Any alternative to Radio Shack sound level meter?

Old Radio Shack sound level meters are easily purchased on Ebay.
https://www.ebay.com/p/1300619695
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There are several iPhone apps that work very well and are free or a few dollars.

RTA Audio Pro, , RTA    ( also spectrum analyzer)

Decibel X, dB meter Pro    (can be calibrated)

+erik_squires

I'm willing to bet a new calibrated USB mic is more accurate as well then a radio shack old style one. specially used. 
 
or just the app one may be enough for your purposes. 

that said you can get professional quality ones form various places that supply the film-studio-recording industry. you may get one from an industrial safety equipment sales place as well. 

I’m willing to bet a new calibrated USB mic is more accurate as well then a radio shack old style one. specially used.

Oh, they are good, but also kind of a PITA to use. I mean, I have 2. Still got to drag out the stand, the cable and the software.

I mean, if you are making speakers or attempting to measure the decay rate of the room, USB mic is the way to go. If you just want to say calibrate a HT system or integrate a sub, the cheap one I suggested is the way to go, and they actually DO have better range in Hz compared to the SPL meters and with the calibration file are really good.

Of course, none of these will come with OSHA certs. :)

Skip the meter. Play music with good bass at the volume level you care about the most. Because the perceived loudness of bass is volume dependent, while meters are not. Also the level you get with a meter is only good at whatever frequency is playing. Your ears on the other hand will balance and average it all together. Meters cannot do this. So play music, make very small adjustments, and repeat until satisfied. 

Which btw will not happen with "a" sub. The day you get four though, watch out! Bliss!