My theater Noise Floor: Quick Study

[Matthew J Poes]

I finally decided to download AudioTools onto my phone and calibrate my iMM-6 mic to measure transmission loss and noise criteria. It will be a while before I can do transmission loss, but I did a quick NC study using the NCB standard. NCB stands for Balanced Noise Criteria.

As a quick background, an anechoic chamber would measure below 0. Paul Barton recently told me that NRC measures 20dbs below human hearing perception, as a point of comparison. A really quiet studio would have an NCB level of around 10-15dbs. A really quiet theater would typically fall between 15db and 25db (depending on the heroic levels one went to in order to achieve this level).

A normal room is going to be around 30-35db when it’s quiet. With background tv, people talking, etc. this level would quickly jump to between 55 and 65 dbs.

Now the iMM-6 still is not a lab grade microphone and it has clear noise issues. This is causing a raised high frequency noise floor suggesting that my room is noisier than it really is. My guess is that above about 2000hz the noise measurement is mostly mic capsule noise.

First the room immediately adjacent to my theater. A good reference since this is the noise level I was keeping out of the theater. My basement is noisier than other rooms in my house. NCB 33, meaning it matched the 33db curve based on the 125hz band.


Now for the theater. This shows that I exceed the NCB 25 curve at 4khz and get a score of NCB 26. I happen to believe this is mic noise.


More interesting may be the reduction in the Lf noise floor. This is largely HVAC noise and it’s clearly much lower in the theater. In fact the ambient noise is below the threshold of hearing by about 100hz.

For anyone with an iPhone and $10, you can try this for yourself. The HF response of the iPhone is rolled off so you will get false results at 16khz. For this test it doesn’t matter. This is an RTA test set to one octave (that’s the standard) and using a noise curve overlay. I used NCB. I also shared pics from my room using the internal mic because I wanted others to be able to share. The iMM-6 looks basically the same but with the 16khz bar raised to be flat with the other bars.

Ok I’m editing to add the iMM6. Silly to mention it and not share the measurement.

 

[Matthew J Poes]

Oh one last thing. If anyone knows what electrical item would be buzzing at 60hz in every room I measure. That would be great. I’d love to kill it. It shows up on every measurement I’ve ever taken of noise floor. It isn’t interference and it’s not speaker hum because it’s in other rooms. I’m guessing it’s HVAC related.

 

[Tony V.]

Well, it could be your hot water tank or furnace? if not that then maybe a freezer or if you have florescent lights the ballast will humm

 

[Matthew J Poes]

Well, it could be your hot water tank or furnace? if not that then maybe a freezer or if you have florescent lights the ballast will humm

Yes but....it’s in every room I measured. I have Led lights in most rooms and the hot water tank is in the basement. It’s also has rather Than electric. That’s why I can’t pinpoint it with certainty. It’s so common i thought it was interference on my laptop, but since it’s on my phone too, it can’t be that.

 

[Tony V.]

Gas hot water is what I have also and when its heating it gives off a very low frequency till it turns off and I can hear it throughout the house at varying levels.

 

[Matthew J Poes]

Gas hot water is what I have also and when its heating it gives off a very low frequency till it turns off and I can hear it throughout the house at varying levels.

Oh interesting. I’ll have to look into that. If that is the case I suspect the only fix would be to mount it on spring isolators which I bet would not be cheap.

 

[Tony V.]

Its actually the sound coming from the chamber inside as the flame heats up the water. all you can do is back off the gas supply so it reduces the flame somewhat.

 

[Tony V.]

Matt, did you ever figure out what the noise was?

 

[AudiocRaver]

Ground loop?

When we were moving into our last house in Nebraska 12 years ago, I was trying to track down a hum and ended up pulling all of the fuses, including the main fuse, so the house was completely electrically "dead," using a guitar's pickup and a battery operated preamp and headphones as a detector, and could never get rid of it. Finally figured it to be a ground loop via the grounding cold water pipe which ran directly over my studio area, and could only reduce it by being careful how cables were routed &/or using toslink to eliminate ground connections and additional loops. They are not always easy to track down, and there are usually not many options for treating it. Just an idea.

 

[Matthew J Poes]

Matt, did you ever figure out what the noise was?

No but I haven't tried looking much since then either. I got busy. I'll try again.

It isn't a system ground loop as this was room noise, measured on a battery powered phone that is in no way connected to an outlet or anything connected to an outlet and the audio system was completely off and powered down. It was just ambient noise.

I think to test if it's related to the boiler or HVAC I will need to ensure that are completely powered down and then take measurements again.

My own theory is that it's hvac noise related to the blower and/or humidifier. If that is the case then I will look into a decoupling Bellow mount for the main duct and return and into the idea of lifting the HVAC onto decoupling spring mounts. That's more expensive than it should be so I will wait to confirm its the issue.

If it is the flame in the hot water tank I'm not really sure what I will do. It could get a decoupled mount as well assuming that is ok.

One thing I didn't do when building the theater was test the cement floor with accellerometers. I can still do that in an adjacent exposed section and probably should. In a really top notch soundproof room the cement would have been hacked out and a new decoupled foundation floor poured. Since that would have cost a small fortune I decoupled the subfloor instead and did it very cheaply. I think it didn't work.

 

[AudiocRaver]

We have plasma tweeters - sweet! - how about super-quiet plasma water heaters?

 

[Matthew J Poes]

We have plasma tweeters - sweet! - how about super-quiet plasma water heaters?

I'm assuming ozone problem?

 

[Matthew J Poes]

Ok so I bought a much quieter measurement microphone and was able to redo this. I knew the mics were limiting factors before, but I wasn’t sure to what extent.

Well the new mic has an equivalent noise floor of less than 18dB. It is quiet enough that my sound interface is actually the limiting factor now. I’m waiting for a new Motu 828 to arrive. In the mean time, here is what I got with a Behringer sound interface and the new mic.



That’s a PNC value of 17 (NCB came in at 15, but is not shown). PNC is a more conservative measurement that places more weight on the low frequencies in the noise floor. My dominant noise floor issues are HVAC related, so I wanted to account for that.

I’m impressed!