Multiple Drivers to Increase Efficiency

I have asked this question myself before as I am sure others have but never gotten a conclusive answer. Many people on this forum and other places when asked increasing efficiency of a speaker say something like "don't worry, just add more drivers".

So my question is does this really work? Lets say to a given amplifier I connect one driver. It has an impedence of 8 ohms and a rated sensetivity of 87 db 1w/1m. Now lets say I take four of these same drivers and use them in two diffrent ways. One is to wire them in a series parallel combination so that they still present an 8ohm load. The other is to connect them in parallel so they present I beleive a 2 ohm load. Would either of these combinations be more efficient than the single driver?

I think the problem with this question is that it is somewhat ambiguous. What I am trying to get at is how loud will the speakers be in my room. I have a relatively low powered amp, so would adding more drivers give me more volume or just a less strained sound cause each driver is doing less work?

So my question is does this really work?
For low frequencies, say below about 300Hz, it works really well.
Lets say to a given amplifier I connect one driver. It has an impedence of 8 ohms and a rated sensetivity of 87 db 1w/1m. Now lets say I take four of these same drivers and use them in two diffrent ways. One is to wire them in a series parallel combination so that they still present an 8ohm load.
This will result in a little under 6dB increase in On-Axis efficiency for the same amount of total power.
The other is to connect them in parallel so they present I beleive a 2 ohm load.
This will also result in 6dB increase in efficiency but because of the lower impedance, assuming the amplifier is not current limitted, you'll get another 6dB raise in SPL because the extra power being supplied to the speakers.
Would either of these combinations be more efficient than the single driver?
Yes, both.
Comb filtering is where certain frequecies or more commonly most frequencies above a certain point develop peaks and nulls within the listening area so that you might be sitting in a particular area in the room that is a null at say 1kHz this is to say that in that exact spot in the room because of the effects of comb filtering related to the particular speaker setup, any 1kHz tones are acoustically cancelled out and you will not hear them or they will be very quiet and partially or fully out of phase. If you mount the drivers physically very close to each and keep the array to low frequencies (I suggested below 300Hz earlier in this thread) then you shouldnt have trouble with comb filtering. Off axis, some degree of comb filtering is inevitable and most of the acoustical efficiency gain is lost getting worse the further off axis you move.

Thanks that clears up comb filtering for me. Would the loss of efficiency as you move off axis apply in a single driver speaker as well? If not could you possibly explain why there is a diffrence.

I feel this need to build a high efficiency speaker and have been going back and forth between using a pro driver and gaining efficiency but losing bass extension, or using hifi drivers and getting the bass extension and possibly getting the efficiency by using multple drivers. Then sometimes I just start thinking about wheather I even need a high efficiency speaker or if 90 db sensetivity might be good enough.

markkanof said:
Thanks that clears up comb filtering for me. Would the loss of efficiency as you move off axis apply in a single driver speaker as well? If not could you possibly explain why there is a diffrence.

It does also apply to single drivers but to a lesser extent. The difference is that with an array you are dealing with the effects of comb filtering and the addition of multiple acoustic waves.

I am talking only multiple woofers. Meaning there will be a tweeter, but only one of them. Also from what I have learned from AudioFreak there may be a midrange as well but that is undecided at this time. He mentioned that multiple drivers work well below about 300 hz.

It will be for domestic application, a home theater to be exact. As for quality, I guess that depends on your definition of quality. I don't claim to be an audiophile mostly because I am still young and not had the oportunity to own or listen to many expensive systems. I am usually quite pleased with the klipsch or polk products at the local electronics store. So that would be my definition of quality. And since I am using this for movies and not much music absolute accuracy is not necassary. Basically I like the idea of the amp not having to put out a lot of power to create a lot of output from the speakers. And also the ability to these speakers to fill any room I may have in the future with plenty of sound. Maybe it is overkill and I should just build regual efficiency speakers, but there is something about the concept of high efficiency that keeps pulling me back to it.

Just another point of view (hopefully) to make the multiple driver efficiency thoughts a bit easier and to explain somewhat more in detail why it is true what said AudioFreak:
Remember: Efficiency is the relation of what You put in and what comes out - hence regarding speakers - the relation of (electrical) input to (acoustical) output power.
In order to see clearer and to understand where from comes the increase of efficiency in multiple driver arrays dont let`s mess up things by comparing at the same time single to multiple driver efficiency also in regards of different input power (caused through eventually changed total impedance of the multiple driver connections) and therefore let us assume we put in a multiple driver array (regardless if serial, parallel or serial/parallel connected) the same amount of electrical power as in a single driver.
What happens let`s say in case of a 4 driver array (drivers are electrically and mechanically identical and if serial/parallel connected in equally valued impedance groups).
Regarding electrical power each driver consumes 1/4 of the power of what the amp. delivers. The acoustical output power of each one driver of the 4 driver array is one forth of what would output the single driver alone. Hence as we have 1/4 of input power and 1/4 of ouput power in relation to the single driver, we have no increase in efficiency so far.

This is true ONLY as long the 4 drivers are FAR AWAY from each other (many meters) AND as long the wavelength is SHORT IN RELATION to the drivers distance.

All this may appear very trivial up to now (and actually is) but to get a better imagination about what happens with efficiency in multiple driver arrays to view things this way may help to better understand what`s going on.

Now, what about acoustical output power and wher does the increase in efficiency from multiple drivers arrays come from actually?
The answer is from "acoustical coupling". What does this mean?
If we are going to shorten the distance between the 4 individual seperated radiating drivers and put them increasingly further together until they are united (e.g. in one box), depending of the wavelength of the radiating frequency (as lower the frequency as better the "coupling") the sum of the moved air increases with decreasing distance compared to what the 4 drivers individually could move if they were still seperated.
The effective radiating diaphragm area is increased this way and therefore comes the increase in efficiency.
Long thread, simple conclusion: The efficiency of multiple drivers increases as narrower they are mounted and as lower the
frequency. Thus this effect is frequency dependend also and the reason why in general this happens for low frequency drivers only.
This is somewhat hard to understand (at least it was for me) and it needed quite some time until I "got" it.

All clear-sights removed now??
Ok Cristoph, for a holiday in Jamaca, can you now explain co-entrant horns with multiple drivers at different frequencies resulting in a virtual single driver a la Renkus-Heinz... ( grins!)
Hi pinkmouse,

that`s what I call a question!:confused:
Though I`m not so familiar with horns I don`t see any reason why the same rules of the coupling effect should not apply to this as well (and I doubt that this is going to be a "virtual single driver").
I have to go to bed now but will think more thoroughly about it and come back about it later (am I right that You already know the answer...?).:)
By all means: what`s the frequency range of the named horn?
With "different frequencies", do You actually mean that all the single drivers are working in different frequency ranges at the same time (crossover) or do You mean they are doing all the same but viewed regarding versus frequency??
What is the size (diameter of radiating area) of the single drivers?

And for the holidays: if You don`t mind, at the moment I would prefer them in Brazil :D
Cocolino could you or someone else elaborate on the closeness of the drivers. I assume this is why the arrays are only to be used for lower frequencies, because the drivers must be closer together than the length of the wave they are producing. Does this closeness refer to each drivers closeness to the one nearest it, or for example in a four driver array, the top driver must be within a certain distance of the bottom driver? Also, what is a good distance to high frequency cutoff ratio. For example I believe the length of a 500hz wave is about 27 inches, so how close should the drivers be to each other. I am asking both if there are formulas to figure this out, but I suspect there are not, so I am also asking what is a good approximation. Could the speakers be 13.5 inches of each other about half the wavelength or should they be closer say one fourth the wavelength, etc. I think my question is clear now, so I will leave it at that.



2002-01-19 9:25 am
Hi Mark,

One of the ideas behind multiple stacked drivers is to try to create a cylindrical wave form. Look at this link to a MacIntosh related site.

Here are some quotes from that site to help explain the thought.

Multiple tweeters can be used to increase power handling, but using several tweeters that cover the same frequency range is normally a bad thing to do. A short column of closely spaced tweeters is very directional along the long axis, but what if the column runs from floor to ceiling (theoretically to infinity)? Then the column is not vertically directional, except beyond the floor and ceiling where output is not wanted. The near field is extended by the cylindrical radiation pattern. Horizontal dispersion is, of course, wide. This column arrangement is also referred to as a line source or line array.

The first column was constructed by cutting off the top and bottom of the round mounting plate for some Philips 1" soft dome tweeters. This kept the center to center distance as small as possible. The tweeters were mounted on a 10" wide board. The width was selected to support the lowest frequency required of the domes. A column of 25 could just be fitted between the floor and ceiling of our listening room. The tweeters were wired in series-parallel. Ideally, the column should be continuous without any spacing between the active tweeter diaphragms. Unfortunately, flat diaphragm or continuous ribbon tweeters exhibit high distortion at high listening levels compared to 25 or more individual soft dome tweeters. I also made a column of 36 cone tweeters to use for comparison. The 36 tweeters have much closer diaphragm to diaphragm spacing, but did not have as high a power handling or as good dispersion.

Ultimately, you have to try your concept in real life and see what it sounds like.

Good Luck

Rodd Yamashita

Sometimes I irritate people on this forum because I tend to operate from a base of fact rather than opinion. There are a lot of folks that want to jump into a forum and input their two cents worth. That they don't know what they are talking about has never registered with them. In addition, people go off on tangents and start injecting things into a thread that really have nothing to do with the question that started the thread. Things can often go downhill rather quickly and the thread becomes useless. Notice the injection of horns into this thread; this does nothing in terms of providing an answer to your question. Before you know it, someone is going to suggest that you pay attention to the directionality of your speaker cable.

An opinion that has recently been presented is the concept of a cylindrical wave front that some believe can be launched by a vertical line array. I'm not going into detail here but I do want to say that acoustical waves do not combine in such a manner; the concept of a cylindrical wave created by multiple drivers is an illusion - you do not want multiple tweeters and an array of multiple mid-range drivers must be approached in a reasonable manner.

I want to offer help, in this area that I am very good at, but want to avoid getting dragged into the ******** that's bound to result because of my comments. If you want answers to your questions, I'm glad to give them to you. I'll even construct a web site complete with diagrams, if need be. I'm an email away. But I'll be damned if I am going to get involved when some idiot is waiting in the wings ready to derail you.

Ok so I was a bit off topic, but the use of horn loading is a valid method of increasing efficiency...

I will start a new thread when I get some drawings of the renkus box together, as I think it is a valid method of construction, sounds good and the theory is a mystery to me!

As for getting into VLAs, it works well on big PA systems in difficult venues, but is not a solution for domestic use.

********, me... (grins). Seriously Markkanov, Bill is right, discussions do tend to go of at tangents, so you do need to pick carefully through any advice you get, to sort the wheat from the chaff.

As for your spacing question, I would just put the drivers as physically close as you can, it will do no harm.


Notice Bill, I haven't mentioned cables once!