[JeffB]

If one assumes an imaginary 12" woofer with 87db sensitivity, then does the following hold true.
87db -- 1 watt
90 db -- 2 watt
93 db -- 4 watt
96 db -- 8 watt
99 db -- 16 watt
102 db -- 32 watt
105 db -- 64 watt
108 db -- 128 watt
111 db -- 256 watt

If so, why is there a general assumption that bass requires lots of power.  There was a fairly recent thread on this, unfortunately I can't find it.  I remember people recommending 300 watts to 1000 watts for bass.  Velodyne  has a new sub woofer with 1250 watts x 2 driving a 12" and an 18" driver.

Is it to handle the dynamic peaks?  If you're listening at 90 db with 2 watts can the music demand 111 db peaks and 256 watts?

Or does sensitivity follow a different power curve in the bass frequencies?

I ask partly because I have been looking into SET amplifiers and possibly the Clari-T at about 10 watts.  I realize that if I want deep bass, convention tells me that I'll need a subwoofer with a big amp.  The SET amps are often paired with single drivers that extened down to between 40Hz and 55Hz and this appears to work fine.  However, the question arrises at what frequency does one start needing more power.  Can a SET amp really drive a speaker at 40Hz.

An amplifier needs to be able to handle the peaks and it needs to be able to generate the peak current quickly.  Are the bass peaks, somehow different than the high frequency peaks?   Does having more watts available give one any indication of speed?

I can appreciate that moving more air requires more power, but then there is that darn sensitiviy number and the fact that bass drivers have a larger magnet and thus power also comes from the bigger magnet without necessarily needing more current.

I need enlightenment.

[WerTicus]

i have heard 10 wat class a amps with very nice bass.

and i have heard 125wat class amps with very nice bass
and 600 wat class d amps with very nice bass

the difference?  well obviously the 10 watter didnt go all that loud but the other two went plenty more loud than you would really want to listen to .

however the 600wat amp at the same volume was just so much more 'effortless' in its reproduction

i recommend more power ALWAYS MORE it just sounds better. all else equal anyways.

[woodsyi]

Jeff,

Watts don't tell the whole story.  You have to check for slew rate and damping factor.  Basically high slew rate means amps can move the cone fast and high damping means amps can stop it fast.  Here is a good article on output impedance of amps, i.e. Damping Factor, that will give you some idea of why you need more "real" current to drive the woofers without distortion.

http://www.classic-audio.com/marantz/mdampingfactor.html

[JeffB]

Thanks woodsyi, the article was very informative.  However, it really proves the saying, "The more you know, the less you know."

In regards to my question, the article states that larger drivers have more reactance, which makes them harder to control.   If the speaker's impedance drops, the amp will have to supply more current to maintain the correct voltage across the speaker.  Since Power = Volts * Current, more watts are needed to control larger speakers.  However, more watts frequently come with more distortion and even though the amplifier can now control the speaker, the amplifier is giving the speaker the wrong directions.  Arg!!!

[Rob Babcock]

The ear also doesn't perceive sound in a linear fashion and is more sensitive to certain frequencies than others.  For example, 100 dB would be ear splitting with a 1.5 klhz tone, but the same volume would be barely audible at 20 hz (although it would rattle the room).  That's the Fletcher-Munson curve, and it's why many components have a "LOUDNESS" button.

Also consider that generally beyond a certain point the woofer will begin to roll off in the deep bass, meaing you'll have to apply more power to compensate.  The response curve of a woofer measured with 1 W input won't ncessarily hold up at full power.

[doug s.]

amps capable of good current output are also needed to control woofers.  which is why i got a better bass response w/a pair of 75wpc electrocompaniet amps driving a pair of vmps larger subs (in a *big* room), than when i was using a pair of original-iteration adcom gfa555s, which are rated at 200wpc.  (and, the adcoms are not slouches for subwoofer amps, by any means.)  the ec's are rated at >80a current output, while the adcoms are rated at ~20a output...

doug s.

[ctviggen]

I always wondered how they derive these numbers.  For instance, the amp I'm using for my subs is rated to 39 amps maximum.  But since I have that amp (and two other amps) on one 20-amp circuit breaker, I can't see that I could ever get 39 amps from my sub amp.  It must be an instantaneous value, and if it isn't an instantaneous value, then it's kinda of useless, as the amp will blow the circuit way before it hits its maximum amperage.

[Kevin P]

The raw driver sensitivity doesn't tell you much about the system.  You need to look at the power needs when the driver is installed in the acoustic system (box).  

Also... the driver's sensitivity rating is based upon a 1W input.   It changes due to VC heating and it drops significantly with higher power levels.   You have to measure the system (box + driver) under load to determine when you have power compression issues.  

Higher power is most often needed when you are using a small box & equilizing to extend response.   Even then sometimes you are driver limited due to excursion rather than power limited.  It depends upon the application though and you have to design with knowledge of where you need power and why.

[markC]

I always wondered how they derive these numbers.  For instance, the amp I'm using for my subs is rated to 39 amps maximum.  But since I have that amp (and two other amps) on one 20-amp circuit breaker, I can't see that I could ever get 39 amps from my sub amp.  It must be an instantaneous value, and if it isn't an instantaneous value, then it's kinda of useless, as the amp will blow the circuit way before it hits its maximum amperage.

That's the transformer @ work. Example: primary 120v @ 6A: secondary 35v @ 25A. Just a rough example, but you get the idea. This would be ~ a 700va tranny. Again crude example.

[jselectro]

Sorry deleting incorrect info

[dwk]

JeffB, the figures given for your imaginary woofer are totally wrong.

Uh, nope - JeffB is right, you are wrong.  10dB increase requires 10x power, so 3dB requires 2x power.  The *perception* of volume does require a ~10dB increase to 'seem twice as loud', though.

[jselectro]

dwk You are absolutely correct my prior posting is wrong. I have deleted it. You have to consider the inverse square law regarding distance to speaker however. I hope that is correct!
Thanks