John, I personally think the important indicator for S/N is mic self noise in dBA. This is an equivalent A-weighted SPL of the noise floor so should be independent of the sensitivity of the mic if it is measured correctly. The lower, the better.

I find this one of the most important specs when choosing a mic, but some disagree, not sure why. There was some rumours around that certain mic manufacturers were spiking the results by ensuring that the mic self noise shape was the same as the inverse of the A-weighting curve, but to me, its still an A-weighted result and hence, if its a low dBA its quiet.

Some of the new generation mics from Neumann and AKG are less than 10dBA, the MKH's have always been quiet at around 10dBA.

The SPL at clipping minus the self noise gives the effective dynamic range of the mic. I think the figures you are quoting is 1Pa of pressure on the diaphram at 1kHz (single freq), ie 94dB minus the self noise at that frequency, hence around the 70dB odd.

Very interested in other's views on this, its an interesting topic.

David
Thank you very much for your reply. I know that low noise is very important, but my main concern would be the relationship between sensitivity and self noise on the assumption that with high sensitivity the noise will be less apparent as it will allow me to use lower gain.

On the other hand, I can understand your point of view on the grounds that self-noise constant (I know this is an over-simplification) but sensitivity varies according to frequency, angle and so many factors, so that mics with similar self noise and sensitivity specs can sound different from each other in terms of apparent sensitivity. I would imagine that in the real world, maximum SPL might appear different as well for the same reasons. I'm thinking of the difference between an ideal hypercardioid and an ideal omni shoved in front of a wide sound source. I know that mics with similar pattern types will also have pattern variations; U87 in cardioid and single pattern AT4060 and AT4047 do not share the same polar pattern in my opinion. Anyway I'm beginning to stray from the point, but I can see your point of view -unless I took it up wrongly :wink:

John

A-weighted noise specs have become very deceiving, driving condenser mic manufacturers to create products designed to test well.

Kind of like the mandatory testing game we're playing in schools these days. Just because a student tests well does not mean s/he actually has critical thinking ability.

Recordista
I think the analogy is a good one. I remember when I was in school some people attended extra classes in the evening for 'exam technique', where they discussed exam papers from previous years. I don't think they learned any more about their subject.

John

John Stafford wrote: I know that low noise is very important, but my main concern would be the relationship between sensitivity and self noise on the assumption that with high sensitivity the noise will be less apparent as it will allow me to use lower gain. John

I have convinced myself that self noise is independent of mic sensitivity, as long as you "assume" the preamp gain noise is not as significant, which it usually isn't with quality preamps. Because self noise is an equivalent SPL, then it will make no difference if your preamp is set on 40dB or 60dB, you still have a lower limit of an SPL that you can record and not an electrical amplitude of some sort. Mics measure SPL not millivolts. This is why I look at self noise with great interest.

Regarding mic manufacturers making things test well, I am not as cynical as some. There is a limit to what they can do without stuffing up the sound quality, so even if they make the self noise test results good, if the mic sounds bad to do this then it will not sell.

Personally, I'm not too concerned with self-noise ratings until I put a mic up and it becomes a distraction.

For example, ribbon mics don't have much self noise as they are a passive mic. However, your preamp better be quiet when you have the gain cranked as that is where you'll notice the self noise.

Some condensers are very quiet, most notable that I've recently used are probably the Sennheiser MKH series mics and the MG M930s. All of those similar quality mics, though will be very quiet (ie Neumann, Sennheiser, MG, Schoeps, DPA, etc...).

A lot of the older tube mics out there sound fantastic, but the noise floor is a bit higher. You listen to the old analog recordings from 40-50 years ago and you aren't thinking noise, rather you hear an orchestra that is recoreded very well.

Much of the issue is also based on what you are recording. If you are recording solo lute or clavichord, noise floor can be a big issue. An orchestra playing Strauss, though, it becomes less of an issue...

Oh well... my rambling $0.02.

--Ben

95% of the time or so (for me), environmental noise is a bigger factor than mic noise. I don't have a nice quiet studio to work in, and my bigger projects are location recordings.

FifthCircle wrote: A lot of the older tube mics out there sound fantastic, but the noise floor is a bit higher. You listen to the old analog recordings from 40-50 years ago and you aren't thinking noise, rather you hear an orchestra that is recoreded very well. --Ben

I was listening to one of those wonderful old Decca recordings, with Janet Baker singing Dido. While this is a recording I have known and loved for a long time, I had never really noticed until recently the level of noise. It such a great record, and sounds amazing. All I ever noticed was a beautiful recording.

As Karl points out, environmental noise is a bigger problem in the real world.

The reason I mentioned noise in the first place is that there's a particular venue where everything has to be mic'ed from a distance. The choir is at the front of the church and the organ is at the back. They don't want mic stands all over the place, so placing a single stand in the middle is the best compromise. I was thinking of using a single stereo mic. I'm afraid that the noise of something like a C34 would be very apparent in this situation.

This is one of those situations where there is no easy answer given that quoting a figure like 72 dB is an over-simplification. Anyhow, the results will probably be incorrect unless the mic is new or in excellent condition.

John

Yep, I agree with Karl.

Very few halls anywhere are truly quiet enough to reveal a mic's self noise and certainly very few playback systems are really all that accurate to portray a mic's self-noise either.

Of course, if taking into account the self-noise, one must also consider the sensitivity. A highly sensitive mic with a high noise floor will be far less noisy potentially than a "quiet" mic with very low sensitivity. But again, I don't find this to be much of an issue. Even on relatively unsensitive mics with somewhat high noise specs (look at the M50 as an example), the noise rarely poses a large problem. You are much more likely to hear the noise from air-handlers in the concert hall or the stupid trumpet player emptying his spit during the horn solo in Tchaik 5 (yes, I'm bitter.)

Of course, the A-weighting refers to a specific frequency response (which I don't currently recall the exact numbers and I'm feeling too lazy right now to look it up) but compared to C rating, it lops of low frequencies and high frequencies to concentrate on the critical mid-range. Some of the most frustrating noise I've actually ever heard from a microphone is in the lowest of frequencies (50 Hz and lower) which isn't measured by the A-weight scale.

So, in all my random babbling above, I guess my point is - I aim for the lowest possible noise floor in any situation and I've never really had a problem with the mic getting in the way of that. In almost every situation, it's environmental variables which cause the problem.

My .02

J. 8-)

Cucco wrote: Of course, if taking into account the self-noise, one must also consider the sensitivity. A highly sensitive mic with a high noise floor will be far less noisy potentially than a "quiet" mic with very low sensitivity.

I am not sure this is correct. Take 3 mics, with the following specs

Mic Sens (mV/Pa) SelfNoise (dBA)
A 25 15
B 12 15
C 25 20

Now imagine you setup to record a lute with an average SPL of 20dBA. Recordings A and B will record the lute level 5dBA above the noise floor. The fact that preamp A has to be set on 40dB gain and preamp B has to be set on 46dB gain is not important.

If you setup mic C, you will still end up with a recording of noisy lute, even though your preamp is set on a low gain.

This is precisely why self noise is in dBA and not millivolts. All this assumes that the preamp noise is much less significant than the mic noise.

I agree with all the conclusions that self noise in most live classical recordings is a non-issue, due to the miriad of noises in the hall.

But for nature recordings or similar it is vital to get a low noise mic and if its sensitive as well then all the better, but this does not influence the noise floor unless the preamp noise dominates.

David

Forgive me if I'm missing the point, but is it not true that using a mic with the same noise spec. as mic C but with higher sensitivity, the result will be the same as mic C recording a louder lute?

Another thing; I'd like to know is whether the decibel scale and voltage scale are of a similar type. I have always assumed they are not, in which case I think I understand.

I've just bought an AKG C34 on ebay. It has both low sensitivity and seems to have a very high noise floor. I was about to buy a Brauner VM-1, but unfortunately I had to restrain myself as I don't actually need the Brauner, but the stereo mic will solve more pressing problems. Some of my favourite pop vocals were done using the AKG C34 so it has its uses even if it proves to be less than ideal for other purposes!

John

Let's try another approach. The goal, when recording, is to fit the dynamic range of the SPL of the performers into the dynamic range on the digital recorder by adjusting the "gain knobs" on either the recorder or preamp or both.

Now, you could argue that the digital recorder as being a voltage recorder, which it is of course, but its useful to think about it being an SPL range recorder. My recorder gain knobs are actually marked as SPL and not "gain" from 1 to 10.

So lets say you have a small chamber orchestra that plays from 20dBA to 80dBA. You will try to set your recorder to clip at 90dBA, and so the lower recorder limit is perhaps 0dBA.

When the music is playing at 20dBA, mic self noise is easily audible at say 15dBA if comparing any two mics with this spec. All the mic sensitivity issue does is change the gain ratio (pot rotation) on the recorder/preamp to fit the required 60dBA into the same optimium area on the recorder.

For a sensitive mic this rotation of the gain knob will be less than for a low output mic, but the final "SPL on tape" is the same and so the self noise can be compared consistently between mics, independent of sensitivity.

DavidSpearritt wrote:
Mic Sens (mV/Pa) SelfNoise (dBA)
A 25 15
B 12 15

Now imagine you setup to record a lute with an average SPL of 20dBA. Recordings A and B will record the lute level 5dBA above the noise floor. The fact that preamp A has to be set on 40dB gain and preamp B has to be set on 46dB gain is not important.

attempting the math on the above proposed scenario:

20dBA(spl) = .0002Pa
Mic Signal A = .0002Pa*25mV = 5uV
Mic Signal B = .0002Pa*12mV = 2.12uV

Preamp A Signal output signal = 20log(5uV * 100/.775V) = -64dBu
Preamp B Signal output signal = 20log(2.12uV * 200/.775V) = -65dBu

Both pre-amps have approximately the same signal output level.
However, pre-amp B has twice (6dB) the noise gain as pre-amp B due to it's higher setting. Therefore A's output will have a better realized signal to noise ratio than B for the same signal output.

Conclusion: Mic-pre gain is relevant to the final recorded signal to noise ratio. To get the full picture, both Sensitivity AND Self Noise should be evaluated. One is not more important than the other, rather they are inter-related.

Zilla wrote: Preamp A Signal output signal = 20log(5uV * 100/.775V) = -64dBu
Preamp B Signal output signal = 20log(2.12uV * 200/.775V) = -65dBu

Both pre-amps have approximately the same signal output level.

However, pre-amp B has twice (6dB) the noise gain as pre-amp B due to it's higher setting. Therefore A's output will have a better realized signal to noise ratio than B for the same signal output.

Conclusion: Mic-pre gain is relevant to the final recorded signal to noise ratio. To get the full picture, both Sensitivity AND Self Noise should be evaluated. One is not more important than the other, rather they are inter-related.

All true, but I did state all along in my discussion that preamp gain noise is assumed to be less significant than mic noise, and in practice I have found this to be so.

When I hear a noisy recording, I am hearing mic noise in the final recording, not preamp noise. The most often effective solution, change the mic, not the preamp.

I wonder what the effective SPL of preamp self noise is, in a typical high quality preamp.

DavidSpearritt wrote: ...but I did state all along in my discussion that preamp gain noise is assumed to be less significant than mic noise ... I am hearing mic noise in the final recording, not preamp noise.

My analysis of your fictitious scenario made the assumption of an ideal, noiseless preamp. But to be technically accurate, the final recording will have BOTH mic and preamp noise. I agree with you that mic self noise is the dominating factor with quality equipment. This makes sense because the total mic noise is amplified by the gain of the pre-amp, while only the EIN of the pre-amp's electronics is amplified by the gain. But if you have a low quality pre-amp, it's noise could easily swamp a good mic.

Yes again, your analysis is absolutely correct, the essence of my argument is one of significance, and the point that self noise specs of mics can be consistently compared pretty safely without really having to consider the preamp.

Also in your example, while the preamp A gain noise is 6dB better for the sensitive mic, doesn't the higher mic A output just cancel this out, ie Mic A's self noise is more twice the uV of mic B. Its the location of the noise gain thats moved, either the noise gain is in the mic or the preamp in this case, so the effective noise is still the same ultimately on the recorder.

I must read the DIN standard to see how they measure self noise and try to understand why no sensitivity settings are quoted with the self noise specs.

DavidSpearritt wrote: ...your analysis is absolutely correct ... self noise specs of mics can be consistently compared pretty safely without really having to consider the preamp.

Sorry, but I must be misunderstanding you because I don't see how you can afirm my analysis and still make that statement.

DavidSpearritt wrote: ... in your example, while the preamp A gain noise is 6dB better for the sensitive mic, doesn't the higher mic A output just cancel this out

No. Disregard the preamp from the discussion for now. Mic A has greater sensitivity, so it ouputs more signal for the same given SPL than Mic B. Both mics have the same self noise. Therefore: Mic A has a greater S/N ratio than mic B. Straight up. Any ratio involves TWO values. The self noise figures alone are not enough to make this evaluation (which is my understanding of what this thread is all about.)

DavidSpearritt wrote: ...Its the location of the noise gain thats moved, either the noise gain is in the mic or the preamp in this case, so the effective noise is still the same ultimately on the recorder

Noise gain does not move. It is present in the amplifying stage (ie. the preamp). What ever gain is set, that will be the noise gain at the preamp's input (the mic+the pre's own EIN). Thus, the effective noise changes with preamp settings and is a result of the noise of BOTH devices.

Disregard the preamp from the discussion for now. Mic A has greater sensitivity, so it ouputs more signal for the same given SPL than Mic B. Both mics have the same self noise. Therefore: Mic A has a greater S/N ratio than mic B. Straight up. Any ratio involves TWO values. The self noise figures alone are not enough to make this evaluation (which is my understanding of what this thread is all about.)

But Mic A also outputs more volts for its noise as well. I am interested in your answer to this ... Assuming both mics have a self noise of 15dBA and we put both of these mics into a room with a 20dBA lute. Will you not still have a S/N ratio for both recordings of 5dBA?

DavidSpearritt wrote: But Mic A also outputs more volts for its noise as well...Assuming both mics have a self noise of 15dBA

I don't think so. If both mics have the same self noise then their noise voltage should be equivalant.

DavidSpearritt wrote: Answer this ... we put both of these mics into a room with a 20dBA lute. Will you not still have a S/N ratio for both recordings of 5dBA?

Ah, I think I see how I have been misunderstanding you. You are trying to express the final S/N as a "dBA(spl)" value. Well, I cannot refute the accuracy of your statements in those terms because I am not informed enough about that standard. However, it is my intuition that doing so is possibly an erroneous use of terms.

All I can do is try to answer your question in terms of electronic signals. Again, my attempt at a mathematical analysis:

A Lute produces 20dBA(spl) = .0002Pa

Mic Signal A = .0002Pa*25mV/Pa = 5uV
Mic Signal B = .0002Pa*12mV/Pa = 2.12uV
Mic Noise A or B = 15dBA(spl) = 0.7uV (IEC 179A)

Mic A S/N = 20log(5uV/.7uV) = 17dB
Mic B S/N = 20log(2.12uV/.7uV) = 9.6dB

So unless someone can show the error of my ways, I believe the numbers above demonstrate that our two fictitious mics indeed have different S/N ratios.

Zilla wrote: Mic Noise A or B = 15dBA(spl) = 0.7uV (IEC 179A)

I think this statement maybe erroneous. 15dBA of self noise does not equal 0.7 microvolts in both mics, it will differ depending on their sensitivity.

15dBA in Mic A will produce =10^(15dBA/20)*20E-6Pa*25mV/Pa*1000uV/mV = 2.81uV

and similarly 1.34uV in Mic B

But this is the beauty of having self noise in dBA. The SPL and its S/N is all that's important in the end, how the gain structure works to fit that onto a recorder is not as significant.

When I am referring to S/N for recording, I am always referring to the SPL of the music as the signal and the "SPL" of the noise floor as the noise.

I think you are referring to the S/N of individual devices in the chain, ie the preamp, the first gain stages in the mic perhaps, and because these have much more dynamic range than the mic, then 6dB of gain structure improvement in the preamp does not significantly improve the S/N of the entire system as a black box.

DavidSpearritt wrote: [quote=Zilla]Mic Noise A or B = 15dBA(spl) = 0.7uV (IEC 179A)

I think this statement maybe erroneous. 15dBA of self noise does not equal 0.7 microvolts

Possibly so. But my source was the "Handbook for Sound Engineers, The New Audio Cyclopedia" (1st ed., pg. 333) which gave 15dBspl = .7uV. They then refered to a standard (IEC 179a). As I said before, my understanding of the self noise spl equivalent standard is incomplete, but this seemed to be credible information.

DavidSpearritt wrote: ...self noise ... will differ depending on their sensitivity.

I think you are making a questionable assumption. In my (incomplete) research of this subject, and in the years of my practice in engineering, I have not seen evidence linking self noise's dependency on sensitivity. I do not believe that self noise dynamically "scales" with the mics sensitivity. The noise in a mic is generally the result of thermal electron agitation in the electronic components. The sensitivity generally to the mechanics of the capsule. So noise is dependent on temperature, not sensitivity.

I certainly have room to learn. So if you, or anybody else, has a technical source that validates your view on this subject, please pass it along. I have to say that in all the books I have read, the self noise spl specification is pretty ambiguously explained. And in addition, this subject has been a challenge to discuss clearly in these postings. A nice "writting skills" exercise for me. I now need coffee.....

My understanding is that a lot of it comes from brownian motion of air particles on the diaphram itself and well as thermal noise in components.

If the capsule excitation gets amplified through the electronics responsible for the mic's sensitivity, then you will have more output for self noise in a sensitive mic.

I have some good references on this somewhere and will post when found. Thanks for your input Scott, I agree the subject is never well explained and I think its important for us to make the right decisions when trying to make quiet recordings. Here's a start
http://www.dpamicrophones.com/Images/DM00670.pdf
The MKH mics are stunning, because they are both very low self noise and very high sensitivity, a potent combination.

Also I had a recent experience where my understanding of this issue was tested. We had a Royer SF12, and it was just a bit too noisy for classical acoustic recording. I was convinced by many, that the SF24 would be a significant improvement because "its 5 times more sensitive". Well I bought the SF24 and the noise level is about the same. I can make slightly quieter recordings with it (2dB or so), and I can use more preamps now, and on lower gain, but the self noise, the absolute bottom of the noise floor for a string quartet recording is still about the same. This got me thinking and searching. My good friend John Smyth (EE) showed that correctly it would change by only 1 or 2dB. Self noise is a very important mic spec, somewhat regardless of sensitivity.

DavidSpearritt wrote: My understanding is that a lot of it comes from brownian motion of air particles on the diaphram itself [..]

Gotcha (I think!)
I forgot about that part of it! That changes things , as you're talking about a component of the noise that, in reality, can be treated as an external source, so it is independent of sensitivity of the mic. It's relationship is with the SPL of the source -a bit like somebody standing near the mic going 'hissssssssss' beside the hypothetical mic that has no other self-noise. You still have a noisy lute regardless of sensitivity.

John

Mic Sensitivity Matters

Well, if my underwater acoustics experience of the past 30 years is applicable here (and I believe it is), the way to view it is as follows:

An acoustic transducer's noise, if expressed in SPL units (Pa), must be converted to voltage through its sensitivity (V/Pa) to get its noise voltage output.

Then, amplifying that tranducer's signal by the preamplifier gain will increase the net noise level going to subsequent electronic stages (filtering/EQ, mixing, etc.).

The noise output for two components (transducer and subsequent preamp) can be expressed as:

Noise Out (V) = Ap* SQRT( EINt^2 + EINp^2) )

where:

EINt = self noise of the transducer in Volts = SPLt * Sensitivity

SPLt = self-noise of the transducer in Pascals

Sensitivity = transducer transfer function in Volts / Pascal

EINp = self-noise of the preamp in Volts

Ap = preamp gain (Volts out / Volts in)

The output noise is an RMS sum of the noise voltages at the output of the transducer and the input of the preamplifier amplified by the gain of the preamp.

The goal is to have the transducer dominate the noise equation by having a self noise higher than its preamp. If both the transducer self noise and preamp input self noise are equal, the output noise will increase by 3 dB. The greater the difference between these two noise components, the less the lower one affects the total.

If the preamp noise dominates, then the full acoustic dynamic range of the transducer cannot be realized since the self-noise of the combination is being set (at least partially) by the preamp's noise.

I hope this is readable. If not, I'll try to explain it better.

So, David, have you given up on the SF-24 overall for serious stuff, or is it just a case of a certain mic for a certain project? (I would imagine you can work around the SN issue in certain applications, yes?)

I'm considering getting the SF-24 (or even an SF-12) for use as a center pair on a baroque ensemble in a closed recording session. (NOT a concert per se, but in a closed-off church), along with a few other mics for detail & touch-up. SN does concern me in this case, and I'm wondering just how much it's worth pursuing, esp when I have many other LD mics available to try. (And, I can alter the pattern with the other mics, moreso than just the fixed 90 degree pattern with the Royers...)

Aside from the SN issue, how does the SF-24 stack up for you in other applications? Have you found other uses for it instead? (Sorry to hijack this thread, but it's still related...sorta!....)

David
I must admit that I'm a little disappointed about the SF-12. I always assumed that they had a really low noise floor :cry:

I don't really think that ribbons are the the saviours of the Universe, but I've been considering an SF-12. I've bought two mics in the last fortnight, so I can't really afford to invest in another for a while, but it's on my list.

John

No, the SF24 is a wonderful mic as is the SF12. Its just that for very quiet sources and backgrounds its marginal for noise level.

The SF24 is our number one choice for live string quartets and chamber music, ie with a live audience. If we had to do a "studio" string quartet recording, we would probably use two MK8's or two KM120's, as the 426 is a bit bright.

The noise issue with ribbons is a problem for distant acoustic recording in quiet backgrounds, but the tonal accuracy of them makes up for the shortcomings. I love ribbons and will continue to try to aquire more of them. The new Coles 4040 is outstanding.

Re: Mic Sensitivity Matters

dpd wrote: If both the transducer self noise and preamp input self noise are equal, the output noise will increase by 3 dB.

This is an interesting post, but I don't understand why the above should be the case. 3dB greater than what?

I'd love to hear more about your underwater work. That is something I'd love to do.

Whenever I can get a pair of Senheiser MKH 110s I want to bury them in the back garden and record the underground world, but for me it would not be serious work -just a little present for myself! I'm becoming more interested in recording things other than music.

Unfortunately/fortunately (delete as appropriate :wink: ) I live in the most seismologically boring place on Earth. As for storm recordings, we don't get the opportunity to record those either. My idea of Heaven would be to park myself and a few mics at the end of an airport runway -although the demise of the Concorde makes that prospect less exciting.

Anyway, I've strayed from the topic here, but I'm very interested in your work.

John

Re: Mic Sensitivity Matters

dpd wrote: ...my underwater acoustics experience of the past 30 years is applicable...

When I was a nuke in the navy I had several sonarman friends. Were you ever a squid?

dpd wrote: ...An acoustic transducer's noise, if expressed in SPL units (Pa), must be converted to voltage through its sensitivity (V/Pa) to get its noise voltage output...

Well, I think this is at the heart of why David and my conclusions differ. Though this seems reasonable enough and possibly true, I am not certain that this is exactly how microphone manufactures are deriving their specifications. And even more importantly, whether they all practice the same method of measurement. Do you have any technical references that confirm the above statement?

dpd wrote: The output noise is an RMS sum of the noise voltages at the output of the transducer and the input of the preamplifier amplified by the gain of the preamp.

I would add that the measured frequency range should be included with the noise figure. We usually assume 20-20k, but assumptions are not always accurate. This is one way manufacturers can print misleadingly awesome noise figures without being technically false.

dpd wrote: ...If the preamp noise dominates, then the full acoustic dynamic range of the transducer cannot be realized since the self-noise of the combination is being set (at least partially) by the preamp's noise...

IMO this is most likely the case with the majority of mics, especially tube mics. This is what leads me to believe that it is thermal (not Brownian) noise that dominates, and that it would primarily be static (independently proportional to sensitivity).

dpd wrote: ...I hope this is readable...

YES! Written very clearly. Thanks.

Re: Mic Sensitivity Matters

Zilla wrote: [quote=dpd]...An acoustic transducer's noise, if expressed in SPL units (Pa), must be converted to voltage through its sensitivity (V/Pa) to get its noise voltage output...

Well, I think this is at the heart of why David and my conclusions differ. Though this seems reasonable enough and possibly true, I am not certain that this is exactly how microphone manufactures are deriving their specifications. And even more importantly, whether they all practice the same method of measurement. Do you have any technical references that confirm the above statement?

Just basic engineering. The transducer senses sound pressure, expressed in Pascals (Pa) and generates an electrical signal. Could be voltage or current you want to amplify. Most preamplifiers increase the low-level voltage. Therefore, the transducer has a 'transfer function' of Volts/Pascal. One multiplies the transfer function by the input: Output (Volts) = Input (Pascal) * Transfer Function (Volts/Pascal)

Zilla wrote: [quote=dpd]The output noise is an RMS sum of the noise voltages at the output of the transducer and the input of the preamplifier amplified by the gain of the preamp.

I would add that the measured frequency range should be included with the noise figure. We usually assume 20-20k, but assumptions are not always accurate. This is one way manufacturers can print misleadingly awesome noise figures without being technically false.

Always a good idea to compare things with equal other specs. RMS can be determined from a narrowband tone (sinusoid) or broadband (music/noise) over some bandwidth.

Zilla wrote: [quote=dpd]...If the preamp noise dominates, then the full acoustic dynamic range of the transducer cannot be realized since the self-noise of the combination is being set (at least partially) by the preamp's noise...

IMO this is most likely the case with the majority of mics, especially tube mics. This is what leads me to believe that it is thermal (not Brownian) noise that dominates, and that it would primarily be static (independently proportional to sensitivity).

To be honest, I haven't looked at this. The noise input of a circuit depends on a lot of things: thermal noise (especially for high impedance circuits), juntion noise in the semiconductors (aka '1/f' noise), power supply rejection, etc. In my work, it's the thermal noise of the front end since we need, at times, over 100 megohm input impedances to match piezoelectric transducers. I've not seen what the impedances of a typical condensor mic diaphragm is...

Lastly, regarding the 3 dB increase (relative to either of the noise levles) when two noise levels are equal:

Noise level 1 = 1 Volt RMS, Noise level 2 = 1 Volt RMS

Total noise level = SQRT (NL1^2 + NL2^2) = SQRT (1+1) = SQRT (2) = 1.414

20*lgt (1.414) = 3 dB.

I recently decided that my Precision 8/SF12 combo was not quiet enough for certain things and considered selling the 12 and buying a 24.

I then realized that for less money I could buy a Millennia micpre instead so I auditioned one. Turned out to be only marginally quieter than the Precision 8 but it made the SF12 sound almost like a condenser in the realm of transient response.

This got me to thinking about the different character of the SF12 with different preamps-- I ended up preferring the Precision 8 for choral and voice and the Millennia for instruments. In fact, all the stereotypes of the 2 preamps were confirmed-- the Millennia VERY fast, and almost X-ray imaging and depth; the Precision 8 a little more euphonic and juicy-- a good match with many modern Neumanns, which explains why Neumann is a fan of the Precision 8. The Precision 8 wins hands down in the value and space (1u) category for 8 channels, plus the dual outputs (TRS and DB25).

If I were shopping from scratch I would buy Millennia/SF12 and put the micpre as close to the mic as possible. That would give an additional micpre option for about the same money.

Rich

One of my posts seems to have disappeared. Ah well....

Rich
I was looking on the Atlas site last night and I came across the Precision 8, as well as a few others I want to post about. I hadn't come across this pre. It seems to be extraordinary value for money.
I've been looking at a few reviews, and get the impression that this is a first class pre. Would you agree?

Thanks
John

Yes, and the value for money is amazing-- about $1700 on eBay. I'll have one to sell as soon as I can find a used Millennia-- I want to have 8 channels of each for variety.

Rich

Sonarerec wrote: ,,,as soon as I can find a used Millennia

I have a spare Millennia Quad mic pre available. If you are interested, give me an email.

For some reason I cannot PM so please contact me via info on my website.

Rich

As I mentioned elsewhere, I got an AKG C34 today. The reason I mention it here is that it has low sensitivity and high noise specs, but it doesn't sound that way. I simply don't notice the noise, as it's a mild hiss that remains separate from everyting else.

I have a cheapo Behringer with 'better' noise specs than this, but the noise makes it unusable. I suppose there's cheap crappy noise and then there's posh noise :wink:

John