Rule of Thumb on 3 way driver SPL's.

[2litre]

I have a quick question concerning SPL of multiple drivers in open baffle.

-If I was building an open baffle speaker using 2 or 3 drivers, what would the Rule of Thumb be as to which driver should have the higher SPL and which one should have the lower SPL if all couldn't be had with the same numbers?-

More to the point, I'm looking at building a version of the Visaton NoBox speaker using Pioneer drivers from PartsExpress, specifically the 15" Woofer #290-175 (A38GU40-52F-Q) and the 8" full range #290-045 (B20FU20-51FW). The 15" has an SPL of 96db and the 8" is 94db. This seems OK to me since open baffle speakers are said to be a bit shy on bass. Visaton says the 15" is a 'bass augmentation speaker' and in this application its 20Hz Fs should do just that.

When I searched 'open baffle' in the Full Range forum I found quite a bit of discussion on the Pioneer B20FU20-51FW. I got the impression I may not be satisfied with the high end and might end up adding a tweeter to the set up, hence my question concerning the Rule of Thumb on SPL of multiple drivers. If I needed to add a tweeter, should it be more or less efficient than the 8" full range? For discussion lets use either Pioneer GT-1025 (PE#280-062) at 90db and GT-1005 (PE#270-011) at 94db.

I hope that's enough info to generate a response.

Thank You,

Jim

[simon5]

The tweeter should be more efficient because it's easy to use a couple of resistors to bring it at the same level as the fullrange.

If the tweeter is 2-4 dB more efficient than the woofer, you also have the option of adding a 2nd woofer to correct the situation.

[Cloth Ears]

I'd agree with Simon5 - either more, or the same, efficiency. It's pretty easy to lpad the tweeter down in a passive situation. And if you're active, then it doesn't really matter either way. IMO they should be reasonably close, though, as I think it's a waste if you have to pad a 94dB tweeter to match an 84dB mid (not in 2litre's case though).

[Ricky]

If you're working on a multi range speaker, whether it is 2, 3, or 4 way, always work on the assumption that the woofer has the lowest SPL. The reasons for this are many. The woofer will have reduced SPL if it has a rolled rubber surround as opposed to a foam or linen edge. There is always a trade off and, as Simon5 says, you can always "pad" a unit down to suit.

In 3 way systems, remember that you are going to use a bandpass filter working between 500 and 4000Hz. You may need to equalise this. I have yet to hear a true 3 way system that works. 2 and a half way systems seem to be the order of the day. The tweeter can be any SPL from 4Db upwards from the other drive units. In a 2 way system, 2nd order filters work for the bass with 3rd order filters for the treble. When you design the network, always use a 3rd order target response. Our AK3 uses a 1st order for the bass and 3rd order for the treble and got 5 stars in What HiFi.

Ricky.
www.arcaydis.co.uk

[Salas]

There is something that I almost never see mentioned. Every time a filter with cap(s) is put before a tweeter there is a loss of roughly 1.5 to 2.5 dB from medium uf to 2.2uf range. This is before any resistive attenuation is applied and is stronger in the last octave. Simulators dont account for that. I use a -.5V reference generator for the tweeter in LEAP 5 to balance the loss... The akward old Calsod was wiser in this respect!
Series filters are the most efficient in this respect. They lose less, or nothing if 6dB.
-So please make allowance for the above loss when choosing your TW SPL.
-Mids can be less efficeint since their output is boosted by sidebands energy and its better not to use resistors to pad them down.
-Woofers are the rulers of overall SPL because they face BSC loss and absorption loss from box stuffing depending on what and how much.