Bad Background Hum with SM57 & X2U Adapter

I am getting a very bad hum with a newly bought SM57 with its Shure X2U USB adapter. I have tried to reduce the noise level with Audacity’s noise reduction effect. I changed the noise reduction variously to 24-36 dB from the default 12dB, and kept the sensitivity setting at 6. It did reduce the noise floor but using so much dB effect cut causes the voice quality to digitize and sound weak and distant as well.

This is not a solution, so – what can be causing so much background hum in the line? We are trying to get audiobook spoken word quality for the finished product. The mic is mounted on the Shure AW55 shock mount with its A81WS windscreen and a sound shield on a mic stand for isolation.

I have the X2U adapter mic gain wide open, and have adjusted the Pulse Audio volume input levels in different tests from 46% to 100%. The audacity VU meters read from -24 to -6dB depending on how the PAV input is set but no matter what setting, the noise level is really excessive, almost masking the voice. It usually shows as no better than -54dB, and sometimes worse.

What can be causing the hum in the line to be so loud? Does anyone have any experience with the X2U or in taming the hum in general? Are there any other settings or info you need? I am running manjaro linux 4.19 with updated audacity v2.3.1.
BTW, I am using a Dell latitude with 8GB RAM, seemed like it should be enough. I am using the USB A-B cable that came with the Shure as well.

Thanks for any help you can give.

I have an X2U and I don’t use it, not because it hums or has distortion or anything like that. It’s actually pretty well designed, but it has low volume. I complained about it to Shure. I said I would kill to have just 10dB more volume in this thing. They said, “that’s the way it is.” It’s still in the closet in its box.

Is the SM57 connected with a new XLR cable like this? The outer metal part design just has to get close, but it must have those three pins in the inside.

Are there any adapters or foreign cables between the microphone and the X2U?

Do you have the X2U Phantom Power turned on (you should not with this microphone).

Does the hum change when you touch the SM57 or the X2U?

This is basically what you have, right.

Does it hum in the headphones—assuming you have them plugged into the X2U like in the illustration?

Does your computer have a built-in microphone? Are you sure you’re not trying to record from that?

I have experienced odd hum problems, but usually not enough to drown out the voice.

If your work is a single mono track (one blue wave) drag-select about 20 seconds of you speaking over the hum and occasionally stop for a couple of seconds. File > Export > Export Selected > WAV (Microsoft).

Don’t use punctuation marks in the filename.

Post it on the forum. Scroll down from a text window > Attachments > Add Files.

This is an odd combination of both microphone and interface. It’s not non-functional, I totally expect it to work, but still they’re not at the top of my list. The SM57, for example, is widely revered for its ability to mic drums.


Oh, wait. I’m just waking up. Take out the three-pin XLR cable and plug the microphone straight in like in my illustration (that’s an SM58). Any change?


Do you have this?

This performer has a badly damaged microphone cable. A problem like this should clear instantly when you take the cable out and plug the microphone directly into the X2U.


Hello koz.

Thanks for all your input and questions. We had been going around the room trying different mic positions, locations and recording level settings to try and eliminate the background hum when I looked at one of your questions:

Does your computer have a built-in microphone? Are you sure you’re not trying to record from that?

I checked inside the PAV volume control in Linux and the internal mic was responding so I muted it. Then I had no input signal at all. It should have been obvious…oops… but… so now, the internal mic is muted and all the signal appears to be coming into the recording from the Shure.
I raised the gain inside the PAV control to 100% for the X2U (it was down at 30% or so).
I now have the PAV audio settings as follows:

Built-in Audio Analog Stereo – port set to Microphone (plugged in) – but now MUTED
X2UAdapter Analog Stereo – port set to Analog input, gain was set at 100% for one audio sample file, and 113% (+3dB) for the second file.

Audacity is reading the signal, the is still some base noise but seems less than when the internal mic was active. Here are the two samples, the 100% and the 113% tests.

As to your setup questions:

The X2U has been plugged directly into the SM57 all along, I have no XLR cable here.
Phantom Power has been off the whole time as well.
Hum does not seem affected whether or not the mic or adapter are touched.
Your photo – yes, that’s our setup, occasionally with the phones connected to the computer output instead of the X2U jack, but the X2U signal is much stronger and more revealing. Yes iI can hear the hum in the phones more when they are plugged into the mic adapter jack.

We chose the SM57 after hearing samples online and liking the sound compared to some others, though there may have been better choices in retrospect…

The hum sample you gave is what it was sounding like before, and that same noise, though now less, still seems present.

Let me know what I can try next, and thanks very much for your time and efforts.

I sent the last one through Audiobook Mastering and treated it to 6dB noise reduction.

First three readings and sentence 2/3 down.

Screen Shot 2019-04-02 at 15.15.30.png
The first one, Koz100 has very serious hum. That can be patched, but the corrections delete some musical tones from your voice and is not recommended.

Koz113 is super interesting. Play the first two seconds at good volume.

MMMMMMM_______Test, Check One Two.

Where did that hum come from and why did it suddenly vanish?


Last things first: What that intro to the koz113 clip is, was me hitting record, then pausing almost immediately to get set up with the mic (I was hand-holding it at that time,while trying to run the test). Then I un-paused and recorded the clip. The initial hum is the common hum level we have been getting all along. Why the noise level suddenly went down is strange to me too – I have to investigate that next.

But - if I read this right, with that koz113 clip, after you applied 6dB of noise reduction, we are in the ACX ballpark? If we’re close that’s good news. What I did to get there (and before the noise was really bad, close to the sample you uploaded of that faulty cable), was to MUTE the internal microphone, and raise the PAV input level to 113% (the +3dB boost). That’s it.

What were your settings inside the Noise Reduction plugin, was it 6-6-3, or something else?

What else might I try to possibly eliminate the need for even that amount of NR manipulation? I’d like it as clean as possibly, naturally.
Also, I had previously tried to install some plugins that I found searching the forum ( ), notably the ACX Check you used, and the RMS Normalizer. I used the instructions here ( ) and here ( ) to try and install them. I did manage to get the RMS Normalizer to show in the effects plugins dropdown list, but not the others. What else can I do to get them installed?

And on a related note, where are the DeEsser and DeClicker, etc. plugins found that are referenced in this thread ( )? Will they help us too?

It’s encouraging that we are getting closer to a good sound, though I will have to determine why the weird change on that 2nd file hum intro. That’s still disconcerting.

Thank you again for all your time and attention, we’re obviously newbies and really appreciate the guidance.

First three readings and sentence 2/3 down.

Yup. If you can make that hum go away forever, you pretty much got this thing licked.

Grounding issues wave their hands and say “Here I am” if you know what the symptoms are. Touching and not touching the microphone or interface is a classic shield or grounding issue. Is touching the difference between the first little bit and the rest of the clip?

Is the computer a laptop that can be run on batteries? That can sometimes help, too. There are posters who can only present for small stretches of time because that’s all their batteries will do and there is sound damage when they plug into the wall.

A side note here. About the second or third week of fighting problems like this, it is recommended you drop the computer like a hot rock and record using a stand-alone sound recorder.

You are only at the start of stamping out the wacky sound problems and being on Linux means you have a small subset of computer users to ask for help. Fortunately, Steve on the forum is one of those people.

Noise Reduction is a two-pass operation. Drag-select a second or two of room tone or background noise. Effect > Noise Reduction > Profile. The tool with vanish. That gives Noise Reduction a chance to sniff the noise to know what to attack later.

Then Effect > Noise Reduction > 6, 6, 6 > OK. That’s the gentle Noise Reduction of the Beast. ACX is not a fan of noise reduction and 666 can reduce background noise without being obvious what it’s doing. In your case, I would probably push that up a little to 8, 6, 6. You can go up to 12, 6, 6 and that’s the limit. At thats amount of reduction, the voice tones start to be damaged.

All these tools and effects have side issues much like medications. There is no “Make the noise go to zero” without destroying vocal tones—at least not for the home user, so starting with a clean studio is super important.

What else might I try to possibly eliminate the need for even that amount of NR manipulation?

Stop using the SM57 and X2U. The SM57 is a legacy microphone with slightly quiet delivery. It’s great with drums. Nothing wrong with it and I used one for years, but I did it with a sound mixer, not a home user interface like the X2U. My X2U is in a box in the garage because it has low volume and slightly high frying background noise (fffffffffff). It will always require a little sound rescue. I think adding the noise reduction in this case is justified. Your voice with the correction combination I used should be good to go.

The ACX goal is pure, clear voice with no distractions and they say so over several pages of instructions.

And on a related note, where are the DeEsser and DeClicker, etc. plugins found that are referenced in this thread ( viewtopic.php?p=365352 )? Will they help us too?


The SM57 is a dynamic (moving coil) microphone and doesn’t have the crisp, gritty distortion that the home user condenser microphones do. You win here.

What else can I do to get them installed?

After you install the tools into Audacity, you have to enable them. Effect > Add/Remove Plugins and Analyse > Add/Remove Plugins. ACX Check is under Analyze, not Effects.

There are only two additions. Everything else is built into the current Audacity.

It’s a New User error to assume a long laundry list of effects and corrections. Mastering is three tools and add Noise Reduction in your case. That’s it. If you can maintain the quality in Koz113, that how you should read. It’s easy to lose the concept that it’s a business and you should produce good work with minimum effort and time.

Now that you’re getting your hopes up, you do have to be able to read out loud. This is theater presentation. The ACX Robot checks your work for technical compliance much like ACX Check does and then the work goes to Human Quality Control who actually listens to your story. This is where you go to die if, like me, your voice is a boring monotone and qualifies as a non-lethal weapon by the State of California.

Another side note. It’s not unusual for people to record in a real studio and only on the second or third book decide to record it at home. Then they find it’s not that easy. They were paying their studio and engineer for a reason.


You can fake out ACX Check. ACX Check requires about 3/4 second of clean, pure background noise somewhere in the piece to measure ACX Noise. If it doesn’t get it, it will measure the quietest thing it can find and if that happens to be you breathing, it will return a stupid high fake noise reading.

Room Tone (Background Noise) is built in to the ACX delivery specification. It’s not optional.

Screen Shot 2019-04-03 at 4.09.18.png
It’s a terrible idea to Generate > Silence in an effort to stamp out a bad noise reading. A recent poster tried to do that. Please know that it’s possible to have ACX Noise come out too good. Nobody’s microphone can do -85dB noise. That’s the sign that someone was playing games with their sound. ACX hates that.


It’s good to know that you think that if we can reproduce that koz113 file quality and apply minimal noise reduction that we’d be about there. I have to check (I’m not at that computer right now) if the grounding was the cause of the hum variation or not. We could run for a bit on battery power and the other day I pulled the charger when we were checking hum and placement but it did not seem to make a difference. I will check that again.

As you say, there will undoubtedly be more issues along the way in editing and post, some I am not even aware of as yet.

However, looking at your recommendations for possible alternatives to the SM57-X2U setup, would replacing the X2U with a different stand-alone mixer or digital interface with more gain help? If so, can you recommend one in a home user’s price range?

Or is the problem inherent in the SM57 itself? So far we have not experienced the hissing (fffff) or mosquito tone problems, but will we perhaps find them after we fix the hum?

If not the SM57, what other similarly priced dynamic mics could we turn to?

I see in your pic what looks like a Zoom H4 digital recorder. If that will record and get us ACX quality then why not go with one? What downsides might it have? Does it give condenser mic type sibilance problems, mosquito noises, inferior overall tone quality, or…? Seems like it would get in the way less when actually recording that a computer interface as well. If it’s easier to run and has cleaner sound then why not just use it? If we did, then would we just import that file into Audacity and do all the editing there?

I’ve seen the effects referenced in that Audiobook Mastering wiki page and do want to use no more manipulation than necessary, both for time and quality considerations. KISS is always better.

Thanks for the tip on the ACX Check location – I do see it in the Analyze dropdown menu so I guess I installed it correctly after all, and had also enabled them in the list as per the instructions. I will test it shortly now that I know where it is. I didn’t even know about the Generate > Silence feature, but I guess I will promptly forget it anyway….

apply minimal noise reduction that we’d be about there.

That does pass technical requirements. The only thing left is theatrical considerations. That’s entirely up to you. There is no acting filter.

Two notes here. In My Opinion, if you find something that works, you hold onto it with white knuckles. This works given you can resolve the hum issue. ACX requires all your chapters match, so there is no changing microphones or effects in the middle of a book.

Nowhere in there did you say you tried touch tests. “This is the microphone system lying on the table.” “I’m touching it now.” “Now I’m not.”
Does the hum go up and down when you do that?

I forgot about this one. If you have a wall-power adapter, how far away from the microphone is it? Disconnect it at the wall and see what happens to the sound. I have a wall power adapter that sprays hum around it like a cloud. Any microphone within about a foot or so starts humming at the Los Angeles power rate. MMMMMMMM.

Gentle rain in the trees background fffffffff sound is audible in Koz113. Turn the volume up during the gap at 14 seconds.

Understanding the noise measurement is handy. -60dB ACX noise means your background sound has to be 1000 times quieter than your voice. No, it’s not easy, and further, the noise measurement just adds up all the sounds in the system that aren’t your voice. Wall power hum, buzz, dogs next door, traffic rumble, air conditioning motors, laptop fans, microphone hiss, USB crosstalk. That’s a partial list. This is why managing background noise isn’t one sentence in the guidelines.

Or, I guess it could be. If you fail ACX Noise, record in a studio.


As above, I would stay with the microphone, but change the interface. There’s a new one from Rode I need to research. I think I like it because they actually publish technical specifications right there on the page. It’s not missing or hidden at the bottom of a collection of adorable animations describing how wonderful it is.

Somebody has to sit through the unpacking review (cringe).

As we go.


They don’t make the Zoom H4 any more. That one is a legacy model and it’s feeling its age. There was a poster on the forum using either an H4n or an H2n. They gave up trying to force their computer to work. I need to look.


There’s an error or omission in the Noise instructions. Drag-select the sample background noise and make that the Profile.

Then select the whole clip or show by clicking just right of the up arrow.

Then Effect > Noise Reduction > 6, 6, 6, > OK.


The poster is using a Zoom H2n. However we have not been able to recommend highly enough using our Audiobook Mastering and they are using their own mastering process.


I’m on the edge of my seat waiting for you to find your hum problem. If it’s unusual, I can add it to my collection of sound problems.


I went back to try to recreate that odd initial hum in koz113, and I cannot get it to generate the overly loud hum and then suddenly drop like that sample did. So that’s still a puzzle… if it happens again I will note exactly what caused it.

I did make a test, though too long to upload, with hands on the mic, hands off, mic back on stand, laptop power brick unplugged from the wall – but none of those seemed to make any difference in the hum level, either audibly or watching the meter.

Here is a new raw sample (test113.wav) with the PAV input still set at 113% (the +3dB boost) and the SM57 on the stand in the ‘booth’ as it were… This time I get a fail here with the ACX Check, not due to the Peak or the Noise Floor, but rather to the RMS reading. Do you see the same? If that’s so, I don’t know how we got from passing with RMS to a too low (right?) level average. I did not change any other mic input or audio settings and was speaking, I believe, at about the same levels at the same distance from the mic.

I do hear a very slight residual noise floor in the new sample, is that the hum or the ‘fffff’, or both at work? But why then does ACX Check pass it, or is that simply an acceptable level? I saved another copy of the file and gave it a 6-6-6 noise reduction treatment and it sounded quieter and did not seem to degrade the voice quality, though it still failed ACX Check. Please tell me what you think on this one.

Regarding the RMS failing on this new file, I have two thoughts as to possible cause:

One - could the SM57 input simply be too low? (But then why wasn’t it a fail before in koz113 but it is now, nothing has changed that I am aware of…?) And if it is too low, would swapping out the rather thick Shure A81 president’s windscreen we have on it for a standard thin pop filter disc help audio input levels and/or crispness and definition? Or do we need a different interface after all? I did raise the master PAV mic input from the 113% +3dB to 126% (+6dB) and it was too hot, peak level was clipping some, and the noise floor failed even with 6-6-6 applied. So I dropped it back down to 113%.

In our previous upload (the koz113 file), was the fffffffff tamed when you applied the 6-6-6 noise reduction? And if I read you right, that’s inherent in the ‘57 and will never really go away. Though isn’t it an easier fix and a better trade-off than trying to eliminate those mosquito tones that a USB condenser can give off? Basically since you noted that ACX is not fond of condensers, wouldn’t we still be better off taming this SM57 dynamic mic?

My second idea for the RMS fail - and I am guessing here as I really am not sure how the RMS really gets measured - is: since there is a fair amount of silence as opposed to talking on the track, does it average it out to be too low a level overall? When I am watching playback I see the VU meters staying between -6 and -24dB in general, so why the low RMS reading? We are trying to assemble a book of quotations right now, one being read after another (and they are short snippets), with 3-4 second break in between each one. Will that cause ACX’s readings to be lower that the actual voice volume really is? If this could be it, or part of the problem, how can we work around that if ACX will reject it based on their measurements? Or am I wrong in this assumption entirely?

As a side note to your previous post, if those Zooms are outdated, are you aware of any current models or their like, that might be worth it? It does seem like it would streamline the actual recording process and allow more attention to be paid to performance, rathe than having to be performer and engineer simultaneously.

But since you suggested that we keep the SM57 (and I’d like to if we can as well) and swap out an interface, do you have an idea if any of these might make a good match? The FocusRite Scarlett 2i2, Rode AI-1 (is this the one you mentioned?), PreSonus Studio 24C, a Behringer… - will any or all of these afford more gain for the SM57? Or do they have their own problems?

There is one more issue that may pertain to the RMS results in our current project down the road: we would like to add in post production a low, background ambient-style music track, possibly bringing it up a bit in between the vocal parts to delineate sections. I read on the ACX site that they frown on music in general but it is not necessarily a deal-breaker, but will it skew their RMS readings even more? What are your thoughts on that idea?

Thank you koz, I know this is a lot and if some of this better belongs in another thread, I apologize in advance…

we would like to add in post production a low, background ambient-style music track, possibly bringing it up a bit in between the vocal parts to delineate sections

In a word. no. You are not producing Radio Theater [Tum Tum Tummmmmmmmm] That’s a “stinger” if you’re keeping count. That music bit signals the listener can go out to make a sandwich while you flog “Alka Seltzer!! Plop Plot, Fizz, Fizz. Oh, what a relief it is!!”

Different from an audiobook. The most music I’ve ever heard in an audiobook is a very brief introduction or signature tune and then that’s it until the next chapter break 20 minutes later. Music is a bad idea if for no other good reason it screws up the quality control system.

Stop trying to read directly into ACX. Nobody can do that reliably. Read so the tips of your blue waves land on average about 50% or maybe slightly less. That’s -6dB to -10dB on the bouncing sound meter. You won’t have to constantly watch your levels and Audiobok Mastering will take care of everything else. Listen live to your headphones. That will help keep your volume even.

Your last post was a hum test. Post a new one and this time go full theater for 20 seconds. Read a chapter heading or the ad campaign from the side of a cereal box. Read it for real like someone was paying you. Follow all the rules. Dead silent for two seconds. (I can hear you moving around in the hum test). Get to the script end and cut some of it off if it goes over about 21 seconds and post it. No effects, filters or corrections.


Like this only we assume you’ll be a lot better at it than I am.

And yes, it passes ACX Check nicely and I didn’t go to a lot of craziness to get it there.

Eventually I need to do one of these that doesn’t sound like I’m learning to read for the first time.

I have one from a restaurant experience buried here somewhere. “A couple sat down beside me at the counter…”


Here it is. This one’s much more graceful. It’s an MP3 because it’s too long for a WAV. It goes off the rails at about a minute.

That’s more the story-teller flow you have to generate.