Question about SPL - help a new member?

Discussion in 'Archived Threads 2001-2004' started by KeithBogert, Sep 11, 2002.

  1. KeithBogert

    KeithBogert Auditioning

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    I have been browsing the forum for some time and have found it to be very educational. Amazing amount of info here. Perhaps I could bother you for a bit more?

    I am trying to understand SPL in subwoofer applications.

    I know that when you double the power to a driver, the volume increases by ~3dB's. I lso understand that adding a second identical woofer can also increase SPL (again 3dB's I believe).

    Here's my question. If I added a second identical driver to a system at a consequece of splitting the power between the 2 drivers (each gets half now), would I end up with:

    a) more SPL than the single driver system with twice the power

    b)less SPL than the single driver system with twice the power

    c) the same SPL as the single driver system with twice the power.

    I just don't know the relationship between cone area and power. Is one more influential than the other?

    Any help is appreciated. Looking forward to being an active member. I will admit that mobile audio is more my bag, but would like to expand my horizons.

    kb
     
  2. Richard Greene

    Richard Greene Stunt Coordinator

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    You're going to hate this answer but it's true:

    If you enjoy listening to one subwoofer at an average of 85dB then you will enjoy listening to two or three or four subwoofers at 85dB. There would be no logical reason for you to increase the average SPL beyond the 85dB you enjoy.
    It's true the maximum SPL would increase but you'd never listen at maximum SPL because you'd probably damage your hearing.


    Maximum output:

    - Add a second powered subwoofer in same corner as first one
    = up to +6dB louder maximum output

    - Add a second powered subwoofer in opposite corner
    = +3 to +4db louder maximum output

    I recommend that each subwoofer driver have its own dedicated amplifier rather than using one amplifier to power two drivers.
     
  3. KeithBogert

    KeithBogert Auditioning

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    Great answer. I don't hate it!

    My question is a hypothetical one however. Your answer is based in reality, but my question is based in theory.

    I'm trying to learn more about the relationship of cone area and power. Ideally, I would disregard the direction of the sub(s) and assume they are both in the same relative position. This cuts down on variables a bit.

    Remeber, if I add the second sub, both subs would be at half the power. Is this louder than 1 sub at full power is the question?

    Typical math sems to suggest the 2 setups would be he same. However, as you bring excursion into play, I think perhaps the 2 sub setup at half power will have an SPL advantage.


    Thanks for the reply,
    kb
     
  4. Dustin B

    Dustin B Producer

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    Fire off an email to Dan Wiggins (Adire Audio CEO) or Mark Seaton (SPL/ServoDrive). Both are members here. And post their reply or direct them to this thread.

    Theoretically I think the answer is C, assuming the single driver can make use of twice the power.

    There are some issues with impedance when running more than one driver of the same source that change things. And in the real world I think distortion and what not on two drivers being pushed a little would be a lot less than a single driver being pushed extremely hard.

    But I have to disaggree a little with Richard. If you are listening to music or a movie that sticks at 85dB or below then yes one sub will be plenty. But most movies and music have a lot of dynamics. Short bursts of huge sound really add to some music and movies. If your one sub can handle these bursts without a problem then I aggree with Richard. But headroom is never a bad thing. I'd rather have a pair of subs that I know I will never take past their limits than a single sub I might take past it's limits.
     
  5. KeithBogert

    KeithBogert Auditioning

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    I throw one to Dan as I have read hundreds of his posts and feel like "know" him better. I'm glad that I'm not the only one who feels a bit unsure.

    Thanks again.
     
  6. KeithBogert

    KeithBogert Auditioning

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    Oh, in the meantime, here is the arguement that got me thinking about the possibility of the answer being (a):

    Let use a made up scenario
    One sub with 200 sq inches of cone area @ 200 watts has 1" excursion = 200 cubic inches of volume. That is for single or for DVC...sub is the same size
    Two subs (4 ohm in parallel to get 2 ohms) with 200 sq inches of cone area each = 400 sq. inches of cone area. Now they are getting 100 watts each, assuming the 2 4 ohm drivers that have identical characteristics of the 2 ohm driver. The cone would be about 3/4 of the way to max excursion which equals 300 cubic inches of volume. This represents a 3 dB gain over the single sub, but the same power was used.
    Four subs with 200 sq. inches of cone area each = 800 square inches of cone area. Each sub is now getting 50 watts and be at half the max excursion that brings it back down to 400 cubic inches of volume or 6 dB more output than a single 2 ohm sub, again same power used.
    It takes about 4X power to double the excursion of a given sub.
     
  7. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Well now that you're in the realm of HT you may find that ppl take highly into consideration: placement, listening position, and amplification. In a car you don't have to worry about listening position or placement, it's all about tuning points and power.

    Acoustic coupling, in the conditions in which it takes place, leads to a theoretical increase of 3dB each time the number of sources is doubled. These "conditions" need to be taken in consideration if you're gonna get some real numbers to play with. Remember that if were talking arrays of subwoofers, there IS a point to which adding another sub becomes detrimental to the output in bass.

    Having the subwoofers on the ground with a wall behind them means a gain of from 3 to 6dB compared to an enclosure flown in a cluster.

    Due to mutual coupling, up to a certain frequency an increase of 3dB spl will be achieved at the low frequencies each time the number of speakers is doubled; since the power is presumably doubled every time the sources are doubled, there will therefore be a further increase of 3dB, thus increasing the total SPL by up to a maximum of 6dB. However, since you will be halving the power to each sub you might only achieve a 3db increase. (WITH COUPLING)

    Another thing... I just thought I'd mention that an amp that outputs, say: 250 watts bridged ain't gonna output 125 watts x 2 channels but you probably knew that.

    Lastly... Coupling can be bad for frequency response in the room. This is why I recommend placing the 2 subs apart in key locations so that cancellation of modes take place and the subwoofers freq response is as close to the anechoic response as possible.

    Hope I didn't get anything wrong...

    Here's an exampe I made with my tempest @ 1 meter (Coupled):

    1 sub with 250w = 113db
    2 sub with 250w each = 119db
    1 sub with 125w = 110db
    2 sub with 125w each = 116db
     
  8. Richard Greene

    Richard Greene Stunt Coordinator

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    Acoustic coupling, in the conditions in which it takes place, leads to a theoretical increase of 3dB each time the number of sources is doubled. These "conditions" need to be taken in consideration if you're gonna get some real numbers to play with. Remember that if were talking arrays of subwoofers, there IS a point to which adding another sub becomes detrimental to the output in bass.

    #################

    RG:
    I don't believe anyone is talking about arrays used for sound reinforcement in auditoriums or stadiums -- just one driver versus two drivers in a home listening room



    Having the subwoofers on the ground with a wall behind them means a gain of from 3 to 6dB compared to an enclosure flown in a cluster.

    #####################

    Haven't seen any subs hanging from ceilings in homes yet either!



    Lastly... Coupling can be bad for frequency response in the room. This is why I recommend placing the 2 subs apart in key locations so that cancellation of modes take place and the subwoofers freq response is as close to the anechoic response as possible.

    #######################

    The frequency response of two stacked subwoofers is almost identical to one subwoofer. Only if the subwoofers are quite a few feet apart will cancellations happen at subwoofer frequencies.

    Use of two subwoofers can at best cancel odd-order room modes between two surfaces. That can be a good thing or a bad thing. A room mode cancelled with this trick has the potential to make the remaining room modes easier to hear and more annoying.
    - If there is only one bass frequency peak that bothers you, use of two subwoofers to cancel (fail to excite, actually) that mode can be very good news.
    - But if there are several audible bass frequency peaks (the most common situation), failing to excite one of the room modes usually makes the remaining room modes more annoying than before.

    -You can reduce the side-wall-to-side-wall and floor-to-ceiling room room modes by using dipole subwoofers.

    - You can reduce all room modes by using lots of bass traps

    - You can reduce bass frequency peaks at one listening position using parametric EQ.

    - The probability of solving bass problems through the clever positioning of two subwoofers (no bass traps and no EQ) is low for an ordinary home listening room. If you want an anechoic-like bass frequency response try 12-18 bass traps or knock down a few walls or place your subwoofers outdoors! Using two subwoofers cleverly positioned is rarely the answer. Two subwoofers will have great difficulty generating output at a frequency whose wavelength is double the distance between the two subwoofers because they will be out of phase at that frequency. And that's just one problem.
     
  9. KeithBogert

    KeithBogert Auditioning

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  10. steve nn

    steve nn Cinematographer

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    I like what two subs do for me. More headroom and more spl at a lower gain setting. I do not have to work them as hard. I have noticed a 4-6 dB bump. I am 100% HT basically and with many openings in my HT area. So imo it does well for me. My one ct-150 does very well but I must admit I like two better and the level I like, I would rather run two stacked at 25% each rather than one at 35%. But then maybe this is just another excuse for my last purchase.
     
  11. Geno

    Geno Supporting Actor

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    I have a question that I hope relates to this topic.

    take a 120 x2 class b amp.

    is it better [greater spl] to:

    1. 1 driver 1 enclosure 1 channel
    2. 2 drivers 1 enclosure 1 channel
    3. 2 drivers 1 enclosure 2 channels [mono lfe signal split before preamp]
    4 2 drivers 2 enclosures 2 channels [mono lfe signal split before preamp]

    if I am missing an alternative, lemme know

    edit: started as new thread
     
  12. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Thanks for clearing some things up Richard,

    I knew he probably wasn't going to do all of the things I mentioned but was just adding some "extra" info on the subject.

    Let me explain what I meant by mode cancellation:

    You start off with 2 subwoofers to cancel out odd order width nodes. Then you position the mains to cancel out the 4th order modes and position the subs to cancel out the 2nd order room modes. This takes care of the most important room modes (1-4).

    As for the Length width modes:

    Line up the front speakers in the 4th order null and put the listening position in the middle of the room where the nulls of the 1st/3rd order room nulls are. This means that there will be a bass reinforcement at the 2nd order mode at the listening position which could be quite noticeable. (You could use the parametric EQ or bass trap to fix this if the final tweaking doesn't help)

    Lastly, the height mode which I'll not forget that may depend on the height of the listener but sometimes they will be close to the null of the 1st and third order room modes.

    That just about covers all the axial room modes so I'd take a response measurement and graph it at this point to see where what frequencies need some tweaking. At least after you've done this you can predict some of the frequency problems. Then just move the speakers around slightly and perhaps not cancel out certain room modes that much or play around with an EQ or a few bass traps.

    In the end, I think a properly "tuned" room this way "can" have a much improved response over just throwing the 2 subs in the corner and trying to EQ like mad.

    My point was to get across that the room and positioning makes a big difference in bass that some people may not realize.

    Here's a simple test: The first order resonance in a room that's 8 feet high is about 74hz. Play a constant test tone at that frequency while moving the spl meter from ceiling to ground. What I tested was that the middle of the room (the null) was about 9 decibels quieter than the ceiling and floor.

    When I said 250 watts bridged ain't gonna output 125 watts x 2 channels I was considering the 2 channel amp in my example. My example was using a theoretical amp that can either be 250 watts, or 125 watts x 2.

    (Man this stuff can get confusing)
     
  13. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    http://www.harman.com/wp/index.jsp?articleId=122
    Excellent paper that got me energized about room modes.
    A link to an excel sheet I made to help calculate room modes and a few other things. (Or you can download Harman's protected excel sheet at their site).
     
  14. KeithBogert

    KeithBogert Auditioning

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    Wow! Great stuff. Thank you folks.
     
  15. Richard Greene

    Richard Greene Stunt Coordinator

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    Chris

    The main problems with moving speakers and subwoofers all over the room in search of the best bass is that:

    (1) The frequencies above 80Hz. are most important and probably should not be suboptimized for better bass

    (2) There are probably going to be only a few alternative locations available for the subwoofers

    (3) The more drivers you have producing sub-100Hz. frequencies, the more cancellations are likely due to destructive interference.

    (4) Any subwoofer not in a room corner will have less than maximum possible output in that room

    (5) Any two or more subwoofers not in the same room corner will have less than maximum possible output in that room

    (6) All that speaker moving will cause a chronically sore back

    (7) If you are going to be using bass traps or EQ or both, why not place subwoofers in the corners for maximum output and then reduce all SPL peaks heard at your listening position down the the SPL of the frequency response troughs, if possible, or at least close?

    In addition you should not place your main speakers or your ears at nodes or anti-nodes (peaks and nulls) if possible -- better sound quality will result from speakers and ears NOT located at nodes or anti-nodes:
    Ears and speakers located 1/3 of the way between opposing walls, or 1/5 of the way, usually sound better when compared with using even fractions such as 1/2 of the way between opposing walls or 1/4 of the way, for that reason)

    YOU WROTE:
    "In the end, I think a properly "tuned" room this way "can" have a much improved response over just throwing the 2 subs in the corner and trying to EQ like mad."

    I disagree. The corner subs will fully energize a room which means lower harmonic distortion at a given SPL compared with alternate sub locations that fail to fully excite some room modes. But the best bass quality is from a monopole subwoofer in a room with 12-18 bass traps to damp bass reflections or a dipole subwoofer with less room interaction (although output will be weak at frequencies below the modal zone = 565/longest room dimension in feet)
     
  16. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    The article I linked has answers in it to the problems listed above. (Well except for #6)

    The impression Richard's reply gives me is:
    "Everyone should place their sub in the corner" and "Playing with placement is bad".

    I acknowledge his advice but I would rather recommend that ppl play with placement and find out the effects it actually has on their own account.
     
  17. Will Orth

    Will Orth Stunt Coordinator

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    Well I will add a few ideas to this post if you have the room for a massive bass cabinet you can make "phase" Diveder Panels that will eliminate the cancelation effects. as we did this 8 years back with #18 15" Drivers with 20,000 watts in a astro van and it worked realy well and broke the World SPL RECORD FOR THAT YEAR!

    What you would need is a massive BOX with output dividers
    between a set of drivers, it looks sort of a horn load app.
    but was breath taking driving down the road and you could not even breathe with it turned up to 159 DB.


    Will
     

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