I’ve been using a mechanic’s stethoscope to monitor first and second crack on the Bullet R2 with very good results, so I wanted to share my setup in case it helps others.
I know this concept has been mentioned before, but here are the specifics of what I’m doing.
I started with a Lisle 52750 Dual Purpose Stethoscope Kit (about $27 on Amazon). I cut the stainless probe down to roughly 2 inches. That shortened probe is then inserted into a high-temp silicone tapered stopper (20mm). The stopper kit I used was a generic high-temp silicone plug set (around $10). I drilled a small hole through the stopper to create a tight, friction-fit seal around the probe.
This creates a clean acoustic path into the drum without picking up excess mechanical noise from the chassis. The result is excellent—I can clearly hear both first and second crack, even on beans that typically have a very faint or subtle crack.
As others have pointed out, determining exactly when to mark FC is somewhat subjective. But to your question, I really didn’t have much confidence in calling FC or SC before using the stethoscope. Between the drum and fan noise, it was just too difficult (for me) to hear them clearly.
With the stethoscope, that’s completely changed. I can now hear even the most subtle “thump,” not just at FC and SC, but even during the drying and Maillard phases. It’s made a noticeable difference in both confidence and consistency when marking events.
That confidence has also carried over into using AI to analyze my roasts and make recommendations for improvement. Having more reliable event markers going in has made that process far more useful—and it’s been a lot of fun.
Haha, controversy isn’t all bad. We don’t all have to agree, but healthy discussion is always beneficial as long as it stays respectful. And I definitely don’t have supersonic hearing (tongue in cheek), so the stethoscope has been a big help for me.
Great question on defining FC. Everyone seems to have their own approach, so here’s mine.
I tend to think of it like stovetop popcorn. I realize corn and coffee are very different and coffee goes through distinct roasting phases, but there are some useful parallels. In both cases, you have moisture turning to steam and causing a “pop,” and they don’t all happen at once. You get a few early outliers—those first sporadic pops.
I don’t consider those initial pops to be FC. For me, FC starts when the pops become more consistent and sustained, but not yet a full rolling crack. That transition point is what I mark.
Curious how others define it—always open to refining this.
I’ve found the active noise cancellation on my Apple AirPods Pro works pretty well for isolating cracks from the rest of the noise floor. Worth a try if you happen to have them or something similar.
This is an amazing idea! I was just speaking to my sister today about 5 roasts i did yesterday, and always have trouble pinpointing when FC starts. I’m going to try this on my next roast!
Squeakie, yes it’s a huge upgrade that takes a lot of the guesswork out of roasting (the timing of the FC and SC). Please post your feedback once you try it out.
Carldebar, Thank ou for this clear post, I am a hobby roaster in Amsterdam and orderded the sthetoscope and plugs on Amazon. My expierence is that with some beans and some roastprofiles, FC is really hard to hear, sometimes I hear nothing, and it must have heppend. Because I determine when to drop on delta time en delta temperature after FC calling FC is really important to me and I want to get more consistency in when to call FC. I do not think it matters when I call FC as long as do it consitently with all roasts. Thank you so much for suggesting an relatively easy solution. I can’t wait to try!
