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Discussion Starter · #1 ·
I think I know the answer to this question but would like more feedback so I can wrap my head around it.

I have pair of LMS 5400 Ultra 18" both in 4cuft. sealed enclosures. During modeling with WinISD I saw that they can take 3200 watts each before exceeding x-max at about 10hz. Now if I add any boost in say the 20hz range then the x-max is exceeded much more quickly with 3200 watts so I reduce power to keep it under control. I am also aware that I can add a HPF as well to control excursion and increase power handling.

Now I had gotten into a discussion with a friend of mine and I had a hard time explaining why my subs would be in danger if I ran 3200 watts into each while at the same time boosting the 15 to 20hz region by 6db. He could not understand why if they could handle 3200 watts that they still couldn't handle it with boost. To be honest I started to get a bit confused by the back and forth as well so I am hoping someone can explain it better.

I am aware that to get 3db more output you need to double the power and that the lower octaves are were the most excursion is used.

I am basically trying to convince my-self that I don't need 3000 watts for each of my sealed LMS 18's since my big room is not providing the gain I would hope for. I have to add a 6db boost at about 18hz to get flat response down to about 12hz. I have added a 15hz HPF so I am flat to about 14 hz in room right now.

I would rather be amp limited than have too much amp and destroy these expensive subs.

What do you guys think?
 

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Elite Shackster
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The power handling of the motor has nothing to do with cone excursion. It is the thermal capacity of the coil in the motor. As you add more current it heats up, and there comes a point when you hit its limit, hence the power rating. Cone excursion for a given amount of power will differ dependant on the cabinet the driver is placed in. Smaller cabinets will restrict cone excursion while drawing more power for higher spls at lower frequencies. A very large cabinet will increase efficiency, and the driver will move easier under less power meaning more low end output. This is why you see gains in the low end performance of subs when the cabinet size is increase (especially so when ports are also introduced).

I always work to the low end figure, rather than boosting. Work to the drivers maximum power handling (actual max power handling is likely a bit over this too, so running upto this figure is considered very safe), and see wht spl you get a 10 hz . Consider this the max spl for this sub and apply cuts to the rest of the response to get it flat (this is how EQ systems work, and for good reason). This way you can use the full power handling without worrying that low end boosting will either hurt your driver or clip the amp (which can also hurt the driver). Of course, you need to ensure the driver is within xmax within its enclosure with your desired input power, and I can see you have done this.

Dont worry about adding an amp thats powerful. The actual power supplied to the subwoofer is dynamic and not fixed. If it were fixed you would have fixed spl output. Most of the time the power being used is very little, only the lowest frequencies at high spls really start to demand great amounts of power. As these kinds of sounds and effects are usually short bursts and have space in between, which is again the quieter stuff, then the VC can happily handle it. Loud bassy and continuous music has the potential to tax the subs more than a film, subs are very good at handling the LFE from movies.

Generally though, boosting should be avoided. Once you get your sub up and running in your room, its response will change. If you eq like I say applying only cuts you will end up with an end result. Any dips inthis response could potentially be nulls, and trying to boost those out will be fruitless and you will just pile more power into the motor for no real gains. This is how drivers can meet their end. Applying only cuts removes this risk. Also, remember that cutting the upper bass will yield the same end result trying to boost the low end, just without all the risky boosting.

One last point. When you apply boosts, your asking the driver for more spl. More spl is generated by increase cone excursion, so this is why you see excursion increase when you apply boosts.
 

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Discussion Starter · #3 ·
Thanks for the reply Dan.

I do get what you have said but I am pretty flat in room until I get to the 30hz range. I have tried cutting the higher ranges to equal the sub 30hz range but the sound is lifeless and much better if I just add a 6db boost in the 18hz range to even it out better. I never try and boost nulls but the sub 30hz range is not a null but instead the normal knee of the sealed enclosure. I have a fairly large room so I am unfortunately not getting much room gain and must compensate.

I have in the past run sealed subs with boost and much more powerful amps than what I should without incident except for the sealed TC3000 15" which I was asking to do too much in a big room, I now have a decent dent on the cone from it bottoming out. I am not going to take that chance with these LMS 18" pair as they are too expensive to be so careless.

You explained your thoughts well to me but then I am a DIY'er, I was trying to get a simpler explanation so I could use for friends who are not into this hobby. It's just hard to get into words easily the effects of boost and power needed and I am sure there are some things I can learn as well which I am always up for.

Thanks again for the reply.
 

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Elite Shackster
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Its quite typical for a sealed sub to begin to drop at about 30 hz. When I apply cuts to level it to 10/15 or 20 hz, I then have to increase the overall gain to get the sub back upto the right level and sound right again. This should in effect be the identical end result to boosting the low end. The deeper you try to go the more max spl you will give up though, so that worth bearing in mind. Your 20hz target is pretty reasonable though.

When it comes to boosting, as long as your sensible, and dont try boost out nulls, it can work fine. Typically, most drivers will have been tested beyond their power rating, and will handle peaks beyond that. Given the nature of LFE, and the fact most deep output comes in quick burst, you can even get away with exceeding the max power handling figure of your driver, as you have noticed. Again though, this requires you to be sensible and monitor things. Be aware of what your sub is doing and act upon any funny sounds.

Its worth knowing most commercial companies need to employ something like a Linkwitz Transform Circuit in their subs. The role of this circuit is to apply the boost to the low end, so the sub performs to the low frequencies without additional EQ by the end user. They do this with a lot of testing though, and work within the limits of the sub, will usually do something to the amps power supply so it doesnt clip, then employ a HPF to protect the low end of the driver, and some protection circuitry to ensure over driving it is pretty difficult, they dont want a lot of returns after all.

When it comes to the simple answer for your original question, well it is simple really. When you apply boosts, your asking for more spl, which asks for more power from the amp and more excursion from the driver. I think your assumption, as you explained it to your friend is wrong, but to discover if that is the case or not, lets look at your modelling.

When your modelling, and you put in a max input power, you are then given a result on the spl chart. If you apply a boost a 20hz, it will show you this boost on the spl chart, but this wont take into account the additional power requirements from the amplifier. For this you need to look at the max spl chart. If you already modelled on full power, you should now see you boost has done very little if anything at all. For the reason behind this you need to look at the Amplifier Apparent Load chart. You should see on here the additional power requirements for the boost you intend to apply. Likely it will massively exceed the capabilities of both your amp and driver, and if its beyond you input power, this is why your max spl chart shows no effect from your boost. To keep your boost in place, you will need to lower the input power till you see the max power your amp can deliver not exceeded on the apparent load chart, which will no doubt have a peak centred on your boost. Stop at your amplifiers max output power. You can then look back at the spl chart and see the overall affect on your subwoofers headroom. What you should end up with is quite a low input power rating but the max power requirements on your apparent load chart equalling that max output ability of your amp.

This is what I mean when I say you need to be careful with your boosts, you need to monitor all aspects of your model. In reality, you will be fine with the boosts applied as long as no content asks for more power from your amp than it can deliver. If your amp can deliver more than your driver can take, you will generally be ok as the driver will take peaks beyond its max rated power for short bursts. As you increase volume (spl), the power needed increases and its where your boost is that will sap the most power. The danger lies in running at high levels with no content where your boost it, you think all is fine, then a peak may occur in the film, and all of a sudden your amp and driver are asked for massive power to keep the 20hz (or whatever your boost was set at) zone level. This is how drivers are cooked and/or destroyed by heavily clipping amplifiers. This is why I say you need to be careful with boosting. Its ok to a degree, but you need to be smart with it. You may find your boost works fine for a long time, then a movie you havent seen before, running at high volume, might just catch you unaware. No doubt this is what caught you out with your TC driver.

Make sure you look at all your models parameters when your looking at applying boosts, so that you can properly monitor its effects. I know this is a bit of a ramble again, but I want to try be clear. If this explanation helps at all, it should clear a couple things up and make the simple answer become clear for the discussion with your friend.
 

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Now I had gotten into a discussion with a friend of mine and I had a hard time explaining why my subs would be in danger if I ran 3200 watts into each while at the same time boosting the 15 to 20hz region by 6db. He could not understand why if they could handle 3200 watts that they still couldn't handle it with boost
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Tell your friend a 6 db boost at a given frequency requires 4 times the amount of power. If the sub is rated at 3200 watts and you feed it 3200 watts, at the 6 db boost frequency the power to the sub will be 12,800 watts resulting in a burnt voice coil. The input power needs to be reduced so the given boost frequency receives 3200 watts. Of course this assumes the sub can handle the extra excursion the boosted frequency would demand.

If he doesn't understand this let us know and we'll try gain.
 

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Discussion Starter · #6 ·
You guys are really helping me out here, I really appreciate it.


.
If he doesn't understand this let us know and we'll try gain.
:rofl2:
 

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For a true understanding of the capabilities of the LMS driver, read Ilkka Risanen's tests on this website. His tests have become the defacto standard for the LMS driver and a whole list of other drivers and commercial subs.

The only way to judge results is to apply the wattage to the subs in their listening environment and make your measurements there. Start with low db and increase volume and/or boost until you reach your specified limits.
Two LMS sealed units should easily pressurize most average sized rooms.

Hang on to your knick knacks.
 

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I have pair of LMS 5400 Ultra 18" both in 4cuft. sealed enclosures. During modeling with WinISD I saw that they can take 3200 watts each before exceeding x-max at about 10hz. Now if I add any boost in say the 20hz range then the x-max is exceeded much more quickly with 3200 watts so I reduce power to keep it under control. I am also aware that I can add a HPF as well to control excursion and increase power handling.
Since WinISD can do parametric EQ, perhaps you could try adding that 6db gain - and then look at what the cone excursion is.

I've spent plenty of time playing with EQ on the low end response - and I've always been really surprised at just how much of a difference it makes in excursion. It's all physics - but it's surprising to compare pre-eq and post-eq results.

I actually tried to model 2 LMS-5400s using T/S parameters from the PE data sheet - and it doesn't seem like it's modeling accurately. With 2 of them in a 5Cuft sealed enclosure and 6400 watts, I'm only getting 25mm of cone excursion. That doesn't seem like that could be accurate.
 

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Mike P. said:
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Tell your friend a 6 db boost at a given frequency requires 4 times the amount of power. If the sub is rated at 3200 watts and you feed it 3200 watts, at the 6 db boost frequency the power to the sub will be 12,800 watts resulting in a burnt voice coil. The input power needs to be reduced so the given boost frequency receives 3200 watts. Of course this assumes the sub can handle the extra excursion the boosted frequency would demand.

If he doesn't understand this let us know and we'll try gain.
Depending on the amp's capability I don't think the power to the sub would be 12800W but you would certainly be clipping the amp. It would be trying to output that power. Square waves put a much higher thermal load on the voice coil than sine waves which would result in a burnt coil. Not to mention distorted sound.

I guess this is the same result Mike gave, slightly different method.

So, you do need an amp capable of 3200W at your boost frequency to get the most out of it (assuming excursion is not an issue )
 

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Elite Shackster
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Mike isnt suggesting the amp in question would output this amount of power, only that this is the requirement for the desired boost under those circumstances. Obviously the amp cannot deliver this so the boost would not be applied. The amps max power is its max power, so your applied boost is only worth the amps max power. Essentially, the rest of the range will run on less power for the boosts effect to remain in place.

The danger comes when you run your sub loud, so that when the boosted area is in demand, the power needed to maintain the correct boost exceeds either the amps maximum power capability, the drivers excursion ability, or both. I think we are all on the same page here though.
 

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Moonfly said:
Obviously the amp cannot deliver this so the boost would not be applied...Essentially, the rest of the range will run on less power for the boosts effect to remain in place.
These are not correct statements.

The boost will be applied and the amp will clip. The amp will not run at lower power for the rest of the range to "keep boost intact".

The camp's gain must be turned down so that it doesn't clip at the boosted frequency.

Maybe this is semantics but I thought the way you worded it added confusion.

Yes, I think we're on the same page.
 

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Elite Shackster
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These are not correct statements.

The boost will be applied and the amp will clip. The amp will not run at lower power for the rest of the range to "keep boost intact".

The camp's gain must be turned down so that it doesn't clip at the boosted frequency.

Maybe this is semantics but I thought the way you worded it added confusion.

Yes, I think we're on the same page.
Its a fair comment for clarity, but if we are to be precise, then we must say the amp would try to apply the boost, but would not be able to within its design parameters. Further, such a massive spike in power required would almost certainly cause the amp to shut down, so the boost still would not be applied.

Its true the amps gain would need to be reduced to as not to exceed its max output capability when boosting, the net result of this is that for most of its operation, the rest of the range will run on much reduced power. You can see this when modelling in WinISD. I ran a model of a Q18 sealed for example. Without boost the max input power is 1000 watts continuous across the range. With a 6db boost at 20 hz, continuous power must be reduced to around 400 watts max, while the boosted range requires the full 1000 watts be available to maintain the boost at the correct level.

Hopefully, there is no confusion :T
 

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I think if you looked at the amount of clipping in the signal you would see the difference between 6 dB boost and 3dB of boost so, technically, the boost is being applied even if you are looking at a clipped signal. It has nothing to do with the max amplitude of the output signal.

Remember, the boost is applied electrically. It is not a 6dB spl boost. It just turns out that way when played through the voice coil.
 

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Discussion Starter · #19 ·
For a true understanding of the capabilities of the LMS driver, read Ilkka Risanen's tests on this website. His tests have become the defacto standard for the LMS driver and a whole list of other drivers and commercial subs.

The only way to judge results is to apply the wattage to the subs in their listening environment and make your measurements there. Start with low db and increase volume and/or boost until you reach your specified limits.
Two LMS sealed units should easily pressurize most average sized rooms.

Hang on to your knick knacks.
I am very happy with the LMS pair for music but the room I have them in now is about 4800 cuft since it opens up to the kitchen so it's not easy to get plentiful bass like my last home. For most people it will probably be more than enough but there not Sub_crazy like my-self:D

I am actually going off in a different direction with this post as opposed to my original post. I was just trying to find a easier way to explain the effects of boost in a sealed box. It's just that you see some sealed build's with a ton of power and a bunch of boost. I was trying to explain why It is not advisable for me to run 3000 watts to my LMS like Warp does with his triple, dual-opposed LMS Monsters. He can pressurize his room easily with each sub not doing too much work as he has 4 more LMS than I do. For me to match the SPL's would be impossible and I would probably destroy my subs. If I put my same subs in a room half the size then the bass would be incredible.

it's just a hard thing to explain as people think that since someone else does it then you should be able to as well.
 

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it's just a hard thing to explain as people think that since someone else does it then you should be able to as well.
If you haven't learned already, you'll find that expectations are a funny thing when it comes to subwoofers... Whether it's volume, fidelity, appearance, rooms, EQ or what-not.

Expectations are all over the place........ and it's a challenge to defy them with logic sometimes.

:eek:lddude:
 
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