My meager understanding of decibels is that it is a measure of loudness. I wonder if there is a quantitative measurement of the quality or pitch of the sound of a muffler. Everyone always wants to know which muffler has the deepest sound. Does anyone know if there is an instrument to measure this. Sort of like a bass vs. treble on a stereo.

 

dB(A) is the setting you should use. It is 'weighted' to be analogous to the human ear and how we perceive loudness. The ear does not hear loudness equally well at all frequencies hence the difference between the terms Linear and Analog. As humans our ears are far more sensitive at frequencies between 1-4 kHz, coincidently about the same sonic range as most of the information contained in the speaking voice. By using the 'A' setting it gives you a loudness reading the same as you 'hear' it to be. dB(C) is an almost linear and flat weighting curve with no EQ adjustments from 20 Hz to 20 kHz. It gives an accurate depiction of sound intensity in the linear world, not as how we hear it in the analog world. dB(B) rating is a mix of both A and C, so unless your measuring speaker frequency response or other frequencies vs. sound levels you would never use anything other than 'A' weighting.
Personally I wouldn't trust the cellphone measurements regardless of app, only because their mics and audio processing circuitry are tailored to pass a narrow portion of voice grade audio within the greater audio spectrum. All the frequencies below 500 Hz and above 4 kHz are severely rolled off. For voice transmission and intelligibility those frequencies are useless and require more cellphone bandwidth, for those reasons the phone companies and manufacturers chop off that portion of the audio spectrum.
As far as meters go I haven't tested the Radio Shack one but I have tested similar $99 SPL meters, and for the most part they're pretty accurate. I found them to be within 1-1.5 dB of our calibrated meter. Hope that answers your question.

 

Yep, there sure is, it's called an audio spectrum analyzer. It will display a graph in the horizontal plane depicting frequency, and in the vertical plane you will see the amplitude or intensity of those given frequencies. Spendy unit though if you just want to to measure exhaust tone and sound level.
A Decibel, or dB for short is just a ratio in which to measure power. Every 3 dB increase is a doubling of power. For example a 1 watt amplifier might be measured at 10 dB, a two watt amplifier would then measure a level of 13 dB (when using the same speaker.) The human ear however does not perceive this as a doubling of audio power. It will seem a little louder, just not twice as loud. For the ear and brain to 'perceive' it to be twice as loud you must increase the power 4 times, i.e. a 1 watt amplifier must be increased to 4 watts to 'sound' twice as loud to our ears. This would be a 6 dB increase in level.
Just an afterthought, those $20 digital electronic guitar tuners might work if you want to measure lowest frequency or 'rumble.' That, and a cheap SPL meter could give you a meaningful frequency and level measurement by which to compare exhaust systems.

 

Excellent information seesaw - pity the app designer was unwilling or unable to do the same.

 

Knowing there are differences....

I think it's a great idea.

Non-scientific, if everyone uses the same app, and tells what device they are using, sitting on the bike, phone at chest level, we could still get a rough estimate.

And as my mo****r implies, I was a safety man for years, and had to take sound level readings with a high dollar unit weekly. I do understand the difference.

But I still like the idea as a rough-good-ol-boys method of comparison.

 

Pretty sure these are going to break the decibel level meter. Got them and just waiting for my build to complete.