What makes NPR’s broadcast sound so crisp and bright, while many local public radio stations have a bassier, boomier tone? Adam Ragusea, host of our podcast The Pub, wanted to find out, so he recently interviewed the guy who should know best — Shawn Fox, senior director of audio engineering at NPR. This is an edited transcript of the interview, which first appeared on The Pub #17.

Current: How did you come up with what’s been described as NPR’s “secret sauce”?

Fox: Oh, the secret sauce! When we were in our very early design phases of our new building, that was one of the things that came up very quickly — making sure that we continue on with the NPR sound. There was a whole detailed analysis of what was that sound? In terms of the phenomenon of being able to hear the difference between local and national audio quality, there are lots of reasons for that. All in all, what our local stations do and what we do at NPR are very close when you compare it to some of our commercial brethren. But yes, you can hear that difference.

Current: What is ingredient number one of the secret sauce? Is it the NPR studio microphone?

Fox: The NPR sound has so many tentacles. If we’re just focusing on the studio side, which was actually the easiest thing, it all starts with the microphone. We use a simple Neumann U87 microphone as the house-standard microphone at all of our facilities. They’re expensive, but that’s what we’ve used for years.

In the new building, we knew we had the old microphones — and microphones don’t die unless somebody really works hard at it — and we had more facilities, so we bought a few more. But it really comes down to the U87 with the bass rolled off.

Current: The bass rolled off? Could you explain what that means?

Fox: Sure. The U87 and most higher-end microphones have two switches on the back. One is a polar pattern, which is the direction of the microphone, and the other one is for the bass roll-off. When the bass is rolled off, you can’t hear the lower frequencies of my voice. The microphone itself takes them away.

And when I flip that switch, you get those lower frequencies. This is what they call flat mode; there is no attenuation.

Current: It takes out frequencies below how many hertz? We’re talking really super-low stuff, like below 150 or 200 hertz or so, right?

Fox: Real low stuff, somewhere below 250 or so.

Current: Yes, but if you’re at a station and kind of frustrated with the bassy, boomy sound you get on your studio mic, and you can’t get the engineering staff to do anything about it for you, one thing that you could do yourself is to look on the mic and see if there is a bass roll-off switch, and turn it on.

Fox: Yeah. I wouldn’t try to focus on trying to fix the studio, because there’s a whole different element of the acoustics with the studio. The reason NPR came to this standard — and this was decades ago — was because most of our listeners are consuming in an automobile or with something else in the background. Back in the day, and even to some degree now, you roll down those windows and hear those low rumbling frequencies. We wanted our voices to get above that so that they could be clear, open and understandable to improve our storytelling.

We came to that conclusion mostly because most of our consumers were listening to Morning Edition and All Things Considered in the automobile to and from work. And now, as more of our content is heard on headphones from iPhones and all the digital sides of that, we discovered that continuing with this is beneficial, because there is still that acoustic outside noise. But it’s not a fix for a studio. That’s the whole other element of why our sound is a little different. We want to make sure that we work with our on-air talent, our reporters and our hosts.

We are fans of being close-miked, and P-pops come into play there. But we make sure that we are within a foot of the microphone and usually a lot closer — close to six inches — in working with any of our on-air talent. That’s another element that goes into it.

Current: The thing about getting closer to the mic is proximity effect, right? The closer you are, the more bassy you are. So why doesn’t that happen to your people?

Fox: That’s where the bass roll-off also comes into play, and we also really work with them so that it’s not head-on directly into the microphone. It’s more to the side.

Current: If you imagine the air coming out of your mouth as a column, you want that column of air not going straight into the microphone but at a diagonal, sort of off to the side of the microphone.

Fox: That would be accurate, yes. That’s where we prefer it.

Current: So going back to special sauce and its ingredients, we talked about a couple of things. We talked about the microphone, and you’re using the U87. In my experience, the most common studio mic at stations is the Electro-Voice RE20.

Fox: The old RE20.

Current: That mic tends to have kind of a warm, bassy, almost gauzy sound to it that, in my opinion, does not work so well on the radio. So there’s the microphone, there’s the roll-off switch on the microphone to make sure that the lowest frequencies come off, there is the performance aspect of talent getting right up on the microphone but talking a little bit to the side of it so that the Ps don’t pop. I would assume that it’s also important to have a nice pop filter on there as well. What’s the next ingredient in the sauce?

Fox: It’s the actual studio itself; it’s the construction of the studio. We have a very low-reverberant studio, and we make sure that there aren’t a lot of solid walls. We want to take out the sound of the outside newsroom. Most of our facilities are all centrally located inside, right in the middle of our newsroom, so we make sure they’re isolated so you don’t hear anything else but the person speaking.

And a quick note on the RE20 — I’m a big fan of the RE20. I do see a lot of stations using it. I never veer away from anybody using the RE20. It’s a fantastic microphone. If we’re trying to be a little cost-conscious, we’ll bring in RE20s. They’re a good equivalent for the Neumann U87. But at stations I do see a lot of people using the RE20 and in that flat position, because that low-end bass attenuation on the RE20 can really be problematic at most stations.

Back when I worked in Detroit, we were an RE20 station. And a lot of on-air talent — and we get a little bit of this here at NPR — likes to sound a little bit more authoritative, and they hit the microphone into the flat position to get that bassy sound. In Detroit, we used epoxy to hold all of the switches into a position so that couldn’t happen again. And, honestly, we do see that occasionally here. We’ll hear something bassy and we’ll run up to the studio and, sure enough, somebody switched it.

Current: That’s hilarious.

Fox: So if you’re a chief engineer at a station that uses RE20s and you’re getting really angry at your on-air host for switching it, just get some Super Glue or epoxy; it’ll be fine.

Current: That’s a relatively easy fix. In terms of getting your studio to sound good so that it’s shielded from outside noise with soundproofing, you also need sound diffusion to make sure that the announcer’s voice is not echoing around the room. Are there cheap fixes that can be pursued? Or if your studio is already built out, are you just kind of stuck with it?

Fox: No, there are some little things. Over the past 10 years, you see more computer screens throughout every broadcast plant. And often that microphone is real close to the computer screen: Depending on how close, you can actually hear some of the electronic interference off the computer screen. The computer screen is the big issue. If it is just a little too close to that microphone, your voice is reflecting off of it.

Current: Right, because it’s a smooth hard surface that reflects sound.

Fox: Yes, this is one of the things I frequently see. Another thing for engineers or anybody at a station to do is to go into the studio, turn the microphone on, crank it to 11. Don’t talk — so you don’t blow your ears out — and listen to the sounds of all the fans that you have in the room. This is one challenge that we’ve had for decades here. You’re never going to get it to zero. You’re never going to get it completely silent. Nor do you really want to, because in order to get it silent you’ve got to move a lot of equipment out, and there’s a lot of cost with that.

But if you’ve got a CPU [central processing unit] right next to the microphone or somewhat close, move it two feet somewhere else and hide it under a cabinet. That helps out quite a bit.

And then there are a lot of studios built for two microphones that will be set up with six microphones for a talk show. There’s not much you can do; just try to space that out a little better. Those are the biggest things that I see on a regular basis.

Current: I haven’t actually visited the new NPR facility yet, but in the old one on Massachusetts Avenue, one of the things that always struck me was the sheer size of the on-air studios, which is counterintuitive because you’d think that the bigger the room the more echo you’d get. But with the right baffles and diffusion and all of that, the size allows all the different mic positions to be spread out really far apart, thus reducing any bleed from one mic into the next.

Fox: Our old headquarters on 635 Massachusetts Avenue was a great plant; it served our needs well for 20-odd years. You saw the large studios, but we consistently had problems with bleed in some of our smaller offline production studios. We resolved that in our new building on North Capitol Street. We made the offline production studios bigger. The actual on-air studios are larger than 635, but we were able to start with a clean slate and really put in a lot of time and effort working with our acousticians and our studio designers to make sure that we had defined what the secret sauce was and that they could design around that.

Current: Are there any other elements of the secret sauce that you haven’t revealed yet, or are those things that you will take to the grave?

Fox: Well, speaking to the public radio community, one of the things that we don’t do is process our signal heading to the stations. We keep it as pure as possible. This has been up for some debate for the longest time, but as we currently stand we don’t compress; we don’t alter our signal leaving the studio to the satellite and hitting the stations.

We know that our over 600 stations and all their listeners have different needs. So we try to give the purest sound to the stations so that they can manipulate it for their market, with varying degrees of success. We have talked about, especially with our newscast unit, maybe starting to slide a little bit of compression into that. We haven’t done it yet, and I don’t know if we will, but giving that pure signal via the satellite to our stations is probably the last element to the sauce.