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Re: 4 Pi Design Questions [message #72885 is a reply to message #72746] Fri, 01 June 2012 19:30 Go to previous messageGo to next message
Nichol1997 is currently offline  Nichol1997
Messages: 21
Registered: December 2009
Location: Virginia
Chancellor
I went ahead and remeasured them based on your instructions (see attached). I took the measurements in my family room and I think that the mic is picking up room reflections. To verify this, I also measured a MTM speaker and one can see how it almost matches the Pi4 response in certain areas (see 2nd attachment, MTM is labeled as BAMTM on the graph).

I also attached a picture of the mic position along with the mic stand that I had to make. The mic is 36" from the baffle and centered on the top half of the JBL 2226 cone.

Thanks for taking the time to explain where to place the mic when taking measurements. On the graph that you posted of your Pi4 measurements, what type of smoothing did you use (1/12)?

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Re: 4 Pi Design Questions [message #72887 is a reply to message #72885] Fri, 01 June 2012 21:35 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18669
Registered: January 2001
Illuminati (33rd Degree)

Yes, now you're getting good measurements. You still see room modes down low, like the one around 120Hz. This is a vertical mode, and is part of what flanking subs fix. Below that, the response is dominated by room modes - both speakers look the same in the modal region - proving that what you measure down low is not the speaker, but the room. But the response up high shows up pretty clearly. This is a useful setup when you just want to see the crossover region, for example.

I don't use any smoothing at all. The difference is mostly due to my measurement being outdoors. Inside, on its back, my measurements look very close to what you have. I have an eight foot ceiling, and I'm guessing you do too.

You can kind of see the similarity in my indoor measurements and yours in the "Vertical Nulls" video in the "Crossover Optimization" thread. See the wiggles down in the ~100Hz region? Those are room modes.

I think the Omnimic measurements made by BigmouthinDC do have some smoothing applied. I'd say probably 1/12 octave. It actually looks less like smoothing and more like the sampling resolution is a little bit lower than my LMS system.

As an aside, there is a difference between reduced resolution and smoothing, but the end result is similar: Detail is lost.

The biggest difference is smoothing will actually reduce the amplitude of peaks and increase the amplitude of troughs. Reduced resolution simply misses some data points, so a sharp spike may be missed. But where a point is recorded, its amplitude is left intact, and the system just connects the dots to make the SPL chart.

Smoothing really takes the edge off a graph and can make a peaky response curve look pretty smooth. Huge spikes just get knocked off. Then again, it isn't necessarily a bad thing, since most published curves are smoothed, sometimes as much as 1/3 octave. That was pretty common in the 1970s and 1980s - if you could find a chart at all, it was likely smoothed to 1/3 octave resolution.

Re: 4 Pi Design Questions [message #72892 is a reply to message #72887] Sat, 02 June 2012 08:56 Go to previous messageGo to next message
mantha3 is currently offline  mantha3
Messages: 72
Registered: May 2012
Location: Lino Lakes, MN
Viscount
Sorry, newbie here.

So are these readings good/desirable compared to what you'd see with the readings on a standard build of a 4 Pi when built to the exact physical dimensions of the design?

I'm following this as I'm thinking of building a set of 4 Pi speakers with the same 7 inch extra height of the cabinet and dual vents on the bottom.

Re: 4 Pi Design Questions [message #72895 is a reply to message #72892] Sat, 02 June 2012 10:30 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18669
Registered: January 2001
Illuminati (33rd Degree)

The measurement above 200Hz is pretty much the same as a standard four π loudspeaker when measured in this manner. And yes, frankly, that's a very good chart. But below 200Hz, it's ambiguous because only room modes can be seen. That's where you would expect most changes due to the box modification to manifest themselves.

This brings us to the hypothetical question, which is, "How much does it matter?" My concern about box mods is always that they might make ripples in the midrange due to internal standing waves. But as long as they're all below the modal range, one might question how much they matter. If we use flanking subs to mitigate midbass and lower midrange anomalies, they'll tend to smooth that range, whether caused by standing waves inside the box or reflections outside, in the room. Still, my take is the less anomalies we can introduce, the better. Less problems to mitigate.

And of course, if we're using subs, we don't need the extra extension the larger box gives, and leaving it stock eliminates the possibility of introducing midrange ripple. The room is dominant below 200Hz, and the biggest peaks and valleys are from standing waves in the room, not in the loudspeaker box. So truly, the best approach is to use multisubs (flanking and distributed) to smooth this range, which will provide both extension and modal smoothing.

My conclusion is this: Since the mods have not introduced any additional ripple above 200Hz, I'd call it "verified". That's the main thing we're looking for here. We don't want any midrange ripple from internal standing waves. Of course, we can't see what it might be doing below 200Hz, but indoors, that almost doesn't matter.

Re: 4 Pi Design Questions [message #73041 is a reply to message #72895] Mon, 18 June 2012 19:50 Go to previous messageGo to next message
Nichol1997 is currently offline  Nichol1997
Messages: 21
Registered: December 2009
Location: Virginia
Chancellor
I took the Pi4 outside to take some measurements (see attached). I used the same microphone stand that you see in my previous post (two-by-four A-frame), laid the speaker on its back and placed the mic at 38 inches from the baffle. The mic was centered and directly above the top half of the JBL 2226 driver. I ran the test a little louder since I was outside and there were birds chirping in the background.

All of the measurement graphs that I posted have 1/12 smoothing. I can post the raw data response if you want to see it.

First time taking measurements outside and I was surprised to see the overall bass level louder than the higher frequencies. I assume this is normal for outdoor sweeps?
Re: 4 Pi Design Questions [message #73042 is a reply to message #73041] Mon, 18 June 2012 21:12 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18669
Registered: January 2001
Illuminati (33rd Degree)

No, that's not normal. Something is wrong. Check your settings.

The spectral balance is the same inside and outside, there just aren't any reflections outside so there aren't any self-intereference notches. The outdoor response curve is much smoother, especially down low. But you won't see any difference in the overall trend, the basic balance between bass, midrange and treble. They are all equal in volume.

Re: 4 Pi Design Questions [message #73048 is a reply to message #61737] Tue, 19 June 2012 07:52 Go to previous messageGo to next message
mantha3 is currently offline  mantha3
Messages: 72
Registered: May 2012
Location: Lino Lakes, MN
Viscount
Is it possible that the Bass levels are higher due to the larger cabinet making bass more pronounced?
Re: 4 Pi Design Questions [message #73049 is a reply to message #73048] Tue, 19 June 2012 07:55 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18669
Registered: January 2001
Illuminati (33rd Degree)

No, a larger box won't make the bass louder, at least not broadly like that. It can make the cutoff frequency lower, or if mistuned, it can create a peak. But nothing like that.

That outdoor curve is the result of a measurement problem of some sort.

Re: 4 Pi Design Questions [message #73051 is a reply to message #73049] Tue, 19 June 2012 19:08 Go to previous messageGo to next message
Nichol1997 is currently offline  Nichol1997
Messages: 21
Registered: December 2009
Location: Virginia
Chancellor
I found my setting mistake in REW. I will have to do the outside measurements again. Wife is not happy about helping me carry the speaker outside again.
Re: 4 Pi Design Questions [message #73054 is a reply to message #73051] Tue, 19 June 2012 21:34 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18669
Registered: January 2001
Illuminati (33rd Degree)

Boy do I understand. I've lugged a lot of speakers around to make measurements over the years. And some of them have been pretty damn big.

http://prosoundshootout.com/Photos/2006/12Pi_pair_side.jpg
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