Rational Acoustics



Dr. J
July 14th, 2010, 12:13 PM
In Bob McCarthy's book, he talks about how the speaker should be measured in the on axis position for EQing. I learned to measure using the ground plane method first and when I measure for the on axis location -- it seems my trace differs by quite a bit. I am sure some of it may be due to ground bounce.

Do you have a preference as to what works best for you?

I do most of my measuring in the free field first.

Arthur Skudra
July 15th, 2010, 12:40 AM
My personal philosophy on placing test mics is to ask: "What are the majority of the people in the coverage area of the loudspeaker hearing"? Obviously only a select few will be perfectly on the centerline of coverage from the loudspeaker and benefit from excellent alignment between drivers and get the full impact of the high frequencies. Unless you're trying to do crossover setup of a loudspeaker or speaker array alignment, I would advise that your microphone positions should be off centerline of the loudspeaker in various places in the coverage zone of the loudspeaker.

In addition to the ground bounce, also consider any adjacent surfaces around the loudspeaker, as these will cause cancellation in your measurement, and vary according to the position of the microphone in relation to the loudspeaker and any reflective surfaces.

PhillipIvanPietruschka
July 15th, 2010, 01:54 AM
Sometimes using the floor as a ground plane can make it easier to obtain clear data in the low frequencies and hence perform sub<>mid/high alignments. But not always; especially in spaces where the seating is a temporary or movable structure and considering the floor beneath the seats a useful and sufficently large ground place is at very least 'complicated'.

I have never felt the need to contemplate its use in any other scenario, though I seem to remember Arthur discussing it's use when aligning a system in a highly reverberant church.

Best,
phillipivan.

lundbergsound
July 15th, 2010, 02:07 AM
There is no absolute right or wrong; the key is to be able to tell what in the trace is local and what is true for a large area. 6o6 has seen enough traces so that with a single on-axis measurement he can parse the local phenomena (ground bounce, standing wave nodes etc.) and the overall trends and EQ with a single mic position. I use multiple microphone positions to EQ each speaker because the trends are clearly evident, especially in Smaart 7 where you can watch how each responds to EQ in real time if you have multiple mics. The design reference book that Bob wrote for Meyer several years ago talks a bit more about using multiple mics for each speaker.

Regarding ground plane measurement, I don't always do it, but it can be handy for aligning subs (like Phillip said) and a few other tasks. Just make sure nobody inadvertently kicks the mic!

Harry Brill Jr.
July 15th, 2010, 12:38 PM
There is no absolute right or wrong; the key is to be able to tell what in the trace is local and what is true for a large area. 6o6 has seen enough traces so that with a single on-axis measurement he can parse the local phenomena (ground bounce, standing wave nodes etc.) and the overall trends and EQ with a single mic position. I use multiple microphone positions to EQ each speaker because the trends are clearly evident, especially in Smaart 7 where you can watch how each responds to EQ in real time if you have multiple mics. The design reference book that Bob wrote for Meyer several years ago talks a bit more about using multiple mics for each speaker.

Regarding ground plane measurement, I don't always do it, but it can be handy for aligning subs (like Phillip said) and a few other tasks. Just make sure nobody inadvertently kicks the mic!

Agreed. I rarely put the mic on the floor because I'm just used to seeing the ground bounce. Smaart 7 will average all the traces together from multiple mics. If the mic stand height is about the same you will see the ground bounce very clearly. While the venue is empty, this is quite audible, but when the bodies show up, it's no longer an issue.

Dr. J
July 15th, 2010, 02:03 PM
Thanks for the input guys.

I measured quite of bit last night starting out with my typical stack setup which consists of two 18 single boxes with a top on it (per side). I went through the ground plane method which I use quite a bit and got a nice smooth trace with a low end build up. I know it will sound decent in most clubs I run sound in.

I then ventured off into the on-axis position that McCarthy talks about. My top is up on a pole and my subs on the ground spaced about 5.5 feet apart. I use four 18 single boxes but in this case I just used two boxes. I did the top first and my trace changed quite a bit once I went on axis. However, I could see some of the same dips & peaks that I saw on the ground plane. I knew those may be realistic enough to apply the same type of Eqing in that region.

My high end 11k on up to 13k ramped waay up. I had to use a shelf to bring it back down. It does this on ground plane as well.

On my subs -- I had to make a decision on what box to align the top to. I used the closest sub and aligned the top to it. I knew from an earlier post that Harry said all other subs would then be out but at least it would be aligned to one close enough. I tried it and it looked ok.

When I fired up the other sub -- so now I have two subs running -- it totally threw the alignment off. It is just like turning the amp up. the XO point shifted and my sub level (magnitude) increased a lot.

I made the decision (tell me if it is wrong) to just measure it the way I run it at a gig. Only one side of course thinking that it will transfer to the other side once I hook it back up. So I aligned the top to two single 18 subs spaced 5.5 feet apart.

At the gig it will be four single 18 boxes spaced 5.5 feet apart across the stage and a left and right TOP. This was all done outside in the free field of course since I don't have an anechoic chamber out back.

I will then compare my saved free field trace and compare it to the venue when i get there.

What else can I do?

Thanks for all the help!

Harry Brill Jr.
July 15th, 2010, 02:35 PM
Thanks for the input guys.

I measured quite of bit last night starting out with my typical stack setup which consists of two 18 single boxes with a top on it (per side). I went through the ground plane method which I use quite a bit and got a nice smooth trace with a low end build up. I know it will sound decent in most clubs I run sound in.

I then ventured off into the on-axis position that McCarthy talks about. My top is up on a pole and my subs on the ground spaced about 5.5 feet apart. I use four 18 single boxes but in this case I just used two boxes. I did the top first and my trace changed quite a bit once I went on axis. However, I could see some of the same dips & peaks that I saw on the ground plane. I knew those may be realistic enough to apply the same type of Eqing in that region.

My high end 11k on up to 13k ramped waay up. I had to use a shelf to bring it back down. It does this on ground plane as well.

On my subs -- I had to make a decision on what box to align the top to. I used the closest sub and aligned the top to it. I knew from an earlier post that Harry said all other subs would then be out but at least it would be aligned to one close enough. I tried it and it looked ok.

When I fired up the other sub -- so now I have two subs running -- it totally threw the alignment off. It is just like turning the amp up. the XO point shifted and my sub level (magnitude) increased a lot.

I made the decision (tell me if it is wrong) to just measure it the way I run it at a gig. Only one side of course thinking that it will transfer to the other side once I hook it back up. So I aligned the top to two single 18 subs spaced 5.5 feet apart.

At the gig it will be four single 18 boxes spaced 5.5 feet apart across the stage and a left and right TOP. This was all done outside in the free field of course since I don't have an anechoic chamber out back.

I will then compare my saved free field trace and compare it to the venue when i get there.

What else can I do?

Thanks for all the help!


Great work. When you signal align one sub, then turn on the second you will have coupling between the 2 subs. The maximum wil be around 6dB. If you simply turn your sub drive down by about 6dB (to taste) you should be close again. The main issue I was talking about was the signal alignment (time alignment) though, not the level matching through the crossover. They need to be treated together of course.

Dr. J
July 16th, 2010, 12:18 PM
I see..... Thanks Harry & to all others who chimed in. :D

Dr. J
July 16th, 2010, 12:52 PM
Oh I forgot to tell you my observations from yesterday's session. I feel comfortable with ground plane measurements and have ventured out into on-axis measurements (Free Field first). I don't have Smaart 7 (Yet) but I started messing with averages. I did a on-axis snapshot and then a off-axis (45 degrees) snapshot of my tops on poles (D.A.S. Rf 12.64's) and then averaged them together.

I had a rough time working with STATIC traces averaging together so what I did for fun was instead of moving the mic repeatedly -- I left the mic on axis about 10-12 foot out & then found the off-axis position of the speaker by turning the cabinet on the pole and marking it. I never re-ran delay locator since I left the mic in one spot and only turned the box instead.

What I noticed from my Averaged ONAX & OFFAX Static trace is that when I ran a live trace -- the mid point (approx 22.5 degrees) looked a lot like the averaged traces from the ONAX and OFFAX. It seemed so close that I just used the mid point 22.5 degrees with a live trace running (way easier) and made my parametric changes in that position until I smoothed it out.

Just an observation and I haven't listened to the system yet to see if the work I did paid off but I will today for sure. Hopefully this isn't something totally against your experience but hey -- I will measure anything at least once to try it out.:D

Not trying to re-invent the wheel here or anything. I am new to this and I listen to you guys very closely trying pick up GEMS here and there.

I guess the easy thing to do and probably a big time saver would be to just buy Smaart 7. :D

Hopefully soon.

Harry Brill Jr.
July 16th, 2010, 11:54 PM
Oh I forgot to tell you my observations from yesterday's session. I feel comfortable with ground plane measurements and have ventured out into on-axis measurements (Free Field first). I don't have Smaart 7 (Yet) but I started messing with averages. I did a on-axis snapshot and then a off-axis (45 degrees) snapshot of my tops on poles (D.A.S. Rf 12.64's) and then averaged them together.

I had a rough time working with STATIC traces averaging together so what I did for fun was instead of moving the mic repeatedly -- I left the mic on axis about 10-12 foot out & then found the off-axis position of the speaker by turning the cabinet on the pole and marking it. I never re-ran delay locator since I left the mic in one spot and only turned the box instead.

What I noticed from my Averaged ONAX & OFFAX Static trace is that when I ran a live trace -- the mid point (approx 22.5 degrees) looked a lot like the averaged traces from the ONAX and OFFAX. It seemed so close that I just used the mid point 22.5 degrees with a live trace running (way easier) and made my parametric changes in that position until I smoothed it out.

Just an observation and I haven't listened to the system yet to see if the work I did paid off but I will today for sure. Hopefully this isn't something totally against your experience but hey -- I will measure anything at least once to try it out.:D

Not trying to re-invent the wheel here or anything. I am new to this and I listen to you guys very closely trying pick up GEMS here and there.

I guess the easy thing to do and probably a big time saver would be to just buy Smaart 7. :D

Hopefully soon.

When you are ready to buy shoot me an email. Address in my signature. Also you should consider the class. Where are you located?

Ivan beaver
July 17th, 2010, 09:46 PM
My personal philosophy on placing test mics is to ask: "What are the majority of the people in the coverage area of the loudspeaker hearing"? Obviously only a select few will be perfectly on the centerline of coverage from the loudspeaker and benefit from excellent alignment between drivers and get the full impact of the high frequencies. Unless you're trying to do crossover setup of a loudspeaker or speaker array alignment, I would advise that your microphone positions should be off centerline of the loudspeaker in various places in the coverage zone of the loudspeaker.

In addition to the ground bounce, also consider any adjacent surfaces around the loudspeaker, as these will cause cancellation in your measurement, and vary according to the position of the microphone in relation to the loudspeaker and any reflective surfaces.


I agree. Unless measuring for a spec sheet, I always place measurement mics off axis and in the "bad" positions.

Regarding the ground bounce-I pay attention to the phase trace for any possible reflections and if I see something "weird", I do further investigation as to why.

If I see something that doesn't look "right", I try to figure out if it is the acutal loudspeaker-or a mic position (floor/wall bounce) or something else.

As the old saying goes-if you don't already know the answer to a question (measurement), then how do you know your measurement is correct?:rolleyes:

Dr. J
July 19th, 2010, 10:48 AM
Yeah a class or two would help a bunch I bet. Harry, I am in central Illinois. Peoria, IL. to be exact. I see you are teaching in Indy soon. I should try and see if I can make that but it doesn't look good according to my schedule. It would be a 4-4.5 hours for me to drive that. Anymore Chicago or St. Louis dates in the future?

I didn't get to measure at my gig (a real bummer) because the place was loaded with people. This was way early in the day too. However, I did play music thru the system and toggled between my ground plane, onAX & OffAx presets in my DR260. Once again -- I found myself liking the ground plane sound better as a starting point. It just seemed clearer from the start.

The subs across the stage to me sounded really nice. The coverage was very smooth at almost any place I stood. Even the tops sounded that way. The audience seemed very pleased and I received several comments. I considered it a great gig & had tons of fun.

Can't wait to get SMAART 7 plus some training under my belt from the classes then I may be set for a while. Probably not.....:D

This stuff is addicting! I am actually a guitar player who decided that my guitar tone has to stop sucking out front from sound guys with bad sounding systems. What I learned is that most sound guys in this area -- do not even know what a flat frequency response means or how to tune their system. It is graphic EQ all the way and lots of them. I couldn't deal with all the constant fooling with the little sliders & the fact that the whole sound of the system hinged on that one piece of gear (I thought). I knew something much more was missing.

Thanks guys!

Harry Brill Jr.
July 20th, 2010, 12:41 AM
Cool!

Dr. J
July 20th, 2010, 11:32 AM
Sorry Harry -- I should have looked further down to place these discussions in the proper category. No problem.

adam
July 20th, 2010, 12:10 PM
Sorry Harry -- I should have looked further down to place these discussions in the proper category. No problem.

Actually I was following the threads and it didn't dawn on me until today to move them. No harm no foul. Besides it gave me a chance to explore the move/redirect functionality of the forums; which I seldom use. :)

Dr. J
July 23rd, 2010, 10:21 AM
Harry or anyone else -- the spacing on the SINGLE 18 boxes seemed very smooth and I think it was a huge improvement over the left and right configuration.

Is there any particular spacing for DUAL 18's? My brother-in-law has four DUAL 18's and has them all coupled together across the front of the stage. This is double the amount of subs I have and as you can imagine a ALOT of sub in such a small amount of space.

Sometimes he runs two Duals coupled in the center and the other two duals are place upright (Left & right) so he can get his tops up on them up in the air.

Thanks!

Harry Brill Jr.
July 23rd, 2010, 08:20 PM
Spacing rules are the same regardless. The distance is measured center to center.

James Woods
August 11th, 2010, 01:22 PM
Jamie added a nice presentation to the Smaart class. I usually give it the morning of the third day, before doing system measurments with the class. Apart from going over the method of aligning a system, he drives home the concept of a "Reference Mic Position".

The Reference mic position can be found by measuring both on and off axis a loudspeaker or array, and averaging the measurements. You then use the mic position that closest matches the average for doing the system EQ. This is something Dr. J has already figured out on his own. With Smaart v7 you can do this averaging live with multiple mics and find the reference mic position really fast.

Dr. J
August 13th, 2010, 11:34 AM
Jamie added a nice presentation to the Smaart class. I usually give it the morning of the third day, before doing system measurments with the class. Apart from going over the method of aligning a system, he drives home the concept of a "Reference Mic Position".

The Reference mic position can be found by measuring both on and off axis a loudspeaker or array, and averaging the measurements. You then use the mic position that closest matches the average for doing the system EQ. This is something Dr. J has already figured out on his own. With Smaart v7 you can do this averaging live with multiple mics and find the reference mic position really fast.

James -- are you saying that what I observed is actually a good way to do it? I was hoping that I wasn't trying to re-invent the wheel because I am far from being an expert here. I do however, like to try things and see for myself why something works or doesn't work.

It is hard to do static trace averages (for me) so after averaging my static traces and capturing it -- I used a live trace position that seemed to line up with my averaged static & EQ'ed it from there. Here is the kicker: I was hoping it would sound better than my ground plane measurements but once I fired the system up and toggled thru my "ground Plane", "On-Axis" & then On-Ax / Off-Ax average -- I felt that the ground plane sounded better.

I chickened out on doing the whole show with the averaged preset. I will go back and do one now so that I know. I only tested it with music off of a CD and not the band live.

Thanks for noticing the discussion and I appreciate your advice!

I really need Smaart 7 and some classes.

adam
August 13th, 2010, 11:48 AM
It is hard to do static trace averages

We agree! In fact how you average static traces has been redesigned for v7.1.

Dr. J
August 24th, 2010, 10:35 AM
One other question guys if you don't mind. I went back and did some more measuring using averages and I feel like I found a good spot to set the mic. What exactly does this do that the ground plane or ONAX position doesn't do?

I kind of gathered that really all you are doing is making the sound coverage more EVEN throughout the space rather than sounding great at the mix position. Am I on track here?

I did this with just my TOPS. I am not sure if you do this with subs as well BUT that would seem difficult to do because of their size and the way they disperse sound anyway. Thanks for the help.