Subwoofers vs Bass Traps

caesar

Active Member
May 31, 2010
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#1
Does the use of a pair of subwoofers reduce the need for bass traps, due to wave excitement/ cancellation properties, or are the two complementary?
 
May 30, 2010
13,978
50
48
Portugal
#3
Does the use of a pair of subwoofers reduce the need for bass traps, due to wave excitement/ cancellation properties, or are the two complementary?
Although subs can be used as bass traps I think you will need a few of them strategically placed with independent equalizers for each sub for such purpose. I tried it with two large subs, but a very large properly tuned membrane bass trap showed to be subjectively much better sounding.
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
3,527
3
38
Monument, CO
#5
Unless the absorbers are very thick and/or there are very many of them they won't have much affect at subwoofer frequencies. Both subs and traps is best.
 

JackD201

[WBF Founding Member]
Apr 21, 2010
10,996
10
38
Manila, Philippines
#6
Unless the absorbers are very thick and/or there are very many of them they won't have much affect at subwoofer frequencies. Both subs and traps is best.
You can always open a door or a window ;)

If one wants to use subs for cancellation I would say you'd need a 3rd unit or even a fourth. Bass traps need not be thick. They can be large without being thick. One third of my ceiling is mounted by frequency specific bass traps with narrow Q. They can also be smaller if you use tuned membranes instead of pure air velocity damping. There's more than foam, fiberglass and rock wool out there.
 

JackD201

[WBF Founding Member]
Apr 21, 2010
10,996
10
38
Manila, Philippines
#8
Just sayin' there's many ways to skin a cat and circumstance would dictate or best narrow the options rather than the other way around. These days there are ways to make structures as extreme as green houses acoustically sound by passive means without sacrificing light.
 

dallasjustice

Member Sponsor
Apr 12, 2011
2,090
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Dallas, Texas
#9
Dallas, so what is your definition of "done right"?
Which one? Room treatments? Multiple subs? Which room? Which seated position? Which frequencies? How many subs? Do you want to eliminate width modes, length modes, oblique etc. Is this a real question about a real room? Or are you just freestylin'?

If you are asking these questions in an effort to help you improve your room, I suggest you post the following to help others help you:

Room dimensions
Speaker position
Seated position
At least a 1/12 smoothed per octave SPL/phase plot
 

RBFC

WBF Founding Member & Super Moderator
Apr 20, 2010
5,121
0
36
Albuquerque, NM
www.fightingconcepts.com
#10
Simply put, I believe you need enough sub(s) to achieve the SPL and frequency extension you desire, coupled with room treatment appropriate for the dimensions, along with some DSP to integrate them with respect to time/phase.

Lee
 

caesar

Active Member
May 31, 2010
2,933
0
36
#11
Which one? Room treatments? Multiple subs? Which room? Which seated position? Which frequencies? How many subs? Do you want to eliminate width modes, length modes, oblique etc. Is this a real question about a real room? Or are you just freestylin'?

If you are asking these questions in an effort to help you improve your room, I suggest you post the following to help others help you:

Room dimensions
Speaker position
Seated position
At least a 1/12 smoothed per octave SPL/phase plot

Thanks. I am only interested in the seated position and it's an open floor plan. Speakers are 4-5 feet from the walls. I am just trying to get the general principles straight. As you know, there are many opinions about this stuff. I will have an expert put a couple of subs in, but I want to make sure I generally understand what's going on.
 

dallasjustice

Member Sponsor
Apr 12, 2011
2,090
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0
Dallas, Texas
#12
Thanks. I am only interested in the seated position and it's an open floor plan. Speakers are 4-5 feet from the walls. I am just trying to get the general principles straight. As you know, there are many opinions about this stuff. I will have an expert put a couple of subs in, but I want to make sure I generally understand what's going on.
Will the expert bring a mic, ADC and a laptop with REW loaded over as well?
 

caesar

Active Member
May 31, 2010
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0
36
#14
Gentlemen,

So when a sub is operating, does it affect the frequencies above the crossover point, say the range from about 150 to 300Hz, or is it exciting the room only below the cross over point?
 

dallasjustice

Member Sponsor
Apr 12, 2011
2,090
0
0
Dallas, Texas
#15
depends

Gentlemen,

So when a sub is operating, does it affect the frequencies above the crossover point, say the range from about 150 to 300Hz, or is it exciting the room only below the cross over point?
It depends on the crossover frequency and slope.

It also depends on where you position the woofer. There are better and worse positions to place a sub with regard to room modes.
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
3,527
3
38
Monument, CO
#16
Crossovers are not magic brick walls that stop all sound above and below. There is a roll-off set by the crossover's slope. I have seen that slope range from 12 dB to 24 dB per octave for sub crossovers. 12 dB/octave means that, for a crossover of 80 Hz, the sub is about 12 dB down at 160 Hz. That is about half as loud (little less, given sub frequency loudness contours are closer than in the midrange). That also means the main speakers are about half the loudness at 40 Hz, assuming they can still respond at that low a frequency (the roll-off of the speakers adds to the overall roll-off). With that same second-order 912 dB/octave) response the sub is down 24 dB at 320 Hz, again assuming the sub itself is flat that high (most are not). An 18 dB/octave slope is about 18 dB down an octave away, about 1/4 the volume. An octave is a factor of two in frequency (or one-half if you are going down instead of up).

At the crossover point, sub and mains contribute about equally (do't want to get into nit-picking details). That is why getting the phase right at the crossover point is so important, and since the crossovers and drivers impact phase helps explain why physical distance is often unreliable for setting the phase or delay of the sub relative to the mains. You want the signals from the mains and sub(s) at the crossover frequency to be in phase (add) at the listening position. As you move away from that frequency it becomes less important as one of the other rolls off and contributes less to the sound. Getting the phase wrong at the crossover is likely the reason folk say a sub didn't help or didn't sound right.

Room modes is a different though perhaps related question. If you want to position a sub to help with those, which are set by physical dimensions, then you want to place a sub in the room null to help overcome the room mode. You still have to align phase to everything else, but in general a sub placed in the wrong place will not help with a room mode no matter how you fiddle with its phase.

HTH - Don
 

dallasjustice

Member Sponsor
Apr 12, 2011
2,090
0
0
Dallas, Texas
#17
Well said. +1

The part about placing the sub into a dominant null is great trick. Unfortunately, your speakers won't be in the same position.

The only thing I would add is that phase and time are not the same thing. You might have a flat phase response at the crossover but the bass can still sound slow and unnatural if the time domain isn't dealt with. Time is tricky though. You can't just measure time with an impulse response and be done with it. The impulse response at these frequencies doesn't look much like an impulse response. However, there are ways to iteratively back into a good time domain response. I'm no expert at this but I've seen one do it many times so I know it can be done.
Michael.


Crossovers are not magic brick walls that stop all sound above and below. There is a roll-off set by the crossover's slope. I have seen that slope range from 12 dB to 24 dB per octave for sub crossovers. 12 dB/octave means that, for a crossover of 80 Hz, the sub is about 12 dB down at 160 Hz. That is about half as loud (little less, given sub frequency loudness contours are closer than in the midrange). That also means the main speakers are about half the loudness at 40 Hz, assuming they can still respond at that low a frequency (the roll-off of the speakers adds to the overall roll-off). With that same second-order 912 dB/octave) response the sub is down 24 dB at 320 Hz, again assuming the sub itself is flat that high (most are not). An 18 dB/octave slope is about 18 dB down an octave away, about 1/4 the volume. An octave is a factor of two in frequency (or one-half if you are going down instead of up).

At the crossover point, sub and mains contribute about equally (do't want to get into nit-picking details). That is why getting the phase right at the crossover point is so important, and since the crossovers and drivers impact phase helps explain why physical distance is often unreliable for setting the phase or delay of the sub relative to the mains. You want the signals from the mains and sub(s) at the crossover frequency to be in phase (add) at the listening position. As you move away from that frequency it becomes less important as one of the other rolls off and contributes less to the sound. Getting the phase wrong at the crossover is likely the reason folk say a sub didn't help or didn't sound right.

Room modes is a different though perhaps related question. If you want to position a sub to help with those, which are set by physical dimensions, then you want to place a sub in the room null to help overcome the room mode. You still have to align phase to everything else, but in general a sub placed in the wrong place will not help with a room mode no matter how you fiddle with its phase.

HTH - Don
 

dallasjustice

Member Sponsor
Apr 12, 2011
2,090
0
0
Dallas, Texas
#19
I'm not saying anyone is a shyster. You first need to describe your system. Do you own a DAC and a windows computer? If I were you, I would buy my own test gear. That way you don't limit yourself to the local-jokel. You can then shop in a much bigger pond to help you find the right person to dial-in the subs.

The way I integrate subs is, I think, ideal. However, you may not have the right gear to do what I do.


Dallas,

So from a high level, what should the guy be measuring? What should he be looking for in the measurements?

I want to make sure I weed out the shysters
 

caesar

Active Member
May 31, 2010
2,933
0
36
#20
I'm not saying anyone is a shyster. You first need to describe your system. Do you own a DAC and a windows computer? If I were you, I would buy my own test gear. That way you don't limit yourself to the local-jokel. You can then shop in a much bigger pond to help you find the right person to dial-in the subs.

The way I integrate subs is, I think, ideal. However, you may not have the right gear to do what I do.
As I mentioned in the other thread, no computer and no dac. My goal is to have an intelligent conversation with the installer and to keep him honest.
 

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