air con inverter, static noise

Guys
i was wondering if anyone would be able to to help or point me in the right direction.

I recently have installed an air conditioning system into a recording studio. We have discovered that when the air conditioning inverter drive starts the compressor, static noise can be heard when they are trying to record.

Can anyone give us any advise on what should have been done to have stopped this from happening in the first place?

Is there any thing that i can do to reduce or stop noise transmitting?

Is there something that should have been done when installing the recording equipment to stop noise interference from the air conditioning?

This is not my area of expertise, but as i am the installer i have to do something.

Unless the air con has a fault or doesn't meet interference requirements, I wouldn't think you would be responsible to do any fixing.
It sounds like the studio needs to add a descent power filter somewhere, or possibly something in the studio is not earthed or shielded properly.
The studio should also get someone qualified to test their equipment as loosing ground in audio gear is common and very dangerous. People have even been electrocuted by microphones.
Statistically, more electrocutions happen with audio gear (like band equipment) than any other electrical appliances.

Also, try contacting the air con manufacturer to see what they say. Could be poor design or assembly.

How close is it physically to the recording equipment and lines? Is the static very irregular, or does it have some pattern to it? Are they using wireless mics?

The noise is a 10Khz squeal and from the guitars only. The noise is only heard on start of the compressor . The Air conditioning is outside of the property and not inside the studio, behind brickwork and a few layers of sound board about 2metres. dose this help any further.

I was thinking that changing my cables for screened cables could be useful. I also heard about noise filters, but i am unaware of what they are or what they do.

the equipment is good quality well made Japanese manufacture. (Daikin VRV)

And thanks guys, your input is most appreciated.

Well, if it's from the guitars only, I would think it's not ground noise. Most likely the pickups or strings are acting like an antennae, and picking up all the extra EMF when the compressor starts. Again, just my speculation.

Magnetic pickups detect any strong magnetic field from motors, transformers, contactors.

Apart from other things that I do, I just happen to be a guitar teacher as well.
Finding that it's the guitars is a good start.

"Most likely the pickups or strings are acting like an antennae"
Pickups or strings will never act like antennae unless they loose a ground.

"Magnetic pickups detect any strong magnetic field from motors, transformers, contactors"
Magnetic pickups will never detect any strong magnetic field from motors, transformers, contactors. It would be impossible for a band to play anywhere if they were that sensitive.

Sorry guys but unfortunately you're both wrong here. band equipment will not pick up any electrical interference if in good condition.

eg A guitar is ground linked from the input socket to the switches, pots, pickups and the bridge. The bridge on an electric guitar is metal so the strings are earthed as well. If the guitar looses ground the strings might and often does, pick up the local radio station.

Steve, is it only 1 guitar amp causing the problem ?
If so then it's either the guitar itself has a bad or no ground, the guitar lead bad or no ground or the amplifier bad or no ground. If it's the amp then it's dangerous.

If it's more than 1 amp then either both amps have faults or the building wiring will need to be checked.

Meatpie

Meatpie

Thanks for the reply, it is more that one amp causing the problem. What you say makes good sense as I have just been informed that the studio is only on a temporary supply, and is still waiting to be connected properly, I cant see that I can do anything until the correct supply to the building is connected. I am unsure if the current temporary supply is good enough. However all the information and knowledge you guys have supplied me with has been useful so far and will post any further information as I find out more.

"However all the information and knowledge you guys have supplied me with has been useful so far and will post any further information as I find out more."

OK thanks steve, it could be useful for many to hear the results of this.

I have no experience with electric guitars so I can't comment.

How can the guitar pick up detect the tiny field made by the string vibrating, yet reject ambient electromagnetic radiation?

I know that audio amplifiers can pick up EMI, I have a pair of amplified speakers which buzz every now and then when my mobile phone is nearby.

I think the filtering on both the amplifier and air conditioning, needs to be improved.

Hero999 said:

...
How can the guitar pick up detect the tiny field made by the string vibrating, yet reject ambient electromagnetic radiation?
...

Click to expand...

Which is what I was trying to say, but I got stomped on. I have evidence to the contrary. If you hold a Weller soldering gun (the kind with the big transformer inside) within 3ft of a live guitar pick up, the hum/buzz produces as much output as the vibrating sting.

That wasn't a rhetorical question but a genuine one, for all I know there could be a way to prevent a guitar pickup receiving interference whilst keeping i operational.

"Which is what I was trying to say, but I got stomped on"
If I have offended you then apologise. There was no offence intended.

My previous answers were refering to normal circumstances. I don't think this studio would be set up in the middle of an engineering workshop.
I thought I might test what was mentioned in post #11. So I used the biggest transformer I could find. My arc welder. It did pick up the hum when the guiter is closer then 18", but not very loud. At 1 foot I could play over the top of it and not hear the hum. Having said that, it would also depend on the quality of the equipment. One would have to assume that a studio is going to use the best they can get.

The strings themselves will pick up noise easily including magnetic, if it's audible. By placing your hand on the strings to stop them vibrating, will stop that noise being relayed to the pick ups.
If the strings are not held, the noise from the amp will cause the strings to vibrate, which is then amplified and played back to the guitar again and the sound will get louder and louder (feedback). The musician will turn the guitar volume off or unplug at the amp when he/she's not holding the instrument.

Try this test, next time you are near an accoustic guitar, from a few feet away sing a constant note for a second. The guitar will faithfully reproduce the same note and play it back to you.

Meatpie

"How can the guitar pick up detect the tiny field made by the string vibrating, yet reject ambient electromagnetic radiation?"
Yeah, I agree with your thoughts. I think the best answer here is : The field made by the string is actually not so tiny, it's designed to work in close proximity and the pick up is very well shielded. The pick ups and the strings have a common ground inside the guitar which will tend to close the loop and keep it local (If you know what I mean).

"I know that audio amplifiers can pick up EMI, I have a pair of amplified speakers which buzz every now and then when my mobile phone is nearby"
I have noticed this with my phone also, when it transmits back to let big brother know where I am. I think it's the older amps that probably don't have the filtering for a relatively new frequency like a mobile phone.

"I think the filtering on both the amplifier and air conditioning, needs to be improved"
Yes, this would definately be the next thing to look at once the equipment and the building is checked and found to be faultless.

Meatpie

PS In a studio, the guitar will be going through the mixing desk, unless they're after an accoustic recording. This brings about the possibilty of the noise actually being heard from the main PA.

From what you've said it sounds like your arc welder uses a mains frequency 50Hz or 60Hz transformer, not a SMPS.

The inverter in the variable frequency drive is an SMPs which puts out 10kHz when starting. I think the pick up might be more sensitive to 10kHz than 50Hz and the human hearing certainly is.

Hero,
Of course my welder uses a mains transformer. I was testing what you said in post #11. And you did say hum not squeal.
"If you hold a Weller soldering gun (the kind with the big transformer inside) within 3ft of a live guitar pick up, the hum/buzz produces as much output as the vibrating sting"

And yes I know It's 10kHz we're talking about, I already read that in post #4.

"I think the pick up might be more sensitive to 10kHz than 50Hz"
10kHz is so high it's almost inaudible. The pick up is designed to be most sensitive to the frequencies that the guitar can play. Also, music equipment does actually have filtering. I've never known a guitar that can play a note that high.

Meatpie

I can hear 10kHz pretty well.

The pickup might self resonate at 10kHz which would explain it.

Ah, I get you, Self resonate. you might be right. All by itself.

Are you being sarcastic?

I'm serious, any coil has a parasitic capacitance which can resonate with the inductance of the coil and the frequency at which this happens is known as the self resonant frequency. Of course, it doesn't resonate all by itself, it needs some external signal, it's just a coil will amplify frequencies near its resonant frequency.

I could be right, I might be wrong, it's just a theory.

Then Steve should test the theory by starting the air con with the amp on and the guitar unplugged, right. ?

Oh and the pickup is not just a coil, it also has a magnetic center.