One vs two subwoofers? - AVS Forum | Home Theater Discussions And Reviews
Forum Jump: 
 3Likes
  • 1 Post By jake51
  • 2 Post By CherylJosie
 
Thread Tools
post #1 of 12 Old 08-28-2015, 10:59 PM - Thread Starter
Senior Member
 
jake51's Avatar
 
Join Date: Jun 2005
Location: Denmark
Posts: 432
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 106 Post(s)
Liked: 18
One vs two subwoofers?

I watch movies at no where near reference level (that is just insane)
I play action movies loud... but not that loud
What are the benefits of having two subwoofers at reasonable sound levels?
phildaant likes this.
jake51 is offline  
Sponsored Links
Advertisement
 
post #2 of 12 Old 08-28-2015, 11:12 PM
Member
 
zeuspaul's Avatar
 
Join Date: Feb 2003
Location: California
Posts: 163
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 73 Post(s)
Liked: 82
I also listen at low bass levels. When I added a PSA V1800 to my SVS PC 13 Ultra it evened out the bass. I know that bass is not supposed to be directional but I could tell where it was coming from. Perhaps it was because the SVS was a couple of feet from my chair just behind me and near my left shoulder. With the addition of the PSA I am now immersed in bass and can't tell where it is coming from. Overall it is just better bass.

PSA MTM-210T front, MTM-210C center
PSA MTM-210T surrounds, PSA MT-110 rears, Atmos / Canton bookshelf x2, PSA MT-110 x2
PSA V1800 , PSA V1801
Marantz SR7009 AVR & Emotiva A-100 stereo amp
Panasonic 65ST60 Plasma, Oppo BDP 93, DirecTV, HBO, Starz, Showtime
zeuspaul is offline  
post #3 of 12 Old 08-28-2015, 11:30 PM
AVS Forum Special Member
 
Bob7145's Avatar
 
Join Date: Dec 2006
Location: Dollars, Taxes
Posts: 2,092
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked: 36
An additional sub won't increase the volume much but it will even out room nodes and help reduce peaks and nulls in the bass frequencies where with a single sub the bass would be more or less noticible from different listening positions possibly creating boominess on one side of the couch and lack of bass on the other.
See Harman White Papers about sub placement up to 20 or more subs.

https://www.google.com/url?sa=t&rct=...2uQO9Q&cad=rja

https://www.google.com/url?sa=t&rct=...b-hLZ5Yr1MztIA

Last edited by Bob7145; 08-28-2015 at 11:48 PM. Reason: add link- Getting the Bass Right, add correct link
Bob7145 is offline  
 
post #4 of 12 Old 08-28-2015, 11:31 PM
 
lovinthehd's Avatar
 
Join Date: Dec 2007
Location: OROR
Posts: 16,242
Mentioned: 50 Post(s)
Tagged: 0 Thread(s)
Quoted: 4588 Post(s)
Liked: 4783
FWIW here's how additional subs can translate into additional dBs http://www.diyaudio.com/forums/multi...-loudness.html

Bigger advantage is usually to smooth room modes https://www.google.com/#q=smoothing+...e:avsforum.com
lovinthehd is offline  
post #5 of 12 Old 08-29-2015, 03:31 AM
 
fatbottom's Avatar
 
Join Date: May 2003
Location: UK
Posts: 5,866
Mentioned: 10 Post(s)
Tagged: 0 Thread(s)
Quoted: 1420 Post(s)
Liked: 520
Quote:
Originally Posted by zeuspaul View Post
I also listen at low bass levels. When I added a PSA V1800 to my SVS PC 13 Ultra it evened out the bass. I know that bass is not supposed to be directional but I could tell where it was coming from. Perhaps it was because the SVS was a couple of feet from my chair just behind me and near my left shoulder. With the addition of the PSA I am now immersed in bass and can't tell where it is coming from. Overall it is just better bass.
You should have identical subwoofers.
fatbottom is offline  
post #6 of 12 Old 08-29-2015, 04:48 AM
 
lovinthehd's Avatar
 
Join Date: Dec 2007
Location: OROR
Posts: 16,242
Mentioned: 50 Post(s)
Tagged: 0 Thread(s)
Quoted: 4588 Post(s)
Liked: 4783
Quote:
Originally Posted by fatbottom View Post
You should have identical subwoofers.
Ideally isn't quite the same as should. If it works....
lovinthehd is offline  
post #7 of 12 Old 08-29-2015, 05:00 AM
 
fatbottom's Avatar
 
Join Date: May 2003
Location: UK
Posts: 5,866
Mentioned: 10 Post(s)
Tagged: 0 Thread(s)
Quoted: 1420 Post(s)
Liked: 520
Quote:
Originally Posted by lovinthehd View Post
Ideally isn't quite the same as should. If it works....
Having my SBU13 and SB12+ together will "work" but both have different phase and FR characteristics.

Do it properly not bodge job.
fatbottom is offline  
post #8 of 12 Old 08-29-2015, 05:09 AM
 
lovinthehd's Avatar
 
Join Date: Dec 2007
Location: OROR
Posts: 16,242
Mentioned: 50 Post(s)
Tagged: 0 Thread(s)
Quoted: 4588 Post(s)
Liked: 4783
Quote:
Originally Posted by fatbottom View Post
Having my SBU13 and SB12+ together will "work" but both have different phase and FR characteristics.

Do it properly not bodge job.
@zeuspaul ...do you have a bodge job?
lovinthehd is offline  
post #9 of 12 Old 08-29-2015, 08:17 AM
Member
 
zeuspaul's Avatar
 
Join Date: Feb 2003
Location: California
Posts: 163
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 73 Post(s)
Liked: 82
Quote:
Originally Posted by lovinthehd View Post
@zeuspaul...do you have a bodge job?

I did not *do it properly*. The first sub was installed using Audyssey. My AVR has two sub outs but they are not independent. I have no measurement software, no computer near the TV and the AVR doesn't calibrate two subs. I don't have an option of moving the subs around. There is only one place they can go. It is not a dedicated theater room.


I just plugged in the second sub and adjusted the gain. It sounds great to me! I am sure the set-up is not perfect. I was concerned because I read a lot about the difficulties of two non identical subs. I guessed I might have to just use one at a time. For daytime TV use I do use one but when the source has bass I turn on both subs and they sound great!

PSA MTM-210T front, MTM-210C center
PSA MTM-210T surrounds, PSA MT-110 rears, Atmos / Canton bookshelf x2, PSA MT-110 x2
PSA V1800 , PSA V1801
Marantz SR7009 AVR & Emotiva A-100 stereo amp
Panasonic 65ST60 Plasma, Oppo BDP 93, DirecTV, HBO, Starz, Showtime
zeuspaul is offline  
post #10 of 12 Old 08-29-2015, 11:47 AM
 
lovinthehd's Avatar
 
Join Date: Dec 2007
Location: OROR
Posts: 16,242
Mentioned: 50 Post(s)
Tagged: 0 Thread(s)
Quoted: 4588 Post(s)
Liked: 4783
Quote:
Originally Posted by zeuspaul View Post
I did not *do it properly*. The first sub was installed using Audyssey. My AVR has two sub outs but they are not independent. I have no measurement software, no computer near the TV and the AVR doesn't calibrate two subs. I don't have an option of moving the subs around. There is only one place they can go. It is not a dedicated theater room.


I just plugged in the second sub and adjusted the gain. It sounds great to me! I am sure the set-up is not perfect. I was concerned because I read a lot about the difficulties of two non identical subs. I guessed I might have to just use one at a time. For daytime TV use I do use one but when the source has bass I turn on both subs and they sound great!
You can only up from there then
lovinthehd is offline  
post #11 of 12 Old 08-29-2015, 12:06 PM
AVS Forum Special Member
 
CherylJosie's Avatar
 
Join Date: May 2008
Posts: 1,375
Mentioned: 18 Post(s)
Tagged: 1 Thread(s)
Quoted: 732 Post(s)
Liked: 297
Adding additional subwoofage can actually make the response worse if done improperly.

If you are happy with the sound of your system there is little reason to change it, but here is a pseudo-technical explanation of why you might want to add one or more subwoofers even if the one you have is already loud enough. I did it.

There are about a billion threads on multiple forums explaining the details but I summarized the high points for you here. You can do searches on any of the terms you have questions about and find abundant additional info.

Harman has a good white paper on using multiple subwoofers and room mode calculator.

Adding a second identical subwoofer of equal power adds 3dB of power gain.

You can hear the difference of 3dB easily if your receiver allows you to select relative volume setting and displays dB units on the volume control. 0.5dB is barely audible change to most people; 3dB is audible and readily detected but not overwhelming. 10dB is roughly doubling/halving of perceived loudness.

There is something called 'mutual coupling' that happens when the distance between the subwoofers is equal to or less than a quarter wavelength. Mutual coupling between physically separate subwoofers usually only applies outdoors or in a large venue where room boundaries are physically distant from each other. There is an additional 3dB of power gain for 6dB total gain. The transition to mutual coupling is gradual rather than abrupt.

http://www.1728.org/freqwavf.htm

To find the frequency that your dual subwoofers will get the full benefit of this mutual coupling gain, enter four times the distance between them into the wavelength-to-frequency calculator at the link above. For 8' separation the mutual coupling is fully established at ~35Hz and below.

The mutual coupling frequency is actually the frequency where physically separated drivers reinforce each other regardless of difference in distance to the listener because phase differences are small enough to prevent cancellation e.g. combing and lobing.

Above that frequency where mutual coupling ends, there is frequency-dependent and direction-dependent destructive cancellation (combing and lobing) from phase difference between the drivers if they are not equidistant to the listener.

Manufacturers often take advantage of mutual coupling between closely spaced drivers within a single speaker cabinet to improve efficiency over a broader range of frequencies than is typically possible with separate cabinets.

Mutual coupling between physically separate subwoofers is largely negated in a typical residential dwelling by reflections off the room boundaries that alter the bass response radically with phase-shifted delayed reflections and room resonances, even when subwoofers are placed very close to each other or stacked.

Subwoofers can have boundary gain below a certain frequency (similar to mutual coupling) from being acoustically close to a boundary, they can have boundary interference above a certain frequency (cancellation with phase-shifted reflected energy) from being acoustically distant from a boundary, and they can have either gain or cancellation from stimulating any acoustically proximal resonant room modes that involve reflections from multiple boundaries.

Room modes create standing waves (physically static interaction between waves travelling in opposite directions) at anything between gain or cancellation depending on the listening position within the path of the room mode that is stimulated. The axial modes are resonances due to perpendicular reflections between parallel walls; the tangential modes reflect off four surfaces and oblique modes reflect off six surfaces. You can see spatial illustration of room modes here. Click on the buttons in the show: column to see the spatial illustration for each mode.

Multiple modes exist at all locations within a room but they are only stimulated when driven at a node and not stimulated when driven at an antinode.

I hope I got that node-antinode thing correct; I sometimes confuse the two.

The modes can get very complicated if the room is not rectangular but unless the deviations are radical i.e. nonparallel walls, major discontinuities, or curved surfaces my limited experience has been that the modes are still fairly predictable. For average small listening room the transition to modal behavior happens at ~300Hz and maybe a little lower say as low as ~200Hz for very large living room or dedicated home theater, and in that case mutual coupling between subwoofers never really happens at all in such enclosed space because the modal behavior dominates the response throughout their operating range.

http://www.acousticfrontiers.com/201...xplained-html/
http://www.soundandvision.com/conten...nd-tell-part-1
http://www.soundandvision.com/conten...nd-tell-part-2

Note that all of these considerations apply to the listening position as well as the subwoofer placement. The transfer function between speaker and listener is bidirectional; that is the principle behind using the subwoofer crawl to find an optimized subwoofer placement.

For systems with smaller speakers with higher crossover frequency, the subwoofer placement might be more constrained due to the localizable nature of upper bass (above 80Hz). It can be distracting to hear the bass from vocals in your center speaker redirected to a subwoofer next to your sofa but at least for the main listening position left-right symmetry can largely ameliorate this effect.

Boundary gain at very low frequencies (below where modal behavior ends) from reflections off all boundaries simultaneously is called cabin gain. For an approximately sealed room, cabin gain translates into a direct pressurization of the space. The gain is approximately 20dB (4x louder) per decade (frequency/10) and is very effective way of getting big bass from small speakers in a car particularly with all doors and windows closed. Cabin gain occurs at much lower frequencies in a typical residential listening room and is helpful for infrasonic tactile/visceral sensation in high performance systems. Cabin gain is unlikely to have effect at audio frequencies in a large venue and does not exist at all outdoors.

All reflections except boundary gain have these effects at multiple discrete frequencies per reflection, i.e. 1x, 2x, 3x, 4x etc. so the response can get very complicated to predict based on analysis alone. Simulation may be required to predict the response and the accuracy of the modelling is limited by how well the room and its objects can be described mathematically. Subwoofer placement and integration is therefore usually done by ear or with measuring equipment instead because most listening rooms are neither rectangular nor devoid of objects.

In the lower bass where room modes are sparse the deviations in response can be drastic.

Uneven bass response is largely due to too few modes in a smaller room, not too many. As the room size increases, the modal density at lower frequencies also increases, filling in the non-resonant gaps between resonances and eliminating modal phase cancellation nulls. In the case of larger performance venue it is probably more advantageous to group subwoofers for mutual coupling at subwoofer frequencies rather than distribute them to directly stimulate specific room modes.

A good graphical representation of the frequency distribution of modes in a typical home listening room can be found here (actually those modes are much more evenly distributed than those in my own room).

There is one more factor to consider and that is whether any of the boundaries resonate significantly. In my listening room it seems the floor resonates and adds a strong tactile sensation because it makes the whole sofa shake. This is considered desirable and leads many people to build home theater upstairs rather than in the basement on a concrete slab. Resonating room boundaries also alter the predicted response some.

At various frequencies the output of a subwoofer in a room can have anywhere from additional ~+30dB to -infinity dB of gain due to reflections and resonances. The actual output at the listening position depends on the shape of the room, any reflecting or sound-absorbing contents in the room that have an effect at bass frequencies (such as large tables, bass traps and sofas), and the placement of the subwoofer and listening position within the room.

When the variations are large the frequency response sounds colored. The broader the range of frequencies that are affected by gain or attenuation and the more drastic the variation between frequencies, the more perceptible the coloration is.

The major advantage of multiple subwoofers is to stimulate as many of the differing room modes as possible to produce more even bass response across a larger listening area than is possible with a single subwoofer. Additional output is seldom a factor but it can help especially with 4 or more subwoofers in a large room or when substantial output at very low frequencies is desired.

Another option is to stimulate as few of the lower frequency room modes as possible to avoid creating any strong resonant peaks in the response. In that case, total output can be significantly lower so more power or more subwoofers may be required to get acceptable levels.

A combination of resonant and non-resonant placement may also be used.

Note that at higher bass frequencies so many room modes exist and are evenly distributed at closely spaced frequencies that there are few to no issues with frequency response due to resonances.

It is possible to have better bass with non-equal phase and gain and frequency response between multiple subwoofers of differing design; in fact the phase gain and frequency response even between identical multiple subwoofers will nearly always be at least slightly different and usually drastically different due to placement of the subwoofers and listener across all the seating, and that is sort of the whole point of using more than one sub in a 'small' room (less than full sized commercial theater) that exhibits strong modal response.

Also, deliberately changing the phase gain and frequency response of multiple subwoofers is a commonly used advanced technique to smooth out the bass. Since I first posted this comment, I have added a third subwoofer (Hsu VTF-1 MK2) and am in the process of tuning it narrow band to fill in a 30Hz gap in my room where there are no room modes to stimulate.

So it is not necessary to have identical subwoofers but it may make integrating (matching them up with each other) easier in some cases, particularly when the room is symmetrical and the placement is also symmetrical. Most people recommend identical subwoofers to simplify the integration.

smellyfungus and zencarver like this.

Last edited by CherylJosie; 02-18-2016 at 05:46 PM. Reason: clarified mutual coupling vs modal behavior
CherylJosie is offline  
post #12 of 12 Old 09-02-2015, 10:03 PM
AVS Forum Special Member
 
CherylJosie's Avatar
 
Join Date: May 2008
Posts: 1,375
Mentioned: 18 Post(s)
Tagged: 1 Thread(s)
Quoted: 732 Post(s)
Liked: 297
Here is a frequency response study of dual SVS PB10-NSD/ISD in asymmetrical living room.
(note: the driver and port are both on the front of the cabinet)

My front sound stage is on the 25' wall and the room is only 12' deep by 8' high.

My living room is asymmetrical:
  • the left end of the rear wall is only 14' from the right side wall
  • kitchen to my left, hallway diagonally behind me to my left
Due to the asymmetry, subwoofer response varies drastically with position.

Here is the explanation of the plots:
  1. Both subwoofers in rear of room
    • positioned under endtables next to sofa (nearfield)
    • facing front, drivers far from walls
    • level matched to 75dB using receiver's initial setup pink noise signal
    duals improve smoothness and high end
    huge dip at 35Hz disqualifies from consideration
  2. Both subwoofers in front of room
    • positioned just outside front l/r mains
    • facing rear, drivers somewhat near front wall
    • level matched to 75dB using receiver's initial setup pink noise signal
    • sub phase is inverted for better integration with the rest of the speakers
    duals are worse than left subwoofer alone
    huge dip at 90Hz disqualifies from consideration
  3. Both subwoofers in front of room
    • positioned in the corners
    • from 7.5" away the left sub faces a built-in bookcase that is perpendicular to front wall
    • from 7.5" away the right sub faces the right side wall
    • level matched to 75dB using receiver's initial setup pink noise signal
    • sub phase is inverted for better integration with the rest of the speakers
    duals improve high end but worsen low end
    shelving-like attenuation in high end disqualifies from consideration
  4. Both subwoofers in front of room
    • positioned in the corners, drivers 7.5" from left bookcase/right side wall
    • both subs facing down supported by bricks at the corners
    • level matched to 75dB using receiver's initial setup pink noise signal
    • sub phase is inverted for better integration with the rest of the speakers
    duals improve high end slightly but worsen low end
    poor frequency response disqualifies from consideration
  5. Both subwoofers in front of room
    • positioned in the corners, drivers 3" from left bookcase/right side wall
    • both subs facing down supported by bricks at the corners
    • level matched to 75dB using receiver's initial setup pink noise signal
    • sub phase is inverted for better integration with the rest of the speakers
    duals improve high end significantly but worsen low end
    extended frequency response makes this a candidate for consideration
  6. EQ of graph 5
    • Graphic EQ smooths frequency response somewhat but still wavy
    • Low end is colored by room modes (bars at bottom of graph)
    • 20Hz bump is due to floor resonance near lowest room mode
  7. EQ of graph 5 vs. Harman ~-4dB/decade preferred target
    • more than +/-5dB variation from target
    • usable xo=150Hz
    this is the setup I used for a couple of weeks
    • good l/r symmetry with high xo
    • works well for center speaker imaging with xo=120Hz
    • bass response is colored but has no huge peaks or nulls
  8. Left subwoofer in rear of room
    • positioned against rear wall left of center
    • facing sideways into left rear tower from 3" away
    Right subwoofer in front of room
    • positioned in the corner
    • facing sideways into right side wall from 3" away
    duals improve high end with but increase 20Hz bump
    level matching causes nearfield (rear) subwoofer to 'loaf along' at low output
    • level matched to 75dB using receiver's initial setup pink noise signal
    • cannot hear rear subwoofer anywhere but at main listening position
    • sub phase is inverted for better integration with the rest of the speakers
    • rear sub phase is inverted too for better integration with front sub
    • rougher response with level match due to phase inversion between front and rear subs
    gain matching improves response dramatically
    • combined output set to 80dB with receiver's sub setup pink noise signal
    • gain set to second tick on both subwoofers
    • no phase inversion required for integration because gain matched rear sub is producing more output
    • bass sounds good all over the room
    smooth frequency response makes this setup a candidate for consideration
  9. EQ of graph 8 vs. Harman ~-4dB/decade preferred target
    • less than +/-5dB variation from target from 25Hz to 90Hz
    • usable xo=120Hz
    this is the setup I am using now
    • fair l/r symmetry with high xo
    • works well for center speaker imaging with xo=120Hz
    • bass response is smooth
    • tactile is superb
  10. Graph 7 (original EQ with front subs) vs. Graph 9 (current EQ with diagonal subs)
    • two solutions optimized for different goals; extended vs. smooth frequency response
    • requires measurement equipment (REW and UMIK-1)
    • requires physical labor (I asked a neighbor for help)
    • requires time patience and imagination
    • requires experience and education or at least advice from an expert
    • the rewards speak for themselves

Now that you see how it is done I hope you have a better idea what is possible.
Attached Thumbnails
Click image for larger version

Name:	subs position rear facing front.jpg
Views:	307
Size:	78.8 KB
ID:	922010   Click image for larger version

Name:	subs position front facing rear.jpg
Views:	274
Size:	75.1 KB
ID:	922018   Click image for larger version

Name:	subs position front facing side.jpg
Views:	318
Size:	76.0 KB
ID:	922026   Click image for larger version

Name:	subs position front facing down.jpg
Views:	266
Size:	77.9 KB
ID:	922034   Click image for larger version

Name:	subs position front corners facing down.jpg
Views:	274
Size:	73.4 KB
ID:	922042  

Click image for larger version

Name:	subs position front corners facing down eq.jpg
Views:	281
Size:	64.3 KB
ID:	922050   Click image for larger version

Name:	subs position front corners facing down eq vs target.jpg
Views:	352
Size:	64.7 KB
ID:	922058   Click image for larger version

Name:	subs position left rear facing front right front corner facing down.jpg
Views:	275
Size:	76.7 KB
ID:	922066   Click image for larger version

Name:	subs position left rear facing side front right facing side eq vs target.jpg
Views:	379
Size:	68.0 KB
ID:	922074   Click image for larger version

Name:	subs position front corners vs. diagonal corners with eq.jpg
Views:	286
Size:	69.7 KB
ID:	922082  


Last edited by CherylJosie; 09-02-2015 at 10:30 PM.
CherylJosie is offline  
Sponsored Links
Advertisement
 
Reply Subwoofers, Bass, and Transducers

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off