# How much headroom is needed?



## thewire (Jun 28, 2007)

If the amount of headroom is a direct indication on how much one may be able to cut or boost a frequency, is the general idea still do not boost more than 3dB no matter how much there is available? Is this small amount because of room modes? How much headroom might be needed to boost 6.5dB or more? How much might I be able to boost with 12dB of headroom (before cuts) for example? The level on each subwoofer is exactly between 12:00 and 3:00.


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## Wayne A. Pflughaupt (Apr 13, 2006)

thewire said:


> The level on each subwoofer is exactly between 12:00 and 3:00.


Here is some reading on the subject, but if you're running your subs that high, I'd say you're close to being maxed out, unless you've had to boost up the level that much to compensate for a low incoming signal.

Regards,
Wayne


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## bobgpsr (Apr 20, 2006)

Enough to get 105 dB SPL with your main speakers and 115 dB SPL with your subwoofer (THX rated movie reference *peak* output level). Amplifier wattage, room size, and speaker SPL efficiencies figure into the calculation.


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## thewire (Jun 28, 2007)

Wayne,

Yes I am turning up the level on the subs because it is -15dB from the XA2 (I think) when all my speakers are set to small. Then there are at least a couple of eq cuts I have to compensate for also. Excellent post in that link thank you. The one in the waterfalls sticky was also very good. Are you saying that with my very low signal it may be possible to do the boost? Is this similar to boosting an analog subwoofer output?

DVE - lets say this is correct for right now.

*Speakers set to small* for -15dB from the subwoofer output and location was front wall center with the driver 8 feet and 8 inches away from the mic. Level on the XA2 is 0. 

72dB at -30dB FS with the 8.5 cut at 50Hz. For this I needed to flip the volume switch to variable and turn up the gain past 3:00 to get 75dB. 

Speakers set to large for -10dB from the subwoofer output.

Volume set to THX: 75dB at -30dB FS with the 6.5dB boost using two filters at 60Hz and a 8.5dB cut at 50Hz.
73dB with only the cut at 50Hz. Again I have to turn on the variable level and adjust the gain.

Most speakers I needed to set to -1 or -2 on the XA2 to achieve the 75dB there.

Then I compare these levels to the THX optimizer. The ones from the Star Wars trilogy are nice because they repeat forever. 

The maximum (big no no so I turn this back while the level stayed the same) gain on my subwoofer was 73dB with the PEQ engaged using a 6dB cut at 50Hz with speakers set to large. It is reading 2dB lower than DVE.

With speakers set to small this went to around 67dB or 65dB I think.

Most speakers I set to around -10 on the XA2 player except my surrounds at -12 which is 75dB to 76dB depending how I tilt the mic. I did not test these with the boost.

Now I try two (one damaged being replaced) with one in each front corner at very close to the same distance with the PEQ disengaged. The THX setting is really high on the damaged subwoofer and each set to that total over 75dB. I turn on the variable switch and turned them down to each read at 72dB for a total of 75dB. The maximum I get from the good sub is 74dB and if I do each to that I get about 86dB. 


bob,

I have already met the spec for the 105dB on the mains. They play nice until around 111dB and go to about 118dB as full range speakers so they should be even better set to small. Getting the center channel setup correctly was as difficult as the first time. So if I am able to playback to reference and the boost is not clipping the amplifiers to soon I am alright?

One of these subwoofers is THX select spec for a room 2000 cubic feet or less and my room is slightly over at 100 - 250 cubic feet larger. One sub went well into 89dB and began to peak at around 92dB. With two (one is being replaced arrived damaged) there is 100dB peaks average and some 90dB ones that I sometimes do not even hear and this has me concerned whether the dip at 60Hz is to blame. I have been able to correct the dip in the past with one subwoofer but it had difficulties at high levels when I was watching DTS. The boost brought the THX switch setting from 73dB to 75dB limited bandwidth pink noise on my HD-DVD/DVD copy of DVE when my other speakers were set to large (meaning -10dB instead of -15dB from my player) . The amplifiers for the subwoofers are only 200W RMS at 4ohm each but the drivers are rated for 400W. My other speakers all use separates with 1335 watts. I wanted to be sure I had that part already covered since I was not sure what size room my next HT was going to be. I ended up needing to get one more amplifier to bridge both my left and right speakers amps so I am glad that I went that route. These SPL readings were all C-Weighted (slow) uncorrected.


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## thewire (Jun 28, 2007)

This is an example of the filters that boost the dip at 60Hz to an 80dB target that I could turn down to 75dB.









These are an example of the filters needed to cut to get the 75dB target with no boosting. Both are for the center seat location. The boost required is more now without being able to effect the signal by sitting down in the center seat but I overlooked mentioning this before since I consider a third subwoofer at the original location. :duh: 









It looks like to much to boost right now.


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## Wayne A. Pflughaupt (Apr 13, 2006)

> Are you saying that with my very low signal it may be possible to do the boost? Is this similar to boosting an analog subwoofer output?


The simple answer is, if you run out of headroom you'll know. The sub drivers will bottom out and make rude noises during demanding scenes. If your subs are DIY and you're using outboard power amplifiers, you'll see their clip lights come on. If your subs are store-bought, many have built-in limiters for protection. If you hear the bass volume suddenly "suck out" or temporarily reduce in volume, you're overdriving it. 

Regards,
Wayne


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## thewire (Jun 28, 2007)

The subwoofer has never bottomed out.


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