Limitations of a 5.5" driver on an Open Baffle?.

kbgl

Member
2010-04-03 1:25 pm
How low can a 5.5" midbass with 4 mm x-max be used on an OB? My room is small, and SPL for music is usually limited to 90 dB. I want to use it as a mid in a 3-way crossed at about 400 hz. I also have some 8" drivers that I could use. I would like to cross to a dome tweeter at 2k. This will be a passive speaker used with a sub.
 
Wouldn't the 8" be enough for a small room ? I use 2 x 6,5" in isobaric and they fill the room with bass . It depends on the drivers you have ,or you are going to obtain .
Some use GOOD 8" for mids in OB ,even crossed at 2500 Hz .Lots of work for obtaining a flat freq response in the crossover network .
X-max of a driver is only a part of the story...it is the result in linear pistonic motion of various forces and springs ,but it doesn't say anything of the sound ,or useful application of the driver on a baffle. Indeed ,it is important for woofers in OB ,since the acoustic load is nearly null and there isn't the air-spring effect as in sealed boxes .
 

kbgl

Member
2010-04-03 1:25 pm
Crossed at 400Hz I'd think it'd be alright, with 12dB or more electrical roll-off. The mid covers a quite a small bandwidth though from just 400Hz to 2Khz. The woofer with a 4" fullrange in a 2-way system may work better.

I have two different Peerless HDS midbass drivers to choose from. One is the one used by SL in the Pluto speaker. 850488 I think. The other is more current. I don't recall the model number, but it is the GFC (glass fiber cone) version. I think the GFC sounds better than the 488 as a mid, but the 488 is a couple dB more sensitive, and can take more abuse I believe.

I have a couple Peerless 8 inch woofers that I could use. I refer to them by the last 3 digits of their part numbers. One (***136) is a low Q and good for a 2k x-over, the other (***709) is higher Q and really needs a midrange. These have an f3 in a sealed box of about 70 hz and 45 hz respectively. I know that goes out the window without a box behind them. The 136 is in a speaker right now, so I would have to rob it. (Something I do all the time!) I prefer to use one of the 5.5" drivers. I'm pretty good with passive x-overs, and can measure.

Did you have an issue with using a dome tweeter due to off axis response differences. I'm trying not to spend any money. I have a lot of parts on the shelf.
 

StigErik

Member
2008-02-21 12:30 pm
You didn't say anything about baffle size. The bigger the baffle, the lower you can go, but it will lead to worse off-axis response, but generally a baffle 2x the size of the driver can work well.

I run two 5" without any baffle at all down to 500 Hz - the cones barely move at all. I'd expect 400 Hz is easy with a baffle some 20-30 cm wide.
 

kbgl

Member
2010-04-03 1:25 pm
You didn't say anything about baffle size. The bigger the baffle, the lower you can go, but it will lead to worse off-axis response, but generally a baffle 2x the size of the driver can work well.

I run two 5" without any baffle at all down to 500 Hz - the cones barely move at all. I'd expect 400 Hz is easy with a baffle some 20-30 cm wide.

The baffle I have in mind for a first try would be 11" x 30", possibly with the top 3rd tappering down to about 8".
 

kbgl

Member
2010-04-03 1:25 pm
have you any thiele small parameters? you could try modelling it in edge software just using the size and tweaking the baffle to see what you get

Here's the specs on the 488.

* Power handling: 110 watts RMS/155 watts max * Voice coil diameter: 1" * Voice coil inductance: 1.0 mH * Nominal Impedance: 8 ohms * DC resistance: 5.7 ohms * Frequency response: 62-5,500 Hz * Fs: 61.1 Hz * SPL: 89.2 dB 2.83V/1m * Vas: .27 cu. ft. * Qms: 2.07 * Qes: .41 * Qts: .34 * Xmax: 4mm * Net weight: 2.4 lbs. * Dimensions: Overall Diameter: 6", Cutout Diameter: 5", Mounting Depth: 2-5/8", Magnet Diameter: 3-1/2".

Here's the GFC

*Power handling: 30 watts (IEC 2685 18.1) *VCdia: 1" *Le: 0.33 mH *Impedance: 8 ohms *Re: 6.0 ohms *Frequency response: 66-7,000 Hz *Fs: 66 Hz *SPL: 87 dB 2.83V/1m *Vas: 0.24 cu. ft. *Qms: 3.0 *Qes: 0.58 *Qts: 0.49 *Xmax: 4.55 mm *Dimensions: Overall diameter 6", 5.275" across flats, Cutout diamter 4.75", Depth 2.75".
  • Truncated cast aluminum frame
  • Glass-Fibre Composite (GFC) cone with half-roll rubber surround
  • Long-throw voice coil
  • Shorting rings for reduced distortion
  • Under-spider and voice coil ventilation
 
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In this speaker the 5.5" is taken down to about 300 hz. Even at high levels there's hardly any excursion. I actually think that it's a good adea to use a slightly wider baffle or a larger driver for two reasons: 1) greater maximum output and 2) more omni-like behavior at higher frequencies which might facilitate a smoother crossover to a single monopole tweeter.

OB3W - drieweg open-baffle dipool van formaat
 

john k...

Member
2004-08-10 2:50 am
US
It is not so much the max SPL that will be the issue. It is the baffle size (width) which will determines how high the x-o point can be. Once the baffle size is set then you can determine how low the 5" can go. 400 Hz with 4mm Xmax should not be a stretch in any case. If you are serious about 90 dB max, then you could easily go as low at 200 Hz on a 6" wide baffle, which is about what you would need to have smooth response to 2k Hz.
 

twest820

Member
2009-06-24 10:49 pm
I would like to cross to a dome tweeter at 2k.
Back to back dome tweeters, I would hope. I've tried 6.5s crossed at 2kHz to front firing only domes and, IMO, it sounded like cr*p.

A single 8" should work well to 150 Hz at least.
Well, probably somewhere between 100 and 500Hz, depending on baffle size, usable xmax, and SPL versus distortion preferences. :p The challenge I think is more on the high end; 8s where directivity is maintained and breakup deferred is high enough in frequency for the OP's 2kHz cross to a dome tweeter to work well are hard to find. The Selenium 8W4Ps I'm using are the best tradeoff I know in this space but getting decent results out of them has required crossing 1.6-1.8kHz LR6 with a 20dB notch around 3kHz to suppress breakup. Since the OP will be using passive crossovers this would be tricky to pull off and their 2mm xmax means they're not really sufficient for the OP's 90dB SPL target (RMS I assume).

For cone drivers in this application I would favor a 6.5, specifically the SB17NRXC35-8, though I'd like to get my hands on a pair of BG Neo10s if they ever come back into stock.

Power compression and derating to usable xmax as a start. For the latter I've taken to using xmax / 10 as a starting point for clean SPL, though I've some data which suggests that might not always be conservative enough.
 
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