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post #1 of 5 Old 06-13-2013, 05:11 AM - Thread Starter
 
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I'm a little confused on this one. If I have a speaker and add more woofers in parallel would that increase the sensitivity? Or it would just mean each driver "sees" less power, less work per woofer? I assumed that adding drivers to a speaker would increase the sensitivity.

I discussed this with an EE and he said that sensitivity would not go up. If a single speaker is at 90 db at 1 watt and you add another single at 90 db it would still net 90 db total but the power would be divided. Is that true or gobbledegook?
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post #2 of 5 Old 06-13-2013, 05:47 AM
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Depends on your amp.

Assuming we are talking about one 8 ohm woofer vs (2) 8 ohm woofers in parallel (4 ohms)

If your amp puts out the same power into 4 ohms, as 8 ohms, then the gain is 0db. If your amp doubles it power into 4 ohms(which some amps do), then you should gain 3db. Most amps are somewhere in-between, putting out more at 4 ohms than 8, but not doubling, so a 1 or 2 db increase in sensitivity may be typical.

http://www.sengpielaudio.com/calculator-spl.htm

Be sure your woofers are 8 ohms before you try this. If you put (2) 4 ohm woofers in parallel, this makes 2 ohms which most amps wont handle well, and may even damage some amps.

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post #3 of 5 Old 06-13-2013, 05:47 AM
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Quote:
Originally Posted by Heinrich S View Post

I'm a little confused on this one. If I have a speaker and add more woofers in parallel would that increase the sensitivity?

Using the common definition of sensitivity which is SPL for a given applied voltage, the usual answer is yes.

It is possible to misconfigure the additional drivers so that they cancel other than add.

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Or it would just mean each driver "sees" less power, less work per woofer?

That requires a series/parallel configuration. A pure parallel configuration reduces the system's efficiency which will then draw more power from the ampliifer for a given voltage.
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I assumed that adding drivers to a speaker would increase the sensitivity.

The law of conservation of energy always applies. From a general engineering standpoint there is no actual over-all increase in SPL for a given applied power. However, many secondary effects can take place. One is the reduced impedance causing more power to enter the speaker system from the amplifier for a given output voltage or volume control setting. Adding more drivers in-phase will somewhat narrow the speaker's directivity, which means more SPL on-axis, but less SPL off-axis in most cases.

Depending... depending... depending.
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I discussed this with an EE and he said that sensitivity would not go up.

He was probably thinking about the law of the conservation of energy, which is valid in its way even though paralleling speakers usually causes the speaker array to draw more power, become more directional, and create more SPL on axis.
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If a single speaker is at 90 db at 1 watt and you add another single at 90 db it would still net 90 db total but the power would be divided.

Not true if the drivers are connected in parallel, radiate into the same space, and you drive them with a typical amplifier that provides a very low-impedance source.

If one speaker driver were an 8 ohm load, then the two drivers in parallel would make up a 4 ohm load which would double the power draw right there. If the two speakers were radiating into the same general space in phase then they would be more directional than one driver which would also provide an additional increase in directionality and might add a dB or more SPL on-axis at the cost of off-axis sensitivity.

Just to further complexify things, the directionality and impedance effects would depend on frequency. The impedance effects would depend on the enclosure and the mounting of the two drivers in the enclosure. The directionality effects would also depend on the room, location in the room and the distance from the listener to the speaker.

Acoustics can make simple seeming questions end up with complex answers.
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post #4 of 5 Old 06-13-2013, 08:49 AM
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i thought i'd always read that for a given amount of system power, with all other contributing and detracting factors aside, simply doubling the cone area yielded a theoretical 3 db gain. doubling cone area and doubling system power (eg. as you would theoretically do by paralleling two 8 ohm drivers on the same amp channel that you originally had a single 8 ohm driver wired to) resulted in a gain of 6 db. the situation in which you gain nothing is wiring in series. you pick up 3 db for doubling cone area, but lose 3 db for cutting system power in half by doubling the impedance seen by the amp channel.

in addition to being pretty sure that this is what i've always read, this is how winisd and unibox both model things. add a second driver but keep total system power the same (ie. each driver is seeing half the power) - gain 3 db. add a second driver and keep the amount of power seen by each driver consistent (ie. double system power) - gain 6 db.

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post #5 of 5 Old 06-13-2013, 11:45 AM
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Originally Posted by absolootbs View Post

i thought i'd always read that for a given amount of system power, with all other contributing and detracting factors aside, simply doubling the cone area yielded a theoretical 3 db gain. doubling cone area and doubling system power (eg. as you would theoretically do by paralleling two 8 ohm drivers on the same amp channel that you originally had a single 8 ohm driver wired to) resulted in a gain of 6 db. the situation in which you gain nothing is wiring in series. you pick up 3 db for doubling cone area, but lose 3 db for cutting system power in half by doubling the impedance seen by the amp channel.

in addition to being pretty sure that this is what i've always read, this is how winisd and unibox both model things. add a second driver but keep total system power the same (ie. each driver is seeing half the power) - gain 3 db. add a second driver and keep the amount of power seen by each driver consistent (ie. double system power) - gain 6 db.

The problem is that it is difficult to simply double the cone area. For example, you are probably going to also change the mass of the cone, and the compliance of its suspension at the same time. For other reasons, you are probably going to end up changing some details of the voice coil and magnet assembly.

So if you are looking for brain exercises that have very little to do with reality, go and double the cone area and think hard about what that will do - for what good it will do you.
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