Thanks for alerting me to the bass roll-off filter setting. I've never carefully checked what frequencies it affects. I've kept it ON, assuming that it wouldn't affect the birds I'm recording.
However, a couple of nights ago I was standing by my bedroom window hoping to hear a Great bittern (Botaurus stellaris) that I know is present some 4.4km (3 miles) from my home. The great bittern's deep, loud 'boom' is strongest around 167Hz and its sound can travel very far. A neighbour of mine has heard it from his house, but the conditions need to be ideal. It was a calm, quiet night, but I didn't hear anything. I then brought out my ME66+K6 combo and headphones to hear see if it could help me pick up the sound. But I still couldn't hear it. So I still lack this species on my yard list.
However, I just now found a good graph in the K6 spec sheet at the Sennheiser site showing the effect of the bass roll-off filter:
It shows that the bass roll-off filter begin to kick in already at 1000hz.
At the Great bittern's frequency of 167Hz, it produce a -6dBV reduction. At 100Hz, a -10 reduction. I don't have a good intuitive sense of how much this means in real life. But for my quest to hear a distant bittern through my bedroom window, any reduction at these frequencies seem less that ideal!
Hereafter, I will turn the bass filter OFF.
Thank you for reminding me of the frequency at which the filter kicks in. I had seen that graph when looking for information on the K6 online, but had forgotten the details. I will be mindful of the frequency at which the filter kicks in, when I try to record bitterns or rails. I might also try turning it off, in general, just to experiment with the results.
I went to a local park with woods and a marsh on Sunday to test the mic, and it worked very well with the Olympus recorder. I'm relieved to find my Ebay purchase functions well. I was able to record several of the common local winter species, including a hammering downy woodpecker. I also found an unexpected house wren near the parking area uttering various rattling calls. I have to say that getting more familiar with the editing functions, and the amplification and noise reduction effects, in Audacity also is a big help with getting the best out of my recordings.
Now, I need to find some places to record birds with fewer people milling around, talking, and fewer aircraft overhead. Not easy, around here.