Measuring low (~18 dB SPL) sound levels with a relatively cheap microphone

Question

I have been tasked with designing a sound level meter (I am an electrical engineer) that can measure accurate sound levels down to ~18 dB SPL. However, part of the task is that my microphone should be relatively inexpensive (perhaps <= $150).

There are a number of other specifications I need to consider when looking at mics, but as a high-level opinion, is this possible or is my boss crazy?

Everything about the electronics/amplifier design, etc, I am fine with, but I've never done much high-performance stuff with microphones before. From the myriad of specs I've seen, hardly any mic in my price range will be useful for signals below ~25-26 dB SPL.

Thoughts?

[Edit: Each octave band from 125 Hz - 8 kHz (my freq range) can be band-limited and measured separately, which should offer some small noise improvement from the 20 kHz BW in most microphone specs.]

Answer 1

I'm no expert, but I expect that is what you pay for. If you've not looked already, it might be worth checking the B&K microphone page, just to see what is already out there. They are pretty much the standard for professional measurement equipment like this, and if you need to ask how much they cost, then you can't afford one! But the fact that only a few of their microphones go down to 10db suggests that this is perhaps more specialised than your boss realises.. If you need a microphone to be very sensitive, yet also strong enough to measure very loud sounds, then that is difficult just in terms of manufacture as you effectively need something which is very delicate and sensitive but also very strong and resilient. On the B&K page they have one mic which goes down to 2.5db, but the upper limit is 102db - that's the trade-off. Also, probably all the capsules you're looking at are designed for regular recording tasks where being able to record loud sounds is actually more useful than quiet sounds, as this is more the regular use.

As I say, I'm no expert when it comes to sound measurement, so this is just an opinion form someone who has been using microphones for regular recording purposes. Let us know if you do find anything though, as it may be of interest to DIY mic builders here.

Answer 2

Mark Durham already mentioned the Bruel & Kjaer mics which just aren't happening at $150 but not knowing your design specs here's a wild suggestion:

Maybe you'd be able to combine a regular measurement mic capsule and a contact mic that's carefully calibrated (I don't even know if this is possible since it might be temperature / humidity sensitive and whatnot).

Obviously this won't work like any old regular SPL meter but depending on what you want to measure it might be worth looking into. If for example you need to measure very quiet objects that can be placed on a larger platform then attaching and calibrating a contact mic to that platform might provide a way to do it.

If you do find a way I would love to hear about it, sounds like an interesting challenge!

Good luck!