good while quality of LF (better transparency, reduced bloat) has improved as have mids and highs.

Do you have any suggestion for optimizing the sub in the vertical dimension of the room? Room H: 106.5”. My Mains are a 2.5 way. The top LF driver covers from 2900 all the way down whiel the bottom one does only from 300 down. Might it be beneficial to put the sub on a stand such that the core of the sub is at the same H of either the lower woofer or the midpoint between the two woofers?

Or, does the prime system apply to room H? 20% of 106.5 is 21.3”. Would putting the center of the sub at 21” above the floor sound good? BTW, that height would be fairly in line with the lower woofer.

Hexspa
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Using prime numbers and room dims to place speakers Dec 26, 2022 18:04:38 GMT

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Post by Hexspa on Dec 26, 2022 18:04:38 GMT

Dec 26, 2022 7:34:07 GMT ferrofluid said:

Thanks for the reply, Hexspa. Do you know if there is a setting for me to be notified when someone posts to this thread?

You are correct to link the prime numbers approach to the harmonics. There person said it was based on a much more involved calculation and that using prime numbers was distilled version. In your examples, is there no 1/7 or 14% harmonic? Just curious.

Two developments since my initial post:

1. Eight days ago I added a Kef KC62 sub. In the first 48 hours I couldn’t hear a difference when flipping the phase switch and sub placement was limited by a short RCA. But with a long RCA now and about 200 hours of break-in, this evening I heard a clear difference between the phase settings. 0 degrees is way better than 180 in the current position. I have no idea if that could flip (no pun intended) with the sub in a different location. I will ask my dealer.

2. Even before the sub arrived, I revisited the prime 5 (length of room) after removing some remaining acoustic stuff resting on the floor near the front wall. With that removed, the prime 5 spots sounds extremely good. So I left my speakers there, moved the seat forward in increments and added the sub.

I am interested in revisiting the prime 3 spot again to see if I prefer it or prime 5, now that the sub is in play.

My Lyngdorf integrated amp has 6 built in subwoofer XO’s to choose from. I’m using butterworth 2nd order and have played with XO at 38, 44, 50. I high pass the mains to see nothing below the XO point and the Low pass the sub to see nothing above it. So no overlap.

Now that the phase is set correctly, I have turned the sub volume way down and the perception of LF depth stays just as good while quality of LF (better transparency, reduced bloat) has improved as have mids and highs.

Do you have any suggestion for optimizing the sub in the vertical dimension of the room? Room H: 106.5”. My Mains are a 2.5 way. The top LF driver covers from 2900 all the way down whiel the bottom one does only from 300 down. Might it be beneficial to put the sub on a stand such that the core of the sub is at the same H of either the lower woofer or the midpoint between the two woofers?

Or, does the prime system apply to room H? 20% of 106.5 is 21.3”. Would putting the center of the sub at 21” above the floor sound good? BTW, that height would be fairly in line with the lower woofer.

Profile/edit profile there you'll see notifications for when you log in or via email

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Perhaps a bit pedantic but if the sub's toggle is 0 or 180 degrees then you are 'inverting the polarity'. "Flipping the phase" is a colloquialism and also inaccurate, afaik, because phase is frequency-specific whereas polarity is for the whole waveform.

Regardless, I think the listening position locations I listed are for just that purpose: listening position. Your sub is a different animal altogether. An obvious example is that putting your sub in eighth space (wall-wall-floor/trihedral corner) gives 'room gain' which is basically free bass. This may be desirable - however - you would absolutely never have your main listening position in an eighth space corner.

With that out of the way, you will virtually always get better bass with more than one sub. The reason is because your room is also divided into polar zones. For example, any given dimension of your room supports a fundamental mode at double that wavelength. If I remember correctly, a standard construction in the US is 8' ceilings which itself is 140Hz wavelength. But, due to what I'm saying, your fundamental axial mode for the room's height is 70Hz. Half of that dimension will be positive polarity and the other half negative and in the center will be a pressure null. For this reason, if you put a sub in the center (energizing both polarities equally) you will cancel the first and third modal harmonics. I forget why this is but Mr. Anthony Grimani has some good presentations on youtube explaining this. I will note that he is a home theater specialist and, in my opinion, does not offer entirely accurate information for home studios; specifically regarding the amount and type of absorption that he recommends. From his sub-mains integration, he says that the polarity of the sub(s) should be whichever produces the best result at the crossover. This makes sense because both speakers and sub(s) are playing there so the phase coherence (remember - phase is frequency-specific!) is important. In my current room, I did not high pass my mains so the polarity of the sub mattered for more than just the crossover region. Actually this is a good strategy for linear bass since your mains will act like mini subs and possibly give you a better response. I don't hear too many people mention this but it makes sense; it's worth trying anyway.

In summary, I am moving rooms right now and I marked off the 20, 32 and 45% lengths and widths but just my regular standing height and put the sub at 1/4 width against the front wall and on the floor (the previous location I had which produced a linear result). From there, I moved the sub one width of itself at a time until I found the most even response with no significant nulls. Of course, I used a measurement mic and REW because I would never do this by ear. Ordinarily, you do not want any speaker or listening to be equidistant to two or more boundaries since this will compound the cancellation. Indeed, the sub is on the floor but slightly forward of the front sliding glass door and even further from the left wall and, naturally, very far from the right wall. It is not quarter wavelength and I doubt that it is exactly 20% (though maybe it's close to that). It is really more like a corner placement but pulled out a little bit. In the end, you will need to iterate and measure to find the best sub location. If you absolutely must do it by ear, place the sub at your listening position and crawl around to put your head at possible locations. Start in the corner and move along the front wall until you find what you like. Those with experience note that a single sub is more localizable than two but if it weren't then even a side wall placement would technically work. Again, the best bass response comes as a result of multiple subs but you can still get a good result with one.

---

Since you care enough about your audio to spend that much on a sub, here are my notes from the various practical resources I found regarding bass reproduction in small rooms. Be aware that this approach recommends getting your bass right first then adding your mains on top of that. After all, bass quality accounts for 30% of listener preference. I make no guarantees about its accuracy or legibility. It is for my own use and home studio-oriented. I'll answer questions but use at your own risk:

Dimensions

Damping

Listening positions
First find listening position
Probably around 20,32,45%
Not 50% (1st, 3rd harm)
25% (2nd harm)
16.7% (3rd harm)
12.5, 37.5% (4th harm)

Subwoofer Placement First
Quarter placement?
Half distance cancels 1,3
Corners gives max bass
Not more than 60cm from boundary
~2’, ~143Hz null (C#3)

Speaker placement
<5cm<avoid<1.1m
~78Hz null (D#2) (below crossover)
0.5-1m from ears
Experiment with toe in/listening window
Aim for unequal boundary distances

Crossover
Where speakers can play
Can’t cross over below speaker response
Above modal issues
Let multisub handle bass, get speaker out
Check at 86-100dB
Distortion, power compression, limiter issues
-3dB from midband
good starting point, consider above factors
LPF 4th order Linkwitz-Riley (LR4 24dB/oct)
Two 2nd order butterworth (squared)
HPF 2nd order butterworth (LR2 12dB/oct)

Tuning
Four microphone positions
Have stands for each mic
Nearfield sub levels
Inter-subwoofer delays
Doesn’t always make a big change
Rear/front/side/indiv. 2ms increments
Inter-subwoofer levels
Experiment indiv. level differences at MLP
Back/front/side/indiv. 2dB increments
-checkpoint-
-have established max level, min variation-
EQ subwoofer group, some individual
Channel levels and delays
Crossover point and slope
Check polarity of subwoofer group
Also indiv. subs if necessary
Should be no dip at crossover
Listening
Balance by ear
1dB increments

EQ
Spatial averaging
Two ear average
Each mic by each ear ~7” apart
Use stereo mic clip
1/6th oct mains
1/12th oct subs
Parametric ideally
Gentle bass slope, minimize peaks
About 4dB shelf @ 80Hz vs 1kHz
Start with lowest, loudest peak to kill
Might need to iterate

(General)
Send LFE and bass management to sub
Don’t use separate subwoofers
Low pass LFE to 80-90Hz
REW has subwoofer optimization routine
Also has psychoacoustic smoothing
~120Hz localization issues start
Aim for biggest peaks 20-40Hz w sub
Free gain that you can eq down (headroom)
Watch out for nulls around crossover
LR filters are -6dB @ cutoff, even-order
Soft knee
Butterworth are -3dB, even or odd
Hard knee
Sealed boxes are 12dB/oct
Ported are 24dB oct
So ported sub, sealed mains
Basically, what’s important is to have symmetrical filters
A sealed box is 12 so adding a 12 is 24

www.avsforum.com/threads/guide-to-subwoofer-calibration-and-bass-preferences.2958528/

www.audioholics.com/subwoofer-setup/subwoofer-placement-the-place-for-bass-part-1/subwoofer-placement-the-place-for-bass-part-1-page-4

Test tracks:
www.svsound.com/blogs/subwoofer-setup-and-tuning/the-essential-subwoofer-bass-music-test-tracks-and-demo-list

Last Edit: Dec 26, 2022 18:51:04 GMT by Hexspa

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