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Discussion Starter #1
I was reading that movie soundtracks are mixed to play at 75dB with up to 40dB added to the LFE channel. That is a potential for 115dB, total.

The model for a sub I am building says it produces 109dB with a Dayton 240W amp. It would take 1000W to produce a 115dB signal in this setup.

Seems like the amp ought to clip when the bad guys start shooting the dooms day machine. I don't read much about this being a problem, so I was wondering what the rest of the story is.
 

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Discussion Starter #3
Thanks, mike. I still don't understand. The amp don't know about room gain. If the amp tries to produce a +40dB (115dB starting at 75dB) signal and it can't muster the 1,000W necessary will it clip?

I'm guessing the amp can produce more wattage than the 240 RMS @ 1% THD it is rated for. I don't suppose it will get to 1,000W. If I'm not missing something, then I should lower the sub channel to gain some overhead. Running the bass 5 or 10 dB hot as is often recommended would be a bad thing.
 

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Certain parts of DVDs will exceed some amps power capabilities when played loudly but not on a (sustained) basis. All amps have a potential to provide more wattage than stated but . . . . only for brief moments. Amps will safely operate (for a while) with clipping but it's not recommended.
 

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Discussion Starter #5
Are you saying that this particular example will clip and that this is not recommended?
 

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There is no real standard for what is the maximum db level for movies but the studios are supposed to keep it to 75db with peaks of 115db. The problem is that with the uncompressed audio formats there are some movies that will task any sub because there are frequencies below 10Hz that have some seriously high SPL.
This is why alot of people recommend more than one subwoofer in a room to smooth out the frequency response and to give alot better room gain.
Another thing to consider is even using a 1000w amp the driver may not be able to handle it and hit its max excursion long before that.

How large is your room and do you have any large openings to other rooms?
 

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Discussion Starter #7
Yep, the design used in this example would be well beyond Xmax before 115dB is reached. I didn't want to focus on that because I'm trying to understand one thing at a time.

I was hoping for an engineering based explanation of what is going on. I guess simple observation indicates that it doesn't matter. If it did, "speakers in a box" wouldn't last a week, but they do.
 

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In my mind I just use those numbers as a reference as to the dynamics of the movie. Every setup will be different, from a all in one boxed system to a higher end floor standers and an IB sub. How loud you like it and how loud you want it to be all play roles in actual output.
 

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75db is just the calibration level to which you match all of your speakers and your sw system to in order to be level matched and ensure that you are hearing the levels somewhat close to what the studio intended. It has nothing to do with the level that you will actually hear during movie playback.

Whatever volume setting that your system is set to when you calibrate it is considered "reference level". That means if you start watching Batman The Dark Knight and you have the volume set at this same level there may be combined peaks of up to 120db or more. (105db from any one speaker channel, up to 115db from the subwoofer channel , possibly 120 or more if you redirect all of the bass into the SW channel and you are running a bit hot like most people do, and then figure in that any combo of channels and the SW may be peaking at the same time). It will be loud, but there will still be parts that are barely 70db. Most people don't watch at this level very often. Usually it's -20 to -10db for me from that level, which would give 100-110db peaks during the movie.
 

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Discussion Starter #10
I appreciate all the information. I am not trying to set reference levels. I watch movies with "normal" sounds around 70-75dB. The sub is set 3-5dB above the mains. I don't care if that is a reference for anything. I like it.

Along comes LFE and adds up to 40 dB to the sub whenever it feels like it. On paper, this should clip the amp modeled in the original example. I just want to know if there is clipping every time there is a loud effect and if this is harmful.

An other way of asking the same question is "do you need a sub capable of producing 115dB in order to watch movies with normal levels set around 75dB without damaging the system?"

I know I can turn it down to gain some headroom, but that ain't the question.
 

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Clipping is not good no matter what speakers your using. Clipping an amp or the maxing out the excursion on any speaker is also very damaging if not turned down right away.

If your running your levels at 75db you do run the risk of clipping but that is so dependant on what speakers, amp and room size your using its not a cut and dry answer.
 

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Discussion Starter #12
It's finally sinking in about room gain. If I measure my listening level at 75 and have 6dB room gain, the amp is only putting out wattage for 69dB. Duh.

So, if I get a 40dB boost, it goes to 109 dB (69+40), not the 115 I was thinking. Walla. I'm glad I left a lot of head room in this design because I sure didn't consider this up front.

So, I looked at what would happen to Xmax if the amp produced 500W for a short period of time. Still safe. It will handle 550 before reaching Xmax. I guess a blind hog found an acorn.

I suppose attention is paid to this when designing speakers, but I haven't noticed much talk about it. I probably just missed it.

Still, there seems to be a lot of designs that could have this theoretical clipping (or Xmax) problem. I suspect it is not too damaging in light doses and probably can't be easily detected.

Thanks for the help.
 

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If you're using a software program like WinISD to design your speaker or sub and you're using the amps output of 240 watts as a guide, that's a good thing. Just remember that the "Amplifier apparent load power (VA)" will not represent a constant state of power draw. It changes as the frequency changes. Most power draw occurs at the lowest frequencies.
 

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Discussion Starter #14
Thanks Phil. I am trying to digest that bit of info. UniBox has a chart for Peak Current and Peak Voltage. If I cypher those numbers will I get the wattage produced at a particular frequency?

Looks like setting Max Power to 240W in UniBox produces 380W at 22Hz (F3) and 65w at around 37Hz. This seems to be factored into the Peak Cone Excursion curve. Is that correct?

Now, is the amp capable of surging beyond those numbers? If so, is there a "rule of thumb" to estimate the max it can do?
 

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Unless I'm mistaken, the discrete sound channels are mastered at 85db with 20db extra headroom - LFE is to be boosted by 10db, so it becomes 95db. Never heard of 40db headroom above master volume, that sounds like a mistake.

To your question though, if a small amp and small driver are fed a signal from your processor demanding more from them then what they can handle, several things can take place.

If the amp can't muster the power, it will clip. It will send a severely distorted signal to the driver and this should be audibly distinguishable. Overdriving your amp a lot can lead to permanently damaging the amp (frying internals) as well as the driver.

If using a commercial sub, there are probably limiters built in, so the input signal going to the plate amp will be reduced through circuitry before being amplified - in this case neither the amp nor driver get to see the full strength of the signal and will only produce a lower output level.

If the amp can faithfully amplify the full signal strength from your processor, but your driver doesn't have enough excursion capability to move proportionally, it can bottom out. The coil will move beyond the gap and you may here a loud clank. If the suspension is very stiff or the enclosure it's in is very small, there may be enough resistance to that motion to keep it from bottoming, and all that will happen is you will get output compression. This means the driver should have created let's say 100db of output, but it really only did 97db. If you have enough excursion capability and there isn't enough resistance to movement, another problem can be thermal handling. If there isn't enough metal in the coil, you can fry the driver at higher power levels.

If none of those constraints rear their ugly heads, your subwoofer can faithfully reproduce the signal being fed to it from your processor.
 

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Discussion Starter #16
Thanks Steve. Does Dayton include these limiting circuits? Are there any schematics to make one?

I got the 40dB gain from taking the worst case from all the conflicting info on the net. This is complicated by my lack of understanding of the issues. It could be incorrect.

105dB is often quoted as THX max for regular speakers with +10, or 115dB for the LFE channel. Then, if the amp has SUB output instead of real LFE output, ya get another 5dB from the regular bass.

Seems like an amp with a SUB output could generate 120dB and be within the THX specifications.

I probably don't understand it, but I don't really care to set reference levels. I do care about how much the SUB channel may be boosted during normal listening and I care that my sub will safely reproduce it at my listening level.

So far, I'm learning a lot of stuff, but not able to determine a "safe" listening level for a specific combo. Seems to depend on unknown things.
 

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The model for a sub I am building says it produces 109dB with a Dayton 240W amp. It would take 1000W to produce a 115dB signal in this setup.

Seems like the amp ought to clip when the bad guys start shooting the dooms day machine. I don't read much about this being a problem, so I was wondering what the rest of the story is.
Don't confuse the dB figures with the output of the Dayton amp OR the capability of the sub to convert watts to sound. The loudness level of your sub in dBs will be a result of the amp's output capacity measured in watts and your subs capability to convert those watts into sound. An average HiFi driver might have an output level of 88dBs with an input signal of 1 Watt measured at 1 meter. Compare that with a PRO driver which might have an ouput level of 98dBs with the same signal and measurement protocol.
The THX levels that have become useful standards are still arbitrary. You don't HAVE to listen at those levels. You won't break any laws or equipment if you continuously choose to listen at a 83dB level with peaks limited to 109dBs.
Remember too that speaker manufacturers like to advertise their products high wattage capacity as do the amp manufacturers who like to claim that their 240 watt amp can easily do a 1000 however briefly.
 

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What we're really talking about is the Dolby Digital standard, not so much THX. Each channel is mastered at 85db with 20db of headroom. The LFE channel is to be boosted by 10db. When you factor the LFE level plus redirected bass below 80hz coming at the sub, yes, in a worst case scnerio, if you are listening at a master vlume level of 0 (extremely loud), the sub can be asked to produce ~120db at your seat.

Will this ever happen in your case? Very likely not.

You first need to calibrate your system. Set your master volume level to 0, pull out an spl meter, and use either your processor's built in tones or tones from a disc like Avia or DVE. Adjust the individual channel levels to either 75db or 85db based on what disc or processor you are using (this will be mentioned in the literature). You are now calibrated to reference.

When you watch a movie, note what volume level you feel comfortable with. For most people, this is somewhere between -10 and -15. This means you are listening at 10 to 15db less then reference, so your speakers could be asked to produce up to 90-95db and the sub 105-110db, worst case scenario.

Would I personally feel comfortable with a 10" sub and a plate amp? No. But you gotta go with what you can afford and what you can live with in your room. How big a priority in your life is your audio system?
 

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Here's a list of around 50 movies showing a range of 16dB in peak LFE level.

No doubt there are movies that are beyond these extremes, but assuming a maximum range of 20dB would be reasonable

This would suggest that if you callibrate your system for the average level, you would need 10dB of headroom to cater for the different mixes that are out there.

Popular movies that are mixed at the average level include:
War of the Worlds
Pearl Harbor
The Incredibles
Batman Begins
These were measured on Region4 DVD's, Dolby Digital, Small speakers (bass from mains redirected to LFE channel)

If you adjust your system so that your sub just gets into trouble with any of these, and then back off by 10dB, you should be fine.
 

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Discussion Starter #20
Good to know what is really happening with the LFE channel.

I started out trying predict the listening level where clipping would occur. Mostly an academic exercise. In other words, how high can the volume be set to avoid clipping when the LFE channel maxes out. Nothing about reference level, just predicting the level it takes to clip the woofer.

No progress there, but plenty of other information. I guess there are too many factors to reach a valid estimate.
 
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