Efficiency of Box

[Doug]

Doug:

Um, are you saying it won't matter if the mid freqs are attenuated by the baffle step? I'm not clear on this.

Thanks. [/B]

No, I'm not saying that it dosen't matter if the mid frequencies are attenuated by baffle step or not, as it is a loss, and basically any loss is bad. But having a narrow baffle moves the point of where baffle step occurs up in frequency so the only attenuation you have to worry about is in the tweeter. If I'm not mistaken, this is "one" of the reasons that the tall narrow enclosures have become so popular in recent years. Any comments from anyone?

Remember, with passive crossovers, you always have to attenuate the frequencies above where the baffle step starts as you can't add boost on the low end to meet the high end because passive components only produce a loss. Not like active networks where there are gain stages.

Doug

 

[Doug]

This is only true for a perfect SS amp (which cannot exist). SS amps with very stiff power supplies get close. A tube amp will gain you no extra power. With some amps you will actually get less power (ie i think the Zen is an example -- if we go from 8 to 4 ohms, and certainly an OTL tube amp -- the latter will put out more power if you series the speakers).

So in practice you get anywhere from almost a 3 dB gain to a loss due to the different amp outputs into a lower impedance -- it is very amplifier dependent.

So what amp do you have?

dave

BTW, Doug's analysis (other than the note above) is very good. It is always important to consider baffle-step -- there is a comprehensive article in the Wiki on this subject.

Hey Dave, I agree with all of the above, I was just trying to keep the explaination in simple terms ( not that I think any of you are simple LOL )

BTW, that article on Wiki is great and I will be reading it again. I am ashamed to say that I had not looked there before.🙄

Doug

 

[Mos Fetish]

Dave

Perreaux made (and still make) some of this planet's greatest MOSFET power amps and preamps. They are a New Zealand company started in the '70s and my amp was made in the '80s. Last year I had it tested by an audio engineer and it's parameters are still all spot on.

Here's a little info:

http://www.audioadvisor.com/store/productintro.asp?IntroType=1&MFGID=182

The 2150 is rated at 200w into 8ohms (.009% THD from 0.25w to rated power) and 400 into 4ohms.

Now, re your calcs on the baffle step--um, I must be getting this wrong. If I have a 300mm baffle aren't all freqs up to that length radiating into halfspace and are therefore suffering NO diffraction loss?

Frequencies approx 1150hz and below, which are longer than 300mm (according to my calculator) would suffer diffraction loss. Or, have I got this backwards?

Mos 😕

 

[Kanga]

Mos

Below the baffle step frequency the sound will radiate in all directions, so you will have a 6dB theoretical (approx 3dB actual in room) reduction in output at the listening point. At low frequencies sound tends to be omnidirectional, and so sound can wrap around the baffle to go in all directions. At frequencies above the baffle step, sound is not so omnidirectional, and so it is concentrated in front of the speaker where you are listening.

The baffle step frequency is proportional to the width of the baffle, but is not the frequency at which the wavelength equals the baffle width. The formula is f3 = 115/W, where W is the baffle width in meters.

See http://sound.westhost.com/bafflestep.htm for more info.

Following my experiences, you may want to reconsider an XO point of 500 Hz. Although there are commercial designs around that successfully use XO frequencies around this point, you will need good XO design and very good driver matching to make such an XO point work well. If you are using two 6.5" drivers in a vented enclosure, you will get pretty good bass extension and output, and a sub may be all that you need. Any reason you want to cross so high?

As Dave said, if you are going to cross that high, then its a good opportunity to cross at the baffle step frequency, which will mean that you can adjust for the baffle step loss in the transition between the bass and mid drivers.

Mick

 

[Mos Fetish]

Mick

Thanks for your reply. I was obviously confused by how to calculate the bs frequency.

Let me redo this scenario. I have 2 x 165mm drivers which will fit onto a 230mm baffle quite nicely (assuming 25mm MDF sidewalls). The bs freq for a 230mm baffle is 115/.23=500hz.

Now, the two drivers wired in parallel (plus acoustic coupling) are going to give me a 6db gain in sensitivity. Right? However, I'm going to get a 3db cut below the bs freq (500hz, my crossover freq).

So, 87db + 6 - 3 = 90db.

Am I closer to the truth? Cause if it's around 90db, I'm just about where I want to be.

Mos.

 

[Doug]

Now, re your calcs on the baffle step--um, I must be getting this wrong. If I have a 300mm baffle aren't all freqs up to that length radiating into halfspace and are therefore suffering NO diffraction loss?

Frequencies approx 1150hz and below, which are longer than 300mm (according to my calculator) would suffer diffraction loss. Or, have I got this backwards?

Mos 😕

Mos;

Your calculations are correct. A frequency of 1150 Hz has a wavelength of .299713 meters. OR 1150 Hz is .983311 ft. approx.

What bass drivers in particular are you using?

A crossover point of 500 Hz does seem to high for a 3 way design. There are pro speaker systems ( 2 way ) out there that cross over at 500 to 800 Hz to a horn driver for the mid/highs though.

I just realized that I had the half space / full space thing backwards, ie: above the baffle step frequency when most of the energy is radiating forward is called half space. Below the baffle step frequency when the energy is radiating both forward and backward is called full space. Sorry for the confusion. and to answer your question, yes, frequencies up to the BS frequency have the loss.

Doug

 

[Nappylady]

*sigh* JUST BUILD THE DAMNED BOX with an L-pad on the tweeter and see how it sounds! Many people don't like "perfectly flat" sound anyway--I know I like to have some room gain in my sound, and a nice boost around 60-80hz.

Seriously. I don't want to sound harsh or anything, but build the thing, then tweak the crossover--if you don't get it right the first time, NO BIG DEAL. 🙂

Right now I'm working on getting my current project to where it looks good and is a decent size; originally the finish I'd planned was oak with a very dark, very red stain, and since trying that, and having it come out absolutely horribly, I'm going to try black artificial leather or nagahide (sp?) and hope it turns out better. If you get it wrong the first time, NO BIG DEAL, just try again. And believe me, crossover tweaking is no big deal, when compared with getting a @#@$#%! finish to come out right.

Good luck.

 

[planet10]

Perreaux made (and still make) some of this planet's greatest MOSFET power amps and preamps.

Right after posting last nite i went and read a steropile review of one of their current integrateds.

The bs freq for a 230mm baffle is 115/.23=500hz.

Now, the two drivers wired in parallel (plus acoustic coupling) are going to give me a 6db gain in sensitivity. Right? However, I'm going to get a 3db cut below the bs freq (500hz, my crossover freq).

The 500 Hz is where the BSD loss is at -3dB... (ignoring room gain) the loss will continue down until it is -6dB and then flatten out.

So if you consider an XO at the BS frequency (or a little lower) the woofers will essentially be radiating into 4 pi space and will be down 6 dB from their 2 pi space efficeincy -- in your case you can consider them to be 87 dB (ie the 2nd driver compensates for BSD).

dave

 

[Doug]

Baffle step

Mos;

Here is a interesting read regarding baffle step.

http://www.t-linespeakers.org/tech/bafflestep/index.html

Doug