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Old 7th May 2008, 08:52 PM   #1
Vivek is offline Vivek  India
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Default Need sub advice

Hi all,
I have built two subwoofers with Infinity car drivers for HT. Each driver is rated at 300W RMS, 4 ohm and 90dB. Vas is 78L and I have put them in 62L sealed boxes.

Some people say since they are 90dB, they are pretty efficient and do not need too much amplifier power. On the other hand, if I use a lower powered amplifier, it may be driven into clipping in case I drive them too hard. Could anyone give some inputs as to how things really work?

Thanks,
Vivek.
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Old 8th May 2008, 04:43 AM   #2
GM is offline GM  United States
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Greets!

Both are true statements, so the variables are what peak SPL is desired and/or how much power is required to use up its ~linear cone travel.

For example, if 110 dB/m peaks are enough, then the sub only needs 2^(110-90/3.01) = 100 W and hopefully it has enough ~linear excursion to handle this amount of power. With two subs acoustically close together and wired in parallel, then you only need ~50 W/sub since they are ~3 dB more sensitive. This will require an amp rated at 2 ohms though if only one amp is used.

FYI, the DD/DTS LFE channel reference peak SPL is 115 dB at the listening position and if all the other other channels are set to 'small', then in theory it needs to be able to handle another ~7 dB or ~122 dB plus any losses over distance.

Bottom line, when in doubt, better to have too much power than not enough.

GM
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Old 8th May 2008, 05:29 AM   #3
Vivek is offline Vivek  India
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Hi,
The woofers have an Xmax of 12mm. I have been using them with 70W amplifiers till now. The problem is that I have to crank up the bass boost on the LFE channel all the way up to get some thump. That was what set me thinking about how much power may be needed.
Now if you are saying for 110dB peaks, only about 100W may be enough, then I suppose I do not have to go for very high powered amps. I think getting 110dB peaks can be pretty loud, don't you think?

Vivek
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Old 8th May 2008, 04:04 PM   #4
GM is offline GM  United States
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Every time you add +3 dB of boost, amp power requirements double and excursion increases ~1.4142 times for that BW, so you may need a lot of power to keep from clipping the amp. Of course if your amp has a clipping indicator and it never lights up, then it's powerful enough.

GM
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Old 8th May 2008, 04:17 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
I think that 90dB to 110dB sensitivity/SPL conversion is for mid band.
Do you subtract the response droop of the speaker/box combination at low frequency from the 90dB sensitivity?

If the box/driver is 10dB down at 30Hz then you need another 10dB of drive to get your 110dB of SPL.

That would equate to 1kW not 100W.
Which is right?

Taking your actual drivers specs of 90db/2.8v/M and a 62L sealed box for a 78l Vas. The response will be dropping from around Fb (50 to 70Hz) all the way down through the bass range.
Then use EQ (Linkwitz transform) to bring the response back up to the desired "shape".
This is where the big power and big Xmax get used up.

If 115dB at the listening position is the target, then 122dB at 1m is the SPL required. That is a further 12dB above the 1kW input. Now we are talking 16kW for a sealed box that has Fb=~60Hz to get 115dB at 30Hz.
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Old 8th May 2008, 06:17 PM   #6
Vivek is offline Vivek  India
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1KW? 16KW? I understand that in theory this is needed.
Maybe I should go for the bigger power amps (about 300W) or so to be on the safer side.

Is there an option that I build a lower power amp and LT the subs to get the desired shape? The idea is to try and build an amp that is not very complicated.

Vivek
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Old 8th May 2008, 06:23 PM   #7
AndrewT is offline AndrewT  Scotland
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It's the LT that causes much of the problem.
Sealed virtually demands LT and that in turns demands big power and big Xmax.

There are much more efficient ways of getting there but all have a downside, just in different forms.

Based on the very rough guessimates I gave as an example, 300+300W give 14dB less than 1.6kW.
That gets you to 101dB at the listening position for 30Hz and considerably louder as the frequency rises.

GM,
was my arithmetic correct and my assumption on midband sensitivity?

BTW,
I measured the voltage delivered to a vented bass speaker and compared that to the mains. The bass was running at more than 3dBV below the main amps even though the crossover was at 150Hz.
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Old 8th May 2008, 06:43 PM   #8
Vivek is offline Vivek  India
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So you are saying I just need to go for ample amount of power rather?
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Old 8th May 2008, 07:01 PM   #9
AndrewT is offline AndrewT  Scotland
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You need to balance cost against size (volume) against sensitivity, against power against Xmax against box (driver loading method).

The car audio folk thought power was more important and see where it got them.
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Old 8th May 2008, 07:06 PM   #10
Vivek is offline Vivek  India
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Looks like I am better off looking for home audio sub drivers. It could be a big task in India, where the DIY market is very small and we do not get drivers like in Europe or the US.
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