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Author Topic: Wedges & Transfer Function Measurement: Meas. Mic Position?  (Read 15540 times)

Ivan Beaver

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #10 on: November 09, 2012, 06:42:57 pm »

I am not aware of this widely held belief. I am aware of a widely held belief that says the apparent source will be the first arrival assuming certain other conditions, like near equal level and a limited time differential.

How do you eq the hole caused by reflection back in? All you can do is be aware of it or change the setup so it doesn't happen. You cannot correct a reflection caused suckout with eq.

Mac
+100 This is VERY IMPORTANT!!!!!!!!!!!!!!!

This is a fact that many people seem to want to "overlook".  Sure, you can "appear" to fix the suckout in one place-but what you REALLY do is to make it worse every where else.

And to expand a bit, while it may "appear" that you can "fix" a hole caused by a "suckout" or reflection, IN REALITY you are NOT fixing it.  You are ONLY fixing the display on the computer screen.  If you were to use a finer resolution on the screen (ie no smoothing), you would see that the notch is still there.  But with a smoother resolution, it will "appear" to be fixed-because of the averaging of the display.

Think of it this way-let's say you have a notch at 2000 Hz at a particular location.  The reflection is causing it.  The loudspeaker does not have this notch when away from the boundary.  So it appears as a dip-so you boost 2Khz 6dB.

BUT WHAT DOES THAT REALLY DO?  It boosts the direct signal 6dB AND it ALSO boosts the reflection 6dB.  So the net result between the two is EXACTLY the same.  SO you have done NOTHING!  However on a display with smoothing, it "appears" that the hole is gone-because the AVERAGE around 2Khz is higher than the rest of the response. 

But the hole is still there.  All you have really done is put a 2KHz boost in all other listening positions-which can actually make fighting feedback more of a problem.

You just can't blindly "whack away" with an eq-thinking you are fixing the "issue".
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Ivan Beaver
Danley Sound Labs

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Frank Koenig

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #11 on: November 09, 2012, 07:16:43 pm »

I am not aware of this widely held belief. I am aware of a widely held belief that says the apparent source will be the first arrival assuming certain other conditions, like near equal level and a limited time differential.

How do you eq the hole caused by reflection back in? All you can do is be aware of it or change the setup so it doesn't happen. You cannot correct a reflection caused suckout with eq.

Mac

Mac,

Thanks for the reply. I'm obviously babbling above my pay grade here but I'm trying to learn this stuff.

On your first point, of course you are correct with respect to the time scale under consideration here. However, is it not generally accepted that sound arriving later than the time that corresponds to a comb-filter interval smaller than the critical bandwidth is perceived more as spacial than tonal? [McCarthy, Sound Systems: Design & Optimization, pp.164] For example, if we assume a critical bandwidth of 1/6 octave and a frequency of interest of 1kHz, then sound arriving at the listener more than 8ms after the first arrival would not have a large impact on tonality.

On your second point, I agree that you can't fix a suckout with EQ. But this actually supports the idea of basing the settings on an anechoic measurement since there is nothing you can do about a suckout you might see when you put the monitor on the floor and the mic at the listening position. Whether the EQ is in the speaker processor or somewhere else, it is still EQ, and a losing battle. There might be some value in mitigating floor, or other room, effects with EQ to some extent. All I was saying is that since I'm not smart enough to do that in advance I'll leave it for ad hoc EQ in the field.
 
--Frank
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Ivan Beaver

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #12 on: November 09, 2012, 07:50:48 pm »

For example, if we assume a critical bandwidth of 1/6 octave and a frequency of interest of 1kHz, then sound arriving at the listener more than 8ms after the first arrival would not have a large impact on tonality.


Why would only be interested in 1Khz when talking about the "tonality" of a loudspeaker.  There are a lot more freq that should be looked at.

In your example, the 8ms 2nd arrival would produce the first notch at 63Hz and further notches that are spaced at 125Hz apart.  So the second notch would be @188hz, the 3rd notch would be at 313Hz etc.

So I would argue that notches at those freq WOULD affect the tonality.

Now when you get to the higher freq, the spacing of 125Hz is going to produce so many notches, that they will tend to "mesh together" in our brain.  But at the lower freq-they would still be very much audible.
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Ivan Beaver
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Frank Koenig

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #13 on: November 09, 2012, 08:23:08 pm »

In your example, the 8ms 2nd arrival would produce the first notch at 63Hz and further notches that are spaced at 125Hz apart.  So the second notch would be @188hz, the 3rd notch would be at 313Hz etc.

So I would argue that notches at those freq WOULD affect the tonality.

Now when you get to the higher freq, the spacing of 125Hz is going to produce so many notches, that they will tend to "mesh together" in our brain.  But at the lower freq-they would still be very much audible.

I agree. I just gave the 1kHz example for clarity (apparently it didn't work). I was not claiming that an equal-level addition of an 8ms copy would not affect tone at lower frequencies.

To get back to the original problem, I see three approaches:

1. Base the settings on an anechoic measurement, averaging over the required range of angles.
2. Base the settings on a "real floor" measurement being careful to ignore uncorrectable artifacts.
3. Build 20dB of boost at 300Hz into the speaker settings in a pathetic attempt to correct for the existence of the floor. Well, maybe not such a good idea.

Signing off for tonight.

--Frank
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Ivan Beaver

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #14 on: November 09, 2012, 08:39:03 pm »

I agree. I just gave the 1kHz example for clarity (apparently it didn't work). I was not claiming that an equal-level addition of an 8ms copy would not affect tone at lower frequencies.

To get back to the original problem, I see three approaches:

1. Base the settings on an anechoic measurement, averaging over the required range of angles.
2. Base the settings on a "real floor" measurement being careful to ignore uncorrectable artifacts.
3. Build 20dB of boost at 300Hz into the speaker settings in a pathetic attempt to correct for the existence of the floor. Well, maybe not such a good idea.

Signing off for tonight.

--Frank
4: Use a monitor that couples well with the floor and doesn't have much of a "bounce" off of the floor.
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Ivan Beaver
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Jim McKeveny

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #15 on: November 10, 2012, 05:59:18 pm »

Shane-

Have you taken readings from other biamp wedges, hopefully from a known manufacturer?? We can overlay measurements A. v. B.

I appreciate the quest for hard copy on relevant parameters, but my first test on your fresh purchases would be:

Do they all sound the same? If not, why not? Match them.

Get them to sound good in the environment they will be used in: On floor (half space sorta)..

Get them to sound good with a mic & vocal - on axis. (58 or better).

Then measure.

What we all desire is:

Consistency across power response (1W/1M & 100W/1m).

Smoothest phase response.

Least dramatic impulse response.

Minimal group delay.

Consistent polar response. Esp @ crossover.

Now you are having fun!

Monitors function in an acoustic toilet for the most part. A consistent and solid tool, even if it lacks "ideal" parameters for high fidelity, can be a winner.

Keep us posted.



 


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Shane Ervin

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Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #16 on: November 11, 2012, 12:09:00 pm »

We're expecting some warmer weather in the next few days.  The pics show the area where I can do some outdoor measurement work.  (You'd almost think the previous owners of this place wanted to test speakers in their backyard like I do!).

In reply to a few questions / comments so far in this thread:

- JBL loaded, mirror-image wedge pair
- NOT a CD horn.  (Another side discussion might be in order, just on merits / drawbacks of CD horns in wedges)
- horns are oriented vertically; great rejection of "adjacent guy's mix" when deployed straight along front line for two musos.  Obviously best when mirror image horns are "out"   
- ~ 20 years doing duty on festival stages; met rider for various acts
- I bought the last two left in inventory; only 2 to which I have access to measure
- relative to an equal (i.e., normalized) horn gain setting and same amp/NL-4 cable used, one wedge's woofer is 1 dB less sensitive (w magnitude resp cursor on each unit's trace - themselves very similar)
- more detailed match-oriented Smaart measurements remain TBD (e.g., horn / overall match)
- easy to make sound good without Impulse Response / T.F. / measurement equipment.  In their first deployment (under my stewardship), I had them in a rehearsal studio for 6 months as a left/right pair for a hard rock guitarist / vocalist in Montreal. I created a MiniDrive preset by ear, the day I delivered them to their room
- later, still before I got the chance to measure them, I used them, one each for two lead singers in the front line.  (w those parametric notches for use with my 57, as described in a previous post)
 
Happy Remembrance Day

Happy Memorial Day

Lest We Forget
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Shane

Newer: QSC PLD 4.2 PLD 4.3; QSC TouchMix-16; UAD Apollo 16; Sennheiser G3 IEM wireless
Older: JBL SR 4738 / SR4718; BSS MiniDrive; Klark Teknik DN-301; Yorkville AP4040 AP2020; Motion Labs Rac Pac;

Ivan Beaver

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #17 on: November 11, 2012, 12:50:01 pm »


- horns are oriented vertically; great rejection of "adjacent guy's mix" when deployed straight along front line for two musos.
This is another common misconception in audio.  Let's say the horn is 90x40 and is 12" in the 40 dimension (and that is outside of thorn flare-NOT the outside of the flange).

This would give pattern control down to around 2Kz.  If the horn is smaller-then that freq would be higher.  Of course if the horn had a wider coverage pattern (say 60), then that number would go down a bit.

So below 2K or so-the pattern starts to get wide.  The front loaded woofer has essentially no pattern control-so what is spilling over into the other musicians area is a lot of low and mid "mud".

Only the upper part of the highs is controlled by the horn.  The lower part of the highs still coming out of the horn are still spilling into others areas.

The only way to get pattern control down lower is to use a larger horn or a wider horn.

For the same physical size-a wider horn will have pattern control down lower.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Shane Ervin

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Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #18 on: November 11, 2012, 01:06:24 pm »

Only the upper part of the highs is controlled by the horn.  The lower part of the highs still coming out of the horn are still spilling into others areas.

+1  Merely assumed that would be understood, but yes, it's best to remind folks, and avoid feeding any misconception.
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Shane

Newer: QSC PLD 4.2 PLD 4.3; QSC TouchMix-16; UAD Apollo 16; Sennheiser G3 IEM wireless
Older: JBL SR 4738 / SR4718; BSS MiniDrive; Klark Teknik DN-301; Yorkville AP4040 AP2020; Motion Labs Rac Pac;

Ivan Beaver

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #19 on: November 11, 2012, 02:03:59 pm »

+1  Merely assumed that would be understood, but yes, it's best to remind folks, and avoid feeding any misconception.
And the same thing goes for ALL speaker cabinets.
When a manufacturer states a particular coverage pattern-it is NOT across the entire operating bandwidth (DESPITE what some manufacturers suggest------).

The physical size of the horn is what determines the lower limit of the control.  And having a "rotatable" horn that is small-is an excersize in futility.  But it does make the user "feel better" about what they "think" they are doing-while in reality they are only affecting the top octave or two only.

Pattern flip is something that most are unaware of-or simply choose to ignore-even though it IS happening.

Take look at the attachment.  This is the manufacturers data on the SAME loudspeaker.  Notice how the coverage is very different between 630=1800 &  8Kz.

What IS the correct spec this cabinet should have?   It depends on what freq you look at-but most all the others are very different.

ACTUAL Loudspeaker patterns is something most users fail to consider. Yet they talk about it all time as if they do. :(

They take a "simple number" and just "assume" that it is overall true of the device in question.

When you start looking at the "whole picture", it gets a bit more blurry and much more complicated.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!
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