pressure graident -- Harvey?

[hk_runner]

Harvey, I've read through your monumental thread before asking this question on pressure gradient mic.

For simplicity, let's say we have a 1 kHz sine wave that reaches first the front and then the back of the diaphragm with a say, 90 degrees phase delay. I plotted the graphs and found that if the "front" wave and the "back" wave are of the same amplitude, the resulting difference (or sum, it doesn't matter) will also be a sine wave (albeit of a larger amplitude).

But if the front wave and the back wave are of different amplitude (I expect the back wave to be weakened a bit?), then the difference will NOT be a sine wave. The observation is valid even if the phase difference is some other value.

It follows that a pressure gradient mic (which measures the difference, or sum, of the front and back waves) gives an INHERENTLY distorted output signal. Its output signal is a different waveform. Is it the reason that pressure gradient mics are less accurate than pressure mics? Am I thinking along the wrong track? Thanks.

[Harvey Gerst]

Yes. Kinda. Sorta. Almost. Not exactly. Actually, there are several ways to make a cardioid pattern, depending on the mic's basic design. The RCA 77DX uses little vanes and a mechanical switch that opens up little chambers inside the mic. Multi-pattern condenser mics use different polarizing voltages and polarity reversing to create the different gradient patterns. Leeme see if there's a simple way to explain it a little better.

Gimme a few days to think about this.

[hk_runner]

Many thanks. I read again your post of 05-10-2001 in the huge thread and thought that the human ear should be a pressure transducer -- unless the Eustachian tube leaks air :-).

It is natural to suppose that a mic that is a pressure transducer should be more capable (than a pressure gradient transducer) of recording sound that is more agreeable to the human ear.

I accept that a phase delay is necessary for a pressure gradient transucer to avoid pressure equalisation, but it seems that the pressure gradient thus measured does not closely resemble the original waveform.

P.S. Can anyone let me know how I can be allowed more time to write a post? I log on every time I start to write my post, but often find myself logged off by the system by the time I submit my post. I then lost my submission and had to write it all over again. (In fact this very post is a re-write, too.) If I can't be given more time, at least let me know what the allowed duration is, so that I can time my writing.

[Fed]

hk_runner,
I just type away and before posting I ctrl-a to select it all then ctrl-c to copy so if there is a problem I can just log in and paste what I typed in. Not perfect but works for me.

cheers

[hk_runner]

Thanks, Fed. I'll do so in future to play safe.

Before I learnt this trick from you, I tried to print my post from the Internet Explorer. But the computer hang up.

[hk_runner]

Say we have a live, unmagnified (no mic) solo voice singing classical arias. For simplicity, let's assume there's no reverberation in the room. The audience normally sit at the far field (say 20 feet from the source). The quality of sound picked up by the human ear at 20 feet is different from the quality of sound that would be picked up by another human ear at say, two feet. (This difference in quality is not a result of reverberation, which does not exist in our example here.)

Now someone tries to record it. To record solo voice, recording engineers normally put a mic at near field (about 2 feet), KNOWING that a 2-feet source-mic distance can NEVER recreate the quality of sound heard at 20 feet, no matter how they magnify and process the 2-feet signal. In other words, no 2-feet recording can sound like a live experience from 20 feet. Question 1: why do engineers still consciously choose a 2-feet distance instead of 20-feet? Why don't engineers always put the mic at where the audience sit (20 feet)? (This question has puzzled me for years.)

Question 2: in theory, if we could make a mic that resembles the human ear, we should put it at 20-feet instead of 2-feet?

Question 3: is the choice of source-mic distance dictated by the technical limitation of the mic, rather than the need to recreate the sound quality heard by the human ear in the far field?

If so, is the mic limitation related to whether the mic is a pressure mic or a pressure gradient mic?

Our ears are (I suppose) pressure transducers. If our ears can work at 20 feet, I suppose that a pressure transducer mic (such as an omni) can also work at 20 feet.

Thanks in advance.

[hk_runner]

I think I found the answer to my question in this web page.

http://www.tonmeister.ca/main/textbo...ustics/07.html

Quite a good read.


Somehow I have to type in this url word by word. Copy and paste don't work.

[littledog]

Perhaps that is the reason why, for classical music recordings, the mics are often 20' away (and not 2'). If spot mics are used closer, they are still usually combined with more distant mics.