Yes. Microphones designed for close in work (telephones, handheld voice mics, etc.) are not very good for picking up weaker or more distant sounds. They are made intentionally insensitive so as not to be overdriven by close in sources.
The ear is pretty good, but being attached to the human body, it comes with its own noise sources. In a very quiet environment, my breathing and heartbeat drown out sounds that could be picked up by a sensitive microphone.
Paul Hovnanian mailto:Paul@Hovnanian.com
Just been playing with some cheap (ECM-66B) microphone inserts and found they have a pretty poor performance compared with the human ear. I can hear some very quiet sounds in my room: the ticking of my clock and distant barking dog. This sensor can pick up sounds at this level but when the signal is amplified enough to hear them, there is a lot of noise with it.
The S/N ratio is quoted at 40 dB, and sensitivity at 60 dB/microbar.
It seems okay where it is next to or nearby a person's mouth, like a hand held microphone or a telephone. But if you stuck a pair on a dummy head and listened through an amplifier, you would not get a good sense of being in the room.
So, do microphones get much better than this?
Or is the ear such a marvellous organ that it is yet to be beaten?
On Tue, 31 May 2005 02:33:49 +0100, Pooh Bear wroth:
Of course, the ear is supported by quite a bit of additional signal processing. If you looked at the nerve signals between the ear and the brain, I suspect that any useable information is thououghly buried in the "noise".
The way the ear hears is not exactly understood. The fact that it can apparently 'outperform' test instrumentation in critical istening tests in certain tests but not others perhaps adds to that puzzle.
Note that the ear hears partly by means of many cilia that are quite high Q 'tuned circuits'. I can't imagine a simple model of how humans or other animals hear. For example the ear can extract a signal that's 'buried in the noise' if it's of narrow enough bandwidth.
The dynamic range is certainly poor, but adequate for some applications.
It is sensitive enough it can pick up feint sounds, only buried in noise.
I was surprised by the amount of difference a simple plastic funnel made. Pointed out the window at a bunch of trees, the birdsong from them was much louder. Might be fun to play with a parabolic microphone :-)
How much does a reasonable quality transducer cost? I've seen mikes for several 10s of £, but much of that cost must go to make the robust housing for the transducer.
I suspect it has noise cancelling mechanisms. After all, it is buried in a mass of blood vessels and right next to airways.
As an aside, what is the noise heard when a sea shell is held to the ear? Usually explained as air currents, I find that hard to believe. I guess it is sounds coming from the head/ear itself, resonating and reflected back into the ear.
FWIW, I once built a remote microphone for my handycam, incorporating a junkbox electret, a BC547 (as "frontend"), a 4558 (one half used) - to create a AGC mike amp with line level out (9V battery fed). The thing could "hear" things that I could ony detect with my "bare" (then still quite good) ears after noticing it on the headset. It would even feedback over thesound leaked out of the earphones.
In other words, I guess that with sufficient power line filtering and an adequate gain distribution - it may not be that bad.
Professional conndenser mics use larger capsules and polarising voltages that produce more output for the same SPL. Hence the s/n is better. And they use quiet fets ( or toobs even in very old ones ! ).
If the bias voltage is 20 times my 9V battery, signal should be 20 times greater.
And that will be roughly multiplied by the area increase.
Hmm... I'm tempted to see what kind of output I'd get from two sheets of tinfoil (say 30 x 30 cm)held under tension very close to each other? Not the most practical stage mike, but it does provide a large sensing area. I imagine it would be pretty directional too.
Okay, it is pretty small so not got a great sensing area. And for £3 I did not expect much.
I went out and bought a dynamic mic insert for £5, and this is labelled as having 76 dB sensitivity. It is a lot bigger and heavier.
I made up this preamplifier
which is all-discrete so should have low noise.
However I didn't find it was much improvement. There is noticeable hiss and buzz.
Hiss seems to be coming from the first transistor (BC109, common base) and the buzz from the mic insert, because the buzz disappears when the signal input is shorted to ground. The hiss and buzz disappears when there is no signal to the other transistors.
Circuit constructed on a bit of veroboard - not ideal but fairly neat and compact.
So, is this mic insert also crap, and if so how much would I expect to pay for a good one?
Also, is this BC109 transistor just too noisy?
Or am I going to have to make a much more complex pre-amp like this one: