A theoretical question regarding bass isolation in an apartment

[autoteleology]

I am in the process of designing my home theatre / studio to isolate my subwoofer from my next door neighbor in my apartment, with whom I share a single wall with on one side of my room.

The thought exercise I am considering that my entire room layout hinges upon is as follows:

Suppose, in an open area, you have a ten foot by ten foot wall of infinite density.

If a subwoofer is directly against this wall, how isolated from the sound of the subwoofer will you be, if standing on the other side of the wall?

The plan is to use heavy industrial steel racking that spans the width and height of my apartment wall, filled from floor to ceiling one foot deep with brick, and then sealed on both sides with several hundred pounds worth of metal sheeting, with one or two layers of acoustic blankets hung from the ceiling, then placing my subwoofer against this wall and hoping that the bass reflects outwards and upwards back towards the listening position and not through the wall.

If the wall does a good enough job of reflecting sound, this should, theoretically, be a better system than having the subwoofer on the other side of the room (thirty feet away), due to the fact that the wall, being closer, should block an essentially infinite "field of view" before reflecting off the room, rather than a limited field of view of whatever the base of an imaginary pyramid leading from the subwoofer to the wall would cover in area, plus the attenuation from the inverse square law due to the distance.

Thoughts?

 

[Steven Holt]

then placing my subwoofer against this wall

First, don't do this, place the sub 12 to 18 inches away from the wall. You risk 'bloating' if you place it directly against the wall.

hoping that the bass reflects outwards and upwards back towards the listening position and not through the wall.

No, afraid not. A subwoofer radiates omnidirectional, very much like a lightbulb when you turn it on. And those LF waves will pretty much go thru anything (sadly, including walls).

You have to be careful when using a subwoofer in an apartment, I'm sure there have been evictions because of it. My only advice is to decrease the Level on the sub control panel, or buy a pair of speakers with a large woofer (JBL comes to mind).

Finally, a subwoofer is gonna be a subwoofer, no matter the room placement.

 

[DVDdoug]

Suppose, in an open area, you have a ten foot by ten foot wall of infinite density.

If a subwoofer is directly against this wall, how isolated from the sound of the subwoofer will you be, if standing on the other side of the wall?

I don't know exactly but bass has long wavelengths and it goes-around objects. And all sounds reflect off of walls.

plus the attenuation from the inverse square law due to the distance.

Again, in a room sound doesn't attenuate as much as in free-space, especially the bass.

filled from floor to ceiling one foot deep with brick, and then sealed on both sides with several hundred pounds worth of metal sheeting, with one or two layers of acoustic blankets hung from the ceiling,

Mass is the main ingredient (or one of the main ingredients) in soundproofing. I assume you've done some research... Acoustic blankets won't help much with bass.

And note that "acoustic treatment" is different from "soundproofing". Acoustics treatment reduces reflections in the room but the walls will still vibrate with the bass transmitting sound to the other side. Air leaks will also allow sound through.

 

[blueone]

I did this once, using a small sub in a second floor apartment. Your only hope for not annoying your neighbors is to keep the volume down. To be more specific, average sound levels of 75db at your listening seat or less. So-called isolation feet, in my experience, are useless in suppressing transmitted sound from subs.

 

[312elements]

I am in the process of designing my home theatre / studio to isolate my subwoofer from my next door neighbor in my apartment, with whom I share a single wall with on one side of my room.

The thought exercise I am considering that my entire room layout hinges upon is as follows:



The plan is to use heavy industrial steel racking that spans the width and height of my apartment wall, filled from floor to ceiling one foot deep with brick, and then sealed on both sides with several hundred pounds worth of metal sheeting, with one or two layers of acoustic blankets hung from the ceiling, then placing my subwoofer against this wall and hoping that the bass reflects outwards and upwards back towards the listening position and not through the wall.

If the wall does a good enough job of reflecting sound, this should, theoretically, be a better system than having the subwoofer on the other side of the room (thirty feet away), due to the fact that the wall, being closer, should block an essentially infinite "field of view" before reflecting off the room, rather than a limited field of view of whatever the base of an imaginary pyramid leading from the subwoofer to the wall would cover in area, plus the attenuation from the inverse square law due to the distance.

Thoughts?

My first step would be having a dialogue with neighbor and establishing how big a problem this actually is.

My second step would be decoupling the subwoofer from the floor. Doesn’t have to be expensive or exotic but we want to minimize exciting the floor and transferring that energy across to the neighbor.

Rather than brick, I’d stuff 75% of the cavity with uncompressed rockwool leaving room at the back of the cavity.

It’s not likely going to be sound proof and I don’t know what your SPL goals are here, but I think that’s likely to yield the best reasonable results. Once complete circle back to the neighbor and have more dialogue to establish how loud you can turn it up without causing problems.

 

[win]

I'm excited to see this infinite density wall once it's built, please share

 

[autoteleology]

Just to be clear, I'm not trying to get perfect sound attentuation here, and I'm aware that subwoofer radiation is omnidirectional. I'm just trying to get *enough* attenuation to use my subwoofer at a reasonable volume without bothering the neighbor.

There are a couple of key details I should point out.

One of the tricks that I am using to make this work is that I am, essentially, sitting directly underneath my subwoofer because I am placing it beneath my sitting position, in order to game the inverse square law.

The second is that I only need to isolate this single wall because my house is very long and narrow and the wall that connects my room to the neighbor is the only shared wall between our two housings. My room itself is 11 x 25 (I just measured).

Mathematically, this totally works, theoretically, according to the following calculation (assuming the reflector works with a single reflection and then the omnidirectionality of the bass waves throws any control out of the window:

An extra 6dB of attenuation at low frequencies is a LOT, not just from a normal volume standpoint, but especially if you look at equal loudness contours, where the falloff with low frequencies is severe, and gets increasingly severe the lower the volume:

So it's not just like, a total crackpot theory. It pans out with math, IF I can actually properly execute the system. Big if, of course.

The real questions here are:

How much mass do I actually need in order to build a proper reflector (even if it is just for a single reflection)?

Will brick and steel actually work to reflect sound? Does the reflector need to be airtight / solid, or is just having enough mass in the way enough?

 

[kemmler3D]

Thoughts?

Welcome to ASR! I admire the ambition here.

I am not sure everyone has fully read your post because it's pretty wild, but I am not sure it would be enough.

Unfortunately your ceilings and floors will also transmit sound to the neighbor. A lot of sound will convey from the air through those materials and on into your neighbor's dwelling.

True isolation (room within a room) is the only really effective way I know of to prevent bass from travelling between spaces.

Now, will a giant steel / brick wall between you and your neighbor HELP relative to doing nothing? Yes, hard to imagine it won't. It depends on how the dwellings are constructed and what physically connects them.

Will it allow you to play your subs as loud as you want at all hours? No.

 

[312elements]

One more thing that likely goes without saying but DSP. Most subwoofers once they interact with your room are going to create great big peaks in the in room response sometimes to the tune of 20db. Taming those peaks reduces the feeling of excessive bass in the room and prevent them from being transferred to your neighbor.

 

[Multicore]

The solution is headphones.

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[ace_xp2]

A bass shaker can make this at least more enjoyable for you. You don't real low end response with one, but you can get more of a total impression of the whole sound signature.

 

[Speedskater]

Hang your sub-woofer with nylon ropes from that metal frame work.

 

[kemmler3D]

Does the reflector need to be airtight / solid, or is just having enough mass in the way enough?

Airtight-ness helps a lot if you're trying to block sound. If there is any free air path between the sub and the wall, you may as well not have the bricks there at all. Consider how much the sound changes from having a door slightly cracked to completely closed.

Anyway, the main thing preventing this plan from being foolproof are the surfaces other than the wall, i.e. floor / ceiling. The sound is only really contained if it's contained in 3D, if that makes sense.

However, your plan of sitting basically on top of the sub is a good one. This will ensure you're playing it at minimum SPL to begin with, which proportionally reduces the sound going to your neighbor's space.


Something a lot easier and cheaper than building a giant brick / metal wall is using very nice headphones at night.

 

[autoteleology]

I did some additional research on how sound reflection works and I realized that short of being able to rebuild the room, this pathway was a dead end, because reflection depends heavily on having a seal between the source and the destination.

However, it occurred to me, after finding some additional acoustic modelling tools... would it be possible to make this work using absorption?

To start, I made a diagram of my room in Photoshop where every pixel corresponds to one square inch. The dark grey paths are where the doorways to the room are located, horizontally (with inch measurements of the doorways and the ends of the room separated by them):

I have a huge pile of producer blankets from VocalBoothToGo hanging around the room (pun intended), but as most people familiar are aware, hanging these on the walls mostly results in over-deadening the highs and midrange with little to no bass reflection mitigation. However, it is also commonly known that hanging them with an air gap between the blanket and the wall, and hanging them in a way that results in the fabric forming waves, drastically increases the low-end performance of these blankets.

So, my thought was.... what if I took this idea to its logical conclusion, and utilized the excess space in my room to create enormous, floor to ceiling absorptive bass traps?

Using the tools at http://www.acousticmodelling.com/porous.php and https://amcoustics.com/tools/amroc?l=739&w=343&h=244&r60=0.6 (this link will show you the modes inside my room), I modelled a design that would not only cover or block the low frequency modes in the room as much as possible, but would also block soundwaves as low as possible without interfering with the usability of the room. This is what I came up with:

At first glance, this probably seems like a ridiculous and obscene waste of money and space, but once you see the numbers, you'll get it.

Note: I estimate that the polyester-filled producer blankets, hung perpendicular to the source of sound, achieve a flow resistivity of somewhere between 1500-3000 Pa.s/m^2, after doing some research on the measured flow resistivity of various types of acoustic material, but I don't have exact numbers from the manufacturer so I'm guessing to some degree.

Here are the numbers for the mid-room diffuser:

Yes, really. Loosely layered together in that space around my furniture, this wall of blankets achieves what appears to be some truly mind-boggling numbers for sound absorption across the entire frequency spectrum. Tuning the flow resistivity of the blankets to find the required density to achieve this number will be a minor task, but it is absolutely achievable with the materials I have.

The numbers on the bass trap design that surrounds my speakers are also quite impressive:

Straight front to back absorption across the wall:

Side to side absorption across the wall:

Side to side across the corner trap:

And front to back through the corner trap:

On paper, unless there are factors I'm unaware of, this system appears to absorb the vast majority of unwanted sound transmission and reflection in my room for not a lot more than $1,000 in materials, most of which I already have on hand to create the system.

Thoughts?

 

[kemmler3D]

I am sure this will make a pretty big difference - if you're willing to fill half the room with bass trapping, it will amount to something.

However, it won't prevent bass from being conducted through the floor/ceiling into the walls and onward. Acoustic treatment mostly only pertains to the sound quality inside the room. Even if you absorb all the bass incident on the walls, that's only about half of the surface area of the room...

I also don't know if I totally understand the end goal / use case here.

Is the idea to play a subwoofer loud at all hours and not bother the neighbor?

How loud and late do you want to play the subwoofer, and how is your relationship with this neighbor?

I play two subs with no bass trapping at all, pushed up against the wall shared with my neighbor. I stop using my subs around 8PM and nobody seems upset with me.

Most of this depends on the situation with your neighbor... this solution might be way too much, or it might not be enough.

 

[Steven Holt]

I also don't know if I totally understand the end goal / use case here

Kemmler, IDK what the hell the guy is trying to do here, except chase a Unicorn. In his above equation he is assuming, on the RHS that the R(2) value is 0, or damn close to it. IT WILL NEVER BE ZERO, as you so well pointed out. You can minimize the impact of a sub to an extent, you will never cancel it out barring walls, floors, and ceilings made out of lead. I hope he takes your advice.

 

[kemmler3D]

Kemmler, IDK what the hell the guy is trying to do here, except chase a Unicorn. In his above equation he is assuming, on the RHS that the R(2) value is 0, or damn close to it. IT WILL NEVER BE ZERO, as you so well pointed out. You can minimize the impact of a sub to an extent, you will never cancel it out barring walls, floors, and ceilings made out of lead. I hope he takes your advice.

Your mention of cancelling it out makes me wonder... if you did some excessive room treatments like OP is suggesting AND you set up a DBA that actually did cancel out the bass in free air... well, you still wouldn't solve it because the initial wave is still going to propagate out of the room before the inverse wave can cancel it. And it would cost a lot more than OP's $1K budget anyway.

I think the real answer is headphones and some good spatializer software, if we really must not allow neighbors to hear our bass.

 

[Tell]

First, don't do this, place the sub 12 to 18 inches away from the wall. You risk 'bloating' if you place it directly against the wall.

'Bloating' as in boundary gain? That's a good thing imo, because it's better to use that extra gain, then measure and EQ it down to the right levels which will give less distortion and less wattage used for the same output. Win-win!
Also I don't think (not sure though) that the neighbour will notice any particular difference if the subs are against the walls or 18 inch away from them, so when placed against and EQd down it will probably also make as many dB lower inside the neighbours apartment, so one more win

 

[autoteleology]

However, it won't prevent bass from being conducted through the floor/ceiling into the walls and onward.

I'm aware. I'm not trying to build a soundproof room. I would just build a room inside a room. What I am doing is see how much low hanging and inexpensive fruit is actually available to be collected if one thinks outside the box and challenges common assumptions of how things should work. Is that not the point of Audio Science Review? People speculate about and experiment with science and others review it?

I also don't know if I totally understand the end goal / use case here.

I'm trying to build upon my understanding of acoustics. Which is why I phrased the OP in the way that I did. Obviously I can't build a wall that defies physics. I was hoping it would make the underlying proposition of what I was asking clearer.

I would like to do things like...

- improve the quality of the treatment in my room so that I can achieve a more consistent and wide-band deadening of the decay of my room
- increase the usable headroom of my system, especially because I have lots of power still left on the table even with my current modest equipment, a Rogersound Speedwoofer 10S MKII, and LS50s powered off of an Emotiva BasX A100 (which I originally only bought to modify in order to push the full output of the amp out of the headphone jack to do extreme bass EQing on a JVC SZ-2000).

I can do my own research and I'm quite familiar with general acoustic principles, and with using tools like EQ, REW and Dirac. I can make my system near-flat with just the ten parametric bands on my MiniDSP HD 2x4, and I'm currently learning how to use convolution engines to make my own FIR filters (because I would like to replace said MiniDSP, as the performance sucks for being $500 after the Dirac license). I may only be in my 20's, but I'm not a novice, I just don't know everything and I'm trying to ask the right people the right questions to get the information required to understand the fundamental principles at hand. I'm far more experienced with headphones than speakers, I was into STAX by the time I was in high school.

Is the idea to play a subwoofer loud at all hours and not bother the neighbor?

No, the idea is to be able to listen to my system louder during the day without it bothering the neighbor. I usually work nights.

I don't have any issues with my neighbor, but once I have completed a prototype of this system, I plan on showing them all of the science that I've been doing to be respectful of their space, and then asking if I can use my UMIK-1 to do tests to validate the performance of the design. I'm sure they'll appreciate the thought.

Kemmler, IDK what the hell the guy is trying to do here, except chase a Unicorn. In his above equation he is assuming, on the RHS that the R(2) value is 0, or damn close to it. IT WILL NEVER BE ZERO, as you so well pointed out. You can minimize the impact of a sub to an extent, you will never cancel it out barring walls, floors, and ceilings made out of lead. I hope he takes your advice.

Why are you being rude? If you know something I don't, I would like to hear it, so I can make something better, instead of being talked down to.

 

[kemmler3D]

I'm aware. I'm not trying to build a soundproof room. I would just build a room inside a room. What I am doing is see how much low hanging and inexpensive fruit is actually available to be collected if one thinks outside the box and challenges common assumptions of how things should work. Is that not the point of Audio Science Review? People speculate about and experiment with science and others review it?



I'm trying to build upon my understanding of acoustics. Which is why I phrased the OP in the way that I did. Obviously I can't build a wall that defies physics. I was hoping it would make the underlying proposition of what I was asking clearer.

I would like to do things like...

- improve the quality of the treatment in my room so that I can achieve a more consistent and wide-band deadening of the decay of my room
- increase the usable headroom of my system, especially because I have lots of power still left on the table even with my current modest equipment, a Rogersound Speedwoofer 10S MKII, and LS50s powered off of an Emotiva BasX A100 (which I originally only bought to modify in order to push the full output of the amp out of the headphone jack to do extreme bass EQing on a JVC SZ-2000).

I can do my own research and I'm quite familiar with general acoustic principles, and with using tools like EQ, REW and Dirac. I can make my system near-flat with just the ten parametric bands on my MiniDSP HD 2x4, and I'm currently learning how to use convolution engines to make my own FIR filters (because I would like to replace said MiniDSP, as the performance sucks for being $500 after the Dirac license). I may only be in my 20's, but I'm not a novice, I just don't know everything and I'm trying to ask the right people the right questions to get the information required to understand the fundamental principles at hand. I'm far more experienced with headphones than speakers, I was into STAX by the time I was in high school.



No, the idea is to be able to listen to my system louder during the day without it bothering the neighbor. I usually work nights.

I don't have any issues with my neighbor, but once I have completed a prototype of this system, I plan on showing them all of the science that I've been doing to be respectful of their space, and then asking if I can use my UMIK-1 to do tests to validate the performance of the design. I'm sure they'll appreciate the thought.



Why are you being rude? If you know something I don't, I would like to hear it, so I can make something better, instead of being talked down to.

Cool, I understand better now. I think your idea might even be overkill for in-room sound quality, and as you note they'll have a big impact on high frequencies as well. It's not considered optimal to have extremely short decay times across the board, you end up with "headphone" sound at some point.

In passing I would just note that it's not exactly rare knowledge here that very deep / far from the wall absorptive treatments can handle bass frequencies well. It's just that almost nobody is willing to devote that much space to room treatments. People just usually cite that as a reason that porous absorbers don't work for bass... they do work, but not in a way most people will commit to.

For treating low-mid and true bass frequencies, you should also look into BAD panels and VPR traps. One uses binary diffusers as facing on (usually) rockwool panels, this effectively absorbs into the 100hz range and lower without over-absorbing high frequencies, with the added benefit of diffusing a fairly wide bandwidth.

VPR traps don't absorb appreciable amounts of treble but can work down into the 20-30hz range if you build them right. They work because large metal sheets have tons of resonant modes extending pretty low down. You glue the metal sheet to a piece of foam which damps the vibrations.

I recently priced out a DIY 4 x 8ft VPR trap and if you're not too concerned about what kind of foam you use, it can be done for maybe $150.

Both have the advantage of not taking up copious cubic meters of space in your room... but if you do go that route I would be very keen to see before / after measurements.

I would also strongly recommend checking out the Cox + D'antonio book if you are really interested in this stuff: https://www.amazon.com/Acoustic-Absorbers-Diffusers-Theory-Application/dp/0415471745 - when I was your age I was selling acoustic foam online for a living ... I would have done well to learn something worthwhile on the subject back then. I only recently read it and I learned quite a bit even from a quick skim.