after further testing
I have this issue since I purchased the RTL510,
did many experiments and can now narrow the problem down to the distance between the edge and the RTL510,
in my case it works better if the distance is around 600mm, 640mm reduces the volume, with 740mm (flyout mount) no more sound
I reported it to Garmin and suggest everyone else with this problem should do the the same (see Garmin response below), Garmin can not fix it without good user feedback
-------------------------------------Garmin Response------------------------------------------
The Threat alert tone is different than the other alert tones and could be harder to hear for some people. I do think the difference in the type and location of the mount will make some difference here.
We will be looking into escalating this to see if the tones could be configured and turned up or down. Additionally, we encourage you to suggest to other forum participants to contact support to report their feedback on the alert tones as this will give the issue more traction.
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Now this is really funny. Granted, it sounds totally illogical - connection is digital and it either works or it doesn't. However, this is also my story, absolutely. I've started hearing radar alerts much better lately - previously I would miss half of them in the traffic noise (that's even after the tone has changed to the triple tone) and now I can hear most of them very clearly. And as I've read your message I've realized that the only change I've made was replacing my stem with a shorter one. Used to be 130 mm and now it's 90 mm. Exactly 4 cm difference. Same Edge mount, no other changes. How is this possible. Very strange.
I can,'t understand why that should be the case if it is communicating with the 1030 and showing traffic there is no reason why the signal for a preceding alert not to be heard. I have the same problem with my Edge 1000 although there is no audible alert when I hold the 510 next to the 1000. But I put that down to Garmin discontinuing the 1000 and not being bothered to fully support it with new equipment.
I have finally had some success,
made a backplate up for the garmin flyout mount that fits into the spare battery connection of the flyout ,
it has the width of the edge 1030 in my case and is right underneath the 2 speaker holes, with about a 2-3 mm gap between the edge and the backplate.
anyway it works, now have to design something slicker to make it look good
Your 1030 has 2 speaker holes? Mine has 4 very small holes for sound. Perhaps you are referring to the other hole to the left which is for the barometer?
I'm certain your approach would not work for me since unfortunately, I can barely hear the alert while holding the 1030 up to my ear.
Sorry, you are correct, it has 4 small holes,
anyway it improved the sound on both of my bikes, could not hear it at all on one of them, because the flyout is further away from stem than on the other, try it and you will hear the difference, I was sceptical as well before I tried it