The rear wall - friend or foe?

[spiritofmusic]

I've been listening in my dedicated space for over a year now, and so much is improved. Warmth, texture, soundstsging and imaging especially. Critically, slap echo is controlled, and voices have a natural intelligibility that can only benefit a calm musical presentation.

My only caveat is a somewhat harder to pin down bass presentation. However even this is starting to come together w careful use of acoustic treatments, changes that have upped neutrality and thus enabled me to zero down more easily on my sub bass settings. And I'm still playing a bit w spkrs positioning.

My main emphasis is now on the rear wall. My room is 18' wide and 9' high at midline apex, and extends on the right side to a depth of 38', on the left (where my lps abd cds are stored) another 10', to 48'.

As things stand, my spkrs are 8' from front wall, I sit 10' from them, leaving 19-29' behind me.

I have the possibility to dry wall partition at any point, and have considered the halfway point, 24' depth, which would mean a new rear wall 5' behind me.

The primary purpose would be to enable my subs to energise the space a bit more fully, and also allow me the possibility of running some horns I'm interested in w chance of better quality, but lower power SETs, 1.46W 46, 3W 45 or 20W 211.

So, the questions I'm posing are:

Could I lose some of the easy acoustic I have, even if the resultant 18x24 space is still larger than most?

Is there any acoustic advantage to having a rear wall several feet back, or is it always an advantage to in effect have no rear wall (as I do now w over 20' behind me).

Will halving my room be guaranteed to cause greater bass loading?

Will halving the room guarantee that lower power SETs have an easier time (would be powering 100-103dB horns AND field coil woofers)?

Views awaited.

 

[Mike Lavigne]

While in theory reducing the size of your space could reinforce the bass energy, my guess is that the effect would be very unpredictable.

If I had to guess you might end up liking it more with less room volume. But the risk is causing more significant bass issues than you now have with more challenging solutions.

I say enjoy where you are at in your beautiful little corner of the world.

 

[spiritofmusic]

Mike, good to hear from you.
You've been much too quiet on these pages recently, I'm taking this as you obsessing less and enjoying more.

The reason to consider this is the potential move to high efficiency horns/field coil bass, combined w going to much lower power triodes, ie from my current 70W 211s w 300W Class D bass, to 1.25W 46, 3W 45 or 20W 211.

These speakers are 100-103dB, conservative estimate, thru the woofers as well.

You can see why I might want to downsize on size, esp since the resultant 24x18x9 would hardly be a small room.

 

[DaveC]

Bass will have the most significant effects... if you halve your space the associated transition frequency will raise by a factor of two. This can actually be good or bad depending on the system and room acoustics... with a larger space you'll get less "room gain", or room gain will occur at only lower frequencies, but you can add more woofer.

As far as higher frequencies and spatial perception, ideally you want as much space as possible between your listening position and the back wall. This is because of reflection times and how our brain perceives them. For a serious room I think 5-6 ft would be a minimum.

Also, IMO you need more efficient speakers than 103 dB to make low power SET work. My horns are in that range and a ~6.5W SET is not enough to get the speakers to full volume cleanly, and that's just the midrange and tweeter horns. A 211 would be more ideal.

 

[spiritofmusic]

Dave, "associated transition frequency will raise by a factor of two".
???

 

[DaveC]

Dave, "associated transition frequency will raise by a factor of two".
???

http://www.acousticfrontiers.com/2012314the-schroeder-transition-frequency-explained-html/

 

[Ron Resnick]

I think it would be a sin to truncate your rear wall so that it is only 5 feet behind your ears. (I wish my rear wall were going to be 10’ and not 5’ behind my ears.)

I think the best rear wall is a rear wall that is so far behind your ears that it has essentially no acoustic effect.

I stand by the advice I gave you two years ago when I visited: erect a wall at the 38’ line to turn the rear “L” annex into a clean-shaped, closed box, and use this room as an equipment room and LP storage room to which your balanced power box and turntable and preamplifiers are re-located.

 

[spiritofmusic]

Ron, your opinion on the rear wall may very well be right. Since I'm currently blessed with in effect no real rear wall (w my rear boundary being 14-24' behind me), as you say, why compromise things unecesarily .

The good thing Ron is that I have a chance to hear these high efficiency horn/field coil woofers off both 1.25W 46 and 20W 211 amps in a room that is 3x bigger than mine, and I'll know when I hear things what will be practical for me.

Yes, I really don't want to hamper what is presently exemplary in my sound by adding unneeded rear reflections.

However siting my gear in the rear alcove is not practical due to the massive runs of spkr cbls that would be needed.

I may get around to siting gear to my side and just behind me, since the SETs I could end up with are ok for moderately long spkr cbls distances. The balanced transformer is going outside the room at some point.

 

[Folsom]

Where are the subs located now, what direction do they fire in?

It’s common to try to set up subs like speakers but often it is fruitless. Changing up this may fit all your needs.

Don’t build a wall. Reflections are so far away they provide ambiance is anything. You got it good.

 

[sbo6]

Some very good advice above. My experience measuring and listening to quite as few rooms tells me that the new rear wall optimal location is further back than 1/2 way leaving you more than 5' behind your ss but not completely open. This would yield the benefit of greater room pressurizing without much of the rear wall room mode challenges especially for very low freq. I would think even a few more feet back (than 5') would go a long way (say, ~8 1/2 feet from rear wall to SS). I would also use a room mode calc to aid in avoiding any multiples in dimensions and nasty room modes. Since your room isn't a rectangle it won't predict every challenge but it should provide good guidance.Your questions answered below:

Could I lose some of the easy acoustic I have, even if the resultant 18x24 space is still larger than most?
- If by easy acoustic you are referring to lack of reflection due to the room size, possibly. However, if you keep the room at 24+ feet deep and well treated I doubt it would be an issue.

Is there any acoustic advantage to having a rear wall several feet back, or is it always an advantage to in effect have no rear wall (as I do now w over 20' behind me).
- Well, you hit on it somewhat - some professionals state having 2 speakers in free space / without boundaries is the best experience but, your ability to load a room or at least your listening position is significantly diminished. So it depends on your listening preferences. However even at 24' deep you would have a nice sized room but, again you will encounter low frequency modes more than now.

Will halving my room be guaranteed to cause greater bass loading?
- In general, yes. It's physics, you will have more bass and more pressurization since it's a smaller space. However, depending on low freq. modes you may or may not like what freqs are loading your room.

Will halving the room guarantee that lower power SETs have an easier time (would be powering 100-103dB horns AND field coil woofers)?
- If you remain seated at the same distance to your speakers, it probably won't make much of a difference other than bass impact.

 

[JackD201]

The rule of thumb is to get as close to AT LEAST half the wavelength of the lowest note your system can produce as you can. This means less corrective measures down the road. I would recommend getting speakers that can give more bass over coaxing more bass with room gain everytime because as said by Mike and Dave....highly unpredictable.

I say keep your room volume and make sure your rear wall is diffusive rather than absorbtive or reflective. Your rear wall will then be friend and not foe.

 

[sbo6]

The rule of thumb is to get as close to AT LEAST half the wavelength of the lowest note your system can produce as you can. This means less corrective measures down the road. I would recommend getting speakers that can give more bass over coaxing more bass with room gain everytime because as said by Mike and Dave....highly unpredictable.

I say keep your room volume and make sure your rear wall is diffusive rather than absorbtive or reflective. Your rear wall will then be friend and not foe.

The problem with that approach is - if you have true full range speakers and / or subs 20Hz = 56' and 16Hz = over 70'. Also, if you don't shrink your room I'd recommend keeping the back 1/2 diffusive since the delay is so drastic. My 2 centavos!

 

[Ron Resnick]

Ron, your opinion on the rear wall may very well be right. Since I'm currently blessed with in effect no real rear wall (w my rear boundary being 14-24' behind me), as you say, why compromise things unecesarily .

The good thing Ron is that I have a chance to hear these high efficiency horn/field coil woofers off both 1.25W 46 and 20W 211 amps in a room that is 3x bigger than mine, and I'll know when I hear things what will be practical for me.

Yes, I really don't want to hamper what is presently exemplary in my sound by adding unneeded rear reflections.

However siting my gear in the rear alcove is not practical due to the massive runs of spkr cbls that would be needed.

I may get around to siting gear to my side and just behind me, since the SETs I could end up with are ok for moderately long spkr cbls distances. The balanced transformer is going outside the room at some point.

Marc, I would never suggest long runs of speaker cables. The amplifiers stay right near the speakers. I was advocating long interconnect runs between your line stage preamplifier and your amplifiers — exactly what I am doing.

Think about it how many posts and how much thought and effort and money is focused on vibration isolation of turntables and front-end components because they have to be placed either between the loudspeakers, or in the line of fire of one of the loudspeakers. You are one of the very few people who has the luxury of placing the front-end components in an acoustic vibration isolated room. I think that conceptually elegant and technically superior configuration is more than worth the long interconnect runs.

 

[DaveC]

Marc, I would never suggest long runs of speaker cables. The amplifiers stay right near the speakers. I was advocating long interconnect runs between your line stage preamplifier and your amplifiers — exactly what I am doing.

Think about it how many posts and how much thought and effort and money is focused on vibration isolation of turntables and front-end components because they have to be placed either between the loudspeakers, or in the line of fire of one of the loudspeakers. You are one of the very few people who has the luxury of placing the front-end components in an acoustic vibration isolated room. I think that conceptually elegant and technically superior configuration is more than worth the long interconnect runs.

How do you know vibration isolation in the way you describe will have positive results? From what most have experienced it's difficult to predict. It could be possible that one of the reasons people like tubes and vinyl is the feedback via vibration. In fact, Herbie's tube dampers "tune" the tube to have subjectively pleasing effects. Most popular footers aren't purely isolation devices, they "tune" the device to produce pleasing results. Soft footers that isolate make the music sound dead and boring. Perhaps a certain amount of "tuned" feedback will produce better results vs complete isolation?

The other issue besides an expensive, long run of IC cable is power, grounding and the possibility of ground loops as a result of the physical layout of the gear being separated. Your IC cable is generally going to be grounded to chassis at both ends and the AC power ground will be attached to the amp chassis, then run back the AC power cable and back to the panel forming large loop area. Magnetic flux in the loop area causes electrons to flow and this is not what we want. Also, the amps and preamp's grounds will be at different potentials due to leakage currents from AC power transformers and other things, and the noise voltage formed by this is a function of resistance, so the increased distance means you need a much heaver gauge ground to minimize noise.

A more ideal AC power and grounding would look like all the AC power lines being run together, being the same length, and being terminated at single receptacles placed close together. Each receptacle would have power distribution plugged into it and each power distributor would have a ground post tying them all together with a ground strap. IMO you're likely to have a system with less noise in it this way vs the way you describe as well.

 

[Ron Resnick]

I agree that a certain amount of “desirable” vibration may be part of a turntable’s design, possibly in terms of its suspension. But I take as intuitively obvious the proposition that we do not want our turntables bathed in random, loudspeaker-generated vibrational energy.

AC and grounding are different issues which are inapposite to what I’m talking about here.

 

[JackD201]

The problem with that approach is - if you have true full range speakers and / or subs 20Hz = 56' and 16Hz = over 70'. Also, if you don't shrink your room I'd recommend keeping the back 1/2 diffusive since the delay is so drastic. My 2 centavos!

True, true. Marc is almost there at 20Hz. I'm down to 10Hz and so I needed quite a bit more trapping inn the form of tuned resonators. Easily done. I'm down with you on the half diffusion, my rear wall diffusor is about half.

 

[spiritofmusic]

Guys, thanks for your in depth responses.

My subs are in the main spkrs and downfiring, so the only flexibility I have is in moving my Zus anything up to 9' from front wall, and I'm experimenting here.

Re not having a rear wall too close to my listening position, and far back to cope w lower bass waves, then effectively I shouldn't be thinking about building this wall. The rear boundary is a combination of wall and shelving, so there's already some diffusion.

In some ways it's neat to have the gear in a walled off alcove at the back, but the disadvantages of running ultra long unbalanced interconnects to the power amps rules this out, added to the fact that I hate the idea of dropping the stylus on an lp, and having to run out of this space to the seat. No go. But I may relocate gear to my side/slightly behind seating position.

My hunch at this point, w everyone's input, is to leave things as is. I remember how much I was taken by the vast impvts listening to music on day one compared to my old room (where my rear wall was 3' behind me). My instinct is not to threaten this.

However, I do know this is potentially restrictive on the option I've been considering of much lower power tubes w new higher efficiency horns.

 

[Folsom]

Well, here is a thought. Buy subs now, first.

You have to be pretty lucky to have a fullrange speaker play the bass perfectly to begin with. You cannot move the speakers about or it’ll throw off the main part of the music. Further, most horn setups don’t play really low either or it is limited for the same reason.

You could get a SWARM setup to give you that low end ambiance that give immersive experience, and will help out horns. They are placed about the room so at least one will be near you. Their profile really allows them being placed about. Having subs next to the speakers may give you the same results as the speakers... moving them about is def the key to absolute sub performance. There are of course lots of options, and knowing what horns you want can help figure that out depending how low they play. Also you may want something that sounds intigrated since horns are pretty fast in nature, a bit unlike some typical dynamic speakers. The units that work the best in subs is not always as much if a dollar to performance ratio as speakers, amps, etc.

 

[fbhifi]

One of the most important, if not the most important, aspects of listening room design is the avoidance of room nodes.I have found, after building three dedicated listening rooms, that the most effective way to eliminate or greatly minimize room nodes is to follow various Length, Width and Height formulas.
The one I’ve had the greatest success with is Cardas Golden Ratio. Look it up on the net and then plug in your current room width and ceiling height. It will tell you the optimum room length.

 

[spiritofmusic]

FB, what figures do I fill in if my height varies from 9' at midline apex to 4' at side walls, and if one half of my room is open to an annex area, effectively a 10' difference in depth R to L?