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-   -   What is 2 pi and 4 pi (http://www.diyaudio.com/forums/multi-way/21119-2-pi-4-pi.html)

 EternaLightWith 4th October 2003 11:37 PM

What is 2 pi and 4 pi

Does it mean that the speaker is radiating only forwards (2pi) versus front and back (4 pi)

Someone once said that PHL speakers where great efficient speakers radiating in 4 pi space.

At the time, I didn't quite get what he was trying to say.

Any ideas on this?

David

P.S. The topic of convo. was the PHL 3451 which has an effic. of ~5%

Of course I later looked at the PHL 3860? that is at ~6% effic. wow!

 catapult 4th October 2003 11:59 PM

Quote:
 Does it mean that the speaker is radiating only forwards (2pi) versus front and back (4 pi)
Yep. Think speaker mounted in a wall so the sound only goes forwards -- 2pi--half a sphere. Anything that tries to go back is reflected by the wall so the SPL is twice as loud.
Quote:
 P.S. The topic of convo. was the PHL 3451 which has an effic. of ~5%
That sounds pretty high. That's probably a 2pi number.

 1/137 5th October 2003 12:10 AM

solid angle

look in your math book...for the term solid angle
rt

 roddyama 5th October 2003 12:34 AM

The efficiency of the driver is a measure of how well (or poorly) it converts electrical power to acoustic power. The power output of a driver is independent of the size of the soundfield it is radiating into. That is to say that if a driver has a certian efficiency, it will radiate the same power into a 2pi soundfield as it does to a 4pi sound field. The SPL increases when radiating into the 2pi sound field because the same acoustic power has been concentrated into a smaller volume.

Typical cone type drivers will have efficiencies in the range from as low as 0.1% to as high as 3% for some of the larger pro drivers. 5% and 6% speaker efficiencies are usually reserved for the horn loaded drivers.

 EternaLightWith 5th October 2003 01:28 AM

reflection, huh....

wouldn't that be a no-no?

COme through as distortion, etc. as it's being reflected through the cone..

Needs a nice layer of dynamat of the back.

David.

I'd rather absorb the backwave

100dB @ 1m..... It's not like PHL drivers are hurting for efficiency.

 Ryder 5th October 2003 02:06 AM

http://www.trueaudio.com/st_spcs1.htm

Cheers
Craig Ryder

 Timn8ter 5th October 2003 01:06 PM

Good article.

Boundary Gain + Pressure Gain = Room Gain

 EternaLightWith 5th October 2003 07:14 PM

Kinda off topic, but....

Since it's my thread :angel: j/k :D

I'm looking at 3 drivers for my TL. I'm making a wide midrange TL... no standing waves either. They all have differences in a variety of area. I'm looking for detail and accuracy the most. There are some things about MJK's program that I was hoping some of you members could expand on....

P.S. The drivers in question are PHL's 3020, 3040, and 3450 drivers.

1. The F3 of the driver in TL will be 100 Hz. If I'm comparing two drivers excursion graphs and one driver takes 1.2 mm to do 50Hz, while the other takes only .8 mm, yet at 100 Hz they both take .5 mm to do the job, are they both the same if only looking at excursion. ie. Does it matter what the excursion characteristics are past the cutoff point?

2. If one driver has a Xmax(Linear) of 2.5 mm and takes 1.2 mm to do 100Hz and the other one has an Xmax(linear) of 2.0mm yet will do the same 100 Hz at 0.8mm, which is more linear?
ie. Is it just that linearity go up as excursion decrease, or it the ration between excursion and Xmax important too.

3. Impedance... I got some advice from Dave (planet10) to flatten the impedance of the TL system while still maintaining a horse saddle shaped curve. I respect Dave's opinion but I'd like to know why, as well. (I'm like that. I drove my parents/teachers nuts.)

ie. Does impedance past the f3 point of your driver/enclosure matter?

My understanding, is that it wouldn't because your speaker isn't going to be reproducing those regions, so it is invisible to the amp.

I'd have made a better system by making the first impendance hump as small as possible and not worry about the huge impedance mountain 50 Hz farther down.
Or is there another aspect that is tied to impedance that I'm missing.

So, assuming a happy 4Ohm driving amp, one would want to keep the impedance from rising too much after Zmin. Right? Cause typically I see the impedance rise and spike from 200 Hz - 100 Hz, this would mean the amp is working harder and may not be able to put as much power into that region.

4. Pet peeve. I hate it that MJK's graphs don't go passed 1 kHz. He told me it would be very hard to accurately describe a TL past !kHz. Or something to that effect. WinISD doesn't either. That just stinks, because I noticed that different drivers do different stuff as they approach 1kHz. and weird stuff at that.

Over the last few months the general trend I've seen was that as the TL got longer, the bigger the dip from 900Hz - 1kHz. Typically, with PHL drivers, 12-14 in. TL gave a rising dB slope in that region while 16 in. onward gave a negative slope. at 20 in. we're talking a 5 dB decrease. My fear is, what about after 1kHz? Is it just a dip there? My hunch is that yes it's just a dip there and by decreasing the TL length, you're changing the standing wave properties (tuning?) and just moving that dip farther up the graph (2-3kHz region).
So what if I build the box and buy the speaker, and find that it's all downhill from there? It's not like I'll just say "oh well" and buy 4 more different driver and enclosure combos to try out till I get the right one...

5. I'm confused about the differences between, "Far Field TL system and IB SPL response" versus " Woofer & Terminus Far Field SPL" What's the difference? Do they both apply to me? They both have very different responses...

6. Seems like I can't change acoustic impedance or phase very much. THe phase part will change with oodles of stuffing, the the SPL response is worthless... nice impulse graph though.

I think that's it...... for now

David

P.S. MJK & Dave, what do you think of the PHL 3020? Take a look at it sometime. It seems to be a TL's soulmate. Tweaked right, the impedance is more consistantly low, the respense curve is ruler flat.....

I'm probably going with the PHL 3040 though. A nice compromise between the powerful 3450 and the elegant 3020.

 kelticwizard 7th October 2003 05:17 AM

2 Attachment(s)
This is an excerpt from DB Keele's article in the Journal of The Audio Engineering Society in April,1974: "Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement". These are actual measrements. From this, we can draw two conclusions.

A) The 4p situation, (full space) gives much worse bass response compared to the 2p, (half space) measurement;

B) The nearfield measurement system, (that is, putting the microphone an inch or less from the cone or port), yields very similar readings to the 2p, (half space) readings.

 EternaLightWith 9th October 2003 04:42 PM

Some more questions

In regards to MJK's TL files, I'm a little fuzzy with the driver position ratio and the TL Geometry numbers (ie. n_closed>1 & n_open>1).

Trying to relate the Offset driver sheet's parameters with the TL sectional sheet's parameters.

I was doing it at 6 (driver position ratio) for awhile and leaving TL Geometery at default (n_open>4 & n_closed>9)

If my understanding of these parameters are correct, then this would not work for me.

I'm putting a 10 in driver into a 7:1 Sd ratio TL with an optimal length of 14 - 16 in.

SO.... I was trying (1 & 5; 3 & 5; and a 4 & 4) this I believe gave my a driver positioning of 1/3; 2/3; & 1/2 respectively.

This would give me some flexibility while still respecting the size restrictions of drive and TL length.

David

P.S. Sticking with Sectional TL spreadsheets for now, as they are the most accurate and I'd be better served to master one rather than jumping between different ones.

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