Industrial Noise & Vibration Centre

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How to use Tonal Noise Analysis - it is Best Practice...

How to identify noise tones, their source(s) and the best control methods

If there is the possibility of tonal noise from noise sources (a very common cause of complaints), any environmental noise investigation or report (e.g. BS4142) that does not include narrow-band frequency analyses is, by definition, not fit for purpose. Consequently (confirmed by surveys) around 90% of consultant noise reports are inadequate. The key reasons are:-

  • Subjective impact: human perception of tonal noise, hums, drones, whines etc is fundamentally narrow-band. Consequently, the only effective way to evaluate the effects of tonal noise is to use narrow-band analysis
  • Objective evaluation: narrow-band noise analysis provides an objective value closely correlated with the subjective impact and tells you exactly how much frequency-specific attenuation is required. Note: low-frequency tones (< c 300Hz) cannot be reduced effectively using conventional silencers or barriers and don't contribute much to dB(A). You must find out if they are there
  • Source identification: tonal analysis provides an instant and precise diagnosis of the source(s) of the tone(s). This takes minutes compared with days or weeks (or never) without this analysis. Octaves are of no use and 1/3 octaves are of very little use - and then only if you're lucky...
  • Noise control: spectrum analysis and interpretation also reveals the precise cause of the problem within each source, an invaluable aid to determine the optimum mitigation technology

Given these benefits and the fact that there are dozens of perfectly adequate and free frequency analyser apps available for smartphones, why is narrow-band analysis absent from most BS4142 noise reports?

In addition, this form of analysis massively reduces the resources spent on complaint investigation and cuts the time taken to resolve those complaints from months or years down to a few days or weeks.

The following is a simple, practical guide to the use of tonal noise analysis.

How to carry out narrow-band frequency analysis

There are a number of options.

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Spectrum Analyser app
Live analysis in the field
  1. use the built-in sound level meter software, provided you have a suitable (and expensive) SLM. This provides calibrated levels.
  2. use one of the many free frequency analyser apps on your smartphone. Whilst not calibrated, the frequencies and the tonal level above background (which are all you need) are as precise as on an SLM
Record sound samples for post analysis
connect sound level meter to recorder
  • SLM: if you have a suitable meter, record audio files (16-bit or higher resolution). To reduce file size, choose the lowest available sampling rate (e.g. 11kHz). The maximum frequency available from the recording is c half the sampling rate. At 11kHz sampling, for example, the maximum frequency is c 5000Hz which is fine for environmental noise.
  • SLM to audio recorder: use a low-cost audio recorder (from c £100 - must have a 16-bit wav file option) to upgrade any SLM. Connect the AC line out on the SLM to the recorder (simple cable), disabling any built-in auto gain control and set the sampling rate as in 1 above. In conjunction with your SLM acoustic calibrator, this allows you to record calibrated sound files.
remote control of ID fan noise
  • Audio recorder: as described in 3 above, but use the inbuilt microphones. Uncalibrated.
  • Smartphone: app audio clips are fine, as long as the file format is 16-bit or higher. There will be a roll-off in amplitude below c 200Hz that varies between phones, but this is not usually an issue. Uncalibrated.
Send us your sound or video samples for analysis

Simply email us your recording(s) and we will carry out the frequency analysis for you. These can be in any audio format or even short smartphone video clips from which we extract the audio.

How to interpret narrow-band frequency noise signatures

Anyone can diagnose the cause(s) of complaint-causing tonal noise, quickly, simply and at no cost.

We have the expertise to use sophisticated analysis and noise signature fingerprinting not only to identify the culprit plant, but also the precise cause of the tonal noise - e.g. out-of-balance, fan blade pass, compressor tones, gearbox gear mesh, worn bearing, blower or pump harmonic, vibratory sieves or feeders, gen-sets, acoustic or structural resonance, burner or combustion noise etc. You can benefit from this expertise by using our email analysis and diagnostic service to get both the diagnosis and the costed noise control options.

However, there is just a handful of common tonal noise sources that anyone (even noise consulants) can identify from the frequency signatures.

  • fan blade pass = number of blades on the impeller x fan speed + harmonics
  • synchronous electric motors: c 49Hz, 24.8Hz, 17Hz + harmonics (for 50Hz mains frequency) depending on the number of motor poles
  • out-of-balance: rotational speed
  • chiller compressors: usually c 245Hz or 294Hz and harmonics (50Hz mains)
  • mains hum (transformers etc): 100Hz and harmonics, with 200Hz often the dominant component. Sometimes multiples of 50Hz.
  • gen-sets: multiples of 25Hz (50Hz mains)

Important note: mains hum and gen-set tones are all at exact multiples of 50Hz and 25Hz as they are locked to mains frequency, whereas fixed speed synchronous motors run at slightly lower frequencies. This is a useful diagnostic tool as, provided you use a frequency resolution of < 0.5Hz, a 50Hz tone is electrical whereas a 49Hz tone is mechanical (a motor).

Frequency analysis resolution - fan source identification example

fan narrow band signature analysis

Noise complaints caused a large industrial site. We requested an off-site smartphone recording plus recordings close to suspect items of plant by email.

  • Analysis: the problem is a tone at 298Hz + harmonic at 596Hz
  • Diagnosis 1: the tones matched those of just 2 of the site fans with 12 blades running at 1490rpm. The other fans were not implicated
  • Diagnosis 2: more detailed analysis of the off-site noise with 0.2Hz resolution showed that both peaks were actually double tones as each fan is running at a very slightly different speed
  • Diagnosis 3: the complaint causing tones only contribute 1dB(A) to the overall noise level, so the dB(A) noise reduction specification was not appropriate
  • Noise control BPM: conventional silencing would be extremely costly, would reduce fan efficiency and would be ineffective at reducing the tones, so complaints would still be likely. Aerodynamic control would permanently kill the tones at a fraction of the cost and without affecting efficiency

This whole process is a simple, fast and effective way for anyone to determine the precise cause of complaints. You can then either search our online case studies for potential best practice solutions or you can send the data to us so we can then add the costed noise control options.

Narrow-band tonal analysis - right and wrong fans

narrow band tonal fan noise hum signatures

EHO threatened abatement notice due to noise complaints - the company had bought ineffective noise control (enclosure and silencer). We requested recordings near the fans and in the complainant bedroom.

  • Analysis 1: the problem is a tone at 296Hz + harmonic at 592Hz
  • Diagnosis 1: the tones matched the enclosed fan - 6 blades running at 2960rpm
  • Solution 1: aerodynamic insert inside fan casing (1 week). Tone eliminated, but harmonic remained, even when fan turned off
  • Analysis 2: more detailed analysis showed harmonic was 2 blade pass tones from 2 other fans at 590Hz and 592Hz
  • Solution 2: similar aerodynamic inserts in the other 2 fans

If this analysis had been carried out by the consultant to identify the cause of the complaints, the company would not have wasted money, EHO time and resources and the complaint would not have had to suffer unnecessarily for months.

Multiple source identification using tonal analysis - mills, vibratory screen and fans

vibratory sieve mills and fan tonal noise identification

Complex noise complaints, potentially due to multiple sources.

  • Analysis: the noise complaints were caused by multiple tones
  • Diagnosis 1: 4 tonal sources identified - vibratory screen (11.8Hz + harmonics), 2 filter fans and 2 mills
  • Solution 1: aerodynamic insert inside fan casings, fan tones eliminated. Mills fitted with bespoke silencers to eliminate noise contributions
  • Solution 2: low-cost engineering modifications to the vibratory sieve cut low-frequency tones by 50dB

The complaints had dragged on for many months without identification of the causes and hence no resolution. All the contributory sources were quickly identified using frequency signature analysis and hence the optimum (and very precise) mitigation could be designed.

Generator and chillers tonal source identification - blaming the wrong site

narrow band v third octave CHP and chiller noise analysis

Tonal noise complaints had been attributed by the Environment Agency to noise from just one site due to the use of 1/3 octave analysis only. In fact, the problems were actually caused by 2 tones close together from 2 different sites. This initial misdiagnosis led to very long delays in mitigation and a huge waste of time and resources.

  • Analysis: the noise complaints were in fact caused by 2 tones that sometimes show in the 80Hz 1/3 octave band as shown above
  • Diagnosis 1: the 2 tonal sources were identified. 75Hz from a CHP generator set (this is a classic gen-set tone) and 71Hz from a chiller on another site
71Hz 75Hz environmental noise tone identification
  • Solution 1: tuned silencer fitted to the CHP engine exhaust, eliminating the tone (18dB attenuation)
  • Solution 2: this would entail low-cost aerodynamic modifications to the chiller fans that would also increase fan efficiency and reduce running costs

Not using narrow-band frequency analysis as the basis for the investigation of this tonal noise complaint proved to be very costly. The Environment Agency spent considerable time and resources with multiple site visits and noise measurements. The company also invested time and resources in its attempts to track down and mitigate the problem.

75Hz generator diesel engine noise reduction

When consulted, analysis of the recordings we requested quickly showed that half the noise problem was a 75Hz gen-set tone (solved by fitting a tuned silencer) and the other half to be a 71Hz chiller fan blade pass hum from an adjacent site.