Helicopter Noise from mics at DVinfo.net

Go Back   DV Info Net > The Tools of DV and HD Production > All Things Audio

All Things Audio
Everything Audio, from acquisition to postproduction.


Reply
 
Thread Tools Search this Thread
Old March 2nd, 2012, 04:36 AM   #1
Major Player
 
Join Date: Feb 2011
Location: San Francisco, CA
Posts: 416
Helicopter Noise from mics

Several times I have experienced "helicopter noise" coming from my mics while setting up. Sort of a pumping in-and-out white noise with a frequency approximately 4Hz. In all cases it has gone away after I fiddled with connections, gain, and flipping the 48V, limiter, and Low pass filter switches, but I haven't been able to narrow down exactly what I did that made it go away. I have experienced this with both my ME66 and cs-3e mics, while feeding the Mixpre.

I have noticed that when it's an issue, it happens only when the gain is turned up past a certain amount. This makes me suspect a voltage supply instability in the mixer. The fact that I have only experienced it while setting up suggests that perhaps it is compunded by cold batteries producing lower voltage, or possibly condensation on the mic?
Tom Morrow is offline   Reply With Quote
Old March 2nd, 2012, 05:56 AM   #2
Regular Crew
 
Join Date: Oct 2010
Location: Copenhagen Denmark
Posts: 70
Re: Helicopter Noise from mics

The Rode NTG 2 did pick up micro vibrations sounding like a helicopter when the internal 48V from a AA battery was used. It simply seems like one AA cannot power the mic sufficiently. When switching on the phantom power inside the camera or a mixer, the sound goes away.
Dominik Krol is offline   Reply With Quote
Old March 2nd, 2012, 08:29 AM   #3
Trustee
 
Join Date: Aug 2007
Location: Portland, Oregon
Posts: 1,177
Re: Helicopter Noise from mics

I would try it without a mic connected to the mixer. Of course, it is never legitimate to test a mic preamp without some sort of termination. A dead short across XLR pins 2-3 at minimum, or more properly, a resistor around 150 to 300 ohms across XLR pins 2-3 makes a proper terminating plug that should be in any good audio kit of bits and bobs.

Some mixer designs have internal feedback issues when operated at high gain. As Mr. Krol suggested for the microphone, I would also test the preamp/mixer with an external power supply and known good fresh batteries. The power supply/battery is an important part of the circuit to stabilize the preamp power.
Richard Crowley is offline   Reply With Quote
Old March 2nd, 2012, 01:34 PM   #4
Regular Crew
 
Join Date: Mar 2009
Location: Maryland
Posts: 42
Re: Helicopter Noise from mics

Quote:
Originally Posted by Tom Morrow View Post
Several times I have experienced "helicopter noise" coming from my mics while setting up. Sort of a pumping in-and-out white noise with a frequency approximately 4Hz. In all cases it has gone away after I fiddled with connections, gain, and flipping the 48V, limiter, and Low pass filter switches, but I haven't been able to narrow down exactly what I did that made it go away. I have experienced this with both my ME66 and cs-3e mics, while feeding the Mixpre.

I have noticed that when it's an issue, it happens only when the gain is turned up past a certain amount. This makes me suspect a voltage supply instability in the mixer. The fact that I have only experienced it while setting up suggests that perhaps it is compunded by cold batteries producing lower voltage, or possibly condensation on the mic?
Doesn't have your mics listed but something to be aware of maybe?

Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC
Dan Ostroff is offline   Reply With Quote
Old March 2nd, 2012, 04:17 PM   #5
Inner Circle
 
Join Date: Sep 2006
Location: Sydney.
Posts: 2,569
Re: Helicopter Noise from mics

Quote:
Originally Posted by Dominik Krol View Post
The Rode NTG 2 did pick up micro vibrations sounding like a helicopter when the internal 48V from a AA battery was used. It simply seems like one AA cannot power the mic sufficiently. When switching on the phantom power inside the camera or a mixer, the sound goes away.
The Rode NTG-2 is designed to work equally well powered by a charged battery or 48volts.

If you use 48volts most of the time don't leave the battery in it and check it has a suitable charge before installing it.

If it's your NTG-2 you're talking about, make sure a battery hasn't leaked and corroded the mics battery terminals ..
then try it with a new *name* alkaline battery, not a cheapie from the East.

Or if it's registered for its 10yr warranty with Rode, email support in your area to get it repaired.

Cheers.
__________________
30+ years with our own audio and visual production company and studios.
Allan Black is offline   Reply With Quote
Old March 3rd, 2012, 04:46 AM   #6
Trustee
 
Join Date: Apr 2008
Location: Cornsay Durham UK
Posts: 1,941
Re: Helicopter Noise from mics

This is sometimes also known as "motor boating" as it can sound like an outboard engine.

It can be caused on some mic's if you have the battery in place and also try to connect 48 volt phantom power from an external mixer at the same time.

It can also be caused if the phantom power is too low or too high on some mic's but can also happen if the earth +ve and -ve balanced connections are intermittent and the power is not being fed to the mic correctly.

One other possible other problem may be if several mic's are being used on a battery mixer and it doesn't have enough power to feed all of them together.

I must admit though that I haven't heard this sound for a few years as most modern mic's and mixers tend to be more tollerant than they used to be but the most common reason is that battery and phantom, is being applied to a mic such as the NTG2 at the same time.
__________________
Over 15 minutes in Broadcast Film and TV production: http://www.imdb.com/name/nm1044352/
Gary Nattrass is offline   Reply With Quote
Old March 4th, 2012, 02:17 AM   #7
Major Player
 
Join Date: Feb 2011
Location: San Francisco, CA
Posts: 416
Re: Helicopter Noise from mics

Thanks. That SD Sound Note looks like what I'm experiencing:

Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC

Quote:
Remedy

With microphones prone to this condition, several things can be done to minimize and/or prevent the condition.

Engage the high-pass filter.
Use 48 V phantom even if not required.
Power the microphone(s) from an external phantom supply.
Power the mixer from an external, DC power supply.
I think next time this happens, I will try the experiment of plugging the Mixpre into an AC adapter. That should show me whether this is indeed the issue I'm seeing.

Perhaps it only happened while setting up because I had the gain too high before I trimmed it.
Tom Morrow is offline   Reply With Quote
Old March 4th, 2012, 05:16 AM   #8
Trustee
 
Join Date: Apr 2008
Location: Cornsay Durham UK
Posts: 1,941
Re: Helicopter Noise from mics

That's very interesting Tom and I recall hearing this a lot more in the old days (25 years ago) when transformer balanced inputs were more prevalent.
__________________
Over 15 minutes in Broadcast Film and TV production: http://www.imdb.com/name/nm1044352/
Gary Nattrass is offline   Reply With Quote
Old March 4th, 2012, 10:30 AM   #9
Inner Circle
 
Join Date: Nov 2004
Location: Baltimore, MD USA
Posts: 2,323
Re: Helicopter Noise from mics

Quote:
Originally Posted by Tom Morrow View Post
Several times I have experienced "helicopter noise" coming from my mics while setting up. Sort of a pumping in-and-out white noise with a frequency approximately 4Hz. In all cases it has gone away after I fiddled with connections, gain, and flipping the 48V, limiter, and Low pass filter switches, but I haven't been able to narrow down exactly what I did that made it go away. I have experienced this with both my ME66 and cs-3e mics, while feeding the Mixpre.

I have noticed that when it's an issue, it happens only when the gain is turned up past a certain amount. This makes me suspect a voltage supply instability in the mixer. The fact that I have only experienced it while setting up suggests that perhaps it is compunded by cold batteries producing lower voltage, or possibly condensation on the mic?
Hello Tom,

It has been noticed that some very current hungry mics will exceed the current capacity of the battery causing motorboating. This may not happen until or unless the headphone amp on the device is turned up A LOT. That extra drain on the power supply puts the power supply into oscillation.

Regards,

Ty Ford
Ty Ford is offline   Reply With Quote
Old March 6th, 2012, 09:28 PM   #10
Major Player
 
Join Date: Feb 2011
Location: San Francisco, CA
Posts: 416
Re: Helicopter Noise from mics

I had a chance to sit down and experiment with this helicopter noise. I found that I can get it to occur reliably by plugging in either an ME66 or cs-3e shotgun into my SD MixPre, and turning up the gain dial all the way (max). When I turn the headphone dial up to approx halfway (give or take a notch or two depending on mic position, low cut, and limiter switches, and power source), the motorboating starts occuring. It sounds like the background noise cuts out periodically; first at low frequency then at faster frequency as I turn the headphone level further up.

Strangely, it occurs more and at lower levels when I plug in with the SD AC power adapter, versus running off of two used AA batteries.

So I think it's not something to worry too much about; I never turn the mic gain that high in practice so this is more of a theoretical than actual problem, and if it becomes a problem I know that the emergency solution is just to turn down the headphone gain.
Tom Morrow is offline   Reply With Quote
Old March 6th, 2012, 09:46 PM   #11
Inner Circle
 
Join Date: Nov 2004
Location: Baltimore, MD USA
Posts: 2,323
Re: Helicopter Noise from mics

Tom,

I think that may well have been a problem with older MixPres.

Here you go: Oscillation with Transformer-Balanced Inputs|Sound Notes|Sound Devices, LLC

I have a Sound Devices 442 and have never had the problem. I run it off a very good external battery.

Regards,

Ty Ford
Ty Ford is offline   Reply With Quote
Old March 11th, 2012, 08:32 AM   #12
Major Player
 
Join Date: Jun 2008
Location: Monroe, NY
Posts: 686
Re: Helicopter Noise from mics

Are you sure it isn't coming from cell phones?

John
John Peterson is offline   Reply With Quote
Old March 11th, 2012, 09:29 AM   #13
Trustee
 
Join Date: Jan 2011
Location: Pennsylvania
Posts: 1,359
Re: Helicopter Noise from mics

Or maybe solar flares?
Greg Miller is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > The Tools of DV and HD Production > All Things Audio

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 09:11 PM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network