Acoustical Treatments Master Thread - Page 7

Exactly, Larry, where the angles hitting the wall are equal.

Scott,

> I was planning on entire front and 42" of side all the way around. <<br />
That's not the way I'd do it. All rooms need substantial bass trappng, and what you describe does nothing at all for the bass. At the risk of sounding like a broken record, see the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

It explains how bass trapping works best in the corners, and there's a sidebar that explains the importance of placing absorption at the first reflection points.

--Ethan

Jason,

> what % of sq/ft needs to be covered with (1") acoutic treatment to achieve correct RT60? <<br />
Unfortunately, that's too simplistic an approach. One-inch thick rigid fiberglass does little for the midrange and below, so already you're starting off on the wrong foot. A complete treatment solution has to consider all frequencies, not just those above about 1 KHz. Also, in most rooms there really is no reverb. What dominates more than anythng else are the individual reflections.

(Sorry, I know this doesn't really answer your question!)

--Ethan

What do you do with a dipole main that has its null aimed aproximately at the side wall?

Thanks Terry. To sum up, the first reflection points need to be treated for two reasons:

1)For high frequencies, you get a time domain reflection that can be seen in the impulse response

2)For low frequencies, you get boundary interference at a certain frequency causing cancellations to occur.

Ethan has great plans for a high-bass membrane trap that is centered precisely where I need it (150-300Hz).

You don't have to worry much about dipole mains and side wall SBIR. Unlike a
monopole, a dipole doesn't become a uniform spherical radiator at low frequencies.

Yup. And they are both dependent on distance from the wall, or more correctly, path difference to your ears, as in Larry's diagram. This path difference determines:

1. For high frequencies, the time at which an early reflection can occur. If this time is greater than 15 milliseconds or so, it does not get merged by the brain into a single spatially-shifted sound image, and you don't have to worry about it screwing up your front sound stage.

2. For low frequencies, the frequency at which SBIR phase cancellation can occur.

- Terry

Thanks very much for the reply. OK< all the way around is out. What about front wall?

I read the FAQ yesterday in it's entirety. I admit I started getting glazed over when I got to the second sidebar and will start over there. I got the gist of the bass traps and got a pretty good understanding of the methods used to control stuff. I got the hows, but still am not sure which I need to plan for as I start construction. Unfortunately I'm still not sure where to go. After seeing a post where someone found out the better ceiling tiles are $$$$ and with my square footage, drywall is getting a rethink. I could place a whole lot of conduit and pay a drywaller to come poke and patch a lot of holes if the poster was correct on cost of good tiles (about $2000 for my room tiles only).

So, what I got out of your FAQ so far is bass traps will be necessary. I also saw your studio pic with the membrane traps. Those look most interesting to me beasue I have 2 little ones and am not to keen on filling the room with semi-exposed fiberglass. At least yours was split between the trp types.

But as I try to get this beast going, I'm still unsure what to plan for.

Above my ceiling is a vary large space, whether it be a drop-tile or drywall. It is made of 21" tall trusses 16" OC. Filling that cavity with fiberglass isn't impossible, but that would be a huge volume of fiberglass. You can't really fill inside the truss members without heroic efforts which I'm not going to be able to do. I could get the spaces between the trusses at least partially filled (13x16" batts maybe). I could potentially pay a insul company to stuff the spaces with 13" thick batts on their sides except where I have mechanicals (ducts).

Would that be one huge bass trap? From your FAQ and the mostly over my head discussions with the engineers and experts here, I gather that:

1) If I drywall the ceiling, the insulation above would be rendered almost moot? If I drywall, I may be better served to double it up, isolate it and get some sound control for upstairs (not a requirement, but if a fringe benefit, ok).
OR
2) If I use a drop-tile ceiling (presume cheap tiles) and insulate above, it will make the ceiling dead for a broad spectrum? giving it the illusion of no ceiling which is apparantly good. But I have a dead floor I'm worried about:

All these questions but one of my main concerns is that I do want carpet. It's a personal thing and we will use the space for more than home theater. The carpet will make the otherwise cold, hard floor more little-human and female-human friendly. If I MUST to keep the floor live from hosing the acoustics I could use one of the prefab wood floor types but I'd strongly rather not.

I'm happy to build the room and add the treatments later. The membrane sealed type traps look attractive to me as a solution if I can figure out which ones to make and where to put them.

The FAQ directs that the corners are the best spots for the traps. My room Link here has 2 corners occupied by small 45deg walls with doors in them. One corner is tight for space. One corner (front left) is wide open for a trap.

How would I best deal with these corners?

Mainly for now I need help in deciding a fundamental basic plan I can tweak later. To cover walls or not, drop tile or drywall ceiling, and all considering I want carpet.

Thanks very much for your and all else's help,
Scott

Thanks Ethan and bpape,

I kind of knew the answer, but was holding out hope.

Ethan,

Does having more than one sub (three (3) in my case) make adjusting bass response exponentially more difficult? As you may know I own Def Tech BP2000's with 15" powered subs in them that serve as my main (L/R). I have set them to "large" on my current reciever (i've tried both large and small, but MUCH perfer the large setting). I could also if it would be better set them to small and feed them the LFE/sub out which would mean they would sync up with the dedicated sub, but currently I do not have them run this way. I also have another (Def Tech) powered 15" sub located in the rear of the room which is feed from the sub out on my receiver (soon to be replaced by a PRE/PRO).

When I take test measurements and attempt to identify where and how much bass traping I need, should I be isolating my mains from my dedicated sub and testing these independently? While that might help me identify which sub is causing a peak at specific frequencies (i.e. mains or dedicated sub), ultimately when I use my system they will all be on, so that would suggest I should adjust my bass response curve in total? Shouldn't it? Just trying to understand. Any suggestions on setup or testing? I included a pic of my Def speaker inputs. Thanks!

-Jason

I am kind of in the same boat as jasplat88, in regards to proper setup to determine accurate measurements because I have NHT T6's and a W1 sub to recreate bass...

The position of a sub in the room will affect the level of excitement of the room's modes. For example, if you have a peak at the seating position, placing the sub in a null for that frequency will eliminate the peaks for that mode. Having said that, placing two or three subs in the room will create a "virtual" sub at some altogether different location in the space. In other words, the three subs together will behave as though there was a single sub in some other location.

Yes, setting up a room with multiple subs is more difficult than with a single sub...although significant benefit can be gained with this method. HOWEVER, you have a difficult problem. You cannot move two of your subwoofers. To do so would place your sound stage at risk. The best you can do is move the single sub around to optimize (favorably) your bass response at the listening position(s). That may leave the sub in the middle of the floor somewhere. Next, using parametric equalization, you tame the remaining peaks at the listening position. You are now left with nulls. These nulls can be addressed with specific band frequency absorption.

Using full range speakers in a small room (residential sized, for example) will simply mean that if you don't like the sound, move 6" and it'll change.

Dennis....thank you for the response. That helps---kind of You are right about my two front subs and the fact that they are pretty much set. I do have the option with my mains (L/R) since they are side firing to have the subs fire inside the room or towards the side walls. I have already tested both of these positions and found better bass for the rear seats by having them fire at the side walls. The front seating location bass did not improve much if any by this change--although I think it got a little more boomy in the higher bass freq's when the subs were firing towards the center of the room.

I have not moved my dedicated sub down there yet, but will do that this weekend. I will also graph my bass response once all subs are down there. After our earlier discussion about the front wall treatment with my particular speakers I have decided to continue testing different acoustic treatments for the highs and mids (1" 703 equilivant), but am more concerned about the bass at this point.

I did not mention, but I am limited on where I can place the sub IF I choose to place it in the rear of the HT. Otherwise, I can put it up front as well, but want to try the rear location first (and possibily settle for a acceptable vs. preferred) bass response curve---aesthetics reasons. I will try to chart out some response curves tomorrow and post them.

-Jason

Terry,

Just out of curiousity, does SBIR repeat across multiples of the fundamental? (ie. If I get cancellation at 220, will I also get it at 440,660,880 etc)

Andy K.

I am trying to find either 703, insulshield or comperable product somewhere in CT. I have spend the last hour calling around and NO one seems to cary anything. Help!! I am redoing the front of my theater and really want to do it right. Any suggestions.

Great thread!

Early on someone asked about perf screeens ...

I'm treating screen wall with 4" 703 mounted 4" off the wall.

Would a perforated screen (mounted approx 4" in front of the 703) improve overall sound quality? Seems like it should, as it eliminates a largely reflective surface over a large fraction of the front wall.

On the other hand, my inuition vis-a-vis acoustics is often confounded by physics.

ps -- if answer is "it depends": room is 18' by 15' by 12'; first reflections off the side walls are being handled by 2" RPG binary diffusor panels or 7" skylines (will decide by ear); rear wall ceiling has soffit with bass trapping.

Yes, it does. However the higher frequency nulls are generally not a problem. They get lost in the increasingly large number of room modes which form the reverberant sound field.

- Terry

Thanks, thats what I thought. If I take a linear plot of my room with ETF, I do see these multiples of the fundamental. However, on the logarithmic graph the multiples are not an issue (you can see a dip at 440 but its small)

This actually has little to do with the low frequency performance of the room...or impacts on modal response. We aim the drivers away from the audience and place a 1" acoustic (fiberglass) panel on the wall the driver points to. The problem is the mechanical noises of the sub, and the slap of the driver against the air creates an artifact of the sub's motion. That artifact can be (a) distracting and (b) fool you into thinking you can localize sounds below 80Hz.

Hi Dennis,

I was hoping you could help me with a theory question.

If I understand Jason's situation correctly two of his subwoofers, which are integral to his mains, are on the right and left main channels. His third sub is a dedicated LFE subwoofer.

In the white papers I've read about using multiple subwoofer to smooth bass response I was under the impression all the subs were operating on the same signal.

My guestion is even if Jason could reposition his mains without adversely affecting their sound stage, wouldn't the fact that his three subs are on three different channels mean there would be little or no effect on smoothing bass?

In other words for purposes of discussion suppose his mains were comprised of a right and left satellite and a right and left subwoofer (with suitable crossovers so the subwoofers integrated well with the satellites). In this example the three subs could be repositioned independently without adversely effecting the sound stage of the satellites. However, would it be reasonable to expect to see improvement in the bass response if the subs were operating independently on three different channels?

If we ran test sweeps on the above configuration in an attempt to test this no doubt we could come up with an arrangement that would appear to smooth the bass response. That might be due to the fact that during testing the same signals are being delivered to all speakers. However, in normal operation when the subs were on different channels would the smoothing effects of opposing subs disappear?

Thanks.

Larry

Here's a quick follow-up on my room and sub situation. I moved down my dedicated sub which I have running through the BFD. The mains (with subs) are still run only feeding full channel to each main (no LFE feed). I placed my rear sub where I would *like* it to be and graphed the response curve. I then used the BFD to adjust for the TOTAL room peaks (i.e. I did NOTHING to change the bass response feed or produced by the front main speakers).

Prior to the BFD, the room was pretty boomy. After the BFD adjustments the bass response is significantly better. A lot of the boominess is gone and I am able to crank deep bass songs and notes much louder without the ringing.

HOWEVER, is it possible, I (and potentially others) *like* <---perhaps it would be more appropiate to say "are use to"---- some boominess? If you look at my response curve, it could still use some work (sorry I only had a couple of hours, and there were at least two peaks being caused by the front speakers becuase no change to the rear sub would impact the response curve. Even with the response curve the way it is now, i turned up the sub after getting it to where it is now, and still would prefer a tiny bit more "boom."

Dennis/Terry/Ethan, I know we have talked about adjusting the response curve, but have you noticed in your experience that some people actually prefer a response curve that is not flat? I might try to run LFE to the front mains and remove the jumper to the lows (which will allow me to run it through the BFD and adjust my curve a bit more). I'm interested to hear your thoughts on the boominess issue and any other observations you can read into this.

-Jason

Edit: How will the "phase" knob on my dedicated sub impact changes to this curve (it is currently set at 180)? The x-over on this same sub is currently set at ~@55Hz.

Yes. For example, the characteristic Bose sound has boomy bass, and some people love it.

It is ultimately a matter of personal preference. However to me, the original meaning of "high fidelity" is something to consider. Audio equipment has improved over the years so as to be capable of much more accurate response -- that is, fidelity to the original sound recording.

But we are currently experience a boom in bass (sorry! ) due to unprecedented ease in reproducing very low frequencies. In the older days of hi-fi, you struggled to get much of any very low bass any way you could. You put speakers right in the corners to get the most room mode boost, regardless of uneven frequency response.

So IMHO, the emphasis on huge bass is in some sense a current fashion. Also, something very important to consider is that psychologically, we vertebrates perform accommodation to sensory stimuli. Without both high and low levels of any stimulus, we tend to just tune it out because it becomes the norm.

A good movie sound mix will make judicious use of loud bass, so that when it is important dramatically, you really hear it, and the movie becomes that much more exciting. The bass should really be there when it is needed, hence the need for power and accuracy in subwoofer response. But to have it cranked up all the time is not necessarily desirable.

Regards,
Terry

The fact that each sub is playing something different is no really relevant to smooth bass reponse. Each sub is still producing frequencies which excite room modes. Now, having said that, if each of three subs is playing something different, you cannot use mutual placement as a means to smooth response between them. Full range speakers are a real problem in small rooms...everyone in the room will hear something different whether discrete bass channels are used or not. The non-directionality of bass makes it rather moot do to this in any case.
---
Your mains and your subs typically have a path difference between the drive and the listening position. At the crossover point (usually 80Hz) the sub and the main speaker are playing exactly the same sound. At the crossover you want the sub to be in phase with the main with respect to the listening position. If you don't, you'll have a peak or null at the crossover point. The phase knob allows you to adjust the phase of the sub to match the mains at the listening position. In your case, they must be in phase at 55Hz.
----
It is true that most do not prefer flat frequency response. We prefer a roll off in the high frequencies and, today, there's a lust for boom. You might do a search on "X Curve" for some background on this area of psychoacoustics.

http://www.screensound.gov.au/glossa...e?OpenDocument

It does come down to preference. Also, most peoples' reference material (music or otherwise) is probably boomy. Currently, fashionable as Terry states Many people listen to music in cars or in areas which aren't close to being accurate. And many people have not heard a reference system. So, when one listens to something in a room with a relatively flat frequency response they will wonder. Where's the bass?

I agree with Terry that you will hear the bass if it was meant to be there. Another consideration is how boosting the bass affects the rest of the frequency spectrum. If you are having a conversation in a quiet room and each person is talking at the same volume level say 75db it is pretty easy to understand them. Now, would you be able to understand them if you were sitting in a Dodge Viper zipping down the road or just at idle for that matter without raiding the volume of your voices. That low frequency rumble is interfering with the intelligibility of your dialogue. The same thing happens when you boost a frequency range. You have raised the overall SPL of the room and this can make dialogue intelligibility and detail tougher to discern.

Now whether you'd rather be in the Viper or able to hear the conversation is your choice. OK, maybe a bad analogy! But, you get the point.

Bob


Bob

Terry/Dennis/Bob...thanks for the responses. I think in my personal experience (as a DJ for serveral years) I was suckered into the "pump up the bass" mentality where you help entertain your audience by helping them *feel* the music. Since that time (that was about 12 years ago) I have become accustomed to bass (although I have never owned a car where you could "feel" the bass a car or two away....but I have always had sub in my cars and in my home system. I think I (and at least many of my friends) prefer some boom <---- and to better define that for me at least....it is the ~35-60Hz boom I really enjoy (especially when it is clear).

Dennis,

Are you suggesting here:


that in my case I would be better off running my mains as "small" and then feeding them the LFE as opposed to running them as large full range? I think you are saying that it would be best to not have full range speakers, but at least for me that's just one more constraint I have to work with.

The x-over set at 55Hz was more by mistake, as before I had taken any measurements it was SO boomy I just turned it down to minimize the output from the sub and I forgot to change it back before measurements. I like running my sub x-over in the 80-90Hz range. I will make that change and re-measure.

I also downloaded EFT5 last night, but since I am not familar with that program yet, I will need to play with it. Can someone please provide some guidelines on what RT60 should be in:

1) A home theater enviroment?
2) A music enviroment?

Bob.....bad analogy...I'd take the Viper

-Jason

I am installing rigid fiberglass on the screen wall of my family room/HT (plus 2' back on the sidewalls from floor to ceiling). For each of the 2 front wall/wall corners I have 2'X5'X4" rigid fiberglass to span the corners and act as base traps. How high off the floor should the bottom edge of the 'traps' be?

Continuing from the trap up to the ceiling, is spanning the corner with the 1" rigid fiberglass for mid/high absorption ok or should it be placed flat on the walls? I save a little material by spanning and I think I am cutting it close as it is. If I span with the 1", should I leave a gap between it and the trap?

Hi Dennis,

Thanks very much for the response.

Aside from perhaps wasting money, is the simple solution to someone who has full range speakers to merely set them to small and set the crossovers sufficiently high?

Thanks.

Larry

Hi Jeff,

Try this thread for a start.

RT60 what is a good value!

If you are refering to two-Channel music then there should be different Reverberation Times. However, I believe that multi-channel music can happily coexist in a room whose acoustics were designed for multi-channel movies.

Larry

Scott,

> if the poster was correct on cost of good tiles (about $2000 for my room tiles only). <<br />
You shouldn't have to pay even 1/4 that much for good tiles! But one alternative is to use thin tiles and lay 6 to 12 inch thick fiberglass batts above them.

> Above my ceiling is a vary large space <<br />
You don't need to pack the entire cavity. R38 batts are one foot thick, and that's plenty.

> Would that be one huge bass trap? <<br />
Yes, but bass trapping needs to be distributed, not concentrated only in the ceiling or only on the back wall.

> If I drywall the ceiling, the insulation above would be rendered almost moot? <<br />
Yes.

> If I drywall, I may be better served to double it up, isolate it and get some sound control for upstairs (not a requirement <<br />
If you don't need isolation you'll do better to skip the sheet rock. But as Dennis Erskine always points out, preventing sounds from getting into a theater room is often just as important as keeping sound from getting out.

> I do want carpet <<br />
That's fine. I have carpet in my home theater too. A reflective floor with absorbent ceiling is more important in a recording studio than a home theater.

> The FAQ directs that the corners are the best spots for the traps. <<br />
Yes, but understand that wood panel traps and fiberglass-based traps work on different principles. Both need to be near the corners, but the exact mounting differs for the two trap types.

--Ethan

Jason,

> When I take test measurements and attempt to identify where and how much bass traping I need <<br />
You already got good advice on sub placement, but I'll add that bass traps do more than flatten the low frequency response. Just as important is the way they reduce ringing, and in a way that EQ cannot.

--Ethan

You are probably better off setting your mains to small and connecting your subs to the LFE output from your BFD. Then I would try to find the best location for your subs and seating if that is a possibility. Before we work with three subs, let's start with one.

1. Put your sub in the corner and move your mic between the corners of the room and take a reading of Pink noise on the LFE channel only. Theoretically, that same corner should be fine. But, I like to double check especially if it is not rectangular. This should show all your room modes.

2. Now move your seating to get the smoothest response possible.

3. Place sub in your seating location and move the mic around the room and find the areas that give the smoothest response. This will be somewhere into the room. Not along the walls.

4. Next place your subs so that if you draw a line from each sub. That line would intersect one of your ideal sub loacations. This is probably a good starting point for your sub placement. Start moving each one individually now and find the best spot. This is the virtual sub technique. Note: if your fronts and seatings is fixed. I'd probably just use this technique first and just take measuments each time you move your third sub and find the best spot this way.

5. EQ out whatever peaks you have left.

Also, when doing this you might want to play with your crossover point a bit. Since the placement affects this. Changing your crossover might yield a smoother response as well. 80 works for the majority of small rooms but sometimes you can fix various problems by altering the crossover point.

About RT-60. Dennis gave great recommendations in the other thread referenced. But, remember RT-60 measures overall room reverberation. You can have a good RT-60 and still poor sound becuase you didn't treat the first reflections. So just having enough absortion to achieve a good RT-60 will not necessarily bring good sound. The absorptions has to be placed in the correct areas.

Larry,

Search Dennis' name. He had a relatively recent thread about why setting your speakers to small and using subs to correct for SBIR effect is useful. The answer to your question is probably yes. Again depends on placement and how the best placement for low frequency information isn't the best placement for the mids and highs.

Bob