# What to if 71Hz is missing in room?

Discussion in 'Mastering' started by jscott, Jan 21, 2003.

1. ### jscottGuest

I have been tweaking out my room, which is rather large, about 27(L) x 22(W) x 10(H) and has various 50 degree angles from about the half way point on the walls up to the ceiling as its built over a garage.

Right now its standard drywall treatment, with carpet. Given its odd shape, I'm not quite sure what to do about my problem, which is:

I appear to have a mode at 71Hz, as you hear nothing at that frequency, which is about -16dB. However, from about 55Hz to 90Hz there is also a substantial bell shaped dip, centering on 71Hz with a 6dB peak again at around 109 or so, and another bell shaped dip of -6dB around 138Hz, which from what I know, seems to make some sense.

I've moved my speakers all around the room on a rolling cart trying all covceivable positions, and located one spot where the dips were less, but probably not a practicle position to monitor from.

They are the HR824s, and I've got them with switches positioned at 47Hz, acoustic space B, and the "0" position for the highend. However, the problem areas do not seem to be effected very much by any switch combination. I know these have been debated endlessly elsewhere on the forum, and despite that, I own them and I don't want to re-debate them. The response from them is otherwise reasonably flat, within ± 2dB above 300Hz to about 19K. AND....I have measured them with precision to make sure they are centered and at various equal distances from the rear wall. Nothing seems to make a difference.

So what are my solutions to this problem? I'm a bit thrown by the odd nature of the sloped ceilings.

2. ### ProlabGuest

Seems to me the acoustic crossover on the passive is around 70 so I would start by putting a dense piece of foam right behind the speakers (close) to force 70 (71 in your case) to get out. It is a phase cancellation from the rear of the enclosure.

This should force the 65 to 75 more forward. Try it and report back.

3. ### Ethan WinerActive Member

Joined:
Mar 19, 2001
Location:
New Milford, CT USA
JS,

> I appear to have a mode at 71Hz, as you hear nothing at that frequency <

This is a very common problem. In fact, pretty much all rooms that don't have proper acoustic treatment exhibit these variations in low-end frequency response.

> I've moved my speakers all around the room on a rolling cart ... the problem areas do not seem to be effected very much by any switch combination. <

Yes, this problem cannot be corrected by moving the speakers or applying EQ. Sometimes that can make the problem a little less severe, but the ultimate goal is to minimize the reflections that create the response peaks and dips in the first place. Waves from the loudspeakers bounce off the walls, floor, and ceiling, and collide with each other and with other waves continuing to come from the speaker. This collision is called acoustic interference, and it creates a comb filter in the air that makes the frequency response vary wildly throughout the entire bass range.

> The response from them is otherwise reasonably flat, within ± 2dB above 300Hz to about 19K. <

The problem is not your speakers! I'm sure they are as flat as their spec sheet indicates.

> So what are my solutions to this problem? <

The best solution is to build or buy real bass traps [not foam rubber]. I wrote an article for Electronic Musician magazine some years ago that describes the problem in great detail, and includes plans for building membrane panel absorbers. Start here:

http://www.ethanwiner.com/articles.html

Then look for "Build a better bass trap" partway down the list.

--Ethan

4. ### jscottGuest

Prolab: Thanks for the replies thus far. I will try the pc of insulation behind the enclosure to see if there is a difference? Pardon my skepticism but I did something similar without much noticeable change, but I will try again nonetheless to make sure.

Ethan: I have read your text before, and I’m still a bit confused. I am to understand there are 3 different versions, and the presence of the standing wave is to be controlled in my case via the low bass trap design, as most of my main issues are below 250Hz – is this correct or are the mid traps also required?

Also you note in one instance that the
But in another you state
So in my case because I have an angled ceiling, and have essentially about a 4’-0” high straight wall – I would prefer the room not be a total dead environment. So my question is, how many walls, and how far apart should the traps be? I seem to be reading some conflicting recommendations into your text or more likely, just still don’t understand how to determine the quantity of traps required?

It would seem that I might possibly be able to do just the rear wall with maybe alternating low and mid absorbers and then do some small foam cubes along the walls near the desk to kill off reflections in the upper registers. But I’m now wondering, how do I know in advance how many of what is required, and do I also mount these on the upper angled ceiling surfaces, etc., etc.?

5. ### Ethan WinerActive Member

Joined:
Mar 19, 2001
Location:
New Milford, CT USA
JS,

> as most of my main issues are below 250Hz – is this correct or are the mid traps also required? <

There are two types of wood panel bass traps. The third "trap" is fiberglass only for the midrange and high frequencies, If you're satisfied with the mid/high performance in your room, you don't need the fiberglass absorbers. Or you can use fewer than I show.

But you do need both types of panel traps. Even though you think the problem is only at 71 Hz., in fact you have standing wave problems throughout the entire bass range. Play some sine waves at various low frequencies through your loudspeakers and walk around the room, and you'll be convinced.

> So in my case because I have an angled ceiling, and have essentially about a 4’-0” high straight wall – I would prefer the room not be a total dead environment. So my question is, how many walls, and how far apart should the traps be? <

Although you don't want the room to be totally dead, that applies only to the mid/high frequency range. You do want as much as absorption as possible at the lowest frequencies. So the best advice is to install as many panel traps as you can fit and afford. You can adjust the overall liveness of the room with fiberglass panels.

Since the upright portion of your wall is only four feet high, you should put the panel traps horizontally there. If you can fit two right next to each other, great. If not, just put one low down on the wall, close to the floor, because that wall/floor boundary is a corner. You could then put more on the slanted walls as explained in my article.

--Ethan

6. ### jscottGuest

Ethan -

I was told tonight that it is possible to take a high powered subwoofer and correct my problem. How? By crossing over the main pair at around 85Hz and use the high watt output of the sub to boost the volume to the point where the 71Hz problem area is relativley flat. Then take a Parametric and cut in the lower frequencies to tweak out the room back to flat.

Now - I'm not saying I will or want to do this, I'm just wondering if this is in fact possible to "force" the wave into the audible range or not?

Secondly - what effect will 6 lbs 705 in 1" thickness do for me in lieu of 703 - 3 lbs. I have found access to both.

Third - in lieu of caulking the panels to the wall, is there an alternate means of creating a sealed trap that will work. For example, can they be mounted away from the wall and be as effective or more so, or, could I fab a soild frame and use a foam weather type seal around the frame?

And then finally - Can you take the plywood outer panel and mount foam to it and use that surface for controlling the upper registers in lieu of building a 3rd style panel?

7. ### audiowkstationActive Member

Joined:
Jun 29, 2001
Although very possible, steep eq does involve phase shifts as does room placement.

Live by the golden rules:

Use thy ears, use reference recordings to see what is real and what is artificial.

Experiment.

8. ### Alécio Costa - BrazilDistinguished Member

Joined:
Mar 19, 2002
go bass traps as soon as possible!
If I were in the Us I would have already bought a dozen of them, maybe put them even towards my bedroom!

9. ### Ethan WinerActive Member

Joined:
Mar 19, 2001
Location:
New Milford, CT USA
JS,

> I was told tonight that it is possible to take a high powered subwoofer and correct my problem. <

You were told wrong. While you can correct for a dip - if it's not too severe - at one or two frequencies in one specific place in the room, as soon as you take two steps to the left or right you'll have a huge boost in level at that frequency.

> what effect will 6 lbs 705 in 1" thickness do for me in lieu of 703 - 3 lbs. I have found access to both. <

Go for the 705, which is twice as dense as 703 and so for a given thickness absorbs twice as well at low frequencies.

> in lieu of caulking the panels to the wall, is there an alternate means of creating a sealed trap that will work. <

You could try to duplicate what we do with RealTraps, but it's tricky and more work. You'd have to build the trap onto a back panel that's at least 2-3 times more rigid than the front.

> Can you take the plywood outer panel and mount foam to it and use that surface for controlling the upper registers in lieu of building a 3rd style panel? <

Nothing must touch the front panel. The only way to do that is to suspend foam or fiberglass in front of the panel using some sort of spacers.

--Ethan