Any tips for blocking out air conditioner droning noise when recording audio (this is for an open legal project, not covert). I'm using an Axis T8351 which works pretty well. Too well actually, I'm picking up the constant, low level hum of the air conditioner. I've tried to physically isolate it from the wall and drop ceiling tile. I'm thinking of putting in a bass blocker/high pass filter on the microphone line to block out the low frequencies.
Anyone with office environment audio experience?
Only thing I can think of is sound proofing the upper area with Foam like musicians to in their band room. Absorbs a lot of the sound and may be able to help you in the same scenario. In this case I would say put a bunch of it on top of the ceiling time the camera is mounted on and see if that absorbs most of the hum.
Good luck mate!
"Studio" foam is only good for high frequencies at the angle of reflection and reducing flutter echo. It does not absorb sound the way a bass trap does, no matter what Auralex tells you ;)
I would be careful with low/high pass filters. The human voice is 'intelligible' at around 1.5K to 5K, in most cases. According to this paper (http://www.kyu.edu.tw/93/epaperv7/089.pdf) the range of an ariconditioner is hits most of that range. A low pass filter would cut out the lower ranges of the office noise, but you will still have the higher ranges, which seem to be rated at the highest dB on the chart.
I found the best option, it has been awhile, was to use a separate microphone, I liked Louroe hardware (www.louroe.com) and adjust the dB rating that activates the mic. Depending on the VMS, you may have some adjustment levels there as well.
It may be best to use a mix of options, adjust dB threshold along with an audio filter that allows for some level of control, like adjusting specific frequencies. For example, something that allows you to increase 2-5k range, while lowering everything else. If I recall, this is called, 'pocketing' in the audio world, creating space for the frequencies you want.
Good luck.
"A low pass filter would cut out the lower ranges of the office noise, but you will still have the higher ranges, which seem to be rated at the highest dB on the chart."
A low-pass filter is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.
Right?
I'm familiar with Louroe and I looked at their items to see if there was a high pass filter option.\
Funny, in doing some testing, I swapped out the mic for a Sony SCA-M30 and the humming noise went away. The price is about double, but the installation options are a lot better and the sound quality is better
I asked Louroe to comment on this question. Here is the response:
"There are a few unknowns here, but generally you want to place the mic as far from an AC vent as possible.
It’s unclear if the source of the noise is the AC vent with air rushing out, the venting above the ceiling, or the sound of the air conditioner itself.
A high pass filter may help reduce the noise but I think it’s unlikely he’ll be able to filter it out without some DSP option and that’s generally overkill for an office environment.
The standard questions we’d ask for this kind of question:
- What are the dimensions of the room? Ceiling height is important too.
- Are you trying to hear the whole room or a stationary segment of it?
- Where is the AC vent or unit in relation to that location?
From there the conversation could head in a number of different directions.
Based on mic placement, it might be better to use a dynamic mic element as opposed to an omni directional element (most Louroe mics and the Axis T8351 referenced in the post are omni directional). "
I'll reply to Louroe with any feedback on these follow-ups. In the meantime they may respond directly to the topic as well.
Based on mic placement, it might be better to use a dynamic mic element as opposed to an omni directional element (most Louroe mics and the Axis T8351 referenced in the post are omni directional).
Maybe he meant directional, as opposed to dynamic? As the former refers to the pickup pattern, the latter to the pickup electronics; and omni-dynamic mics are quite common.
The mic is about 8 feet away from an AC vent. The noise doesn't sound so much like air flowing past it, as it's the very low freq droning noise from the motor.
The room in question is actually huge. It's a cube farm. Typical office ceiling height. I had the mic on the wall and also clipped to the ceiling tile grid. I thought that maybe giving it some shock buffering would help, but even when I held it in my hands, it still did it.
Best option is to have the customer's HVAC company switch out the AC vents to "low velocity" versions. This will eliminate the vent noise you are hearing on the mics.
Mic placement could have an impact here. Low frequencies build up in corners so keep the mic away from them. It is also best to get as close to your sound source as possible so you don't have to crank the gain and introduce unwanted noise. Sometimes the frequency range of the mic is the issue so EQ or another mic is needed.
This Mic should solve your problems, its a bit pricey, but its one of the best out there for recording speech without close mic'ing.
"The Crown® PZM®-10 and PZM-10LL, part of the Pressure Zone Microphone® line, are designed for security, surveillance, and conference table use. Since they do not look like microphones, they are inconspicuous. Each mic’s tubular housing mounts in a 25/32-inch diameter hole in a ceiling panel, wall or table.
Frequencies below the voice range are rolled off to reduce pickup of air conditioning rum- ble. The high-frequency response is boosted slightly to help articulation. Because of their tailored response and PZM construction, the PZM-10 and PZM-10LL will pick up conversations or other desired sounds with extra clarity and definition."
I've had good luck in the past with the Crown PZM mics, but never had to deal with the background noise issue described here.
These are also available in a single-gang mount design (the PZM-11), and a weatherproof version (PZM-11LLWR), which might be useful in an interview room setup because the stainless wallplate also adds some tamper resistence.
"LL" designations in Crown pics are line-level output, the non-LL parts are mic-level output.
LL.. are line-level level output, the non-LL parts are mic-level output.
I'm thinking that if the camera supports line level then that's the way to go, on the theory that the crown pre is going to be superior to the camera's and the added signal level will be less susceptible to electrical noise.
Do you agree?
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