Matching Efficiency

[SMathews]

Hi,
Has anyone matched a woofer that was 6 or more db (db/watt) efficient than a mid/high driver? There seems to be some debate on the damping factor front (i.e. if its effect is of importance or not), but what are the other cons on doing this? Am trying to use some drivers I have on hand. Any help will be appreciated.

SMathews

[SY]

Well, efficiency is not as important as sensitivity. If the woofer is more sensitive than the MR, you've got a few options:

1. Best option- biamp.
2. OK option- use passive EQ between preamp and power amp to flatten out the frequency response.
3. Less good option- use a step-up transformer for the MR.
4. Least good option- use a series resistor in the woofer feed.

If you're really lucky and you've built up a lot of good karma in this life, you might be able to work the crossover point so that the extra woofer sensitivity can be used to make up for spreading loss. This will take intelligent coordination of crossover point, cabinet geometry, and woofer diameter.

[SMathews]

1. Best option- biamp.
2. OK option- use passive EQ between preamp and power amp to flatten out the frequency response.
3. Less good option- use a step-up transformer for the MR.
4. Least good option- use a series resistor in the woofer feed.

Thanks SY.
Options 1 and 2 are out for me.

How about step-down transformer for the woofer in option 3?

Looks like its going to be the "Least good option" using an L-pad. Any pointers will be much appreciated.

SMathews

[BillFitzmaurice]

You have a sensitivity differential, not efficiency- efficiency is expressed as a percentage figure and does not translate to dBs. But as to the crux of the matter your woofer is likely to lose 6dB of sensitivity to the baffle step anyway, depending on the box configuration, so your situation may require little, if any, corrrection. I would not be concerned until I built the box and measured the response and then whatever correction was required I'd accomplish with the crossover.

[SMathews]

Build and adjust. Sounds like a plan. Will need to do more reading for sure. Thanks.

SMathews

[Svante]

Since you will need baffle step compensation, this might actually be a good combination. You will need a ~6 dB boost for the low frequencies, and the different sensitivities might fit this. However, you will probably need to attenuate the high part of the woofer's range, with a normal <parallel LR circuit> in series with the woofer. If this attenuation is 6 dB, the tweeter will fit in nicely at the crossover frequency.

[darkmoebius]

Originally posted by SY
3. Less good option- use a step-up transformer for the MR.

Sy, could you please explain why this is a not-so-good solution?

I've got a similar dilemna in that the high-efficiency single driver kit, the Hammer Dynamics Super-12, I built has a 98db woofer(40-10kHz) and a 94dB supertweeter (>10kHz). Everything sounds great, although the high freq's are slightly underdriven.

The easiest solution would be to simply get another supertweeter w/ greater sensitivity. But this particular design has the tweeter mounted slightly offcenter in front of the woofer(see below) and I cannot find another of the right size to work there.

So, I thought it might be possible to use a step-up transformer to boost the signal to the supertweeter enough to compensate for the 4dB difference. A while back, I found a dB conversion formula and worked it out to be 1:1.4 steup, but now I have no idea how I reached that conclusion.

Would the stepup tranni affect phase?

[BillFitzmaurice]

There are a number of options to smooth the response, transformers being one. But to devise a cure without first finding out what corrective action may be required (if any) by measuring the response of the finished box is putting the cart before the horse.

A transformer will affect phase; everything in the signal path does, right down to the cable, so it's best to minimize components and not try to fix anything until you're sure that it's broken to begin with.

[SY]

Sy, could you please explain why this is a not-so-good solution?

Cost, size, availablity, distortion, radiated field, bandwidth. You have to trade these off- if you don't mind size and cost, you could do well for distortion. Bill's point is right on- see if you have a problem before taking extreme measures.

[darkmoebius]

Originally posted by SY
Bill's point is right on- see if you have a problem before taking extreme measures.

Aaaaah, I see what you guys mean. Perhaps, I should actually do some real world in-room testing to see if there is actually any big dips or humps in response before fiddling with anything.

As with anything in audio, there are multiple tradeoffs. The people who switched out the original 94dB Audax TM025F15 tweeter for a 98 dB Fostex FT-17H have reported much smoother presentation and high/low integration, but had to forgo the quasi-coax arrangement for more traditional mounting.

In my rudinemtary experiments, I found that moving the tweeter off the main fullranger slightly affected the nature of the sound in a negative way. But, I've never done this in a serious manner.

Anyway, I'll see if I can find someone from our local horn builders group that meets at the Edgarhorn's shop can come over do some tests for me.