I'm not really a product review kind of blogger (maybe you already know that). However, when I got an email asking if I wanted to review a quiet lawn mower I said "yes." Why this product? Well, I thought to myself "they won't really ship me a lawn mower, will they?" They did. That's fine because I like to challenge myself with new things.

Just to be clear, this is the Craftsman Quiet Front Wheel Drive Mower. The nice people at Craftsman provided the lawn mower for me to test. In this post, I am just going to look at the sound levels for this lawn mower.

What Is the Sound Level? ————————

Maybe I should first look at sound. Sound is a compression wave in air. Usually, this is produced by some oscillating source that compresses the air. This compression in the air then travels outward. A larger compression would produce a greater pressure in the compression of the wave. This pressure is a measure of the "strength" of the audio signal and it has the typical units of pressure of Newtons per square meter.

Humans (like you - unless you are a robot) do not directly measure the pressure levels for sound. If we (I am also human) did measure the pressure, you could double the sound source and that would double the audio strength level. This is actually a problem because we humans want to be able to hear both very quiet things and very loud things. The only way to do this is to have some type of logarithmic hearing scale. And so we do, it's call the sound intensity in Decibels.

If I know the pressure of some sound wave (P), I can find the Decibel level like this (I will call the sound intensity in Decibels as just I):

Here, the P 0 is the threshold of hearing at 2 x 10-5 N/m2. How about a quick example. Suppose that there is a quiet 10 year old child (yes, this is just a hypothetical example) that produces a sound wave with a pressure of 0.014 N/m2. How loud would this child sound?

Now suppose there are TWO quiet children that produce identical sounds. What would happen to the sound level? The pressure wave would be twice as much. If I put in a pressure of 0.028 N/m2, I get a sound level of 63 dB. So doubling the sound source does NOT double the sound level.

Actual Lawn Mower Data ———————-

But what about the lawn mower? Is it quiet? Well, here is the test that I will perform. I will use a sound level meter to measure the sound level at different distances from a typical mower and the Craftsman Quiet mower. It's that simple.

How do you measure the sound level in Decibels? The easiest way is to use a sound level meter. Here is the one that I used.

There are several sound level apps for the iPhone. I played with them, but I'm not sure how accurate they are. In fact, I'm not sure this meter is accurate. However, since I'm looking at a comparison for sound levels it should work fine.

For the data, I will collect the sound level starting at 1 meter from the center of the engine. I put the sound level meter on a tripod so that it was about 1.4 meters above the ground (near human ear level). Oh, I also recorded the background sound level with the mower off. Finally, I repeated these measurements for moving away from the front, rear and both sides of the mower. Here is the data.

I didn't include the background noise in the plot because it was boring. It varied between about 55 and 60 dB and made it harder to see the lawn mower data. I also measured data from the left and right sides of both mowers - but they weren't that interesting either.

Is the Craftsman Quieter? ————————-

Yes. Just look at the plot of the data. At every distance, it is 1 to 2 dB quieter. Is that a big deal? Let's just do a similar calculation for the sound level of the 10 year old kid. What if I had two of the Craftsman mowers both running side by side. What sound level would this be?

First, I need to find the sound pressure for one mower. Let me say that the sound level for the Craftsman at a distance of 1 meter is 88 dB. I can then solve for the pressure level at this same point:

With a sound level of 88 dB and a threshold of hear at 2 x 10-5 N/m2 this gives pressure level of 0.5 N/m2. With two lawn mowers, the pressure would be twice as much at about 1 N/m2.

Just for comparison, the Toro mower at 1 meter has a pressure level of 0.63 N/m2. That might not seem like much of a difference, but it is a 26% increase in pressure levels.

So, 1.5 dB doesn't sound (pun) like a major improvement. However, a reduction of 1.5 dB at 90 dB is more significant than the same decrease at 60 dB. This is because the dB sound level scale is not linear. Maybe this plot will help. Here is a plot of sound level (in dB) vs pressure level.

Look at the pressure difference going from 70 dB to 71 dB, it's just an increase of about 0.0077 N/m2. An increase from 96 to 97 dB is a pressure increase of 0.199 N/m2. Get it?

Yes. This mower is quieter. In fact, a 1.5 dB difference is quite noticeable. Check out this nice graph at HyperPhysics that shows the Difference Threshold in hearing. Basically, at 90 dB even a 0.5 dB difference is noticeable.

What About a Weed Eater? ————————

My weed eater is way too loud. I didn't have a good way to put it in the proper position and measure the sound level so this it's just sitting on the ground with the sound level meter still at 1.4 meters above the ground.

You have to be 5 meters away from this monster in order for it to be the same sound level as the Craftsman at a distance of 1 meter. Really, I suspect that more lawn mowers are like this weed eater with a much higher sound level than the Toro. The Toro is probably on the quiet end for typical mowers. Maybe I should go borrow my neighbor's noisy mower.

Homework ——–

There are many things to explore. The thing that I thought about first was the decrease in sound level as a function of distance. If a lawn mower created spherically symmetric sound waves, the pressure levels should decrease as 1 over distance squared. However, the sound level vs. distance looks fairly linear. Should it be this way? Make a plot of both pressure level and sound level vs. distance for a spherical sound source (it could be a spherical mooing cow).

What about these iPhone sound level apps? Do they measure sound levels accurately?

What about the human factor? Take some random humans and make a blind hearing test. Don't let them see the mower and ask them which one is louder. I am almost positive they will say the Craftsman is quieter. However, it would be interesting to make other comparisons. How far away would a Toro have to be in order for a human to pick the Toro as quieter?

Can you measure the sound level with a microphone? I suspect that you can, but I didn't check.

How many Craftsman quiet mowers would I need running together so that they have the same sound level at a distance of 1 meter as the noisy weed eater? How many quiet 10 year olds would be the same as a Craftsman mower?