I work for a large foundry in the automotive part supply business. We use the SDT model. It provides both audio output (to the user through headphones - subjective) and a digital readout of the actual decibel reading (objective). We have found it extremely useful for determining proper grease quantity and frequency and we use both the audio and the digital outputs. We also use it for "quick" fault detection and for confirmation. It measures a range from 38 to 42 kilohertz.
Warning, there are a bunch of other reasons why you would get a high ultrasound reading (bad bearing, misalignment, imbalance, etc.) so high readings do not necessarily mean there is a grease problem.
That being said, too much grease or not enough both produce high ultrasonic outputs which the SDT provides on the digital screen - output in decibels. Like I said, both give high readings, but, they "sound different" ... Listen for a while and you will begin to train your ear - there is really no other way to learn.
It works best if you start a program and learn as you go. Try using it on a brand new installation that is balanced, aligned, bearing is new and was properly "packed" with grease - now you know what it is supposed to sound like and you have a "perfect" reading in decibels for that bearing. The reading is only applicable to that bearing though, others may naturally run higher, the sound is generic and applicable to all. Next, listen to a bearing you know is bad, one that you know is over filled, one that is dry, one that is misaligned, one that is out of balance... You get the idea.
Your ultrasonic supplier should be able to provide sound bites for you to listen to but still there is nothing like the real thing and no substitute for experience.
As for the decibel output - trend readings on a particular bearing are all you can go by... I have had ones run as low as 25 and some as high as 45 when they are good... Generally a 10 decibel increase warrants action - try greasing it to see if it goes back down. If it goes up, you have too much grease. If it goes down, you keep adding slowly until it reaches a minimum and begins to go back up. If it does not change - you have something other than a grease problem.