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How to Measure Sound with a Handheld Meter -- Workbench Wednesdays 26

element14 presents
How to Measure Sound with a Handheld Meter

element14 Presents  |  Bald Engineer: James Lewis' VCP Profile |  Workbench Wednesdays

 

 

One of the loudest pieces of equipment on Bald Engineer’s electronics workbench is the Electronic DC Load. James has finally had it with its noise and decided to swap the fan out. To see (and not just hear) an improvement, he uses a Tenma 72-94272-942 handheld sound meter to compare before and after.

 

Supplemental Content:

 

 

 

Bill of Material:

 

 

Product NameManufacturerQuantityBuy KitBuy Kit
Sound Meter 72-94272-942Tenma1Buy NowBuy Now
DC electronic Load 8540B&K Precision1Buy NowBuy Now
  • in reply to 25_cps

      wrote:

     

    That 10 dB reduction in noise output corresponds to cutting the volume in half - notice how if you cancel the 10 and the deci metric prefix, you're left with a one Bel change in sound level - that electronic load should sound 50% quieter going by ear with the new fan.

    I just had a huge "duh" moment! The other aspect is that the frequency of the noise is different as well. The old sound was relatively high pitched. This point makes me think I could have probably made better use of the A and C weighting to measure the difference.

     

    Also, I just recorded some video with the electronic load turned on. I forgot it used to make so much noise. Stupidly simple upgrade, but totally worth the time (and cost of fan.)

  • The A and C weighting modes on the meter indicate how it's shaping it's frequency response.  A weighting attempts to closely match the frequency response of the meter to the frequency response of human hearing so that the results you see displayed on the screen match what you're hearing.  The C weighting curve is much flatter especially at the lower end of the audio spectrum where human hearing rolls off quickly, especially at low volume levels.  Unweighted, if it's available, measures everything across the meter's range of sensitivity.

     

    The interesting thing that hasn't been mentioned in the video or in the comments was the 10 dB reduction in noise output from the electronic load before and after the fan replacement.  Human hearing sensitivity is logarithmic which is why the dB scales are used.  That 10 dB reduction in noise output corresponds to cutting the volume in half - notice how if you cancel the 10 and the deci metric prefix, you're left with a one Bel change in sound level - that electronic load should sound 50% quieter going by ear with the new fan.

  • in reply to beacon_dave

    Three racks with fans made a lot of noise, then you add in four racks of other equipment some with fans and you get a high db reading.

    Once I did not hear the fire alarm during a test and they installed a new bell right in my lab to make sure I could hear it.

     

    DAB

  • in reply to DAB

    "...I still remember the din that the PDP-11 used to put out when you turned it on..."

    Strangely, I think turning the PDP-11 off was more noticeable, as the 'silence was deafening'...  although some of that was probably down to the attached Ramtek and Grinnell image processing units.

  • May be of interest - NTi Audio have  a number of useful webinars on sound measurement.

     

    A, C and Z frequency weightings are introduced in this one:

         Webinar: Sound Level Fundamentals

         https://youtu.be/RiIWvaHRv44?t=907 

     

    Their full series of webinars can be found here:

         https://www.nti-audio.com/en/support/webinars-on-demand

  • Good episode.

     

    There are many things in a lab that create a lot of noise.

     

    I still remember the din that the PDP-11 used to put out when you turned it on.

     

    DAB

  • Great tutorial James. Thanks for the kind words and links to my project.

     

    Frank

  • in reply to kl0s7388

    It was a Noctua NF-A6x25, 60mm. One word of caution, it doesn't have as high of airflow as the original fan. Which is why I did the temperature test, even though I didn't fully load the load. As I said in the video, I rarely use it above 5 amp.

     

    Regarding the noise, it only bothered me for two reasons: 1) it was the loudest thing in my lab and 2) its pitch was extremely difficult to remove from videos.

     

    For a $15 upgrade, it was worth it to me. But, now my power supply seems super loud. image

  • What's the source/part number for the fan you used? I have both the load and matching power supply right next to each other. I don't find the overall noise necessarily objectionable but may want to bite the bullet and just change them both since the fan in the supply is probably the same with the common frame design. Thanks for sharing!

     

    -- Dino

  • in reply to jw0752

    Thanks John! The Tenma manual did not have frequency ranges. I wish it had. Seeing this that made me think of a test rig to sweep frequiences and see how the meter responds.

     

    Maybe my advanced sound chamber will make another appearence in the future.