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NRC - defining and choosing proper bass traps

@Brien Holcombe

Hi Brien... :)
I'm looking into some commercially-made traps to improve the room I'm now in ( SonoLite Bass Traps), and I was wondering, if I shot you some NRC numbers, if perhaps you would be kind enough to translate what these numbers actually mean.
This would help me, as well as potentially helping other RO members, or, for those who may be doing an internet search on the topic.

This seems to be a hot-button subject these days, with the plethora of home "studios" popping up on what is probably a weekly basis, and I thought we might benefit from your wisdom on this...
feel free to use visual aids and hand puppets if necessary - LOL - to help me get this. ;)

Here are the details and NRC's for this particular trap I'm looking at:

3" thick, 2' x 2'. I can get each panel for around $15.

Here is an explanation of the innards: http://www.auralex.com/product/studiofoam-pro/

NRC's

125Hz @ 0.55
250Hz @ 1.01
500Hz @ 1.25
--------------------
1khz @ 1.20
2khz @ 1.00
4khz @ 1.14

Thanks pal, I appreciate you looking at this info for me.
:)
-d.

Comments

Profile picture for user Brien Holcombe
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Brien Holcombe Mon, 02/27/2017 - 19:03

Well, the noise reduction coefficient (nrc) is simple really. Zero means reflective and 1 means fully absorbable at the specific frequency level.

Using your numbers:

125Hz @ 0.55 absorbs at 125Hz better than 50 percent
250Hz @ 1.01 absorbs at 250Hz 100 percent
500Hz @ 1.25 absorbs at 500Hz 100 percent
--------------------
1khz @ 1.20 absorbs at 1kHz 100 percent
2khz @ 1.00 absorbs at 2kHz 100 percent
4khz @ 1.14 absorbs at 4kHz 100 percent

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DonnyThompson Tue, 02/28/2017 - 02:43

Thanks Brien.
Simple for some is a question mark for others, I appreciate you taking the time.
I think this will help people who were perhaps unsure, as well as those who may be doing an internet search on the topic.
:)

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DonnyThompson Tue, 02/28/2017 - 04:37

@Brien Holcombe

Brien...
A follow-up question...
On this pre-made Auralex bass trap, there is a cardboard backing ( the side that would face the wall for mounting and that also holds the absorbent material in place).

Would it still benefit me to leave an air gap between this surface and the wall to increase the efficiency of the trap?...

...or would this be negated because of the this harder surface?

If it was negated, would drilling holes in this harder back-surface help to "open up" the rear to absorb sound from the backside as well as the absorbent material that faces the room, so that mounting it with a 3" air gap would increase the efficiency?

Thanks... :)

-d.

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Brien Holcombe Tue, 02/28/2017 - 10:02

Yes...having a gap of the same thickness of the panel or multiples there of, aid in the efficiency of mid and low frequency absorbtion.

A typical soft faced treatment is already capable of absorbing high frequency as we have seen. Small holes in helmhotz type absorbers are usually aimed at specific high frequency concerns and we aren't concerned with them at this point so it would not benefit you. In fact...since this is a specific design tested to control at specific frequencies you 》could 《 reduce the panels frequency range and void any warranty that the product comes with.

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DonnyThompson Thu, 03/02/2017 - 05:10

@Brien Holcombe , @dvdhawk, @audiokid, @Kurt Foster, @pcrecord,

Update:
I tracked a vocal last night after putting the treatment in, and I think it has also positively affected the room for tracking - at least for vocals - I'm finding that I need to adjust EQ far less than before.
The traps - while my intention was to reign in lower frequencies - also have good BB absorption across the whole spectrum, so I'm not fighting the EQ of the recorded vocal tracks nearly as much as before, which was geberally across the 200hz - 2k range.
The room never presented much problem in the top-end, I wasn't getting any noticeable flutter echo or high end harshness, but I'm sure that the acoustic treatment has smoothed out the room for recording - which is a benefit I wasn't counting on. The room is by no means "square", it's more of a trapezoidal shape, with chimney and closet breaking up parallel surfaces. The ceiling is canted from 7'/6" up to 12'/8". I'm firing my monitors from the short side into the tall side.
So far I'm liking the results - which means that I may just go back and retrack all the vocals, before I attempt a mix of the latest piece I've been working on.
-d.

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pcrecord Thu, 03/02/2017 - 06:14

DonnyThompson, post: 448063, member: 46114 wrote: So far I'm liking the results - which means that I may just go back and retrack all the vocals, before I attempt a mix of the latest piece I've been working on.

Good for you Donny.
Reading this makes me happy to know I'm not the only one who wants to go back and retrack when I get a new recipe that sounds better !! Got to like recording, right ! ;)

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DonnyThompson Thu, 03/02/2017 - 06:26

@Brien Holcombe , @dvdhawk , @audiokid , @pcrecord , @kmetal , @Kurt Foster ( et al)

I've lifted this quote from another recent thread here on acoustics, ( https://recording.org/threads/looking-for-a-low-end-check-please.62140/#post-447736)

Brien Holcombe, post: 447736, member: 48996 wrote: Mix position as you guys have discussed is important...and elevation in the structure can not be over stated...meaning the farther you get from the earth the more flanking can penetrate your environment....like a second story room.

I thought it would be pertinent to this thread as well; Brien mentioned something last week I hadn't taken into account, and that is that the space I'm on is on the second floor.

I'm wondering if perhaps I need to add some treatment that goes lower than the rating of the treatment I have in place now ?... perhaps 6" of Roxul in two of the corners ( the shape of the room is such that there aren't 4 "defined" corners)... something that could get down into the lower frequencies, like in the 50hz to 100hz range.

Or, there is another option - I do have a closet behind me that isn't really being used for anything important - maybe I should consider adding some thick Roxul into this closet, in essence, making one big-assed bass trap ( 8'H x 7'L x 30" deep), which would give me 117 cubic feet of trapping.

The current doors of the closet are typical cheap wood that you'd find in any condo that was built in the 70's... but I could always open the doors when I'm mixing, or, I could add "slatted" doors, like these:

As I've mentioned before, I'm not concerned with isolating the room. Short of occasional vocals or acoustic instrument tracking, nothing will be recorded here that would involve mics.
My main goal is to have a mixing space that is accurate - or at least a whole helluva lot more accurate than it probably is right now. ;)

Thoughts? :)

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Kurt Foster Thu, 03/02/2017 - 06:50

the trapping in the closet behind you is an idea i have toyed with myself in my current "mix room".

i did a very similar thing in a live room i built for an associates home studio, in a two car garage that had a 20' roof peak. the garage sounded good when i started the renovation but you could hear rain hitting the roof when it rained. so i put up some pink stuff between the studs, and covered it all with sound deadening board fiber board. along one wall of the garage there was a shelf that was 4 feet deep that ran the entire length of the room floor to ceiling. we had a bunch of pink stuff batts left over from the ceiling treatment and i just stacked them up on the shelves and then we put up some moving blankets over that. it really tuned the room right up. i did a lot of really good drums tracks in that room and the reputation i gained from that build and the work i did at that studio, gave me the gas to propel me into my next venture which was KFRS, my big studio.

the mixes i have heard from you have always sounded great Donny. my assumption is you know your room and what you need to do to get your mixes where you want. but then i am not a mind reader so for all i know you might be frustrated with your results. still what you do sounds great to me and many others.

all that being said, you are never going to get close to an accurate listening environment until you get in to a room with a lot of space. this is not to say that great recordings haven't been done in less than ideal rooms. if you know your space and you are doing good work in it (which imo you are ) what more do you need?

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DonnyThompson Thu, 03/02/2017 - 07:16

Kurt Foster, post: 448069, member: 7836 wrote: the mixes i have heard from you have always sounded great Donny. my assumption is you know your room and what you need to do to get your mixes where you want. but then i am not a mind reader so for all i know you might be frustrated with your results. still what you do sounds great to me and many others.

Thank Kurt. I do appreciate your kind and supportive comments. :)

The thing is, the stuff you've heard from me in the past hasn't come out of the room I'm in right now.

This is a fairly new setup, I've only been "mixing" ( and I use that term loosely, LOL) in this room for less than a month, so at this point, I don't really "know" my room...( but I sure am hoping to. ;) LOL)

I suppose - like any other pro audio cooker out there ( such as you and the other talented regulars here on RO) - that I'd like to get the space to be as accurate as it can be, but at the same time, knowing that achieving a "perfect" environment is difficult to do in even the most professional of rooms, let alone in my condo's second story spare bedroom. I guess I'm after "as good as I can get" under the current circumstances ( and limitations). I'm not expecting miracles, given the circumstances.

@Brien Holcombe ( I'm wondering if Brien is rolling his eyes right now...:rolleyes: thinking, "I wish he'd shut up about all this and just mix!" LOL)

I dunno... Maybe it's just that never-ending journey for great sound that we all travel. As we all know, there are lots of factors to getting good mixes; how well the tracks were recorded, the gear, skill, experience, environment, monitoring...

But ... if I can achieve even a 25% improvement to my space - and in turn, to my mixes (without selling a kidney, that is), I'd sure like to try. ;)
At this point, any improvement - regardless of the degree - is still an improvement, which puts me one step closer to getting what I'd like to ultimately get... which is a night of wanton passion with Jillian Anderson. Or Susannah Hoffs. LOL.

:)

-d.

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Brien Holcombe Thu, 03/02/2017 - 08:40

Flanking is difficult to overcome. That said there are ways to get away from it. The treatments are helping on the inside of the room as you are witness to.

So let me ask you about the monitors and maybe even where you sit and monitor from.

Even on a concrete floor low frequency can travel up desk feet or lightweight stands and interfere. 703 Owens corning rigid insulation can be used to decouple parts of an environment from the environment itself. With a 2'x2' piece of the rigid insulation topped with 3/4 inch plywood placed under each stand or something of a sufficient diminsion under the monitor itself if they are on a desk should eleminate any interference you may be experiencing.

You may want to consider doing the same thing if and when you track vocals or acoustic guitars.to decouple the.mic and the seating area if possible, from the floor.

As a task for you, have you made yourself aware of electrical usage thru refrigerators washing machines anything that can vibrate, so that when you are working you are not in competition with these machines?

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DonnyThompson Thu, 03/02/2017 - 09:37

Brien Holcombe, post: 448077, member: 48996 wrote: As a task for you, have you made yourself aware of electrical usage thru refrigerators washing machines anything that can vibrate, so that when you are working you are not in competition with these machines?

I have not, Brien, but will now do so.

Brien Holcombe, post: 448077, member: 48996 wrote: So let me ask you about the monitors and maybe even where you sit and monitor from.

As of this writing, I am trying these Eris E8 monitors that Dave Hawk brought me last weekend.
They are actives, and have 8" Kevlar drivers, 1.5" silk domed tweets, and are ported in the front ( the ports are 3/4" high by 7" long... it's not a "hole" but a horizontal "slit" type port.
The drivers have a rating of 75 watts, with all frequencies past the X-Over point ( 2.2k) having a rating of 65W through the tweeters.
I do not know what the cabinets are constructed of.
My current monitor setup puts the monitors at about 34" from any boundary behind them ( well, they're that way NOW... up until Dave came to visit me last weekend, the rear of my old Alesis M1 monitors was much closer to boundaries).
I've since moved the whole rig out from the walls further. My mixing position is at the point of an equal triangle of 32", which is the same distance that the monitors are apart from each other.

FWIW, the "acoustic compensation" switches and pots on the back of the monitors are all set to "flat".

Kurt ( @Kurt Foster ) mentioned something a few months ago in another RO thread, (if I'm understanding him correctly), where he suggested that there are some rooms where using a bigger woofer, like the 8" that these Eris's have, may not necessarily be the best thing for certain spaces... I'm just throwing that out there as a possible consideration, in that I'm wondering if these E8's might be "too much gun" frequency-wise for the space I'm in - but I also realize that it's tough for anyone to say anything unequivocally, because you're not here in the room to listen. I could hook up my M1's again, do a ix or two and see how it translates.

The treatment I've put up has resulted in a "tighter" sounding low end, along with far less difference in different parts of the room... the low end sounds a lot more "even" to me throughout the room. I'm not saying that further treatment isn't necessary, I'm just saying that there aren't the audible peaks and dips in the 100hz to 300hz range now that there was before I put the treatment up.

Brien Holcombe, post: 448077, member: 48996 wrote: Even on a concrete floor low frequency can travel up desk feet or lightweight stands and interfere. 703 Owens corning rigid insulation can be used to decouple parts of an environment from the environment itself. With a 2'x2' piece of the rigid insulation topped with 3/4 inch plywood placed under each stand or something of a sufficient diminsion under the monitor itself if they are on a desk should eleminate any interference you may be experiencing.

This is definitely something I can do... and a great idea. I could probably even pick up a few slabs of sq. foot pavers to set the stands on if you think that would help.

The stands I have are lightweight; I currently have a 8" x 8" piece of 3" OC 703 between the monitor and the stand to help isolate them a bit.
But truthfully, I'm not sure if that's helping or not? I figured it probably wouldn't hurt, but as to its true effectiveness, I can't say for sure.

-d.

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DonnyThompson Thu, 03/02/2017 - 09:49

@Brien Holcombe

Here's a scenario I just thought of... feel free to tell me if its bat - s h i t nutty...

What would happen if I placed an omni directional mic at my listening/mixing position, played some music (something professionally mixed) through the monitors, and then recorded it, at say, -23dbfs ( LUFS), and then presented this mono audio file for analysis?

Is this totally crazy?

Or ...should I just stick to science, and do an actual analysis where the results could be viewed on a graph or waterfall visual?

Either way I'm gonna have to buy ( or rent) something like the Earthworks Omni for this..
I dunno. Probably a worthless idea.

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Kurt Foster Thu, 03/02/2017 - 09:59

DonnyThompson, post: 448078, member: 46114 wrote: The treatment I've put up has resulted in a "tighter" sounding low end, along with far less difference in different parts of the room... the low end sounds a lot more "even" to me throughout the room. I'm not saying that further treatment isn't necessary, I'm just saying that there aren't the audible peaks and dips in the 100hz to 300hz range now that there was before I put the treatment up.

it might sound better but actually be worse Donny. you never know with low end until you do acoustical measurements. you can't trust your ears with the low end like you can with midrange and high reflections.

my opinion is if you are in a room that is too small to allow a half wave length of a fundamental note to develop, then you need to limit the amount of that frequency you put into that space. common sense. you can't put 10lbs of sh*t in a 5 lb. bag. physics. this is why speakers have ports or manufactures like Bose have labyrinths. to accommodate low freq wave length propagation. 70hZ needs 14 feet. that where my 14 foot minimum comes from.

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DonnyThompson Thu, 03/02/2017 - 10:19

Kurt Foster, post: 448082, member: 7836 wrote: my opinion is if you are in a room that is too small to allow a half wave length of a fundamental note to develop, then you need to limit the amount of that frequency you put into that space. common sense. you can't put 10lbs of sh*t in a 5 lb. bag. physics. 70hZ needs 14 feet. that where my 14 foot minimum comes from.

Yeah... I don't have that, pal. Not hardly.

10' from my back to the wall behind me, 5' to my right and left, 4' from my head in a sitting position to the lowest part of the canted ceiling.

I realize that what acoustical perfection is unrealistic - more like impossible.
But, if I can get even small increments of improvement, between the treatment, and me getting "acclimated" to the room and the monitors, that will just have to do until a better option becomes available.

For as much as you guys have been patient and put up with my bitching about investing into a studio being a terrible business idea, I'm not done trying... not hardly.

In fact, I have a big project coming up in the spring ( with RO member Dave Hawk co-engineering the project with me, an album for charity that includes Cleveland's best A-List players)...and while the tracking will be done at another temporary location, I need to get this room as good as I possibly can, because this room I'm in is where we will be mixing the record.
I'll just have to do what I can - without altering construction or going crazy with money - and get used to the space we're gonna be in.

That's not to say that I'm done improving this space... I'm happy - if not enthusiastic - to continue to make improvements with treatment (and/or monitoring alterations).
There will most certainly be a limit to the improvements... I know this - after all, I'm mixing in the second story spare bedroom of a condo that was built in 1972, so I do realize that I'm not sitting on a potential mix engineer's acoustical dream here.

I'd just like to do everything I can to get right up to those limitations, if at all possible. ;)

-d.

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DonnyThompson Thu, 03/02/2017 - 10:26

As a side note, I'd like to thank everyone for their experience, their willingness to help, and their patience with me, while I'm working this all out. ;)

@Kurt Foster @Brien Holcombe @audiokid @dvdhawk @pcrecord @Boswell - and everyone else that has been following my recent threads between the monitors and the room...

Thank You. :)

x