speaker sensitivity question

[anthillrich]

dumb question, but...
How would using two speakers(in parallel, same ohms, proper Z setting on amp) of different sensitivities affect volume. in other words,
if i have a speaker of 98db, and couple it with a less sensitive speaker, say 96db, would the amp have less overall vol than using the 98db by itself ?
thanks,
rich

[Robert M. Martinelli]

dumb question, but...
How would using two speakers(in parallel, same ohms, proper Z setting on amp) of different sensitivities affect volume. in other words,
if i have a speaker of 98db, and couple it with a less sensitive speaker, say 96db, would the amp have less overall vol than using the 98db by itself ?
thanks,
rich

You're talking about dBs alone, and that's not the correct way to put it IMHO.

Things become clearer if you use the proper measurement unit.

The proper measurement unit for speakers' sensitivity ( or efficiency ) is dB/W @ 1 m ( the dB a speaker puts out when fed with a power of 1 W, with the sound pressure meter 1 m away from the speaker ).

Keep in mind that a +3 dB difference is what you get when doubling the amp's power ( all other factors being equal ).

To put it in another way, a 50 W amp connected to a 96 dB/W @ 1 m speaker will sound double as loud if compared to the same amp connected to a speaker with a sensitivity of 93 dB/W @ 1 m ( all other factors being equal ).

Answering to your question is not easy though, because using two speakers involves phase/frequency issues you don't have to deal with when using only one; being the speakers side by side, even if they're in phase electrically, some of the frequencies could reach your ear attenuated due to in-air phase canceling issues.

With two 12" units side by side, all other factors being equal, I'd expect slightly reinforced lows and highs and slightly attenuated mid-highs ( the amp would sound "tighter" IMHO ).

The same would happen using a couple of 98 dB/W @ 1 m speakers, all other factors being equal, the amp would seem ( just a tad ) louder.

( similar "dB vs. frequency" curve as above, translated by 2 dB on the Y-axis ).

Hope this helps

Best regards

Bob

[Enzo]

The efficiency rating is just how much noise they make for a given signal. if you have one speaker it makes loudness X. If you have two identical speakers it will be louder, by I forget, 6db?. If you have two mismatched speakers like that, one will be a couple db louder than the other. Both will be making sound though, so the two will be louder than either one alone. it will just be a little less loud than two of the higher rating.

The less efficient one won't make the other one any less loud. I am assuming the power level remains stable. Kinda hard to do since your speakers alone have a different impedance than when together.

To put it in another way, a 50 W amp connected to a 96 dB/W @ 1 m speaker will sound double as loud if compared to the same amp connected to a speaker with a sensitivity of 93 dB/W @ 1 m ( all other factors being equal ).

It would be 3db louder, yes, but that isn;t doubly loud. Double loud is 10db.

[Robert M. Martinelli]

Sorry Enzo,
you're of course right.
By saying "double as loud" I wanted to say that a 50 W amp connected to a 96 dB/1W @1 m speaker is as loud as an amp double in power ( 100 W ) connected to a 93 dB/1W @ 1m speaker.

My apologies if I expressed the concept in such a misleading way.

( OTOH I'm glad you came to my same conclusions )

Cheers

Bob

[Steve Conner]

For two speakers, you can't add the dB ratings, because dBs are logarithmic. I'll demonstrate the math for anyone interested.

Let's say we have two speakers, of 96 and 99dB/1W/1m sensitivity.

The 96 connected to a 50w amp will give 96+ 10*log(50) = 113dB SPL. Ouch.
The 99 connected to a 50w amp will give 99+ 10*log(50) = 116dB SPL. Ouch again.

If we connect both, then each gets 25W, so they produce 110 and 113dB respectively. But what is the SPL from both speakers combined?

To add dB we must convert them to linear SPL, add them (coherently since both speakers produce the same signal) and convert the result back to dB. So the answer is 10*log(10^11 + 10^11.3) = 114.8dB SPL.

However, according to Bill Fitzmaurice there is a 3dB gain from mutual coupling when two identical speakers are placed within 1/4 wavelength of each other. So we could get another 3dB of bass on top of that, but I expect the effect would stop working by the time you got to the upper midrange.

[anthillrich]

thanks for the replies.
if i look at the math that Steve used as an example, then my amp would be louder using the higher sensitivity speaker alone. 116db vs 114.8 using both.
not alot of difference tho.

I wanted a fuller sound, louder wasn't a priority, but it wouldn't hurt.
So i have been experimenting with this a little. i added another 1x12 to my 1x12 combo, and the difference was not real big. To be fair, i only have one other 12 with matching ohms, so i did'nt have the chance to sample other twelves in this position. im using an old jensen with a newer celestion.
i still would like to try other speakers to be sure, but i dont have that luxury right now.
it seemed the amp did "tighten up" a bit. A little more tame actually . Im not sure i even liked it in a band situation. I think i've gotten used to that "open - loose" sound(sort of on the edge of control).
Dont get me wrong, there's nothing like the sound of a 4x12 roaring away, i love that sound also. It's just not practical for me with my current needs from a rig. Most places i get to play wouldn't have room on the stage for a 4x12 cab.
thanks for the info guys.
rich

[Robert M. Martinelli]

No one expected the difference in loudness to be a big one; remember that 3 dB is considered the minimum variation in SPL the human ear can perceive...

I thought it would have sounded a little "tighter" as I was saying, because of the different directional behavior of sounds at different frequencies. The lows have a greater wavelength and they do get reinforced some, the upper mids around 1.3 KHz are likely to suffer some from in-air phase canceling ( considering the speed of sound in the air is some 330 m/s @25 °C and with the speakers side to side with their centers some 35 cm apart ) the highs come from the inner part of the speakers and they tend to be "directional" so they don't suffer any in-air phase problems IMHO.

Cheers

Bob

[tedmich]

I too have made a small box, but I built a tuned port B4 enclosure, (like this Omni 10) and added a single small horn tweeter on an L pad; the bass response is very good down to ~60Hz and I add back the highs to taste with the L pad. The super cheap Pyle Pro PH25 3" x 3" Horn Tweeter works great as its response tops out at 15k Hz, and its very efficient at 100dB/watt. I did have to reinforce the spider assembly, but what do you expect for only ~$7