How close, is too close = Mic distance from Acoustic Panels

[AcousticDreams]

As many of you may know by now, I tend to question every scenario in the realm of sound. Testing may be the ultimate decision maker, however it is always good to start with some known theories or personal experiences.

In some minor experiments between my friend and I, we have noticed that what works for him, does not work for me. Most of this is due to playing style. The guitar in which I sound the best on...is not the guitar he sounds best on. We both agree upon this after switching guitars back and forth several times. And the same thing for recording technique. The mic distance-angle-placement that works for him, does not work for me. And the difference is not small...but very significant.

While Acoustic Panels are meant to absorb and not reflex sound, I am wondering if they can interfere with the Mics polar pattern causing other effects? This may not be as far out of a question as you might first surmise. Looking at what causes Proximity effect :"Since it is the difference in pressure that causes the mic’s diaphragm to move, the low-frequency sensitivity of the mic for very close sources is greatly exaggerated compared to high frequencies—we hear this as emphasized bass in the micÂ’s output, and call it the proximity effect." Of course Proximity effect is even more complicated than that with some adding the bass response is part of a phase shift.

If Proximity effect is caused by closeness of sound source, then is it not possible that an acoustic panel too close to the microphone could cause some additional pressure levels? Of course, an acoustic panel is not a sound source, however is it not possible that the panels could interfere with the polar pattern?

Which brings me to my Question of: How close is too close to an acoustic panel? Using wide cardiods, it is a possibility that the side panels could create uneven pressure levels. Especially since the distance of the mic to the side panel, is closer than the distance to the other side panel. it is unbalanced.

For my portable panels this might be something to consider. While some may say that I am over thinking this, there is some studies that indicate there could even be a focusing of frequencies. Hmmm? Maybe I need to be back further and spread the wings out. But then again...this lets out more sound.

I am hoping that someone here has done some testing on this subject matter.

[ChuckS]

I think a major consideration is the design of the absorption panel. Some panels have an enclosed/rigid back and some have an open back. I would think panels with an open back would be less of issue (re: distance from mic to panel) because I would think there would be less source sound reflected back to the mic.

[Bob Womack]

The effect you are wondering about is called the "Boundary Layer." Bass travels along boundaries, and rises there as well. This is one of the reasons small rooms can be a problem - you can't get away from the bass rise caused by the boundary effect. You can demonstrate this to yourself: play some music and walk over to a wall. As you approach it, you can hear the bass tilt up. It also works with sound sources: as you move a speaker towards a wall you can hear the bass tilt up through an effect called "coupling."

It also works with microphones. Have you heard of PZM microphones? A designer noticed the tilt-up as the mic moved in very close to the boundary into what is referred to as the "pressure zone." The designer figured out that he could take a very limited bandwidth mic and use the bass tilt up when it was built onto a small, permanent boundary layer. It could be tuned to be a full-bandwidth mic with a hemispheric pickup pattern.

How does that affect you? The closer you place your instrument or microphone to a boundary, including your gobos, the more bass you will pick up. By and large, gobos, ie. acoustic absorbent panels, behave as boundaries on bass frequencies. This factor can either be used by you or you can be a victim of it. Be aware that you will experience the least bass build-up in the middle of a large room without gobos nearby. Of course, for that you work, the room must sound good on its own. It is all a trade off.

Bob

[keith.rogers]

Long ago I watched a video where some guy tested different gobo configurations, and while (IIRC) it was with vocals, and in a good sized space, the most effective placement (again IIRC) was having a "V" both in front and behind the source, with the cardioid pattern mic aligned between the Vs: < -- O >. And, yes, you want to be in the middle of the room. Then, most of the room badness is at least as far from the mic as possible.

If your ceiling is very low and clouds are not an option, you might want to experiment with aiming the mic down a bit, too. And definitely work on optimizing a single mic recording before adding a second.

[KevWind]

Originally Posted by Knives&Guitars

As many of you may know by now, I tend to question every scenario in the realm of sound. Testing may be the ultimate decision maker, however it is always good to start with some known theories or personal experiences.

I am hoping that someone here has done some testing on this subject matter.

First no I have not tested recording close to and then further away to an acoustic panel .
I have always had my mics probably at least 6 or 7 ft way from panels. My current setup and positioning that you can see in videos posted in "Show and Tell" I have the mics at about 8 ft away from the back wall behind the mics and about 4 to 6 ft away from the closest side wall My room is a half vault with the short wall at 8 ft and the tall wall at 12' 6"


BUT I am curious about two things
#1 why would you even consider recording close (lets say less than 3 or 4 ft ) to an acoustic panel ?
#2 What kind of acoustic panels are those ? They look like only maybe 2 inches deep and more like office space dividers..And I am guessing if those are metal frames on the side panels , they would be fairly reflective and probably not in a good way ?

[ChuckS]

Originally Posted by Bob Womack

... [snip]...

How does that affect you? The closer you place your instrument or microphone to a boundary, including your gobos, the more bass you will pick up. By and large, gobos, ie. acoustic absorbent panels, behave as boundaries on bass frequencies. This factor can either be used by you or you can be a victim of it. Be aware that you will experience the least bass build-up in the middle of a large room without gobos nearby. Of course, for that you work, the room must sound good on its own. It is all a trade off.

Bob

Does a gobo that does not have a back panel (the absorption material is open to the front and to the back) also act as a barrier to bass frequencies? If so, at what distance to the mic does it become significant?

[AcousticDreams]

Originally Posted by Bob Womack

you will experience the least bass build-up in the middle of a large room without gobos nearby.
Bob

What is your recommendation on distance away from the gobo's?

Originally Posted by keith.rogers

the most effective placement (again IIRC) was having a "V" both in front and behind the source, with the cardioid pattern mic aligned between the Vs:

If your ceiling is very low and clouds are not an option, you might want to experiment with aiming the mic down a bit, too. And definitely work on optimizing a single mic recording before adding a second.

I was planning on experimenting with the V configuration..with the mic in closer. But this then would block the side angles that the Wide Cardiod would pick up. Defeating the purpose of that polar pattern. And also possibly create pressure boundaries.

I like the Idea of slightly angling down. I am big believer in Mic Angles.

Originally Posted by KevWind

.
I have always had my mics probably at least 6 or 7 ft way from panels.

BUT I am curious about two things
#1 why would you even consider recording close (lets say less than 3 or 4 ft ) to an acoustic panel ?
#2 What kind of acoustic panels are those ? They look like only maybe 2 inches deep and more like office space dividers..And I am guessing if those are metal frames on the side panels , they would be fairly reflective and probably not in a good way ?

1.If you exclude the concept of pressure boundaries, and think of them as solely for their absorption abilities, you would think that placing them closer to the panels would seemingly isolate the mics better. In fact I have seen many recording videos with panels like this inches around a drum set. Further I have seen many companies selling vocal booths. That is the theory of which I thought these panels were based on--to isolate.

Pressure boundaries that might effect the polar pattern is something I did not even think about till yesterday.

While I knew of the problems of reflectivity with walls and surfaces, the thought of reflectivity never occurred to me with acoustic panels. As they suppose to be of a non reflecting nature.

But then yesterday, I wondered if there could be some sort of pressure build up from the panels effecting the polar pattern. I did not know if even this was a possibility. This is different from reflectivity. This has to do with pressure from the surrounding air space.

2. The Panels are from a designer who has quite a reputation for acoustic design-knowledge in the industry. His formulation claims that with acoustic guitar, panels do not need to be 4 inches thick. And his aluminum thin framing is so small that it will not affect the sound. I am not making any claims that this is true....I am only repeating what he claimed. I was only basing my decision on his claims. I sent diagrams & pictures of my room dimensions. And this is what he recommended.

Looking at this mechanically, I would think that my panels are designed to do as he stated. To Block sound from going outward and being reflected back into the microphones. Therefore placing the microphones closer to the panels might block the sound from going out into the room more. In many ways it is the very same concept with close micing. The closer you mic, the less room you can hear. The Closer the mics are to an absorption panel, the less sound enters the room. That was the concept of which I was told.

The very first video , in our wonderful video tutorials for room treatment listed is "Just Two Broadband Panels" And he has the panels very close to the microphones.

As a small time recordist, I have the option to make the room itself completely sound conducive, or to create portable sound improvements. I use Open walled Living room to den for many purposes and needs to be converted quickly. The advantage of this room is it naturally sounds better than any of my other rooms. These retractable panels allowed me the easiest way to convert back and fort when needed.

Again, not saying that the approach I have taken is the correct one. It was only a solution that one acclaimed designer said was a good one.

* This is probably the hardest part for a recording hobbyist to overcome. He can purchase reasonable recording equipment...but making his room sound good is not always an easy task.

[Bob Womack]

Originally Posted by ChuckS

Does a gobo that does not have a back panel (the absorption material is open to the front and to the back) also act as a barrier to bass frequencies? If so, at what distance to the mic does it become significant?

You know, it all depends on the material and the construction. But typically, it isn't all that affected by rigidity. You can move close to a temporary office panel wall with two 1" sheets of absorptive (each backed by a thin rigid liner, back to back) and hear the bass tip up.

Originally Posted by Knives&Guitars

What is your recommendation on distance away from the gobo's?

I don't have one. This is an aesthetic choice for the recordist based upon the room. I work in treated rooms and typically like to seat a guitarist at least three feet away from a wall but it usually starts in the center or, if there is a location that sounds better, there. The point is to be aware of the principle and move according to what sounds best. Expect bass build-up near barriers, and especially near corners because they will feature a confluence of three to four barriers (two walls, floor, and ceiling), depending on ceiling height. Keep your ear open for the influence and use it or avoid it to your benefit.

Bob

[keith.rogers]

Originally Posted by Knives&Guitars

...
I was planning on experimenting with the V configuration..with the mic in closer. But this then would block the side angles that the Wide Cardiod would pick up. Defeating the purpose of that polar pattern. And also possibly create pressure boundaries.
....

You have lost me a bit here. If [with that choice of pattern] you want to capture the room why are you using gobos?

Strictly IMO, the acoustic guitar is not creating a huge sound-space on its own that needs anything other than a cardioid pattern, or maybe two, in any kind of "studio" setting. Room treatment, even a temporary kind, is probably a better way to think about the problem of a small space, because every [reflective] surface is too close to keep entirely out of the mic's pattern, unless you build a box.

[AcousticDreams]

Originally Posted by keith.rogers

You have lost me a bit here. If [with that choice of pattern] you want to capture the room why are you using gobos?

Strictly IMO, the acoustic guitar is not creating a huge sound-space on its own that needs anything other than a cardioid pattern, or maybe two, in any kind of "studio" setting. Room treatment, even a temporary kind, is probably a better way to think about the problem of a small space, because every [reflective] surface is too close to keep entirely out of the mic's pattern, unless you build a box.

Polar patterns are not always about capturing room sonics. Polar patterns can be about capturing the entire Body of the instrument . And this is so with my style harder, louder style of playing. I find Cardiod and Hypercardiod generally lacking for me. And it is not because they are not picking up the room. This has nothing to do with Room sound...it is the instrument sound.

Testing on a few friends of mine, a cardiod pattern works find. They agree. On me, cardiod does not work. They also agree. I choose Wide cardiod to capture all of the guitar. If I had a great room, I would choose an Omni polar pattern. Omni is the sound I like the best. But, I do not have a good room. Wide Cardiod gives me some of the Omni sound, with some of the cardiod rejection.