BFD vs. Bass Traps & best position for target curve

[Blueeyedfrog]

I have positioned my sub and listening position to get the following best room response I can (see graph). However, as you can see it's far from flat. I am waiting for a DSP1124P to arrive at my local music store however, after reading the audio acoustics forum threads, it appears that bass traps would also be very (more?) effective at reducing the dip I have. My questions are:

a) given budget constraints (can only really afford either BFD or DIY bass traps - insulation material is very expensive in Australia), where should I spend my money eq or bass traps?

b) If eq-ing is the better option, where should I place my target (hard-knee) curve? Keep it where it is and try to pull the higher frequencies (eg 52, 62hz) right down? Or, move the target curve up so it falls at the bottom of the line of higher frequencies - touching the 68, 75, 82hz frequencies, then pull the peaks down. The problem with that though is that I'll have a massive dip - so maybe I should go with the room treatments first?

Cheers,
Blue

[Wayne A. Pflughaupt]

IMO, there is not much equalization or bass traps can do for that response. Is this the best curve you’ve been above to come up with from your available sub locations? Do you have a good corner you could try?

Regards,
Wayne

[Blueeyedfrog]

Hi Wayne,

See diagram for room layout. The room is a concrete block garage. Red wedges in the corners are proposed sites for bass traps. White blocks denote shelving (which could be moved). Ceiling height is 2.4m (8'). At the moment I have the sub in the top right corner (not lower down as illustrated). Sub is a dual passive so obviously one PR facing the wall when in corner.

Moving the sub around seems to effect the peak at 24hz very little. However, bringing the sub down either the front or right side walls makes the 32hz dip even lower and also creates bigger variations, especially dips, in the higher 55-80hz range.

LP is about 1-1.5' closer to the middle of the room than the 38% from rear wall mark. Moving the seating further back increases the peak around 24hz somewhat. It also increases the big 32hz dip while moving it up to around 40hz. Centering the listening position (as shown in diagram) increases the 32hz dip dramatically - at present I'm 6" right of centre of the two lower side walls (moving 6" left of centre also reduce the dip).

Overall, the LP seems to effect the large dip at 32hz more than the sub placement.

I must admit that I have not tried moving to, and measuring from, a LP in front of the car. This is because it would necessitate moving the projector and screen, rotating the entire setup 90 deg. to face the top right side wall. What do you think, could the sound be better that way?

Cheers,
Blue

[Wayne A. Pflughaupt]

Try moving it to this corner:

Room%20Layout.JPG

EDIT - just noted you already have it in that corner. Sorry.

Regards,
Wayne

[Wayne A. Pflughaupt]

Have you tried either of the corners behind you? Really hate a rear sub, but this seems like a pretty desperate situation. If those corners don’t do anything for you, hate to say it, but you might have to build a wall where that curtain is.

Regards,
Wayne

[Blueeyedfrog]

Thanks for the suggestion. No I haven't tried rear position(s). Have just tried measuring with the LP in front of the car. The first graph is the best response at the current LP, 2nd graph is with LP in front of the car and sub in centre of right far side wall (less dips than if the sub is in the far right corner). Third graph compares the 2 positions - blue = current LP; gold = LP if HT orientation is rotated 90degs.

Question - which graph/LP is better?

My worry is that the front sound stage will change dramatically if the HT orientation is rotated 90degs - it's slightly squished in there.

Will measure sub at rear positions now...

Regarding constructing walls - what about if I moved the shelving at the top left side of the diagram to create a wall where the curtain is? The shelving (Ikea) is lightweight, is 20" wide and runs up to the ceiling. It has no backing but I could attach some plywood. Would that be rigid enough?

Cheers,
Blue

Cheers,
Paul

[Wayne A. Pflughaupt]

Neither graph is a keeper. The problem is the ~40 dB between your peaks and depression. That needs to be narrowed to something workable, i.e., ~15 dB or less.

Regarding constructing walls - what about if I moved the shelving at the top left side of the diagram to create a wall where the curtain is? The shelving (Ikea) is lightweight, is 20" wide and runs up to the ceiling. It has no backing but I could attach some plywood. Would that be rigid enough?

The problem isn’t rigidity. What’s probably happening is that curtain is an acoustical “hole,” as far as bass is concerned. If that was a wall there, that top left corner would probably give you a response curve that could be easily equalized.

What’s behind the car, hopefully a garage door of some kind?

Regards,
Wayne

[Blueeyedfrog]

Thanks Wayne. I'm glad there is hope.

The curtain is very flimsy cotton.

Yes, garage door is behind car. At present I'm moving the shelving - with all shelves packed full of boxes, and all the boxes packed full of stuff, it should give a fairly solid 'wall'. If it improves things then I could get some heavy (.75" - 1.25"?) MDF and attach it to the rear frame (MDF facing into HT area)...

Haven't tried the sub at the rear yet - don't want it there if I can help it.

Also, what about if I also put some shelves up on the right hand side to create a rectangular shape area? Anyway, will try the shelves positioned on the left first

Cheers

[Blueeyedfrog]

While I'm at it, would it be a good idea to put the shelving on the left at a slight angle to improve things acoustically (I seem to recall non parallel walls being a good thing)?

[Blueeyedfrog]

Ok, I made a kind of wall out of the shelving and placed it on an angle, widening out from the top wall. I think generally with the mains/surrounds the sound is better.

The top graph shows the response for the sub with the new 'wall'. Little improvement - still a big range from peaks to lows; 42db. The bottom graph is with the sub in the bottom right corner behind the listening position. Much flatter, but still 32db between peak and troff. The rear position also drops more than the front position(s) near 20-21hz (about 9db on avarage).

Subjectively, the sub bass seems somewhat quieter with the sub at the rear, but still shakes the seat when required. I think it's better (not sure) - less intrusive? More balanced? It doesn't seem like the bass is coming from behind though, which is good. At present, as per usual, all the speakers are same db using receiver's pink noise. Should I just try to get a balanced curve and then turn the gain up on the sub to compensate?

Anyway, is the rear a better position to start from? If so, which should I try first, eq-ing or bass trapping?

Cheers,
Blue

[Wayne A. Pflughaupt]

While I'm at it, would it be a good idea to put the shelving on the left at a slight angle to improve things acoustically (I seem to recall non parallel walls being a good thing)?

The non-parallel wall thing is more for the upper frequencies than the low, to eliminate the slap back or flutter effect.

Not sure I get this shelf thing. What kind of shelves are we talking about? If they have some kind of a closed back, then they might make a difference - if they were floor-to-ceiling, virtually sealing the entire section where the curtain is. If they’re open-back, then they aren’t going to make any difference.

Your rear-location response looks like a winner. Knock down those peaks at ~25 and 55 Hz with an equalizer, and you’ll be in business. The fact that it doesn’t sound like it’s coming from the rear – absolutely wonderful. Still, even if it was, I think I’d rather have nice-sounding bass that I could localize as being behind me than wretched bass from the front.

The sub probably seems quieter at the back because the 40-50 Hz range has been substantially tamed. You’ll need to re-calibrate the sub’s level, with or without equalization.

Anyway, is the rear a better position to start from? If so, which should I try first, eq-ing or bass trapping?

Equalization will definitely be cheaper. Bass traps will serve a couple of functions. One is to reduce the span between the peaks and depressions so that less equalization is required. This is primarily above ~60-80 Hz, though.

Second, traps will dampen the bass frequencies (again, above ~60-80 Hz), so that there is less signal decay time or “overhang” (also called “ringing,” for some incomprehensible reason). For instance, if you’ve ever heard the difference in reverberation in a bare room before and after carpet and furnishings were added, that’s essentially what bass traps do for the lower frequencies. The audible result should be bass that sounds noticeably tighter. That low frequency dampening isn’t really something that equalization can do to any great extent.

So to summarize, the equalizer will smooth out the worse response irregularities, while the traps will accomplish low-frequency dampening. Both are worthwhile, but the equalization will get the most bang for the buck because bass traps aren’t cheap.

Regards,
Wayne

[Blueeyedfrog]

Hi Wayne,

The shelves are floor to ceiling and also open-backed. However, the shelving allows me create a wall from 20" long apple boxes (full of photos, books, clothes, etc.) stacked on the shelves. This gives me a wall 20" thick (albeit with gaps of 1-2" around some of the boxes). In the future I may attach some MDF to the rear of the shelves and then try the sub at the front again - mainly in the attempt to boost the drop I now have near 20hz. Or is that drop not really worth worrying about?

Ok, I'm happy that the response is much flatter now and thank you so much for answering my questions re eq vs. bass traps for bang-for-buck. I'm now a very happy camper . I have read a lot of your stuff, and others', around the forum which has been really really helpful. How to you find the time to respond to all the threads? Anyway, really appreciate you time.

However, always hoping for more... do you think I can do anything about the drop near 20hz? The sub designer claims it gives 105db at 20hz. Obviously, from what you have said bass traps won't effect frequencies that low. Could I try judiciously boosting the eq 2-3db (I know it's not good to boost but...)? Will it produce clipping? The gain on my sub is only set to about 20% (or does that have nothing to do with clipping?). Alternatively, could I drop the target house curve a little lower overall and if so, by how much?

Cheers,
Blue

[Wayne A. Pflughaupt]

Thanks for the kind words, Blue.

However, always hoping for more... do you think I can do anything about the drop near 20hz? The sub designer claims it gives 105db at 20hz. Obviously, from what you have said bass traps won't effect frequencies that low. Could I try judiciously boosting the eq 2-3db (I know it's not good to boost but...)? Will it produce clipping? The gain on my sub is only set to about 20% (or does that have nothing to do with clipping?). Alternatively, could I drop the target house curve a little lower overall and if so, by how much?

The target curve looks about right, especially for the rear location.

What kind of sub are we talking about here?

Based on what your graphs show, I wouldn’t try any boost to get better extension. If your response was a gradual decline below 25 Hz you could do some boost to get a little more extension. However, your response virtually brick-walls at 25 Hz. Boosting is going to do little if anything for that.

Not knowing anything about your sub, I don’t know if boosting the low end will cause clipping. Some people automatically claim you’re going to overdrive your sub and amp if you apply boost, to everyone who asks, not having a clue as to what sub they might be using. In reality it really all depends on how capable your sub is. For instance, in your room if you have four 18” subs with 4000 watts divided between them, then you could safely boost to the moon. However, if you have a 50-watt 8-incher, then you probably can’t apply any boost. Make sense?

Regards,
Wayne

[Blueeyedfrog]

The sub uses one 12" XLS Peerless driver and two 12" XLS Peerless passive radiators. The box is 50 litres (1.8 ft3) and tuned to 20hz Fb. The amp is a generic 350watt (240 into my 8 ohm peerless driver) similar to one which could previously be purchased at Parts Express - specs can be found here: http://www.myshopping.com.au/PR--164...Remote_Control. The THD seems a bit high <0.1%, but it is a bargain amp.

The designer claims:

"It reaches well to 20Hz, and hits very good SPL levels, considering the enclosure size. This is the most bass I've ever made out of a 50 litre box. Here are the SPL (sound pressure level) results:

SPL 104db @ 20Hz
SPL 109db @ 30Hz
SPL 112db @ 50Hz

All these SPL levels were measured anechoic - this is so room gain will not get in the way of the results.
Indoors, its flat to 20Hz and plays at about 112db throughout its passband. "


Any idea why I'm not getting flat down to 20hz? Should I try increasing the gain on the sub amp (at present its quite low), and decreasing the output from the receiver (at the moment I have all the speaker levels balanced via the receiver)? Or is it the room?

Cheers,
Blue

[Wayne A. Pflughaupt]

You’d have to post a question on our DIY Subwoofer Forum to find out why your sub won’t get down to 20 Hz – that’s not my field of knowledge. Give them your TS parameters and info on the box and the guys there can tell you.

That said, it seems to me like a 12” sub should be able to get down to 20 Hz in a room your size. But then again, that garage thing is a big unknown, not to mention the stairs opening up to who knows what.

Regards,
Wayne