are 3db important?

Not everytime you add a sub...it's everytime you double the number of subs increases 3 db's.  How important, and how much do you need to add?  It depends on where you're starting.  Decibels are on a logarithmic scale, the way we actually hear.  A typical subwoofer with only 1 watt of power input will produce 87 decibels.  You can add just 1 more watt of power and increase the db's by 3.  But you didn't just add one watt, you doubled the power.  Now it's at 90 db.

Now, how do you get 93 db?  You can either double the power or double the sub cone area.  Let's keep this with one sub and we'll also ignore any other loudness gains due to enclosure, porting, or cabin gain.  Just keep it a simple 1-sub system and look at its increases all by itself.  So, you give it 4 watts to get to 93.  At this point you may be thinking "At this rate this sub will be jamming with my little amp!.  93 decibels with only 4 watts!"

Well let's see how this works out:  give it another doubling of power, to 8 watts.  96 db.  16 watts:  99 db.

32, 64, 128, 256, 512, 1024, 2048, 4100 watts............now you're at 123 decibels.

What the...?

Doubling of cone area works like this, too.  When you're starting with only one sub and double the number to two, the increase of 3 db is great.  But then it takes an increase to 4 subs to get another 3 db.  Go to 8 subs to increase just 9 decibels more than the original single sub.

3 db is considered a noticeable increase in loudness.  That is far less than double the loudness, as you would have to increase power X 10, or a total gain of 10 db, to get a perception of double the loudness.  Look at the case above where we increased the number of subs from 1 to 8 to achieve a 9 db gain....we just about doubled the loudness.

IMO, achieving 140 decibels is very difficult with one sub as you can see with the math above.  Taking no other loudness-raising factors into consideration, we had 4000 watts going to one sub and were getting only 123 decibels!   This is where the total system build and vehicle comes into play.  The great ones (like Jeff Chilcott ;)  constantly work with designs and tweaking to add a little more to the output.  Why, just  the most imperceptible flexing of the enclosure will lose valuable db's.  It's an art.

BTW, I've never built an SPL system, but it's something I'll want to do sometime if I can afford the luxury of this kind of playing around.  It's big bucks.

Are 3db important?  It depends on which 3 they are.  If they are the three that make you deaf, they are the most important ones in the world. 

t-roy, if one speaker @ 400 watts can generate 116 db, then 2 speakers @ 400 watts each with all else being equal should generate 119 db.

This is a good thread to post some of the reference posts found in threads from the past that I've saved in 'my documents'....these are some of the words of wisdom that have helped increase my interest in this field:

1.  re: the loudness discussion.  In general, an increase of 1db is by definition "noticable".  An increase of 10db is perceived as "twice as loud."  It often requires 10X the amplifier power to achieve this, or 3000 watts to sound twice as loud as 300 watts (assuming the same sound is being perceived from the same loudspeakers.)  Two sources producing equal loudness added together generate an average perceived increase of +3db.  Thus, two loudspeakers being supplied the same tone at the same relative power level will generate net SPL of approximately +3db greater than either one alone.

DYohn

2.  (Note that what I have written here is a very gross oversimplification and in no way is intended to define how all systems work, but to give you an example to start from.)

First, understand that the majority of music happens within the range of a standard piano, which is 27.5 Hz (A0) to 4186 Hz (C8).   The human ear can theoretically hear any sound wave between about 20Hz and 20KHz, but most humans over the age of 3 cannot actually hear much lower than about 25Hz or much higher than 17KHz.  Most sounds higher than about 6000Hz are sibilance, harmonics and other non-musical tones.

Sound is divided into octaves (and other intervals, but for now let's stick with octaves.)  An octave is a doubling of frequency.  So, if 20Hz is the lower limit of hearing, the first octave would be 20-40Hz.  Second is 40-80Hz, and so on.  Most music exists mainly in the fourth through eigth octaves (160Hz to 5120Hz.)  The area our ears are most sensitive is between about 400Hz to 4Khz, which is called the mid-range, and is actually further divided into lower, mid and upper midranges.  Our ears are MOST sensitive to 3-4KHz, and too much loudness in this area is interpreted as "harsh" or "shrill" or some other negative concept.

SO, now to answer your question.  In general, bass is anything lower than 400Hz and treble is anything higher than 4Khz.  To translate that into speaker terms, here's an idea of how it would work.   Woofers generally handle bass and lower midrange frequencies.  So-called subwoofers are actually just woofers that have been optimized for the first two (or so) octaves.  Since sound above the second octave is directional and in the first two octaves (or lower, which we cannot hear but can certainly feel) non-directional, subwoofers are generally crossoed over at 80Hz.  Woofers handle 80Hz up to somewhere in the lower-midrange, say 6-700Hz.  Midrange drivers handle 6-700Hz through the rest of the musical spectrum to about 4Khz.  Tweeters handle the upper registers and sibilance areas above 4Khz.

There are of course tweeters that can handle the midrange, and midranges that can handle the upper registers.  There are woofers that operate fine through the entire piano spectrum of music.  There are speakers designed for extemely narrow bands and are highly specialized (like subwoofers or "super tweeters.")  And also, just because a speaker can reproduce a frequency does not mean it sounds good doing so.  There are many so-called full-range drivers out there that can indeed reproduce the full range from 20-20Khz.  But they cannot necessarily do so efficiently nor with very much musicality. 

So-called "flat" frequency response means that all frequencies are reproduced with equal intensity.  Many people think this sounds artificial or even bad, as it tends to emphasize those areas where our ears are already more sensitive and to lack authority in those areas like bass or upper registers where our ears have trouble.  That's why most people add woofers to create more bass, or place tweeters near ear level, or use EQ devices to compensate for our human shortcomings.

Anyway I hope this gross overview helped you.  There are lots of web pages out there with additional and much deeper information.  Try doing a Google search for something like "frequency ranges loudspeaker."

Cheers.

(DYohn 5-20-05)

3.  Gotta chime in here because some common misunderstandings about power have been driving me bonkers lately:

You don't need a lot of power to make good speakers sound good.

You don't need to match RMS ratings of speakers to make them sound 'right.'

A doubling of power only yields a 3db gain in SPL.

3db is small.  It takes an increase of 10db for material to sound twice as loud to our ears.

You only use the EXTRA power when you're cranking it past what the weaker amplifier can reasonably produce.

Underpowering DOES NOTHING BAD TO SPEAKERS.

Lets use Master's system, lets say 70x4 and the 35 watt system as an example.  Let's assume the speakers are all 90db efficient.  Let's assume at full rated power, both channels driven, the amplifiers have similar distortion characteristics.

Our question:  how much of a difference does the big bad extra 35 watts make?

Well, 1 watt will produce 90db.  90db is loud.  Prolonged exposure to 90db can cause gradual but premanent hearing loss!

2 watts will produce 93db.  93db is slightly louder than 90db.

4 watts will produce 96db.

8 watts will produce 99db.  Here we almost have a doubling of perceived volume!

16 watts will produce 102db.  Now we're over 100db.  This is quite loud.  Many sources suggest not to listen above 100db for more than 15 minutes or you risk speeding up hearing loss quite rapidly. 

32 watts will produce 105db.  All authorities on hearing loss say that music should not be listened to above this level for longer than 10 minutes.

-----we've now reached the point where distortion will start entering the system via the pushed 35w amplifier ---- however, and this is important too, it isn't like the amplifier just dies at 35 watts or just stops there.  It keeps putting out more power, it just adds some distortion.  Better quality amplifiers will add distortion above rated power at a slower rate.  And we've still got max power, remember.  If the amplifier needs to reach deep for a nice 107db blast, it usually can! -----------

Alright, lets test that big bad 70watts.  Oh, it only adds another 3db.  It can produce 108db within its rated distortion level.  Woopie.  You gained 3db at a point where you shouldn't be listening to music for longer than handful or two of minutes anyway.  Good job. 

From this you might be saying, "then why do I always hear that underpowering harms speakers?"  The author of www.bcae1.com said it best on this topic.  It was something like, "underpowering doesn't harm speakers, idiots with the volume control who clip the amplifier harm speakers."  Check out that page.  Set your gains correctly and know the point at which you can't turn the volume dial and you won't have to worry about this problem.

Also, you may be asking yourself, "then why can't I just use my 20w rms headunit?"  The answer is because not all amplifiers are created equal and manufacturers are dishonest with their ratings.  You simply cannot pack powerful amplification into such a small package.  Period.  With a full-range signal w/ all channels driven, distortion will be much higher than most (almost all) manufactureres list at their rated rms number.  Even then, a good amp will be much cleaner.

(caveat - as class d / chip amps become more popular and better researched, we may see some class d head units that can compete with some separate amplifiers... something tells me that head unit manufacturers aren't really too interested in this though). 

kfr01  1-19-05

4.  Did a quick search, and got this equation for increase in db:

increase in db = 10*log(p2/p1)

where p1 is the reference power, p2 is the new power level, and log is log base 10.

if we plug that in, increase in db = 10*log(400/50)

increase in db = 9.03089

mikew04

A thanks to DYohn, kfr01 and mikewo4 for not minding my reposting of their past material (as if I asked permission). 

Learn from what you read, reference to substantiate it, and then share it with someone else.  Like I say in my sig.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

my setup hit 134.4db with just 450RMS (Rockford Fosgate 225.2 into two HX2's) off of my cars stock casette player (using a line level converter to get the RCA input to my amps) in the trunk of my 325is.

SO why did i say this?  Calculating sound pressure levels is not as easy as a simple equation because obviously my setup does not conform.  I also have heard that doubling the cone area or doubling the power delivered increases sound by 3db.  3db according to what i heard is the limit of perception by the human ear.  What this means is that most people cannot differentiate between volume levels until it reaches a minimum 3db difference.

But to answer the original question a 3db change is harder to achieve the louder it gets but at the same time means more the louder it gets.  Here is a nice little page I found that does some decent explaining:

http://www.mhhe.com/physsci/physical/jones/ol15-5.htm

Nope.  Read what was said in posts above.  1 db is the perceptive level...that's what signiifies the measurement of the decibel.  Otherwise, why make it 3 decibels?  What, then, would a decibel signify?  1/3 of the limit of perception by the human ear?

You also missed some other parts:  the increases in sound pressure level had to do with the driver alone, without added gains from this and that, etc. .....as explained above.  Your actual reading will be higher than what the driver alone will deliver.  The reading you got is quite normal.