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Old March 7th, 2004, 01:00 AM   #1
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Newbie wants to understand: sensitivity, S/N-ratio, self noise

1) Is a 70 dB S/N -ratio good enough for nature recording (low sound pressure: birds, waves etc)?

Does that 70 dB mean that if the recording is 80 dB then the noise is 10 dB or is it more complicated?

2) How should I interpret the sensitivity value? Is a small numbert good?

3) Is there a simple relation between self-noise, S/N and sensitivity? So that I could estimate one using the two others?
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Old March 7th, 2004, 01:37 AM   #2
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Do some reading. To answer your question in a thread would be time consuming and repeating what has already been written at these sources


The AT Glossary offers some explainations for your questions
http://www.audio-technica.com/glossary/index.html

The Audio Technica general site map includes links to all kinds of interesting info.
http://www.audio-technica.com/sitemap/index.html

The Shure site offers info on mic sensitivety
http://www.shure.com/support/technotes/app-sensitive.html

and general audio for video
http://www.shure.com/pdf/booklets/audio_for_video_production.pdf

Cybercollege
http://www.shure.com/pdf/booklets/audio_for_video_production.pdf

Location sound basics
http://www.kenstone.net/fcp_homepage/location_sound.html#link%20two

http://www.equipmentemporium.com/


You get through this and I'll give you some more sites.

There will be a test :{
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Old March 8th, 2004, 12:39 AM   #3
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Sorry, Beas, if I'm testing your authority here - no rudeness intended :-) I just have a soft heart for newbies!
Ralf: I think you'll have to tell us why you are thinking of estimating S/N, sensitivity, and self-noise in the first place - or even why you should want to work out one from the other. Principally, these are issues that you need to consider when acquiring, buying or renting a mic - at which point you must remember that any figures the manufacturer gives in the literature is to be taken with a pinch of salt. The best way to find which mic suits what purpose is to try and use them in practice; or ask in this forum.
Having said that:
1. Any electronic system - cable, amplifier, mic or whatever - inherently has an amount of non-useful but transmittable electrical 'pulses'. This is what you've referred to as 'self' noise - or as it should be called more correctly, system noise.
This is as opposed to the useful and transmittable electrical pulses called the 'signal', which - in the case of a mic - is the stuff which we actually want to record or put through the poice of equipment. The better the manufacture, the lower the inherent system noise. System noise also increses with poor maintenance and age - and in some amps and mics, as a function of operating specs like humidity and temperature.
2. ...which brings us to the concept of S/N ratio - which stands for the signal-to-noise ratio. This is a measure of the difference between the level of the signal, and the level of the system noise, expressed in decibels (dB). Thus, the greater the S/N ratio, the better (or 'cleaner') the signal is, since it is more clearly seperated from the noise component. The S/N affects low level sounds in the sense that if signal levels are low, one would prefer to keep the levels as far seperated from the noise as possible. But 70 db should be good enough for most purposes, if you're following the other basics of recording audio. S/N does NOT work the way you've assumed, as should now be clear! :-)
For nature sounds, I would concentrate more on the directionality of the mic (recording sounds from a specific direction, and rejecting others, like some shotgun microphones do), and frequency response (the relative levels at which various frequencies of sounds pass through the electronic system/mic).
3. Yes, a small sensitivity number, as you've called it, is good - this is the minimum signal level (in dB) that the mic can usually detect. But remember that this has to be seen in context with the purpose you're using the mic for, and the other factors I've discussed above. A very sensitive mic may seem like a good idea, but try using it in a situation where there are sudden and unexpected loud sounds - recording rain when there could be a thunderclap, for example. You're likely to damage the mic, and anything else connected to it in a situation like that. similarly, if you want to record the gentle lap of waves on the seashore, but you have a sentive mic that has very poor frequency reponse for the frequencies you're tring to record, you'll record something all right - but it won't sound like what you hear with your own two ears. So assess what you want to record, and frame your requirements accordingly. And read, read, read!
Well that's Audio 101 for today, folks; hope this helps!
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Old March 8th, 2004, 01:24 AM   #4
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Ram
All due respect but i think anything would be easier to understand than your post. Take it from a wise old fart and break the text with a new paragraph occasionally.

The links that i left are to sites that have spent much time on their explainations. You or I don't have that time to craft the perfect answer to a dificult question.
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Old March 8th, 2004, 05:15 AM   #5
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Understood, Beas: *apologies* :-)
And very nicely put - it didn't sting at all! :-)
Ram
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Old May 18th, 2004, 08:41 AM   #6
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Ok, I did some reading. The kenstone.net link was very informative. Thanks. Now it's about time to go shopping, so I must ask one final question (closer to the end of this post)...

Ram Nagarajan wrote: "Ralf: I think you'll have to tell us why you are thinking of estimating S/N, sensitivity, and self-noise in the first place - or even why you should want to work out one from the other."

The reason: I have a very specific goal in my mind. Instead of having a "clean interview sound" I want to record a certain mood and ambience. I want to include certain sounds in the ambience: natural sounding bird song, the sound of grasshoppers (if possible), maybe distand thunder, the sound that leaves make in the wind and such. Some of these are high frequency and others are low frequency, but generally they all are quiet sounds. To make things even worse, I only have a camcorder for recording (two hands only and dual miking is too complicated for me). This setup is not optimal, and it probably cannot record everything that I want, but I'm trying to make the most out of it.

The camcorder has two XLR connectors and Line/Mic/MicAtt level inputs, but it also produces significant hiss if I turn the audio gain up a lot. It also has a frequency response more like an upside down parabola than a straight line. Thus I would need a microphone that records the low (<200Hz) and high frequencies (10k-20k) at an "usable relative level" (whatever it means...) and also does not require a lot of gain on the camcorder.

I thought that the mic sensitivity would affect the amount of audio gain needed on the camcorder. Thus a more sensitive mic would make the low/high freq sounds louder and recordable, at least to some extent, without the camcorder-induced hiss.

QUESTION:

I have two mics to choose from. Both are from the same manufacturer and both have a S/N of 70 dB SPL, 1 kHz at 1 Pa.

One of them has -36 dB (15.8 mV) open circuit sensitivity and the other has -47 dB (4.4 mV). The one with -47dB sensitivity has a flatter frequency response (about 6dB more at low/high end).

So I guess that the -36dB (15.8mV) mic still requires less audio gain on the camcorder. Am I right? Or is it the -47dB that's better?
I'm a little bit uncertain of which way I should read those numbers...

----------------------------------------------------------------------------

"A very sensitive mic may seem like a good idea, but try using it in a situation where there are sudden and unexpected loud sounds - recording rain when there could be a thunderclap, for example. You're likely to damage the mic, and anything else connected to it in a situation like that."

A "maximum input sound level" of 123 dB on both mics. Should still take the less sensitive mic if I want to get some thunder there too?
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Old May 18th, 2004, 10:13 AM   #7
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It's tough to select mics based on spec. Which mics are you talking about and what is the application.
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Old May 18th, 2004, 12:39 PM   #8
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Tough... yes, I know... If I only could listen to those mics, but no, I cannot. Not easily, at least. Neither mic is locally available.

At the moment I do not have any decent mic. Just the camera mic that is not much worth. And so I am in the process of choosing one to two mics that would offer some versatility and a better sound.

The microphone alternatives are:

1) Audio-Technica AT-825 X/Y stereo mic (mono compatible) + some directional mic such as the AT-897

or

2) the AT-835ST M-S stereo shotgun mic alone.

Those alternative setups cost roughly the same (the 2 mic setup beeing a bit more expensive). I want stereo mics because sometimes (often but not allways) I want to record a wide live sound field:

A) Things happening near or around the camera. I have tried to record such situations with the poor mono shotgun that came with the camera. It resulted in very high and rapid changes in volume. Some *shouting* sounded ok - until the person quickly took a step or two and the shotgun was right in front of the mouth... I have never taken off headphones that quickly! Hurt my ears. So never again a mono shotgun or any other directional mic for close-ups and crowd.

B) Ambient sounds. And I'm not going to build the ambience from ten different pieces. I want to record "the whole picture" at once. If there must be someone talking, then I go closer or mic the ambience first and then the talk and mix them in post.

Both situations would require a cardioid or broader for ambience or crowd, but not an omni mic (because of the camera). Both AT stereo mics are thus usable, and better than cardioid.

The choice is difficult, though.
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Old May 18th, 2004, 12:49 PM   #9
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I want to add one more thing: I *believe* that mic sensitivity is important because of the camcorder - less gain needed. What I do not know, however, is whether -47dB is good enough or not... I'm not experienced enough to say that.
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Old May 18th, 2004, 03:18 PM   #10
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Why do you want a stereo mic? Most use a mono for dialog. Have you ever used a good mono?

I can't help you with stereo but i have a pile of mono clips if you want to listen. I think once you try the stereo it may turn out to be less than you anticipate.

What camera do you have. That will have a bearing on whether or not you have need of preamplification. You mention noise when the gain is increased.

Do yourself a favor and down load some clips and get into a few more disussions. make sure of what you really need. Audio can be an expensive proposition and even more expensive when mistakes are made. We've all made them but I always try to save others the expense that i have had to incur.


have a look at http://dvfreelancer.com/forum/viewtopic.php?t=16
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Old May 19th, 2004, 01:26 AM   #11
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No, I have never used a good mono microphone. Also, I have absolutely no idea of how wide different patterns (cardioid,super,hyper...) are in practice. But I know that the Sony shotgun is sometimes annoyingly narrow. Especially for ambience and crowd and for six people talking around and close to the camera.

Three months ago I bought a Sony PDX10 camcorder and I decided to get a better mic. Back then I believed that there would existist something like a good "general purpose" mic and I also believed that stereo was allways a better option. Then I did some reading, and now I believe that I should in fact have different mics for different situations.

1) One is the monolog/dialog situation. Especially in urban areas, in the street and in other noisy environments a highly directional (and thus mono) mic would be good. Either a shotgun or a hyper cardioid would probably help to reduce unwanted noise... Such microphones would be of general use for dialog, because they would be equally good for dialog on quiet settings, right? A very narrow acceptance angle would also help when using a zoom (the sound stays distant but there is less distraction from outside the picture).

The PDX10 is a small camcorder - very small. Thus long shotguns are not usable - especially not with a fur (for outdoors use in windy conditions)... Thus I am interested in the shorter directional mics like AT897 - 11 inch shotgun, AT4073a - 9.1 inch shotgun and AT4053 hypercardioid

I believe that one of those might be a good one for dialog.

If those are too narrow, then maybe a supercardioid would help... This is where I could make a big mistake and take a too narrow or too wide mic. So, super or hyper or shotgun for dialog? Any good advice?

Thanks for the link. I'll listen to some samples...

2) Another situation are the wideangle shots where the camera is in the middle of and surrounded by everything. I need a mic with a wide acceptance angle to be used with a wideangle widescreen lens. I was thinking of a 220 degree X/Y stereo mic for this job, but maybe a mono cardioid would do the job better? On the other hand I would also want to capture a "beeing there" feel that might be better conveyed by mediocre stereo sound than excellent mono...

And, of course, the ferquency response and sensitivity are important factors for the nature recording tasks...

So, it's going to be two mics. The AT835 stereo shotgun might be toomuch of a compromise. It's neither a good directional mic nor a good wide mic.
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