Isolation is needed to prevent leakage, particularly in the bass region where energy can be coupled through the structure. My media room does a pretty good job with floating walls and ceiling (concrete floor) and 6"+ of mineral wool in the walls.
Can a Room 'Leak' Bass?
- Thread starter Rutgar
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I think what Ethan is saying is that you want bass to "leak" out of the room. Concrete floors or walls are usually not the way to go.
The other point Ethan makes is so often missed. SBIR is a bigger problem than the peaks/ringing that so many folks are worried about. So called room gain can be dealt with through a combination of EQ and treatments. SBIR cannot be fixed with eq and treatments need to be very effective at the precise location and frequency that cause the null. This is one of the reasons I really think everyone should use a microphone to measure their rooms. I've talked to dealers that claim they are great at room setup. I always ask them whether they use a microphone. They usually say they can do it "by ear." Anyone that thinks they can identify the exact frequency of a null "by ear" is smokin' something. It's not possible.
There's also excel spreadsheets that will tell you the distances of your SBIR if you know the frequencies. I think Ethan may know about that. I have one I got from Nyal Mellor and it was very helpful in finding out where the SBIR was coming from.
I was addressing Nightlord's comment about leakage getting him/her/it into trouble with the neighbors due to LF transmission.
I am not sure Ethan said exactly that, but perfectly reflective walls (zero transmission or absorption) would provide isolation but increase the magnitude of in-room modes (and SBIR, natch). I am not sure that is good or bad (I lean towards "good" for isolation is usually a design criteria) but in practice I am not sure there is a large difference in the magnitude of a peak or null whether the walls are concrete or plaster. Note reducing the magnitude in-room by using "flexy" walls means energy is escaping from the room, again not necessarily a good thing. It may be worth highlighting that treating a room acoustically is not usually the same as providing isolation; they are usually independent goals with different techniques used to meet them.
I need to look up SBIR; I thought it applied to both constructive (peaks) and destructive (nulls) interference.
Comb filter effects are quite annoying and one reason I prefer a more "dead" (highly-treated) room than a "live" (reflective) room.
I do use a microphone (Earthworks measurement microphone) and SW (RplusD) to help characterize a room's sound.
Onwards - Don
Your comments are only true if you care more about whether your neighbors can hear your music more than the quality of the music you are hearing in the room. You want sound to escape the room, just like Ethan said.
Hmmm... I'll defer to Ethan, but hold fast to my right to disagree with that statement. I do not believe isolation and in-room treatment to be mutually exclusive; it is most certainly a goal of every recording studio I have had the privilege to work (on either side of the glass), and both are goals of virtually all sound rooms I have helped design. In-house or commercial.
It is true that if all sound escaped there would be no room modes to speak of, but that (an open room) is not the only practical solution.
All IMO - Don
It is true that if all sound escaped there would be no room modes to speak of, but that (an open room) is not the only practical solution.
All IMO - Don
If you cannot play due to disturbing neighbors, then there's no quality of any sort in the room. Perhaps in the US one can be an a** and don't give a damn and just keep on playing, but in this part of the world we do have laws regarding distrurbance.
It's even worse than that.
Even if your ears were so well trained they could identify peaks and nulls accurately, music comes in different keys. So a peak or null at 110 Hz will definitely be evident for music in the key of A, or a related key where the bass sometimes plays A notes. But music in other keys won't contain frequencies that align with the peaks and nulls so they'll never be noticed. This is why proper software sweeps a sine wave through all frequencies.--Ethan
It might be possible to design walls that isolate and also absorb bass. But the usual method of using mass to increase isolation does indeed make the peaks and nulls inside the room worse. High-end studios therefore have massive walls, and they also have even more bass trapping than usual to counter the stronger reflections.
--Ethan
^^^ It is possible to design such walls, but I was thinking of the usual practice of massive floating walls and mucho' in-room treatment, so I think we are on the same page (as usual).
HVAC is another huge issue with isolation; my media room is not on the house system but has its own outside air ducts and a minisplit for climate control.
HVAC is another huge issue with isolation; my media room is not on the house system but has its own outside air ducts and a minisplit for climate control.
Actually your straightforward double drywall with green glue in the middle does a spectacular job of damping LF and providing isolation. Works even better if you use isolation clips and hat channel. Turns the walls into giant bass absorbers in the under 100hz range which is SO difficult to treat effectively using passive acoustic treatment. You still need a few 'bass traps' for the 100-300 range but the peaks and dips are less. At some point I'll post some measurements from my demo room showing the response with 0 acoustic treatment.
I can imagine that works, and I'd love to see real data. But "plain" mass still vibrates at some low frequency, and that passes sound through to the other side.
--Ethan
"Major Studios" are moving to this construction technique. Even in the world of retrofitting, this take place. I cannot 'name names', of course, this is data that +billion $ corporations like to keep to themselves.
From the data I have from Kinetics which is unfortunately unpublished...double drywall by itself actually has slightly lower absorption than a single sheet, but is at least more consistent in it's absorption curve. Both have a couple of areas where they are highly absorptive and others where they are not so much i.e. their absorption is variable over the bass range. Hang some double drywall on iso clips and hat channel and the assembly now has absorption slightly better than a single sheet but instead of the absorption tapering off below 63Hz like it does when you attach drywall directly to the studs it instead increases all the way down to 20Hz.The major differences are <70Hz which coincidentally is the place where conventional bass traps don't work too good (with the exception of pressure based devices like the Modex Plates, panel resonators, etc).
Couple of graphs from my (unfinished) demo room. No acoustic treatment yet so your just looking at 4 subs and the construction technique I used which is double drywall with green glue on hat channel and iso clips.
The frequency response measurement is UNSMOOTHED. The key thing on the waterfall to see is the nice even decay across most of the sub 100Hz range with the exception of a peak around 70Hz which is the height mode. My sub positioning didn't cancel that. Since all listener's heads are on the same plane vertically (more or less) that resonance is a prime candidate for EQing (as the response will be the same for all listeners hence EQ works just fine).
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Couple of graphs from my (unfinished) demo room. No acoustic treatment yet so your just looking at 4 subs and the construction technique I used which is double drywall with green glue on hat channel and iso clips.
The frequency response measurement is UNSMOOTHED. The key thing on the waterfall to see is the nice even decay across most of the sub 100Hz range with the exception of a peak around 70Hz which is the height mode. My sub positioning didn't cancel that. Since all listener's heads are on the same plane vertically (more or less) that resonance is a prime candidate for EQing (as the response will be the same for all listeners hence EQ works just fine).
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It would be more interesting with graphs done for those components individually. I can get green glue over here, and possibly I could talk someone into shipping clips - but we don't have your size hat-channels over here and that's not something I'd pay for transatlantic shipping on. So I'd be more interested in how much is the glue and how much is the 'movable wall'.
And as always... The interesting bit below 20 is off the chart...
And as always... The interesting bit below 20 is off the chart...
Here's another with the back two subs turned off (that's why the big dip appears, since the back 2 are helping fill in the SBIR related dip that comes off the back wall) around 55Hz and some EQ applied. Was just playing around
View attachment 7317
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It would be more interesting with graphs done for those components individually. I can get green glue over here, and possibly I could talk someone into shipping clips - but we don't have your size hat-channels over here and that's not something I'd pay for transatlantic shipping on. So I'd be more interested in how much is the glue and how much is the 'movable wall'.
And as always... The interesting bit below 20 is off the chart...
That would be nice....let me build a room, tear it down, rebuild it, etc!
Why is sub 20Hz interesting? IMO the key area for bass / LFE in movies is 30-70Hz.
Here's another with the back two subs turned off (that's why the big dip appears, since the back 2 are helping fill in the SBIR related dip that comes off the back wall) around 55Hz and some EQ applied. Was just playing around
Nyal,
I can guess your listening position is around 5 feet from the back of the room. Can I ask how many subs do you have and where do you locate them?
BTW, I am inserting a link about SBIR Speaker Boundary Interference Response something many people are not aware of:
http://www.acousticfrontiers.com/Speaker-Boundary-Interference.html
Yep exactly, around 5ft. Good work!
For that first measurement there were 4 subs, at 25 / 75% of room width front and back wall. The rear pair and front pair are not fed exactly the same signal - one pair has some additional delay relative to the other. Kind of a quasi double bass array but with half the number of subs.
The second measurement was just the front 2 subs.
In both measurements the front L/R were also playing.
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