Test what you think you know: Part 2

[Jay Barracato]

A friend on facebook posted a picture of the old bluegrass bass technique of rolling a SM58 in foam and stuffing it in the tailpiece. I happen to think that technique sucks, and I said so backing it up with the statement that blocking the rear ports turns the mic omni. (I thought that was a fair statement in a non technical discussion, I was talking to a bassist, after all)

Another technical person objected to this but never clarified if he thought the term omni wasn't descriptive of the change in pattern that occurs, or if he thought there was no change in pattern.

Any technical discussion of pattern should also include frequency because we know that response also depends on frequency. So there was never any doubt in my mind that "omni" was a simplification, but was it a valid one?

So, having some extra time today, I decided to test it.

First a little mic theory as I understand it:

A common way of mechanically making a mic omni in pattern is to have an enclosed chamber behind the element. This chamber has a fixed amount of air in it; therefore, the pressure and volume change in relationship to each other. This makes the diaphragm react to changes in pressure on the other side. Since sound waves are pressure waves made up of compressions and rarefactions, the vibration of the sound is transferred to the diaphragm regardless of the direction the wave is moving. It may also help to remember work is delta (pressure x volume).

If the rear of the diaphragm is open to the air the sound wave is traveling through, the alternating compressions and rarefactions act on both sides of the diaphragm if the wave is traveling in line with the vibration of the diaphragm. However, a wave coming from 90 degrees to that vibration would act on both sides of the diaphragm equally, canceling out. This creates the figure 8 pattern.

Unidirectional mics are made by adding ports to the air mass at the rear of the diaphragm which increase the amount of time it takes for the wave to arrive. The ports are basically arranged so sound waves arriving from the front of the mic add at the diaphragm, while waves arriving from the rear of the mic cancel at the diaphragm.

So blocking the rear ports would cause the air mass at the rear of the diaphragm to act more and more like a trapped air mass, hence the expectation of omni behavior.

The problem with this simplification is that the open spaces on the rear of the mic ball are not the ports to the rear of the diaphragm. The actual ports are under the mic ball. So part of the question is how much does blocking the rear of the mic ball trap air in the diaphragm chamber?

So I set out to test it...

The first question I had was: What does it mean to be omni? Is an omni pattern the same over all frequencies or is it frequency dependent?

So I started with a test of one of my TR40 measurement mics which is labeled omni








































So the first thing I noticed is that at some high enough frequency, the wavelengths become short enough that the mic body itself becomes a physical barrier to the wave. So omni doesn't mean the same at all frequencies from all directions.

So next I test the pattern of a SM58 at close proximity (1 ft) and farther from the source (3 ft). The source was Smaart generated pink noise run through an Equator D5 monitor speaker.

















































































These pretty clearly show the polar pattern and the difference between the top trace (0 degrees) and the other two traces. (90 and 180)

So what happens to the pattern if I wrap the bottom half of the mic ball? I used a piece of bubble wrap, not quite the same as you would see at a bluegrass festival using an open cell foam, but close enough for my interests. The point is block the transmission of the sound wave on part of the mic ball.

Anyway, here is what I got:








































Interesting, no real directional differences up to about 250 hz, and once again we can see the same effect in the hf as with the omni mic. What really jumps out at me is that 2000k boost which wasn't there in the unwrapped mic.

So lets look a little closer. Here is 0 degrees wrapped vs unwrapped:









































It looks to me like wrapping the mic lowered the proximity effect and added a large boost at 2 k. (That corresponds to what my ears tell me when a vocalist cups the mic)

And here are 90 degrees and 180 degrees:
















































































In both cases, the trace on top over most of the frequency range is the wrapped mic, and they show a response that is much closer to the level of the 0 degree mic, while the unwrapped mic shows the reduction in response we would expect from a unidirectional mic at 90 and 180 degrees. These graphs were from 3 ft, the graphs from 1 ft showed a similar effect that was not quite as pronounced in the frequency range where the proximity effect becomes larger.

So at this point, I think it is fair to say that wrapping the mic changes the pattern of the mic fairly significantly including reducing the proximity effect, taking on attributes of an omni pattern in both the LF and HF ranges, and adding a significant boost around 2k. I have to speculate that feedback problems associated with cupping the mic, result more from this boost than from the omni pattern.

Which leaves the question: What about cupping the mic? In these tests, the mic was wrapped tightly creating an airtight seal. My hand is not big enough to totally surround the mic ball but here are those results:







































Not quite as bad of an effect on the directional nature of the pattern, but still that big boost around 2k.

I reran the whole experiment using a supercardioid mic (Samson Q7) and a hypercardioid (EV PL80a) and got similar results.

Of course there has to be part of this that I am not sure about my explanation, and that was observed in the phase response. Both at 1 ft and 3 ft, I observed the phase changing differently with the wrapped mic versus the unwrapped. The wrapped mic started deviating rapidly from the unwrapped mic right where that 2k boost was, and by time it got to the hf the phase was spiraling badly. Since I set the timing in Smaart with the unwrapped mic, I am not sure I can attribute this to anything in particular but since wrapping the mic made changes to the timing of the arrival of the sound wave at both sides of the diaphragm, it does make sense that the phase response was changed.

 

[Spenser Hamilton]

Re: Test what you think you know: Part 2

Thanks for the post Jay, definitely gives us all some food for thought. :thumbup::thumbup::thumbup:

 

[Art Welter]

Re: Test what you think you know: Part 2

1)A friend on facebook posted a picture of the old bluegrass bass technique of rolling a SM58 in foam and stuffing it in the tailpiece. I happen to think that technique sucks, and I said so backing it up with the statement that blocking the rear ports turns the mic omni. (I thought that was a fair statement in a non technical discussion, I was talking to a bassist, after all)
2)Another technical person objected to this but never clarified if he thought the term omni wasn't descriptive of the change in pattern that occurs, or if he thought there was no change in pattern.
3)So next I test the pattern of a SM58 at close proximity (1 ft) and farther from the source (3 ft). The source was Smaart generated pink noise run through an Equator D5 monitor speaker.
4)So what happens to the pattern if I wrap the bottom half of the mic ball? I used a piece of bubble wrap, not quite the same as you would see at a bluegrass festival using an open cell foam, but close enough for my interests. The point is block the transmission of the sound wave on part of the mic ball.
5)So at this point, I think it is fair to say that wrapping the mic changes the pattern of the mic fairly significantly including reducing the proximity effect, taking on attributes of an omni pattern in both the LF and HF ranges, and adding a significant boost around 2k. I have to speculate that feedback problems associated with cupping the mic, result more from this boost than from the omni pattern.

Jay,

Nice to see some measurements to test theory.

1) Using open cell foam to mount a mic in the tailpiece does not block the ports any more than a thick windscreen. Using closed cell foam for wrapping a mic sucks.
2) The change in pattern created when using the cardioid mic mounted on the surface of the bass is similar to that of a PCC (phase coherent cardioid), the surface of the bass is a boundary, the mic in close proximity to the boundary is not subjected to multiple path length reflections.
3) Neither of the two distance tests remotely resemble an upright bass sound source.
4) "Not quite the same" is an understatement, one would not consider using bubble wrap as a windscreen, and the bubble wrap (two thin pieces of plastic with a variable air pocket) is also not similar to a hand cupping a mic.
5) Yes, wrapping a mic in bubble wrap adversely affects both the pattern and frequency response.

Wrapping a mic in open cell foam and placing in the tailpiece actually works far better than a stand mounted mic for sound reinforcement of an upright bass for the following reasons:
1) The mic position remains consistent regardless of the bass movement, freeing the player to play rather than keep his instrument an exact distance from a stand mounted mic.
2) The bridge position is the loudest point of an upright bass, other than F holes, which have a hollow, boomy sound, and promote resonant feedback. Loudest source makes for more gain before feedback. It is in theory possible to keep a stand mounted mic close to the bass, but in the reality of nearly every upright bass player I have encountered, the mic distance in performance always varied by at least a 2 to 1 ratio, a six dB variation in level and drastic tonal change.
3) The boundary effect eliminates ragged response associated with the multiple surface reflections of a stand mounted mic. Smoother response means better sound quality, as well as more gain before feedback, especially critical when stage monitors are close to the bass.

For the above reasons, and years of comparing on many different jazz, rock and bluegrass shows, I find the tailpiece position to work better than any stand mount alternative.

Art

 

[Jay Barracato]

Re: Test what you think you know: Part 2

Jay,

Nice to see some measurements to test theory.

Art, Thanks for participating in the conversation. I have some comments about yours

1) Using open cell foam to mount a mic in the tailpiece does not block the ports any more than a thick windscreen. Using closed cell foam for wrapping a mic sucks.

I disagree that a 2 1/2 piece of foam is no different than a windscreen. All screens attenuate sound to some degree. There is a big difference between a 1/8 in to 1/4 windscreen and 2 1/2 inches even if the material is similar.

For the record, here is the picture that started the conversation:







































The tape is also adding a barrier to air flow.

2) The change in pattern created when using the cardioid mic mounted on the surface of the bass is similar to that of a PCC (phase coherent cardioid), the surface of the bass is a boundary, the mic in close proximity to the boundary is not subjected to multiple path length reflections.

I am having trouble with seeing where you are going with this. I would not call the surface of the bass a boundary in the same sense as a PCC because the surface of the bass is the source of the sound wave. In other words, your boundary is vibrating. In addition, usually with a PCC the source is placed so the wave is traveling and vibrating parallel to the boundary, not perpendicular. If the surface of the bass is a boundary, then the instrument in any close mic situation is a boundary. I am not sure this is any different than pointing a mic across the top of a snare drum.

3) Neither of the two distance tests remotely resemble an upright bass sound source.

I used pink noise. Everything that is in an upright bass is in the source I used. Because I used the same source for both tests, any differences should be attributed to the only change made: i.e. blocking the ports.

4) "Not quite the same" is an understatement, one would not consider using bubble wrap as a windscreen, and the bubble wrap (two thin pieces of plastic with a variable air pocket) is also not similar to a hand cupping a mic.

I am not talking about a windscreen. I am talking about a mounting method that significantly blocks the port. I also never claimed the wrap was similar to a hand. For the cup test, I used my hand, which I noted is smaller than many and only incompletely blocks the lower part of the mic. I DO think it is significant that partially blocking the ports with my hand produced the same effects that blocking them with the bubble wrap did. And for the record, I have seen bubble wrap used in the stuff the end piece technique, even if camp mattress foam is by far the most common material.

5) Yes, wrapping a mic in bubble wrap adversely affects both the pattern and frequency response.

I am willing to extrapolate far more information from these tests (including several dozen traces with two other mics in addition to the ones I posted.) then you seem to be willing to. I am almost inclined to go cut up a camp mattress in the basement just to show that my model is a fair approximation of the photo that started the discussion.

Wrapping a mic in open cell foam and placing in the tailpiece actually works far better than a stand mounted mic for sound reinforcement of an upright bass for the following reasons:

Don't have to convince me there, but I am not necessarily only comparing with a mic on a stand. There are dozens of easy ways more appropriate mics can be mounted on a bass. As much of sound reinforcement has improved by people trying new things, I see no reason to stick with a 30 year old technique that was a compromise to begin with. (Unless you really need to use a 30 year old Shure Brothers Uniball mic onstage. That happened to me on a side stage at a festival in Kentucky)

For the above reasons, and years of comparing on many different jazz, rock and bluegrass shows, I find the tailpiece position to work better than any stand mount alternative.

Art

Once again, I will concede the tailpiece is a slightly better position than a stand mount; however, I am convinced we can do much better. In those famous, loosely quoted words, "This is not my first rodeo". I have had the privilege to work with most of the top instrumentalists in bluegrass, and talk with them about what they are trying to do with their tone and how to achieve it. All I can say is that I can't imagine any of them arriving on a fly date to a rented instrument and being happy with the 58 in the tailpiece solution. (This assumes that they have set up their personal instrument so this would never be necessary)

 

[Jay Barracato]

Re: Test what you think you know: Part 2

Here is my personal instrument showing a technique I have used with **********, *************, and **************.
[/ATTACH]

Sent from my DROID RAZR HD

 

[David Morison]

Re: Test what you think you know: Part 2

A friend on facebook posted a picture of the old bluegrass bass technique of rolling a SM58 in foam and stuffing it in the tailpiece. I happen to think that technique sucks, and I said so backing it up with the statement that blocking the rear ports turns the mic omni. (I thought that was a fair statement in a non technical discussion, I was talking to a bassist, after all)

Nice to see some measurements to test theory.

1) Using open cell foam to mount a mic in the tailpiece does not block the ports any more than a thick windscreen. Using closed cell foam for wrapping a mic sucks.

I've never had to do it, but I would have considered Art's point axiomatic. Surely that renders the original argument moot, no?

EDIT Just saw the photo you posted, probably while I was replying - that does look like more of an obstruction than I'd imagined.


David.

 

[Jay Barracato]

Re: Test what you think you know: Part 2

I've never had to do it, but I would have considered Art's point axiomatic. Surely that renders the original argument moot, no?
Or to put it another way, why would anyone even think of using a closed cell/high density foam for that application?

Looking forward to any enlightenment you can share,
David.

Because the people doing it don't understand how they are changing the behavior of the mic. I have never heard anyone using the technique actually discuss the type of foam used. I have seen towels used, sweatshirts used, bubble wrap used, both open and closed cell camp pads, and any assortment of packing materials. I have even seen aurelux used, an expensive way to quickly suck.

My opinion, based on my experience, is the resulting sound sucks. If what you want for the bass is a muffled thud, with no definition, no attack, and no sustain, this might be the technique for you.

Some people will trust my experience, others will want some reason. I think I can now say, based on this, that blocking the ports changes the response of the microphone. We can quibble all day about how much different materials will block the ports, but while that is happening I am just going to walk over to my mic box and pull out a pro35 which is my quick solution for a bass with no pickups that the player doesn't have their own preferred method.

 

[David Morison]

Re: Test what you think you know: Part 2

Because the people doing it don't understand how they are changing the behavior of the mic. I have never heard anyone using the technique actually discuss the type of foam used. I have seen towels used, sweatshirts used, bubble wrap used, both open and closed cell camp pads, and any assortment of packing materials. I have even seen aurelux used, an expensive way to quickly suck.
<snip>
Some people will trust my experience, others will want some reason..

Sorry Jay, I think we ended up composing replies at the same time - your photo for clarification wasn't up when I started replying, and I've edited my post to reflect that.

Would never argue against blocking ports affecting the mic, and it's always good to see measurements to back things up - it just hadn't been evident from your opening paragraph just how much the mic might be blocked in that particular implementation of the technique. Your photo cleared that up.

Thanks again,

David.

 

[Jay Barracato]

Re: Test what you think you know: Part 2

David, the "some people" I was referring to aren't the people reading this post. You all can decide on your own.

The "some people" are some of the bassists I work with who think this technique isn't filled with IMHO compromises to the sound of their instrument.

Sent from my DROID RAZR HD

 

[Dave Barnett]

Re: Test what you think you know: Part 2

So at this point, I think it is fair to say that wrapping the mic changes the pattern of the mic fairly significantly including reducing the proximity effect, taking on attributes of an omni pattern in both the LF and HF ranges, and adding a significant boost around 2k. I have to speculate that feedback problems associated with cupping the mic, result more from this boost than from the omni pattern.

I've been trying to convince people for years that cupping the mic creates a cavity resonance, and this is what causes all the problems, not any change in the pattern.

 

[Tim McCulloch]

Re: Test what you think you know: Part 2

I've been trying to convince people for years that cupping the mic creates a cavity resonance, and this is what causes all the problems, not any change in the pattern.

It's a cranial resonance. /nudge, wink.

 

[Art Welter]

Re: Test what you think you know: Part 2

1)I disagree that a 2 1/2 piece of foam is no different than a windscreen. All screens attenuate sound to some degree. There is a big difference between a 1/8 in to 1/4 windscreen and 2 1/2 inches even if the material is similar.
2) I would not call the surface of the bass a boundary in the same sense as a PCC because the surface of the bass is the source of the sound wave. In other words, your boundary is vibrating. In addition, usually with a PCC the source is placed so the wave is traveling and vibrating parallel to the boundary, not perpendicular. If the surface of the bass is a boundary, then the instrument in any close mic situation is a boundary. I am not sure this is any different than pointing a mic across the top of a snare drum.
3)I used pink noise. Everything that is in an upright bass is in the source I used. Because I used the same source for both tests, any differences should be attributed to the only change made: i.e. blocking the ports.
4)I am not talking about a windscreen. I am talking about a mounting method that significantly blocks the port. And for the record, I have seen bubble wrap used in the stuff the end piece technique, even if camp mattress foam is by far the most common material.
5)Once again, I will concede the tailpiece is a slightly better position than a stand mount; however, I am convinced we can do much better.

Jay,

1) 2 1/2 inches of closed cell foam wrapped with tape certainly would be different than wind screens, which do have different high frequency attenuation and diffraction effects. Good wind screen material (open cell foam) hardly has any effect on frequency response in the usual bass region. Some test examples using different materials covering a mic below.
2) An upright bass miced from the bridge position is a boundary, as is a large speaker baffle. A mic close to the skin of a drum would have a boundary, on the side of a drum head as is common, the drum would not form a boundary. The Crown PCC (Phase Coherent Cardioid)mic element is perpendicular to the source, the Crown PZM (Pressure Zone Microphone) element is is parallel to the boundary. Both are just smaller updated implementations of the old EV "Mike Mouse" concept, which is exactly what a bridge mounted bass mic emulates if done properly.
3) Pink noise is a good source, but the proximity (1 and 3 feet) and size of the pink noise source (a small monitor) you used are not similar to a bridge miced upright bass. A dynamic cardioid mic placed on a speaker baffle of similar dimensions to an upright bass would show a completely different frequency response to your tests, and be more similar to what you ostensibly were trying to test.
4) I wrote "using closed cell foam for wrapping a mic sucks", the wrap you were "talking about" but did not post a photo of in your OP is an example of that sucky approach. Similar to people that wrap SM 57 wind screens with tape (blocking the ports behind the element)because they lost the screen retaining clip, people do a lot of sucky sound shit.
5) I wrote "tailpiece" by mistake, the tailpiece is the string attachment point at the bottom of the bass. The correct place to mount the mic is under the bridge, where the soundboard is generally loudest and has the most balanced tone, combining the body resonance and the string tone.

I certainly would not suggest that an SM58 is the ultimate microphone choice for an upright bass, though properly mounted under the bridge it can give quite good results.
There are many microphones that may do better in that position.

What are you convinced is much better?

Art

 

[Jay Barracato]

Re: Test what you think you know: Part 2

Jay,

1) 2 1/2 inches of closed cell foam wrapped with tape certainly would be different than wind screens, which do have different high frequency attenuation and diffraction effects. Good wind screen material (open cell foam) hardly has any effect on frequency response in the usual bass region. Some test examples using different materials covering a mic below.

At this point I have no idea how you are using a windscreen type material to mount and support the mic. I will say if the mic is in contact with the bridge or the soundboard, a major transmission is going to come from the physical contact.

2) An upright bass miced from the bridge position is a boundary, as is a large speaker baffle. A mic close to the skin of a drum would have a boundary, on the side of a drum head as is common, the drum would not form a boundary. The Crown PCC (Phase Coherent Cardioid)mic element is perpendicular to the source, the Crown PZM (Pressure Zone Microphone) element is is parallel to the boundary. Both are just smaller updated implementations of the old EV "Mike Mouse" concept, which is exactly what a bridge mounted bass mic emulates if done properly.

I understand how both PCC and PZM mics work. Since they are based on making the time of travel to the element the same for both direct sound and reflected sound. I do not see how that applies to the sound of the bass in this case because, as I previously said, the source of the sound is the soundboard. If the soundboard is the source of the sound, it can't also be a source of a reflection. The soundboard could act as a boundary to bleed from other sources, but not to the bass itself.

3) Pink noise is a good source, but the proximity (1 and 3 feet) and size of the pink noise source (a small monitor) you used are not similar to a bridge miced upright bass. A dynamic cardioid mic placed on a speaker baffle of similar dimensions to an upright bass would show a completely different frequency response to your tests, and be more similar to what you ostensibly were trying to test.

Sorry, not buying it. You haven't convinced my the soundboard is a boundary, nor how the sounds produced by a bass are somehow different from the sounds in the pink noise. This is a dual path measurement. I am not looking for what the sounds are, but for differences between the two conditions.

4) I wrote "using closed cell foam for wrapping a mic sucks", the wrap you were "talking about" but did not post a photo of in your OP is an example of that sucky approach. Similar to people that wrap SM 57 wind screens with tape (blocking the ports behind the element)because they lost the screen retaining clip, people do a lot of sucky sound shit.

No doubt but I am still not sure what technique you are talking about, because the monstrosity I posted is exactly what is seen at many festivals.

5) I wrote "tailpiece" by mistake, the tailpiece is the string attachment point at the bottom of the bass. The correct place to mount the mic is under the bridge, where the soundboard is generally loudest and has the most balanced tone, combining the body resonance and the string tone.

I certainly would not suggest that an SM58 is the ultimate microphone choice for an upright bass, though properly mounted under the bridge it can give quite good results.
There are many microphones that may do better in that position.

What are you convinced is much better?

Art

I already showed a picture of a bridge mounted EV nd487, which is extremely feedback resistant, produces thundering bass, and can be tilted to dial in the exact ratio of fingerboard to body that you want.

I also like to use small body condenser mics mounted in a couple of different ways but by far my go to is a pro35, atm350, or 9006 either with the boom or taken off the boom.
Here are some of my techniques:

The clip on mics can be attached to the f hole, or the bridge. When attaching to the bridge I often use these small modelers clamps. They are lightweight, can attach in a number of positions, and provides a solid place to attach the clip on mics. I especially like to take the mic element off the boom and just tape it to the clamp. Great sound, low profile, and solid attachment to the instrument.

I have also used the arm from a audix mic clamp rubber banded across the bridge to hold a small body condenser pointed just to the side of the sound post. In the photo it has a regular clamp on it, because that was what was there when I pulled it out of my possibles box, but it works well with the smaller clamps.

Sent from my DROID RAZR HD

 

[Art Welter]

Re: Test what you think you know: Part 2

1)At this point I have no idea how you are using a windscreen type material to mount and support the mic. I will say if the mic is in contact with the bridge or the soundboard, a major transmission is going to come from the physical contact.
2)I understand how both PCC and PZM mics work. Since they are based on making the time of travel to the element the same for both direct sound and reflected sound. I do not see how that applies to the sound of the bass in this case because, as I previously said, the source of the sound is the soundboard. If the soundboard is the source of the sound, it can't also be a source of a reflection. The soundboard could act as a boundary to bleed from other sources, but not to the bass itself.
3)You haven't convinced my the soundboard is a boundary, nor how the sounds produced by a bass are somehow different from the sounds in the pink noise. This is a dual path measurement. I am not looking for what the sounds are, but for differences between the two conditions.
4)I already showed a picture of a bridge mounted EV nd487, which is extremely feedback resistant, produces thundering bass, and can be tilted to dial in the exact ratio of fingerboard to body that you want. I also like to use small body condenser mics mounted in a couple of different ways but by far my go to is a pro35, atm350, or 9006 either with the boom or taken off the boom.
Here are some of my techniques:
The clip on mics can be attached to the f hole, or the bridge. When attaching to the bridge I often use these small modelers clamps. They are lightweight, can attach in a number of positions, and provides a solid place to attach the clip on mics. I especially like to take the mic element off the boom and just tape it to the clamp. Great sound, low profile, and solid attachment to the instrument. I have also used the arm from a audix mic clamp rubber banded across the bridge to hold a small body condenser pointed just to the side of the sound post. In the photo it has a regular clamp on it, because that was what was there when I pulled it out of my possibles box, but it works well with the smaller clamps.

Jay,

1) Open cell foam similar to what is used in windscreens is used for packing material in many microphone shipping boxes. It has very low vibration transmission acoustically isolating it from the bridge and soundboard. The material can simply be wrapped around the mic and put under the bridge, it has no effect on the sound of the mic.
2) The soundboard acoustically amplifies the string vibrations, as well as being a boundary for a bridge mounted mic.
3) The sound of the bass has similar frequency content to pink noise, no argument there. The proximity of the mic in relation to the sound producer is the element of discussion, a non-variable D cardioid has a totally different bass response at one inch from a sound source than at one or three feet.
4) The EV nd487 has lots of low frequency bass boost proximity effect, no doubt it can produce thundering bass. All the techniques you mention are good, though they may not provide as much vibration decoupling as foam. They do provide far more flexibility to find the "sweet spot", given the time to experiment, and the lighter mics will have less acoustical effect on an instrument than a heavier mic.

The examples below show various mics response at the same close distance to a 10" speaker on a large baffle. The RTA-420 is a omni test mic (no proximity effect), the speaker was equalized for roughly flat response for it, other mics were compared. It is interesting to note the 7 dB 35 Hz low frequency proximity effect boost the AT37 has. Also interesting is the Shure Beta 58 low frequency response looks almost the same as the RTA-420, just 3-4 dB less, while the latter Beta 58A and Beta 57 show a greatly reduced low frequency proximity gain. They make for less boomy vocals, but way less LF thunder.

Also noteworthy is the difference in phase response of the condenser mics (RTA-420, AT37) compared to the dynamic mics. When mixing a mic and a pick-up transducer, the two phase responses never are the same, but either a condenser or a dynamic may better compliment the particular transducer .

Art

 

[Jay Barracato]

Re: Test what you think you know: Part 2

Jay,

1) Open cell foam similar to what is used in windscreens is used for packing material in many microphone shipping boxes. It has very low vibration transmission acoustically isolating it from the bridge and soundboard. The material can simply be wrapped around the mic and put under the bridge, it has no effect on the sound of the mic.

We can keep your photos as an example of how to do this is you have to...

But,
Your setup doesn't tell us anything about the effect of the foam and if the foam is causing any differences from the normal mic response. But I am not sure that even matters because my original testable question was not What is the effect of wrapping the mic in foam? it was What is the effect of blocking the rear ports of a directional mic? Given my question, your test doesn't tell me much because I don't see the foam blocking the ports in any way. A better test to tell if the type of foam is a critical variable would be to move your wrap forward until the ports are covered.

In addition, I think you are quibbling over the type of foam. Open cell foam comes in many different densities. The foam used in the picture I posted is open cell (air escapes when it is compressed) but of a much higher density than what you pictured.

Given that the length of the mic ball/diaphragm corresponds to a wavelength of about 4000 hz. I would not expect any type of foam as you have placed it to make a major difference. Actually at that wavelength I would expect the mic body itself to be a larger source of scattering.

2) The soundboard acoustically amplifies the string vibrations, as well as being a boundary for a bridge mounted mic.

We are going to have to agree to disagree on this. I can still see no way that this affects the situation. You keep saying boundary without explaining why you think that boundary is important. My point, once again, is that the boundary serves to make the time of arrival of the direct sound and the reflected sound as close as possible. If the soundboard is the source of the sound, it can't be a boundary as well because the sound is not reflected off of it. I can think of no other significant source of sound around the bridge area that might possible be reflected. The strings themselves make an insignificant amount of sound when compared to the soundboard (try playing an electric upright like a NS designs unplugged).

3) The sound of the bass has similar frequency content to pink noise, no argument there. The proximity of the mic in relation to the sound producer is the element of discussion, a non-variable D cardioid has a totally different bass response at one inch from a sound source than at one or three feet.

Once again I think you missed a basic premise of my experimental design. (This is also reflected in the lack of a control in what you posted) I am looking at the difference in the response that can be isolated as being caused by the single change made. If you change more than one variable, you can't attribute changes to anything. No one denies that proximity effect changes the absolute response, but I am comfortable that the trend in the change observed at 1 ft and 3 ft can be extrapolated to closer differences. I would like to test it, but it is a physical impossibility to place the rear of the diaphragm in a 8 inch long mic closer to the source than that. In order to compare response it is critical that the measurement at 0 degrees and 180 degrees take place at the same distance from the source.

4) The EV nd487 has lots of low frequency bass boost proximity effect, no doubt it can produce thundering bass. All the techniques you mention are good, though they may not provide as much vibration decoupling as foam. They do provide far more flexibility to find the "sweet spot", given the time to experiment, and the lighter mics will have less acoustical effect on an instrument than a heavier mic.

Actually with my nd487 technique the physical coupling is part of the magic. I don't want the mic decoupled from the instrument. With the other techniques there is almost no coupling. When the pro35 is used in the boom, the element is just supported by the windshield, when it is attached to my little modeler's clamps, I attach it by a loop in the cord so the element hangs free.

The examples below show various mics response at the same close distance to a 10" speaker on a large baffle. The RTA-420 is a omni test mic (no proximity effect), the speaker was equalized for roughly flat response for it, other mics were compared. It is interesting to note the 7 dB 35 Hz low frequency proximity effect boost the AT37 has. Also interesting is the Shure Beta 58 low frequency response looks almost the same as the RTA-420, just 3-4 dB less, while the latter Beta 58A and Beta 57 show a greatly reduced low frequency proximity gain. They make for less boomy vocals, but way less LF thunder.

Also noteworthy is the difference in phase response of the condenser mics (RTA-420, AT37) compared to the dynamic mics. When mixing a mic and a pick-up transducer, the two phase responses never are the same, but either a condenser or a dynamic may better compliment the particular transducer .

Art

I have never noticed that really low proximity effect in the pro37 but I usually use the LF rolloff on it. Once again, I think your test shows a comparison of different mics in a particular situation, but it doesn't tell me if the foam has any effect, or what the effects of blocking the ports would be.

I think I have to stand by my original conclusions:

I said in the original Facebook conversation that blocking the rear ports of a directional mic changes the pattern to omni.

While the context was about stuffing the tailpiece of an acoustic bass, the sound profile of the instrument, the proximity effect of different mics, or even the type of foam have nothing to do with the question I posed.

My test shows a pretty positive result.

So to extend this to the bass question, if you block the ports you will change the response of the mic. If you have some sort of acoustic transparent foam, fine; or if you can wrap the mic so the ports are not blocked, fine; but, if you block the ports the response will change.

I will not be doing any experimentation during any live shows. A pro35 is about $129. If you are working with acoustic basses a lot, I recommend you just get one. It is a lot smaller package then carrying around a bunch of foam.

 

[Greg Cameron]

Re: Test what you think you know: Part 2

AI disagree that a 2 1/2 piece of foam is no different than a windscreen. All screens attenuate sound to some degree. There is a big difference between a 1/8 in to 1/4 windscreen and 2 1/2 inches even if the material is similar.

Agreed. On the Pearl Jam '03 tour I went out on, We tried out Audix's new spring steel mic balls on the OM7 and there was a noticeable change in sound relative to the older original "crushable" mic balls. The gauge of the wire used to form the ball was probably slightly different. We wound up sticking with the originals for the time being since PJ runs really hot monitors and keeping them stable with known quantity mics is paramount. Not to mention that the lead vocalist was used to a particular sound of the mic.