Posted on: Feb. 25, 2015
A brief guide on how to evaluate low-frequency noise problems quickly and without spending any money - or simply using your smartphone.
Evaluating low-frequency noise complaints is one of the most common (“bane of my life”) issues that EHOs contact us about. Using the right approach, we can sort most of them within a few days. The following is a brief, practical guide detailing solutions to the most oft encountered initial evaluation problems using a tool that won’t cost you a penny.
This fan extract system was the cause of numerous complaints about a very low frequency hum / woofle / throb. Frequency analysis of audio recordings showed the problem to be a 35Hz tone plus first harmonic. As this did not tie-in with fan speeds etc, we immediately knew that it must be a resonance. One site visit later and the problem was resolved by altering the duct acoustics and inserting an aerodynamic flow control plate to improve the system efficiency. The tones were eliminated and the broadband noise was also substantially reduced at a cost <£1k.
An entirely zero cost way to analyse this type of problem is detailed below.
Low Frequency Noise Complaint Evaluation
Forget dB(A), it is irrelevant as it filters out low frequency sound (-39dB at 31Hz). If I had a £ for every extensive set of logged dB(A) plots carried out as part of a low frequency noise evaluation…
The best option. Record samples with plenty of gain for subsequent listening and analysis. Ideally, we like to get 2 or 3 recordings when the noise is bad and a further couple when it is not there or less prominent. That’s it: no prolonged sets of logged data.
I can’t hear anything on play-back
The problem with very low frequency sound is that you need speakers the size of a wardrobe or an unusually good set of headphones. The best results in practice can often be obtained with a good set of in-ear buds that are well screwed into your ears. However, you can use our free QuickRecord software utility to play-back at a higher speed.
We would usually use a x3 or x5 speed increase to shift a hum at perhaps 40Hz up to 120Hz or above where you can clearly hear hums etc. whilst using kit that ordinary mortals can afford… Install the utility, open a wav file and hit play. Pause, click the button next to play and select your preferred speed, un-pause to listen further. Email or call us if you’d like a copy of QuickRecord – firstname.lastname@example.org. Note: QuickRecord usually runs fine in Windows 7 or 10, but it is supplied “as-is” with no support.
Alternatively, you can download the open source Audacity software which allows you to achieve something similar.
Here, after loading the sound file into Audacity, I’ve selected “Effects” and “Change Speed”, and a 200% increase which will double the frequency on playback (the file duration displayed is halved to match). Don’t save the file!
Is there a “real” hum?
You can use the spectrum capability in Audacity to take a quick look for any tones. Revert back to 100% speed and then select a section of the wav file time trace (left click and drag). Then select the “Analyze” tab and “Plot Spectrum” from the drop-down list of options. This displays a simple spectrum. Note that the log X axis allows you to view the low-frequency end of the signature more easily, but you lose the ability to spot harmonics by eye (they are evenly spaced on a linear X axis). In this case, the dominant 35Hz tone can clearly be seen.
This plot is an overlay of the noise before and after treatment, showing the effect of the elimination of the tones and much of the broadband noise using our own, more sophisticated software.
Contact us if you need help with analysis of a specific problem or the interpretation of results. Or you can simply email sample recordings or data to us (remote diagnosis and control - a no-cost service) The most common causes of low frequency noise problems are fans, chillers, burners, compressors etc. We have developed low cost, elegant engineering noise control measures for all of these sources – take a look at a range of environmental noise diagnosis and control case studies.