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Old 8th October 2003, 06:02 PM   #1
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Default Choosing wattage for woofer, midrange, & tweeter

Hi everyone,

This has always confused me, if I will build a 3 way speaker box (i.e. woofer, midrange, tweeter and passive crossover), what is the common rule in choosing the wattage for them?

Let's say I have a 12" woofer with a nominal wattage of 200W, what wattage must the midrange and tweeter be? Should they all be equal?

A long time ago, I build a 3-way loudspeaker with a 100W woofer, 50W midrange, and 50W tweeter. Is this the right way to do it?

Thanks in advance!
JojoD
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Old 8th October 2003, 06:27 PM   #2
gary f is offline gary f  Canada
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The wattage of a unit can mean many things ( there is many discussions on how to measure the power of a unit) but basically, the higher frequency units don't get as much power as the low frequency units.

The drivers don't need at all to have the same power handling capacity. For a normal (?!) room, with normal (???) pressure levels, 50 or 100 watt is more than enough if the crossover is correct.

In the case of a tweeter unit, I think that the excursion (X Max) is more problematic than power handling...

F
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Old 8th October 2003, 10:05 PM   #3
Emiel is offline Emiel  Europe
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You should take into account the effeciency (e.g. dB at 1 watt at 1 meter distance) of the speakers though. The woofer and the midrange speaker should be about equal, the tweeter is usually a bit more, but can be easily adjusted by a series resistor.

Kind regards,
Emiel
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Old 9th October 2003, 04:10 AM   #4
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Hi,

Thanks for the replies, but isn't tweeters more efficient than woofers? If I use a higher wattage tweeter, will the overall sound be balanced? Considering that I will only use passive crossovers.

Thanks,
JojoD
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Old 9th October 2003, 04:48 AM   #5
navin is offline navin  India
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dave might correct me.....

the music spectrum is such that about half the energy is under 300hz and the other half over it

what compoenets are you looking at. there are too many variables here.
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Old 9th October 2003, 07:10 AM   #6
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Quote:
Originally posted by navin
dave might correct me.....

the music spectrum is such that about half the energy is under 300hz and the other half over it
It varies with the kind of music, but 300 is the generally recognized average figure.

As to wattage, in most cases it is meaningless for most hifi purposes. You would like the woofer to be the least sensitive of the lot and if you can get sensitivities the same, then that is best (with the assumption that you are using a passive XO -- with an active XO it matters not)

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Old 9th October 2003, 08:04 AM   #7
navin is offline navin  India
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it depends dave on teh design.

given that BSC loss affects woofers and midbass most it pays to have them a bit more sens.

say you have a 90db tweeter and a 93db woofer you can apply a BSC ckt and still manage a speaker that is about 89db.

however if you have a 93db tweeter and a 90 db woofer then you have to fist BSC the woofer to about 86db or less and the pad the tweeter to 86db too.

yes 6db is more correct but note i assumed 4db BSC not 6db for above cases. 6db is too much compensation in most cases (unless you have 2 or more woofers) where you have the extra sensitivty. I try to keep the sens above 87db so one does not need 300W monoliths.

If the front bafffle is wider (say 0.34m to accomodate a 12") then teh BSC starts at 1000Hz and ends at 50Hz. However below 100hz room gain comes in too. so one does ot need to compensate for the last ocatve or so which I assume to be 2db.
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Old 9th October 2003, 08:10 AM   #8
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Quote:
Originally posted by navin
iven that BSC loss affects woofers and midbass most it pays to have them a bit more sens.
Good point... so the woofer should be the least efficient after you factor in BSC... since i'm mostly building bi-poles now i often forget about BS.

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Old 9th October 2003, 12:44 PM   #9
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jojo,
You need to understand that the "wattage" of a speaker, as Dave said, is practically meaningless. I've noticed a trend among many to use that so-called speaker rating as the determining factor in performance. Forget all of that! It tells you nothing about how the speaker (actually the driver) is going to react when it's inside a box (or outside). Probably the main information you'll want is the frequency response, the efficiency and the impedence. There's more but that's a start. I suggest you acquaint yourself with these terminologies by reading everything concerning speaker design you can get your hands on. About frequency response: different drivers cover different parts of the audio spectrum better than others, no two are the same. About efficiency (sensitivity): how many decibels will a driver produce when driven with one watt? It won't work well, necessarily, if one driver can produce 86db and another 92db in the same enclosure unless you compensate for that. About impedence: you probably don't want to use a 4 ohm driver in the same system with an 8 ohm driver. These are generalities but perhaps this gives you an idea about what it takes to design a speaker system. Three words, read, read, read.
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Old 9th October 2003, 01:45 PM   #10
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Quote:
Thanks for the replies, but isn't tweeters more efficient than woofers? If I use a higher wattage tweeter, will the overall sound be balanced?
In Emiel's post he was talking about efficiencies, not power - tweeter usually more efficient.

A lot depends on crossover points and the relative power spectrum between each point. For your previous scenario of 100W woofer, then 50W midrange maybe a little large depending on where you cross the woofer, and what sort of music you listen to. If you used say 35W midrange then tweeter may only need to handle 10W, again depending on your crossover point - higher crossover frequency, lower power requirement.

As a bit of a guide, in pro audio concert PA, a 3-way system crossed at 250Hz and 1.2kHz usually runs mids = 3x tweeter power, subs = 3x mids power.

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