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A Question on Speaker Driver Efficiency

I have been tweaking my guitar amps, by upgrading the speakers.

I installed a larger speaker (was 8" now 10") in my bass amp, but I made sure it was very efficient - net result
- not only is the bass much deeper sounding,
- but because the new driver was more efficiant I now play at a lower volume.

So I am now considering upgrading my other amp (i.e. used for my 6 string) and got to thinking about building a new cabinet that houses two speakers.

I know that connecting the speakers in ...
- series will double the impedance, i.e. 2 x 4 ohms would have an onverall impedance of 8 ohms
- parallel will halve the impedance, i.e. 2 x 16 ohms would have an onverall impedance of 8 ohms

But what I have not been able to get my head around is...
- what will each connection method (i.e. series or parallel) have on the "combined" sensitivity rating?
- e.g. if both speakers are rated at 96db sensitivity, will the overall sensitivity change due to the connection method or remain at 96db?

Since I can get 4 ohm or 16 ohm drivers - which connection method would be best? series or parallel?

in case it is a factor
- the amp is 15 watts into 8 ohm
- I am looking at employing two identical drivers each rated at 96db sensitivity
- 96 db (or higher) is the target for the combined sensitivity

Any help is appreciated - Many Thanks Steve
williewonka

Showing 5 responses by toddalin

toddalin's avatar
toddalin
309 posts
 
If the amp puts out x watts at 8 ohms, and you split it between two speakers totalling 8 ohms, each speaker sees the half the power and puts out the same dB level.  When you add them together, the net result is the same.

That said, I would think that, if the net volume in dB for the speakers truly are the same regardless of the impedience, two 16 ohms speakers would be the better choice.

Speakers in parallel work independent of each other.  Speakers in series work together and can have an effect on each other.

Also, speakers with higher impedience tend to have a high Bl, and therefore follow the signal more accurately.
toddalin's avatar
toddalin
309 posts
 
I think you missed the point.

He uses two 4 ohm speakers to produce 8 ohms or two 16 ohm speakers to produce 8 ohms.  Nothing else matters!

If the amp put out 100 watts at 8 ohms, it does so into either of the two noted loads..., because either load totals 8 ohms and the amp puts out 100 watts at 8 ohms.

As for the speakers.  There are two and they share the 100 watts between them.  If they have the same impedience, they share this 100 watts equally, so they each get 50 watts, regardless of whether they are in series or parallel.

Do you think that one will get 25 watts and the other 75 watts or some other crazy combination if they have the same impedience and share the power equally?  That's nuts.

As for Bl, wrong again.  

The Bl is the product of magnetic field strength in voice coil gap and length of wire in magnetic field (T•m), and has a direct impact on how well the speaker reacts to the signal.

A higher impedience voice coil will typically put more wire in the voice coil and therefore has the ability to result in a higher Bl.

But, just look at the TS parameters for JBL speakers of the same type in 4 and 8 or 8 and 16 ohms and this becomes apparent.

e.g., 2265G (4 ohms) Bl=13.6, 2265H (8 ohms) Bl=19.5
2268G = 15.2, H=21.5
2269G = 19.2 H=26.4

etc.
toddalin's avatar
toddalin
309 posts
 
"Speaker sensitivity is measured in decibels per 1 watt per 1 meter, but is usually referred to as just decibels [source: JBL]."

For an 8 ohm load, this equates to 2.83 volts. But for a 4 ohm load it’s 2.00 volts and for a 16 ohm load its 4.00 volts. Because the OP said the speakers have the same "sensitivity," this has been accounted for.

And yes, JBL does put more wire and turns in the gap for their higher impedience speakers leading to a high Bl.

In fact, there are cases where the higher impedience model looses travel (Xmax) because of this difference.

And seeing as how JBLs are the only make of speaker that matter in my household, I stand behind the logic that a higher impedience typically leads to a higher Bl.

As to the effects of inductance, I did note that there is interaction between the two speakers in a series set-up (goes beyond inductance) that does not exist in the parallel set-up, and for these reasons I would choose a parallel set-up.  
toddalin's avatar
toddalin
309 posts
 
The L19 made it’s debut in the 1979 catalog. Seeing as how I’ve been into JBLs since the 60’s, it sounds like you are the newcomer.

I've even developed and make products for use with classic JBLs.

https://youtu.be/pbbS0QYK2_o

http://www.audioheritage.org/photopost/data//500/medium/Resized1.jpg

http://www.audioheritage.org/photopost/data//500/medium/Tweeters1.jpg

I never said that the higher impedience meant that the speaker would have a higher Bl, but allowed for it, and showed several examples to that effect.

You’ve not shown one model that demonstrates otherwise.

As for sensitivity, you are using your definition,. and I am using mine, and both exist in many places on the internet.

How can you know that the manufacturer for the speakers from the OP isn’t using the same definition that I am using?

BTW, when someone types "You’re out of your league," this typically indicates that they are at the limits of their own understanding and just don’t want to show their ignorance by pushing it further or are too head strong to consider another point of view.
toddalin's avatar
toddalin
309 posts
 
I look at PV, Eminence, and others..., and yes, for the same models, the higer impedience version typically has the higher Bl.

So, contrary to what some would say, I guess that JBL is not the only one who does this and the OP is advised to check the specs for the selected speakers for himself.

Obviously, someone else around here hasn't even bothered to look at the various manufacture's similar offerings to confirm or deny their beliefs.
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