Noise Problems with SM7b

H

Hemluth

New member
Hello everyone!

I have a rather annoying problem.
Yesterday I recieved my new SM7b. It although did not really work out the way I wanted it to. There is a quite strong noise all the way from 4-5kHz up to 10-20kHz.
At first I thought it was a preamp issue. I've tried three different preamps (Line Audio 2MP, built-in Mbox 2 PRO and some cheap Begringer thing) coming up with the same result though. I have also tried to connect the preamps to each other (so that each individual preamp need to use much less gain) and the noise was just the same.

For quite speech at a distance of about 40cm from the mic the noise level was around -30db when you elevated the speech peaks to 0. For louder speech and yelling at a distance of about 35cm from the mic the noise level was around -40db when you elevated it so that the peaks were around 0. It is still TOO much. As far as you start compressing or equalizing it sounds like a computer mic.

I have also checked and rechecked that the there are no high frequency noices inside of the recording room.

The board are not allowing me to url to clips if I don¨t post more than 5 posts. Exept that I have plenty of clips to show you if you are interested.

What do you think the problem might be? Was the microphone simply broken when delivered? Are the preamps just bad?

Thanks
 
just out of curiosity, have you tried a different mic? Different cable? Are the switches on the mic set to flat frequency response?

And what's in your channel strip -- dry? Or is there a compressor / exciter / etc.?
 
As you know, the mic requires a good bit of gain to work. The Line Audio pre puts out 60db of gain, which isn't huge, but should be enough for the SM7. I donlt know what the other pres put out. The mic works best, however, on close mic applications, so 40cm for a quiet voice is too distant IMHO. How does it work when set 5-8 cm for vocals? Or just off the grill on a guitar cab?
 
I've tried to do some things closer, around 5-10cm away. The noise gets slightly better, but it's still far too much. Slight compression is enough to make it very noticable.

Trying different cables was one of the first things on my mind and didn't make any difference at all.

Only minutes ago I did some tests with a couple of other microphones and noticed something interesting.
To start with, there are ABSOLUTELY NO noises from my Studio Projects B1. It is so quiet that you could hear neigbours miles away if you put the gain to the max.
But then it is not fair to compair dynamics with condensator mics.

To the interesting part; I tried to put maximum gain on my pair of SM57s. Guess what, there gets noise. Not a constant high one like with SM7b, but a lower more cycling noise, similar to earth noise. I have not noticed this before, and I'm not even sure I've tried SM57s for any low level sounds in this apartment. Can there be earth issues?
I however do not see how an earth issue could create the high noise in my SM7b, and how the SPB1 could be unaffeceted.

And yes, the strips are just clean.
 
Hemluth, it's time for some math. Your SM7B has sensitivity of -59dBV with a source of 94dBSPL at the capsule. Be aware that the capsule of the SM7B is several cm from the front of the windscreen.

If you are talking, that's probably around 84dBSPL. Talk 40cm from the capsule, probably more like 64dBSPL at the capsule.

So we know that the mic's output will be -59 - (94-64), or -89dBV.

Your Line Audio preamp has self-noise of -129dBV. Therefore, the noise will be 40dB below the signal, and that is close to what you have observed. Note that peak and average (RMS) levels are different, that probably accounts for the rest of the difference.

The SM7B was designed as a broadcast mic, and broadcasters put their mouth very close to the microphone, and at such a distance, their voiceover will easily exceed 100dBSPL, giving them better than 80dB signal-to-noise, which is plenty for radio. If you are using this mic for quieter sources, you are likely to be disappointed.


Now, your SP B1. It has sensitivity of -34dBV, and self-noise of 12dBA. Subtract 12 from 94, you get 82dB--its stated dynamic range. Subtract that from -34dBV, and you get its absolute noise rating of -116dBA. This is 13dB higher than your preamp, so the mic's noise will dominate. But its output is also much higher, so irrespective of your preamp's noise, it will achieve its stated 82dB dynamic range.


So the B1 has a functionally lower noise level, with 82dB of dynamic range, compared with 70dB for the SM7B with your Line Audio preamp. Your other preamps probably have inferior performance, which will adversely affect the SM7B, but not the B1.


And the noise from the B1 microphone also has a different character than the noise from your preamp--simply stated, the mic noise is "pink", and the preamp noise is "white". So if you normalize the SM7B and B1 noise levels, the SM7B would probably still sound more objectionable, because of the higher frequency content of the noise.
 
antichef said:
That is to say, get closer to the mic, sing louder, or get a preamp with more clean gain?
Click to expand...

#1 and #2, yes. #3 is nearly impossible. Theoretical noise of a 150 ohm resistor (an ideal mic source; the SM7B is close enough) is -133dBV unweighted, or around -136dBA. The Line Audio pre didn't say if it was a weighted noise measurement or not, but suffice to say that is a good quiet pre, and even if perfect noise performance were possible (it isn't), there is only 4 or 6dB to be gained, when 20dB is needed. Unless the Line Audio specs are just a complete fabrication, but I'll trust them.

As an aside, everyone always says that preamps need "clean gain". No, what they need is low noise, period. Preamps are often relatively noisier at low gain than high gain, you just don't notice because the gain is so low you can't hear the noise.

Also, some preamps have very nonlinear gain controls, where the gain suddenly spikes a large amount in the last tiny turn of the knob, and the noise along with it. That isn't necessarily a noisy pre, just one with a poor gain control.

So don't look at maximum gain, don't think about "clean gain", just think about low equivalent input noise.
 
I also have a brand new SM7b and get very satisfying sound and low noise level when using it with a Joe Meek TwinQ (new model), but that's with 70 db gain. If the mic is not broken, you probably just need more gain.
 
mshilarious said:
Hemluth, it's time for some math. Your SM7B has sensitivity of -59dBV with a source of 94dBSPL at the capsule. Be aware that the capsule of the SM7B is several cm from the front of the windscreen.

If you are talking, that's probably around 84dBSPL. Talk 40cm from the capsule, probably more like 64dBSPL at the capsule.

So we know that the mic's output will be -59 - (94-64), or -89dBV.

Your Line Audio preamp has self-noise of -129dBV. Therefore, the noise will be 40dB below the signal, and that is close to what you have observed. Note that peak and average (RMS) levels are different, that probably accounts for the rest of the difference.

The SM7B was designed as a broadcast mic, and broadcasters put their mouth very close to the microphone, and at such a distance, their voiceover will easily exceed 100dBSPL, giving them better than 80dB signal-to-noise, which is plenty for radio. If you are using this mic for quieter sources, you are likely to be disappointed.


Now, your SP B1. It has sensitivity of -34dBV, and self-noise of 12dBA. Subtract 12 from 94, you get 82dB--its stated dynamic range. Subtract that from -34dBV, and you get its absolute noise rating of -116dBA. This is 13dB higher than your preamp, so the mic's noise will dominate. But its output is also much higher, so irrespective of your preamp's noise, it will achieve its stated 82dB dynamic range.


So the B1 has a functionally lower noise level, with 82dB of dynamic range, compared with 70dB for the SM7B with your Line Audio preamp. Your other preamps probably have inferior performance, which will adversely affect the SM7B, but not the B1.


And the noise from the B1 microphone also has a different character than the noise from your preamp--simply stated, the mic noise is "pink", and the preamp noise is "white". So if you normalize the SM7B and B1 noise levels, the SM7B would probably still sound more objectionable, because of the higher frequency content of the noise.
Click to expand...


um...this is WORD FOR WORD what I was going to say.......WORD FOR WORD!!!!

Its like he copied and pasted it from my head.......EXACTLY I tell ya!!

yea so...anyway...thanks for that MSH....
 
You must log in or register to reply here.
Back
Top