Audible low frequency hum in my condo :(
Old 1st May 2010
Gear Head

Thread Starter
Audible low frequency hum in my condo :(

Hey guys. I'm in the planning stages of converting a room in my condo into a project studio. I catch myself wondering if my efforts will be futile. There is a low frequency hum with a bit of a high frequency ringing quality to it present all throughout my condo except for in the master bedroom. I turned the main breaker off to see if it was electrical, but the hum was still present. I've walked around the house several nights in a row trying to locate it (It's more noticeable at night when there is no other noises). The best I could up with was that it might be from the roof. I am on the top floor of a condo and the ac units are on the roof. My best guess was that the noise from their compressors is causing this humming in which case I'm not sure there is a way to fix it. Do you guys have any better guesses?
Old 1st May 2010
Registered User

In a complete analysis of a space, one of the very first measurements made is that for Noise Criteria. This fundamental evaluation is critical in determining if a space is suitable to be utilized.

Many times, if these preliminary conditions cannot be satisfied, it is often too expensive or complex to re-engineer the space to meet the necessary pre-requisite criteria.

Noise Criteria is a type of sound level measurement. The human ear at low levels predominantly hears mid-frequency sounds, so to measure the equivalent perceived equal loudness contours, an analyzer will apply the NC scales and then rates the room. In general a home theater, for instance, should be NC35 or lower, and NC30 is preferable.

The lower the number the better.

A mic such an Earthworks M30 is adequate for use to NC30 – which addresses 99.9% of typical environments. If one needs work at NC25 or below in the remaining 1/10th of 1 percent (typically in a lab setting), one would need to employ a microphone suited to use near the limits of human perception.

Noise Criteria (NC) is a method to evaluate the low level ambient noises in a room. We care about ambient noise as it tends to mask the subtle details (Foley effects) in a film and as they distract the listener from the material at hand. In extreme cases noise will also mask speech, making it difficult to understand dialogue.

Typically in an interior space, three measurements are made initially. One with the HVAC and audio system off, and one more time with the HVAC system on, and then again with both the HVAC and audio system on (with no test signal, just powered on). From this you will see the responses overlaid relative to the NC contour curves.

In making the measurement, the mic is typically placed at the primary or central seating position on a stand at ear level. The measurement platform must be remotely located in order for it to not contribute to the environment noise. People should not be moving nor making any sound while this measurement is occurring, as even small noises can affect the rating.

By comparing these curves, you can see where the ambient noise is coming from – room, audio or HVAC. Additionally, if greater detail is required, one can use the mic as a sniffer, moving it closer to the bottom of doors, HVAC ducts, fans, etc. to look for sources of noise.

Note: it is not my desire to attempt to evaluate your space – rather this is simply meant as an overview of how such issues are addressed from an analysis POV. From the results obtained, and the sources identified, one must then evaluate the specific situation on a case by case basis and make the appropriate determination as to what is required to remediate the issue(s).
Old 1st May 2010
Gear Guru
DanDan's Avatar

The surest way to find the source is to turn it off and on, hear the difference.
If that is not possible, turn as much as you can off an try to find the path of the sound. It is quite likely entering your space by conduction through the solid structures of the building. This can seem to come from everywhere. The source could easily be in the next apartment. A fridge, a noisy lighting fixture. You will have to try to find out where it is loudest to establish some sort of direction. A stethoscope or even placing your ear directly in contact with a wall, floor, etc. might work. If there are AC ducts try listening closely to them. There must be some reason why the master bedroom is immune. There must be a clue in that fact. Perhaps enhanced ducting, silencers in there.
The louvre face on ducts can have quite a beneficial effect sonically. Are yours all the same, or is the bedroom one different?

Last edited by DanDan; 1st May 2010 at 07:33 PM.. Reason: Question Mark!
Old 1st May 2010
Lives for gear
avare's Avatar

Originally Posted by SAC View Post
If one needs work at NC25 or below in the remaining 1/10th of 1 percent (typically in a lab setting), one would need to employ a microphone suited to use near the limits of human perception..
I think you are confusing A weighted vs NC curves. The NC curve for control rooms from the alphabet soup of organizations is NC 10 with no levels NC15. TheA weighted lef3ls rough relationship between A weighted levels and NC curves is that A weighted levels are approximately 10 points higher.

Old 1st May 2010
Registered User


I am referring to NC curves in this case (relative to the US) - and not the NR (Euro) curves for ambient noise levels.

NC - Noise Criterion

The irony is that these tests are literally among the first performed in a space someone is seriously considering using. In effect, they are a variant of basic Noise Level Analysis (NLA) best practices that precede any attempts at measurement/analysis or treatment.


The larger issue to which I was referring that hopefully might make others aware, rather than simply attempting to locate a specific source here, was simply that few here ever perform an initial evaluation regarding ambient noise - something that is fundamental in the commercial/'pro' markets and may well determine if a space is even worth considering and if any additional steps will even help. In fact, if one ever has the opportunity to work in a large acoustical space such as an auditorium or civic center, you will quickly discover that such issues become critical - and REAL issue to deal with!!! And I literally dare anyone to approach a facilities manager to inform them that they must turn off the HVAC systems for an event as the ambient noise levels are excessive!

And while many spaces luckily qualify, many do not. Many of the spaces mentioned here, if this fundamental analysis were performed, would be eliminated from consideration and the expenditure of a great amount of time and money (with cause) from inception.


And all of this segues into the issue of what we know and do not know - and what gets posted as if it is indeed accurate and subsequently becomes fundamental to the larger group-think.

As many such practices allude to even larger issues that lurk in shadows and corners outside of much of the generic information tossed about here by many.

And that is the fact that MUCH IS KNOWN and understood in acoustical engineering that many are quick to deny here! Much that many dismiss requires not only the desire, but an awareness and familiarity with physics and acoustical engineering! I know that the mention of such is anathema to many who would prefer to imagine acoustics as a black ART and not science, but it IS science, and that is the language of the discipline.

And the field is not so ignorant of various designs as some individuals may be (be they diffusion, resonant absorbers - I mean, how many times will we read that the design of resonant absorbers is not well understood when they are a backbone of applied industrial acoustical engineering! e.g.: Almost every mechanical engine utilizes them! - but then you are not going to find detailed design guides that explain them logically without the use of mathematical relationships based upon determined characteristics, measurements, etc., etc. etc.!) And as such it is not valid for some to issue broad statements tot he effect that such techniques or principles are not understood. Rather it may simply indicate that one might have to pursue such information outside the confines where some are not aware - or ignore-ant - of them.

I have no problem with someone stating that the design or concept behind a particular technology is not understood by them, but it fascinates me that someone who is themselves unaware of the science feels compelled to pronounce and entire discipline of engineering as ignorant of the concept...

And rather than simply pursue brute force cures for fundamental problems, folks would benefit from understanding that there IS a bit more to "acoustics" then simply throwing up a bunch of panels! As there persists a notion that acoustics is some primitive 'black art' that few if any understand based solely on the fact that neither they nor others have explained the significant body of knowledge and best practices that DO exist in mature form.

For instance, the notion that applying facing (...we might use "FSK" as a generic term here) to the surface of an absorptive membrane is "not understood"? Please! Modifying the acoustical impedance of material is WELL understood. Perhaps not by many here, but then such understanding involves not only a conceptual understanding, but math - the mention of which sends most scurrying!

And additionally - as absorptive panels have been mentioned, just today it was mentioned that panels should be at least 4 inches in depth to insure that they absorb a relatively flat power spectrum - and while I won't debate that general guideline - the Larger irony is that most absorptive materials, and almost ALL facings of any kind (cloth, perforated materials, etc.) render the absorptive surface exceedingly reflective at all 'grazing' angles of incidence with perhaps a normal (perpendicular) incidence!

Yet FEW cite, post or even acknowledge the very significant fact that absorption exhibits a VERY significant reflective component especially in the normal (as in average/typical) application where the various sources are oriented at rather extreme (low) incidence angles. Yet many are under the erroneous impression that if you install absorption, that all or most of the the energy in a particular frequency range in question is effectively mitigated and 'removed' - when in fact a large part of that energy is not, and is reflected in a non-linear distribution.

And has anyone even pondered as to if or how this behavior might be identified and quantified? Such should be readily apparent simply by the fact that an acoustic (or any other) impedance is not simply comprised of real values - or even more fundamentally - of what happens by definition in the event of impedance mis-matches! Again, while this may be qualified for acoustical issues, the basic concept is fundamental to the study and understanding of the electro-mechanical world as we know it!

The irony being that even with a lack of such test and informational data, such practical significance is EASILY determined in a practical manner via the use of ETC measurements that provide real time feedback regarding the reflected energy from any applied surface, be it absorptive, reflective or diffusive, providing an accurate comprehensive picture of the actual performance at any given time in any given location relative to any treatment (or lack thereof).

Yet this use provides significant USABLE data available essentially to anyone who takes the small amount of time to simply employ it!

Yet many still 'rely' on incomplete and unreliable NRC measurements that provide little or no practical value - in fact, with their inherent limitations and incomplete profiling of the behavior of a material - they are not much better than tire grading schemes for tire wear and temperature that, AT BEST, provide a comparative ranking among the same manufacturer's product, but are meaningless to compare product between manufacturers!

The failure to acknowledge and to employ such basic understanding as well as the practical and readily available tools - or for that matter, the active campaigns by some against such tools - simply contributes to the perception in the larger forum, as well as those whose are here to learn of the various options regarding acoustics, of acoustics being a rather 'slow' and ignorant stepchild of the engineering sciences!

The understanding is in large measure indeed available - And the tools are available as well - for those who are serious enough to take the time to simply ask and not be satisfied with the seemingly mantra-like pronouncements that little is known in acoustics! As such pronouncements reflect only the provider of such 'insight', and not the state of affairs in acoustical physics.

And at the point where our understanding reaches the limits of the known, we can indeed ponder such wonders - but for the most part, those limits are relatively distant from the boundaries of that which we are dealing here.
Old 2nd May 2010
Gear Head

Thread Starter
Thanks for the post guys very informative, but some what over my head (I suppose I have some background reading to do so I can completely follow). SAC thanks for your suggestions, and background I might add however that I'm not sure if anyone in this thread was questioning the fact that acoustics is a science. I just had a query and given my limited knowledge of acoustics I came and asked others who are more knowledgeable then me what their thoughts were. This does not mean I think its a black art or something that I couldn't learn by getting various physics and engineering books, but since I have a full time day job and am a musician if I were to devote years to studying physics that kind of would be counterproductive wouldn't it be. That would be time that I wouldn't have spend tracking and mixing. While I do desire to gain as much knowledge as I can about physics and engineering that is relevant to producing music that desire is behind the initial desire to record and write music. Thats why forums like this exist so that people like me can come and maybe learn from experience. Someone who for example has devoted a significant amount anothers nt of time to grasping the acoustics or construction or what ever it may be.
While I do think people shouldnt just beg for answers and never look further into it or study it more. I think that there is a point where it is counterproductive to try to achieve total knowledge in all things. I think it can be a long term goal, but for example if Im trying to build the best mixing room for me right now with my given limitations I can't expect to have a full grasp on acoustics in a couple weeks or months even.
Sorry if I got the wrong impression from your second post, but I thought I would explain how I personally feel about posting in forums asking for aid.

DanDan- As far as turning things on and off. Like I said in the OP I turned the main breaker off and still heard it. I put my ear up to the ac duct in the room and it did not appear to be coming from there. The ac duct in the master bedroom is a bit different, but Im not certain that it is coming from the ducts to begin with. I think it is coming from the walls. possibly the ac compressors in the roof that are over my living room area. I'm going to try to use the mic technique to see where it is coming from, but if its something in the roof or next door I guess I will be out of luck, but what can I do Ill just track with that limitation until I move.
Old 2nd May 2010
Lives for gear

Originally Posted by ELECTRICFL View Post
DanDan- As far as turning things on and off. Like I said in the OP I turned the main breaker off and still heard it. I put my ear up to the ac duct in the room and it did not appear to be coming from there. The ac duct in the master bedroom is a bit different, but Im not certain that it is coming from the ducts to begin with. I think it is coming from the walls. possibly the ac compressors in the roof that are over my living room area. I'm going to try to use the mic technique to see where it is coming from, but if its something in the roof or next door I guess I will be out of luck, but what can I do Ill just track with that limitation until I move.
I lived in an apartment like that once. Hum came from everywhere. Aggressive acoustic treatment made it slightly less annoying, but in the end, I just moved. The best solution is to get a detached house in a quiet neighbourhood, of course, but that costs more $$$.

Good luck.
Old 2nd May 2010
Registered User

Originally Posted by ELECTRICFL View Post
I might add however that I'm not sure if anyone in this thread was questioning the fact that acoustics is a science. I just had a query and given my limited knowledge of acoustics I came and asked others who are more knowledgeable then me what their thoughts were.
As was specifically stated, aside from the basic application of noise level analysis to evaluate such conditions in ANY space, the remaining LARGER issue of the response was NOT aimed at you. Hence the statement:
"The larger issue to which I was referring that hopefully might make others aware, rather than simply attempting to locate a specific source here,..."

If you do happen to read a few other threads in the forum you will note that the specific issues referred to have indeed occurred today, and for that matter, occur rather routinely. Topics such as Helmholtz resonator design and the behavior of material facings (FSK) are routinely declared 'unknown/unknowable' by virtue that 'the behavior or design is not understood'.

Its fine if an individual does not fully understand the behavior or design - its a broad science and many behaviors are indeed complex! But it is an error for one to mask their own unawareness of a behavior by simply labelling the subject 'unknown' or 'unknowable' in the scientific discipline at large! And it is this tendency to which I object.

Advice is wonderful - and why we are here - and hopefully many are able to get practical answers or greater understanding. But advice should also be based upon a real understanding, or at least identify itself as a best guess, and where necessary or applicable, acknowledge its limitations as well. As initial questioners as well as respondents can all learn. But the responses should limit their 'answers' to that which is objectively verifiable and identify speculation as such. ...All can benefit from this.

So bottomline, the point is that there are simple existing measurement tools that can address exactly such issues as you presented and quickly determine if time and money are worth being sunk into a problem with respect to reasonable expectations of success.

And an awareness of the existence and use of such readily available tools can serve to assist a much wider audience as might be determined simply by the myriad sound isolation threads that have been posted in the last week or so..
Old 2nd May 2010
Gear Guru
DanDan's Avatar

The different AC duct/louvre in the bedroom, coupled with the lack of noise there still interests me. Is it possible to get the AC turned off? There lies the proof. The engineering needed to isolate compressors from the structure is pretty simple. Mass and Springs. This could be remedied. Even simpler if it is a fridge next door. I would regard this hum as a fault in the building. Perhaps you could report it as such, see if the authorities will step in.

Old 2nd May 2010
Lives for gear
666666's Avatar
If you have big AC units right over you, it would make sense that these would be the source of the noise. But, typically, AC compressors cycle on and off (and do not run continuously), do you note the noise ever coming and going... or getting louder / less loud? Or.... some AC units may have transformers that hum ALL the time (regardless of compressor activity), maybe there's one close by and not "properly" mounted.

Finally you have to ask yourself, WHAT will you be doing in your "project" studio? It's possible that this hum noise, IF relatively very weak, will not cause problems for you when doing project work. When you're mixing, a very low level hum may not be an issue. Sure, it would be nice if it wasn't there at all, but if it's faint enough, once you crank up your monitors it will effectively disappear and not interfere at all. If you are recording fairly loud instruments, same deal... the hum may be relatively so low that it will not be picked up.

I suppose the hum could be an issue if you are often recording very low-level type acoustic instruments with very sensitive mics, but then again if you keep the mics quite close to the source, you still might be fine. Distant miking of low level sources with sensitive mics will be the one thing you definitely cannot do if there is any level of audible hum in the room, but unless your space is acoustically very appealing to begin with (unlikely if it's a small space), you will not be doing "distant" miking anyway.

As a test, maybe set up some of your gear now before even putting up any treatment, and record some things and do some rough mixes, actually observe for yourself whether or not the existing hum will be an actual issue. Play your mixes at another location on some good gear and listen for any microscopic degree of the hum... if you cannot detect any trace of it, you'll probably be fine.

I did a project once at another location where there was a ton of ambient noise, thumps, rumbles, footfall noises, etc, but we were recording mostly acoustic drums and guitar amps (loud sources)... the ambient noises were quite "loud" but never proved to be a problem... they could not be detected at all in the final mixing stage. We got by with what we had. This type of thing would not be acceptable for a "professional studio", but if it's just going to be a "project studio", it might be ok.
Old 3rd May 2010
Gear Head

Thread Starter
Originally Posted by DanDan View Post
The different AC duct/louvre in the bedroom, coupled with the lack of noise there still interests me. Is it possible to get the AC turned off? There lies the proof. The engineering needed to isolate compressors from the structure is pretty simple. Mass and Springs. This could be remedied. Even simpler if it is a fridge next door. I would regard this hum as a fault in the building. Perhaps you could report it as such, see if the authorities will step in.

Thanks for all the suggestions guys.
This might be a silly question but doesn't turning off the main breaker turn off the ac? When I turned off the main breaker I still heard the hum which led me to believe that it was from an outside source. I'm not sure if it is directly related to my internal ac unit. Which while does have a noise of its own when it is running does not seem to have a low freq hum to it. Dandan I rechecked the master bedrooms vent with the power turned on and my ears up to it, and it does seem to have a noise to it, but it is diff from the one heard in the rest of the house a bit of a higher freq sounds more like its electrical rather than a low freq resonance. I've walked around the house a few times and the source seems to be in the far room across the hall from the room where I want to construct the project studio and it appears to either be coming from the roof or the wall its hard to tell. I dont have roof access so I can't check if there is a compressor unit directly over that room. I could ask the neighbors if they have a fridge in that area though. How could it be fixed if it was a noisy fridge?

Thanks for your thoughts 666666 you are right since this is a project studio with a sorted amount of limitations including a small space the low level hum might be a non issue since everything will be close miked. I was just trying to make the best of what is currently available to me. Also the noise does not come and go it is a constant buzz with slight fluctuations in level every other second or so.

Last edited by ELECTRICFL; 3rd May 2010 at 12:49 AM.. Reason: Forgot to add an answer
Old 3rd May 2010
Registered User

Assuming an apartment and a central HVAC supply for the entire building, I suspect that your particular apartment breaker will not turn off the main/central systems. Nor would one want this to be the case.

If you have a building manager or a maintenance supervisor, I would talk to them to engage there cooperation. Some systems are a bit more complex and require more attention than simply cycling the power on and off.

But as far as this being a "fault of the building" and expecting an easy solution, do not expect allot of sympathy for this position, unless you can provide substantiation that codes require an ambient noise floor below experienced levels. And even if you can, I would not expect them to be glad to see you!

Few residential buildings have such requirements. And the solutions are neither trivial nor inexpensive. And remediation can often extend far beyond simple mass-spring tuned resonating isolation - the cost and installation of which are not cheap!

So, even if you can establish a basis for such remediable action, based upon precedence, I would reasonably expect future relations with the management/owners of said property to be a bit chillier than before. And as such, you may quickly discover your noise issues that you will create with neighbors or management may become more critical to them as well. You may well win a battle but precipitate a war. And a Pyrrhic victory,

Rightly or wrongly, such results are common even when fundamental corrective structural issues are identified that must be addressed that enhance the structure of the building! You may find the distinction between the letter and spirit of local rules becoming much narrower.

You might want to step back and examine the larger purposes for being there before you pick a potential battle - and it will most likely be perceived as such if you attempt to make the building management spend substantial amounts of money on issues that are local to only your exceptional interest.
Old 3rd May 2010
Gear Head

Thread Starter
I see what you mean SAC. At this point in time I'm really only trying to remedy the issue with things that are within my control, and not that of others such as things that would involve management to make alterations to the building. Being that I will only live in this location for another year I cannot justify pursuing the route that would cost management any amount of money for reasons such as the ones you included. If I could find the source it might help to remedy it though. If it is as simple as a loud refrigerator that could be fixed. I don't think thats the case though. I was not aware of the HVAC system being connected to the main building. I thought it was on a unit by unit basis. Since I have an ac unit within my condo which is wired to my compressor which is on the roof I was not aware there was another component but then again I dont know much about HVAC systems just yet.
Old 3rd May 2010
Gear Guru
DanDan's Avatar

From what you say it appears that each condo may have it's own AC system.
If that is the case, then I would also have assumed that the main breaker would shut this down. However it may not fully shut it down. Perhaps the air moving fan continues? Perhaps a compressor cannot shut down just like that.
I know little of these systems, and less of US practices, so I can only speak in general. Mechanics sometimes use a variation on the stethoscope to listen for worn bearings or gears. A probe is pressed into contact with a solid and any noise within is loudly revealed. These are simple devices, probably quite cheap or borrowable. That might help tracing the structure borne path.
If you do have your own AC, the units are probably quite small. Floating such a unit on resilient mounts should be quite do-able. Similarly there are floating panels for under fridges. Structure borne sound can be quite mysterious, appearing to come from anywhere and everywhere. A local venue sought my help with a strange loud knocking noise. It was extraordinarily loud and seemed to come from the Ladies' toilets. We gained access and found no source in there. Ultimately the source was found. It was a water pressurising pump which had been dislodged from it's somewhat resiliently mounted position i.e. supported by plastic pipes. It was simply touching some drywall. This large diaphragm radiated an enormous noise into the air and into the building.
A small water pump, quite a distance from the Ladies.
I would keep looking. Get advice on what can be turned off or not. Get roof access perhaps. The problem may or may not be very small at source. You will only know when you find that source.

Old 3rd May 2010
Registered User

Yeah, so much depends upon the topology of the building.

If you can (meaning more precisely, 'if they exist'), see if you can't casually confer with a building maintenance tech. You may be able to learn quite a bit about the systems through casual banter - as well as attitudes about the maintenance and upkeep of the building- as they are often the ones who hear the comments from the management company in the context of problems over time.

Often the chillers/evaporator units are mounted on the roof.

If you do 'luck out' and the source is a locally mounted device, there are quite a number of vendors who source individual vibration isolating 'feet' for not too much money (depending of course upon how large the unit is). The specific design will depend upon the specific piece of gear, and prices aren't too bad if the gear remains in the refrigerator, washer/dryer mass range.

Good luck. hopefully the source of the problem will be minor and relatively easily addressed with simple relatively easily installed isolators.
Old 3rd May 2010
Lives for gear
avare's Avatar

Thanks SAC;

Your writing that you are indeed speciically refering to NC curves makes the relationship between this discussion NC curves and A weighting even easier. I am certain that you know all that I am writing here. The majority of this post is to help readers who have gottne over their last headache learning acoustics.

NC curves have become somewhat obsolete by the NCB curves. NCB curves ae similar to NC curves above 63 Hz, with relatively minor differences. However, significantly they include spectrum data down to the 16 Hz band. An equation has been developed showing the relationship between NCB curves and A weighted SPL levels. The equation is

Db(A)=NCB x .8 + 13

Re-arranging for NCB, the result is NCB 10 is equavalent to 21 Db(A).

(To Others)
Alpahabet Soups

In a previous post I made reference to an "alphabet soup" regarding recommendations. This was in reference to control room recommendations. Specifically, from:

Japanese HDTV multichannel suround forum,
NARAS, and
Tonmeister Surround Sound Forum (aka. SSF).

There is another alphabet soup involving room noise levels. There are various standards for room noise including:


NC (Noise Criteria) was one of the first, if not the first attempt at providing some form of single figure identification for room noises. The curves are based on equal loudness curves of human hearing.It has certain weaknesses, but as a tribute to its creator, the great Leo Beranek, it is still in common use. ANSI is the promolgator.

NCB (Balanced Noise Criteria, go figure) is an improvement on NC curves by Beranek, the most significant difference being the inclusion of 32 and 16 Hz bands. These frequency bands, though not normally thought of as being particularly sound, can have significant impact on the environment by nature of the sounds causing vibration. Also, the standard includes a noise quality factor in it's descriptor. It is an ANSI standard.

NR (Noise Rating) is the latest, and becoming the global standard for noise rating in rooms. It is similar to NCB curves, and is promolgated by the ISO. It has been adopted by several countries as their standard for noise levels in buildings.

PNC (Preferred Noise Criteria) was Beranek's first attempt at improving NC curves. It has become mostly obsolete.

RC (Room Criteria) curves were developed for ASHRAE for designing air handling systems. The curves are straight lines. They go down to 16 Hz, and include a noise qualty descriptor, likie the NCB. but only down to a sound level around 35 dB(A). This is because it is intended for non-criitical applications, and lower levels of nosie are deemed for critical applications where specific engineering is appropriate.

For those of that they were over the aspirin days of studio acoustics and had not thought about required noise levels, welcome back!

Initially AAV,
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