sensitivity rating of pushpull sub

[fergs1]

greeting Ladies and Gentlemen, just wondering if you have two 8ohm speakers at individual sensitivity of 88db/1watt,when used in a pushpull alignment in a ported box presenting a 4ohm load, is the sensitivity increased and if so by how much

cheers fergs

[sreten]

Hi, goes up 6dB/Volt, 3dB/Watt, /sreten.

[Cameron Glendin]

not sure about this but I read somewhere that push pull designs give a better quality bass with higher power handleing but not much efficiecy gain, simply a box 1/2 the size of a single woofer box. 6 db @ 1 w gain if 2 drivers share a box twice the size, 1/2 the impedience.

I have been wrong before but am curious to learn the truth!!!!
I have built a few push pull subs receintly and for quality alone well worth the effort

[sreten]

Hi,

I misread the question, my answer refers to push push, force cancelling.
(Though it is possible to make a sub with two bass units and reverse
the mounting of one of them to try and reduce 3rd harmonic distortion,
this could be referred to as a push pull arrangement.)

Isobaric (or clamshell) obviously the sensisitivity does not increase.
It stay the same - 0dB/2.83V change, but -3dB/Watt efficiency change.
The reduced efficiency is what gives you the same performance in a
cabinet half the size.

/sreten.

[Rademakers]

reduce 3rd harmonic distortion

2nd (even), as the non-linearities of the drivers are opposite.

Due to the lack of distortion this configuration can be percieved as being less loud as a "normal configured " double.

Wkr Johan

[paulspencer]

It's not clear what you are asking, hence the responses are all different. If you are talking about isobarik where only one driver radiates to the room, then you gain nothing in efficiency. The thermal power handling increases, but this increase is used not for extra output but to drive a second driver to give you a smaller box.

If you are talking about any other push pull arrangement where both drivers radiate to the room, then you get a 3db increase in efficiency due to doubling of driver area, and 3db extra output due to double the power handling. The voltage sensitivity on paper will increase by 6 db but this is on paper only (and misleading) since 2.83V into 4 ohms is actually 2 watts. This is why when 4 ohm speakers are given a spec for voltage sensitivity, it is higher. A typical bookshelf speaker might be 87 db sensitive from 2.83v or from 1w, being 8 ohms. Now look at an MTM floorstander using the same drivers. The 1w 1m sens might be 90 db, but since it is now a 4 ohm speaker, the 2.83V rating is 93 db - don't be fooled, it's not really that efficient.

Push pull is not relevant to efficiency, unless by that you are referring to isobarik.

[fergs1]

greetings Ladies and Gentlemen, thankyou for taking the time to answer. I'd better clear up the sub topology even if I don't know what you call it. Firstly it is a ported box with two vifa w-26mr-09-08 that are clamshelled together with one speakers backside is sticking out of the box while the other of course is inside the baffle. What do you call this arrangment and what happens to the efficiency now?
cheers fergs

[e-side]

Originally posted by fergs1
greetings Ladies and Gentlemen, thankyou for taking the time to answer. I'd better clear up the sub topology even if I don't know what you call it. Firstly it is a ported box with two vifa w-26mr-09-08 that are clamshelled together with one speakers backside is sticking out of the box while the other of course is inside the baffle. What do you call this arrangment and what happens to the efficiency now?
cheers fergs

That's an isobarik configuration. When you wire the drivers in parallel, as you mentioned in your first post, then:
- sensitivity will be the same as one single driver, 88 dB;
- Nominal impedance becomes 4 Ohms;
- Power handling for the isobarik configuration will be 2x power handling of one single driver;
- Vas will be half that of a single driver, so the required enclosure volume can be divided by 2. Because it's a ported box changing the clamshell will lead to a misaligned system.

Isobarik is a very inefficient way of using the drivers, unless box size is a priority (usually car audio).

[paulspencer]

Also you might be losing some output, because half of the power is going into the second driver which isn't radiating any more bass. If you go from 8 ohms with one driver to parallel hence 4 ohms with two, then you probably get more power of the amplifier in total, but half of it won't produce any SPL.

Lets say your amp puts out 50w into 8 ohm, but 80w into 4 ohm. Now you have more power available for the amp, but only 40w is actually producing SPL, but in practice this represents a small difference in max SPL