Do I Understand This Correctly?

It doesn't matter whether you connect in series or parallel; that only affects drive impedance.

You won't get 3dB per driver, but at most 3dB per doubling. Four drivers would give 6dB (two lots of doubling). This assumes, of course, that adding a driver affects efficiency at all - I don't know as I am not a speaker expert.

Note that you have to compare like with like: sound power out for electrical power in.
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
It doesn't matter whether you connect in series or parallel; that only affects drive impedance.

You won't get 3dB per driver, but at most 3dB per doubling. Four drivers would give 6dB (two lots of doubling). This assumes, of course, that adding a driver affects efficiency at all - I don't know as I am not a speaker expert.

Note that you have to compare like with like: sound power out for electrical power in.

There are two ways of looking at this, power or voltage.
The voltage method gives 6dB per doubling (in parallel) so 4 drivers give +12dB for the same input voltage. This will draw 4x the power (6dB) so the power method gives only +6dB for the same input power. Amplifiers are voltage sources to a large degree, and speakers are "voltage sensitive", so in my opinion, the voltage method is correct. Think of it this way: If you take a single driver and measure, then add three more in parallel without changing amplifier settings, the SPL will go up 12dB and the power will go up by 6dB.

There is an additional factor that is not as easy to calculate, and that is how the drivers couple across frequencies. The coupling is best at low frequencies and lessens slightly at higher frequencies. You also get frequency dependent directivity effects at higher frequencies when dealing with an array.

Another thing: the effects of output impedance might start to become significant at really low impedances. IF you have a damping factor of 10, the output impedance is 0.8 ohms and the impedance of the 2 ohm speaker might be around 1.4 ohms above and below resonance, and perhaps 20 at resonance, so the amount of power "lost" in the output impedance becomes significant, and varies with frequency.
 
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Ron E said:
Think of it this way: If you take a single driver and measure, then add three more in parallel without changing amplifier settings, the SPL will go up 12dB and the power will go up by 6dB.
If that is what happens then that is a rise of 6dB in efficiency, which is what I said.

If you define 'efficiency' simply in voltage terms then you can get increased 'efficiency' just by using very low impedance voice coils.
 
So..

I'm dealing with an amp that operates @5vdc. The voltage can't increase much BUT there is a difference in SPL between 4 ohm and 8 ohm drivers.. (Shorter battery life/Same basic driver with different impedance)

I understand the resistance is more (lower) with a 4 ohm driver but wouldn't the increase in SPL be power? (Current)

I think you understand. Put another way, 1 of your 8 ohm drivers puts out 89db with 2.83VRMS, and the amp is delivering 1 watt. 2 drivers in parallel will put out 95db with 2.83VRMS but now the amp is delivering 2W. 4 drivers in parallel will put out 101db with 2.83VRMS and now the amp is putting out 4W.
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
If that is what happens then that is a rise of 6dB in efficiency, which is what I said.

If you define 'efficiency' simply in voltage terms then you can get increased 'efficiency' just by using very low impedance voice coils.

Efficiency is defined in %. Sensitivity is defined as an SPL for a given stimulus.

You actually didn't say anything about efficiency ;). I didn't even say you were "wrong", just that there is a lot more to the story that you are leaving out in a very incomplete answer.

As is often the case, the question is about something completely different, as in how to make the best use of 5 Volts and a battery powered device.... As always, the devil is in the details, and without details nobody can answer the question.

In a battery drain application, if you double or quadruple the current, the battery life does not go down by just 2 or 4. Look up Peukert's Law.
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
Two speakers instead of one will result in +6dB sensitivity if they are radiating into the same space. That is: +3dB to power, and +6dB to pressure from within the radiation space, with the difference manifesting in the radiation resistance.

If they radiate into different spaces power would still be +3dB but pressure would remain the same (0dB) from any one fixed location.