Thursday, November 1st, 2012 | by Peterson Goodwyn
Here is Part II of our 3-part video series about the inner workings of the most famous dynamic microphones ever made: the Shure SM57 and SM58.
Your host is Peterson Goodwyn of DIYRecordingEquipment. With him is Shure’s Product Specialist for wired microphones, John Born. The two spent a day at Shure’s headquarters in Niles, IL, discussing how dynamic microphones work.
In Part I, How Dynamic Microphones Work, we saw all the components of the SM57, from the Mylar diaphragm, to the coil, the magnet, and the transformer.
Here in Part II, we learn how Shure engineers a Cardioid polar pattern, and a useful frequency response into the Unidyne III cartridge — using little more than vents, fabric, and air! And a whole lot of complex math.
Tune in tomorrow for Part III, in which the 50 year old secret to Shure’s “pneumatic shockmount” will be revealed.
Hi, welcome back to RecordingHacks. I’m Peterson Goodwyn from DIYRecordingEquipment. I am here again with John Born, continuing our discussion of dynamic microphones, how they work, and their place in the modern world. In our first section we covered a very simple version of how they work, without looking at all at these real-world problems that we’re bound to run into. We finished with the output signal from our transformer… what do we have there, and how does it compare to our input signal, and let’s go from there.
JB: There are some inherent things that we’re going to want to fix in a dynamic microphone… if you look at the coil, and the magnet, and the diaphragm, and you put that stuff together, you’ll get a signal level of some sort. You can improve some things with the transformer, but [making a useful microphone] is a lot more complex than just that.
I was afraid of that.
JB: It’s deep. It’s deep and ugly! But we’re going to go there. The inherent things that are wrong with a dynamic microphone [include that] there is a resonant frequency to the diaphragm. It has a frequency response that isn’t quite desirable — it’s not linear, by any means; it has big peaks and valleys to it. We’re going to want to fix the response. We are also going to want to create a directional microphone, somehow; make a unidirectional microphone, instead of omnidirectional. And the last thing is, a dynamic microphone is inherently an accelerometer. It needs to be isolated from mechanical vibrations. We’re going to need to put some pretty good shockmounting around this product. We’ll go into all those three things. Let’s start with the response.
Fixing the frequency response
There is a resonant frequency to the coil and diaphragm assembly that we’re going to want to basically damp down and make it flatter. The resonant frequency of the diaphragm of an SM57 or SM58 is pretty low, in the 100Hz area. That means there would be a big bump in the [microphone's] response [in the 100 Hz area] that we would want to tame.
And that’s just because of the physical character[istics] of the Mylar and the coil?
JB: Yes. The tuned or resonant mechanical frequency of the Mylar [is] inherently there. So, we need to damp that down, and we do that through a couple ways. The biggest way is by creating a cavity behind the microphone. So we create air volume behind the microphone. We also use some resistance materials under the microphone to help damp down that resonance.
Acoustical resistance materials?
JB: Acoustical resistance materials, yes.
So, what are we talking [about here, something] like rubber or foam or something?
JB: Generally it could be cloth. We use a lot of resistance cloth that is tuned to a specific resistance value. It has a certain acoustic impedance to it. We use other various materials. You can use small gaps of air; you can create resistance that way, by restricting airflow. We also create cavities and volumes of air under the diaphragm — tuned volumes of air, specific values of air — to help damp down those [unwanted peaks in the] frequency response.
So, looking at our SM57… all of this air in here, this actually contributes to the frequency response of the microphone? Is that correct?
JB: Absolutely. It’s all there for a reason. Everything down into the handle, even, and through the transformer.
The volume of air that’s between the closing ring and the transformer, there… when you screw the handle of the ‘57 together, that volume of air is part of the tuning process. The volume of air all through here [indicating the dynamic cartridge itself], inside of the [internal] shockmount, which we’ll get into, is all part of that damping process, [for] getting the proper low end response of the microphone.
So these are not arbitrarily sized cavities?
JB: We don’t just create different potting heights or whatever. It is all specifically tuned to a certain value. Every chamber of air in here is tuned to a specific value and a specific volume, to get the proper response.
And what about, I guess you’d call it the headbasket?
JB: We would call it the resonator cap. And that helps us get more high end response, more detail out of the microphone. And that also uses a volume of air, that is created on top of the diaphragm. Putting this cap on, on top of the diaphragm like this, there is a space of air above the diaphragm, and that space of air is tuned to a particular resonant frequency. [It is] a very high frequency. As the microphone’s natural response starts trailing off, we create a volume of air above it, that is tuned to a specific resonance, that boosts the frequency… it kind of picks up where the microphone starts trailing off.
The more air you have above [the diaphragm], or the bigger holes you have [within the resonator cap], create a lower frequency [boost]. The smaller the air volume, or the smaller the holes in the resonator cap, create a higher frequency [boost]. That is used to extend the frequency response of the microphone.
So those of us who have ever built a control room, or even just tried to treat their mixing room at home, know words like “standing wave,” and “nodes,” and that kind of thing, and know that the minute you put something in a room, you’ve got a lot going on [acoustically] — you’re not in a vacuum any more. So is it fair to say that we’re putting the capsule in a very tiny room with very finely tuned standing waves and that kind of thing, to actually tailor the response of this capsule?
JB: Absolutely. It’s all about chasing resonant frequencies in a dynamic microphone. It’s all about tuning different air volumes and cavities of air to get the proper response — to get the response we desire. Because naturally, a dynamic microphone, and the Mylar [diaphragm] inside of a gap, around a magnet, doesn’t create something that is good-sounding. We have to do a lot of manipulation to materials, volumes, processes, everything, to tune the microphone to get the response that we desire.
Great. Let’s move on to problem number 2: how do we get it so [speaking into the XLR jack] is not the correct way to talk into the microphone? Why are those youtube videos so funny?
JB: Because it’s so wrong!
The directionality aspect: if we were to take that diaphragm, magnet, and coil, and pop it onto a handle, and not create any air volume behind the microphone, we would have an omnidirectional mic — so that all the sound arriving from any direction around the microphone enters the front of the diaphragm and vibrates the diaphragm, and conversely the coil inside the magnetic field.
Creating a directional microphone: we vent the under the diaphragm, we vent around the diaphragm, to allow sound to strike under the diaphragm, or behind the diaphragm.
So I’ve always wondered what this mesh is [indicating the vents at the base of the SM57 capsule].
JB: [Those are] the rear-entry ports of a unidirectional microphone. And that’s an easy way to know if a dynamic microphone is unidirectional or omnidirectional; it if has ports around it, it’s unidirectional. If you don’t see any ports, it’s going to be omnidirectional.
Conversely, if you close those ports up [by clamping your hand around them], you’re basically restricting the air from being allowed to strike behind the microphone, causing it to be omnidirectional — or creating some very weird polar pattern that you may not want. (8:40 ish)
So, the “SM57 Omni Mod!” You heard it here first on RecordingHacks.
JB: Close the ports up, and it will sound a lot different.
It’s only a $5 mod; send money our way …
JB: … and we’ll take care of it!
So, these rear entry ports are specifically set to a particular value. It works in conjunction with what we call the “d,” or the time it takes the sound to go around the microphone…
“D” for “Delay”?
JB: “D” for “delay,” yes. So the delay is the space, or the amount of time it takes for the sound to hit the front of the diaphragm, and then diffract around it, enter the rear ports, and hit the rear of the diaphragm. It’s called the “d,” and it is a specific value. We then restrict the sound using a resistance cloth, which we talked about. What we do is allow the sound to get delayed using [an acoustic] resistance.
So, when you create a directional microphone, you speak on-axis [at] the microphone, you get all the sound entering the front.
So let’s just say 0 degrees, on axis.
JB: Yes. All the sound is entering the front, and vibrating that diaphragm.
As you go around the microphone, to 90 degrees, or directly behind the microphone — say on a Cardioid microphone, the point of most rejection is in the rear, at 180 degrees. As sound enters [from 180 degrees], it is actually going to enter through the rear-entry ports first, when you’re speaking behind the microphone, get delayed, and that delayed value is timed to the same amount of time it takes to go around the microphone and hit the front.
So, when [sound] hits the back, it gets delayed through that resistance [inside the rear-entry ports], goes around the front of the microphone, and essentially they hit the diaphragm at the same time, both on the front and on the back of the [diaphragm].
And thinking about what happens when you’ve got two identical signals hitting at the same time –
JB: That diaphragm doesn’t move.
JB: And you get rejection. Because there is no movement of that diaphragm in the coil.
So it would be as if I had two people pushing with equal force on my hand [in opposite directions], it’s not going in either direction.
JB: It’s not moving anywhere. Which means it’s not going to move that coil, which means there’s not going to be any voltage [created].
Wow. It’s so clever –
JB: So simple!
So simple, right! But it’s so clever to be able to do that without any power, without even getting to the electronic part, we’ve already made [the microphone] directional. That is really fascinating.
JB: Yes. All those resistances and values [require] complex math. But by creating different cavity volumes, and creating different resistance values, or the spacing of the holes… that’s how we tune the microphones.
JB: Tuning the polar pattern of the microphone. Whether it is Cardioid, or Supercardioid, or Hypercardioid microphone. Or, we can close all the ports up, let all the sound enter the front of the diaphragm, and it will be Omnidirectional.
So how does what we’re doing to tune the polar response affect the frequency response? That is to say, if we took away all the porting and all that, made the SM57 with the same capsule, an omnidirectional microphone? How does what we’re doing [to control the polar pattern] affecting the frequency response?
JB: Vastly. They’re interrelated. That is one of the difficult parts of designing, tuning, and making a dynamic microphone. Every single aspect of this dynamic microphone has an effect on something else. So, you can’t tune the microphone’s [frequency] response, and then go [independently] tune the polar pattern, and then go tune [something else]… They’re all interrelated. [If] you change something here, you’re going to probably screw up something over there. You have to look at it holistically. We have to take a step back and say, if I change this part, what’s going to happen with this, what’s going to happen with that…
So there’s no “solo button.”
JB: There’s no solo button when you’re designing a mic.
You can’t mix the snare in solo…
JB: Every part is doing something else. The resistance is doing this and that, the cavity volume is setting this time, or this volume of air… they’re all interrelated. It’s very complex.
And that’s with only two of our three problems addressed. That brings us to the third real-world problem: these don’t typically site here without any shock and get used. They’re all over the place, in front of the kick drum, they’re on a floor, getting mechanically coupled to the floor —
JB: They’re in your hand, moving around. They’re going inside and out of a stand.
So how do we make it so we hear the sound and not the actual mechanical vibration. What’s going on there?
[Tune in for part III tomorrow to learn about Shure's proprietary "pneumatic shockmount," the secret reason why these microphones exhibit less handling noise than the modern competition.]
Peterson Goodwyn is a drummer and audio engineer based in Philadelphia, PA. He runs the popular DIY site for audio engineer gear builders, DIYRecordingEquipment.