Record very low frequency with a modified Rode NT1a?

I don't know what a standard size membrane is able to capture,
but I bet there is usually some form of high pass filter in the membrane signal preamplifier.
Here is the schematic for the NT1a:
http://techie007.3utilities.com/resources/groupdiy/Rode%20NT1-A%20schematic.png
Any suggestions on a modification to extend the low end? Down to 1hz would be nice.

Not sure other than capsule and 1G resistor forming hpf. Say capsule is 50pF, you would need 3G resistor to lower it to about 1hz. But you would need to measure the capsule capacity.

You can also inject signal into '3' connection of jfet and see how circuit measures at the output.

I doubt the capsule it self can get so low in frequency, and it doesn't have to do with size of it.

Maybe increasing the value of the 47 mfd output capacitors could also help a bit.

One of the features of  RF bias condenser microphones is that their LF frequency response can extend right down to DC.
The extended LF responses of the Sennheiser MKH110 and MKH110-1 mics - for example - are mentioned at the bottom of page 3 of this white paper: https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf.

I have always been tempted to try  and record some extreme LF with one of the prototypes of my own RF project  (see here: www.amx.jp137.com)  but I haven't found a suitable source as yet -- (I'm thinking maybe organ pedal notes, for example - but you need an organ with 32' pipes to get as low as 16Hz, and that would probably mean getting permission to record a cathedral organ!).

It's not an expensive mic to build, and the only change I think would be required for extreme LF would be to increase the vale of  C5 and C10 to perhaps 1uF? (The exact LF response calculation would require you to know the impedance of the mic preamp being used ) .

Other than the values of those two capacitors, I think the LF response of the existing circuit should extend down to DC? ... (Yet to be actually tested!)

I have a Schoeps CMC 6 xt U that goes from 10Hz to 40KHz but i have to use it with a filter most of the times. I got it to record Elephant and Tortoise infra sounds.

kingkorg said:

Not sure other than capsule and 1G resistor forming hpf. Say capsule is 50pF, you would need 3G resistor to lower it to about 1hz. But you would need to measure the capsule capacity.

Click to expand...

That would take care of the electrical HPF, but there would still be the inherent acoustic high-pass filtering of the cardioid arrangement. All cardioid mics have an inherent LF roll-off that is generally masked by proximity effect.
A dedicated LF omni capsule is the way to go. Unfortunately, it's a very expensive option.
Alternatively, depending on the application, you may just apply aggressive EQ and hope for the best. 
A Baxendall-type LF section with adequate values should get you going.

abbey road d enfer said:

A dedicated LF omni capsule is the way to go. Unfortunately, it's a very expensive option.

Click to expand...

Do you have any info on construction of these? Maybe we could mod an existing MXL 603 type capsule to get the effect. They are quite easy to modify, and even re skinning shouldn't be an issue. Just by going by intuition i guess the diaphragm should be a bit looser. But i'm just guessing.

kingkorg said:

Do you have any info on construction of these? Maybe we could mod an existing MXL 603 type capsule to get the effect. They are quite easy to modify, and even re skinning shouldn't be an issue. Just by going by intuition i guess the diaphragm should be a bit looser. But i'm just guessing.

Click to expand...

I'm not verse in re-skinning capsules, really.
There are much more knowledgeable people at micbuilders_at_groups_dot_io

I think that the capsule itself will be the most important restriction.
Any pressure gradient capsule will attenuate extremely low frequencies, because of the long wavelength.
At extremely long wavelengths, the pressure at the back of the capsule will be (almost) the same as at the front, effectively nulling out the wanted VLF audio.

I wonder whether Sennheiser used their symmetrical capsule design (described on page 5 of this paper: https://assets.sennheiser.com/global-downloads/file/11061/MKH-Story_WhitePaper_en.pdf ) in their MKH110 VLF mics (mentioned on page 3 of the same paper), or whether they needed to apply other porting techniques to get their VLF responses?.... 

Thanks for that paper Rogs, it's quite useful.

This article pretty much explains why i always preferred F8 ribbons on vocals, where acoustics weren't an issue. They are free of distortions and phase irregularities mentioned. Some call it ''harshness'', i like to use ''weirdness'' in  high end, as it's not really dependent on amount of high end. 

What i don't get is how they achieve omni and cardioid if capsules are symmetrical, but i'll dive into research.

I suppose you should see them as 'two capsules back-to-back'?

What exactly is your application - what do you need 1Hz for?

Knowing this would probably widen the range of possible solutions

/Jakob E.

RuudNL said:

I think that the capsule itself will be the most important restriction.
Any pressure gradient capsule will attenuate extremely low frequencies, because of the long wavelength.
At extremely long wavelengths, the pressure at the back of the capsule will be (almost) the same as at the front, effectively nulling out the wanted VLF audio.

Click to expand...

Does that apply to both cardioid and omni capsules?...

The main difference is that cardioid microphones are pressure gradient transducers, while omni microphones are pressure transducers.
Pressure transducers can (theoretically) convert air pressure down to 0 Hz.
Especially when using a RF principle.

I shall dig out one of my double sided edge terminated 34mm Chinese capsules, and configure one of my DIY RF mics as an omni.
I may also need to increase the value of audio coupling capacitors, to extend the LF range, and then see if I can locate a really LF source to record. (My Tascam is flat down to 20Hz -- hopefully there will still be some useful response below that frequency?)....

I am not sure it will do the trick, you still get two intrinsically cardioid capsules wired to create omni pattern. I believe it would have to be a true omni capsule. But please do report on your findings.

If you manage to record a "sound" wave at 1 cycle per second, how do you plan to reproduce it? There does not exist a speaker or pair of headphones (not that you could hear it) up to the job. Pardon if this is explained somewhere in the thread, but do you just plan to look at it on the computer? Since you won't find such sounds in nature, how to you plan to produce the wave in the air? I think somebody mentioned elephant flatulence, but I'd like to see the data.

In any case, you'll certainly want an omni. Not sure that Rode is up to the task, but I have a Bruel & Kjaer measurement mic that is flat (-2db) down to 4hz!!  8) The low frequency response is really amazing, and satisfying, on things that border on subsonic, like helicopter whumpa-whumpas.

Best of luck.