I am trying to improve the sound insulation in my garage shop, and I gather that Sound Transmission Class (STC) ratings of different soundproofing solutions vary as function of Hz levels of sounds. I have read that the loudest machines like planers and table saws have relatively lower Hz levels, but I can't find any tables offering range of HZ levels to expect for planers vs table saws.

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    Interesting question. I would think that a musical instrument tuning app for your phone might be able to give you a pretty decent idea.
    – FreeMan
    Jul 23 '20 at 22:17
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    @Ashlar I am actually trying to reduce noise moving from garage through garage door to easily annoyed neighbors and HOA.
    – Joe
    Jul 24 '20 at 4:45
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    Sound is energy moving air. Stop the air movement, and you stop most of the transmission. Dampening is important for some things, but for real noise control, you control the airflow. See this relevant Q&A: woodworking.stackexchange.com/q/9682/5572
    – jdv
    Jul 24 '20 at 6:07
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    And no thread on this subject would be complete without the mention of switching to hand tools for as much of the work as you can stomach :-)
    – Graphus
    Jul 24 '20 at 6:36
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    @FreeMan, well someone needed to... and nobody beat me to it so I decided to once again take one for the team and suffer the inevitable ribbing ^_^
    – Graphus
    Jul 24 '20 at 19:30

I have an audio spectrogram app on my tablet, so I took it into my workshop and made a couple of measurements, using the power tools I had setup for a project (table saw, orbital power sander, and trim router; don't have a power planer, sorry). I used all softwood material (18mm pine), kept the tablet about 1m from the tool, and decoupled it from the workbench with a bit of foam.

My workshop is only about 4.5m by 7m, and has concrete floor/ceiling, sandstone walls and glass door/windows all around, so there's plenty of reflections; really not an ideal environment for any type of sound measurements: levels will be off in general (on the louder side), and some frequencies will be attenuated while others are amplified. So I don't know how useful the results will be for your sound isolation problem; different tools (make and model) will certainly sound different, too. But it's at least one data point.

Spectrogram Tablesaw Spectrogram Powersander Spectrogram Router

Here's how you read those graphs (click on them to open full size). The upper third shows the audio spectrum (log-frequency on the x-axis, logarithmic sound pressure spectral density in dB/Hz on the y axis); yellow is current (updates every 25ms), red is max hold for the entire measurement. The lower part shows a waterfall diagram of the spectrum over time: log-frequency is on the x-axis, time is on the y-axis (going up, so further down the graph is older), sound pressure spectral density is color coded using the log color scale shown on the left; higher sound pressure is shown in a brighter color. I've set it up so that the ull time range of the waterfall is about 10s. Since sound level is in dB, because of the logarithm a change by only 6dB is equivalent to double or half the noise.

In the first image, I first turned on the table saw, then added the dust extractor (big shop vac) a couple of seconds later, then did three quick crosscuts in 18mm pine, and finally shut the table saw and the vac off at the same time. There's plenty of sound energy all across the spectrum even with the saw just idling. The cuts add very distinct high frequency noise, you can even see the frequency going down a bit during the cut as it puts load on the motor. The dust collector doesn't register all that much on the graphs, even though subjectively it makes a lot of noise.

Same story in the second image: I first turned on the orbital sander, then the dust extractor, then lightly sanded a piece of pine using 80 grit sand paper. The waterfall shows a strong fundamental at 200Hz (my power is 50Hz), with plenty of harmonics up to about 5kHz. The Sanding action reduces the high frequency concent quite a bit, but adds lots of low frequency energy. Sometimes, you can see the frequency going down briefly as I pressed down a bit harder (I know, really should not do that) until the constant rpm feature in the sander revs it up again.

Finally, the trim router shows about the same characteristics. Looking at the max hold, this is actually the noisiest of the three tools. And I believe you can see me breathing after the router rev'd down.

All in all, the bad news is that all three power tools have energy content over the entire audio spectrum; if you were hoping to get away with some sort of low-bandwidth muffler device, that's not going to work unfortunately. Since there's significant low frequency content, you will need lots of mass if you want to suppress that; curtains will not help you there, but I guess will be somewhat effective in the higher frequency range.

  • what is the meaning of -49 Db at 376 Hz for the tablesaw? I have a good understanding of decibels, but don't understand how they can be negative.
    – Joe
    Aug 2 '20 at 4:32
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    Excellent to actually get some data on this! +10 if I could for taking the time to go out and test various tools and actually record all the data *clap*
    – Graphus
    Aug 2 '20 at 7:35
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    @Joe a value expressed in dB is always relative to a reference - if a signal is lower that the reference, dB values are negative, positive otherwise. Sound pressure level is usually expressed relative to a reference sound pressure of 20 micro pascals (below our hearing threshold), so sound pressure values are usually positive. In the app, I suspect that the reference is a single sine tone that is just loud enough to saturate the tablets' audio signal chain (microphone, preamp, filters, analog to digital converter). Such a signal would probably come out at 0dB, everything else is below that. Aug 2 '20 at 20:25
  • @Joe Since the audio level axis represents a spectral density, the unit should actually be dB/Hz (sound energy per unit frequency) rather than sound pressure level; I'll edit that in. As for what the value of -49dB/Hz at 376Hz really means, I'm not sure - the sensitivity of the microphone in the tablet is unknown, so trying to convert to actual sound pressure level is not possible. The relative information (the saw is 30dB louder at 376Hz than at 1kHz) should be mostly accurate though. Aug 2 '20 at 21:10
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    Yes you can add them as an addendum. While it's not right to not Answer the OP's query, and considered bad form to deliberately choose to not provide one (except in specific circumstances, which are explained) it is fine to add commentary, interesting somewhat-related info or just general advice (e.g. about something that came up in the Comments) once the Q has been Answered. Anything in the realm of "you might be interested in this" ... "this came up in the Comments" ... "of possible use to future searchers".
    – Graphus
    Aug 3 '20 at 6:49

Since you've clarified that you're trying to keep down the noise eminating out through the garage doors, consider a vinyl strip curtain of both the pedestrian and vehicle doors.

Something like this:

enter image description here
Image from strip-curtains.com, found via an eBay link. No endorsement intended or implied

Because they're flexible, they'll absorb a lot of the air movement generated by the tool motors and cutters (the source of sound), but because they're individual strips, they'll allow the wind to push them around some to allow some cooling air flow. Some...

You may have to put up some sort of horizontal track like a vertical blind to move them out of the way when you need to use the garage door for silly things like parking a car. (You'd have to move the tools out of the way, you sure you want to do that?)

  • Do those really work well as sound baffles? I've interacted with them only a little, and haven't noticed one way or another if they cut down on sound transfer so I'm really curious if they'd make a meaningful difference to e.g. the sounds of a planer or jointer working hard. I used to live near someone who used either a circular saw or a small table saw regularly, in their indoor workshop & the sound of their cutting was clearly audible across the local neighbourhood. Even a couple of streets away you could hear it..... not enough to intrusive IMO but HOAs aren't noted for being reasonable :-)
    – Graphus
    Jul 24 '20 at 19:37
  • TBH, @Graphus I can't really say. I was looking at jdv's comment on the OP about sound being nothing but air moving (which it is), and that came to mind. I've worked in our local food-bank and they have them as baffles on the walk-in freezers (in addition to a roll-up, insulated door). I've noticed that not only do they do a decent job of keeping the cold in, they do seem to keep the sound of the fork trucks down, but I wasn't actively checking for noise level. And, HOAs are diametrically opposed to reasonable. (Don't think I could ever live with one.)
    – FreeMan
    Jul 25 '20 at 1:53
  • "And, HOAs are diametrically opposed to reasonable" Yeah, so I've heard — "You have an oil stain on your driveway, you have 3 days to remove it before you incur a penalty..." like WTF?
    – Graphus
    Jul 25 '20 at 6:11

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