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Author Topic: High frequency response of classic microphones  (Read 7382 times)

Brian Campbell

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High frequency response of classic microphones
« on: November 12, 2016, 12:01:25 pm »

While the discussion of high sample rates rages on it occurred to me that some of the most cherished microphones (U47, U67, C12) have a limited high frequency response. I'm fairly certain my U67s roll off dramatically after 20khz. It seems to me recording at 96khz, 192khz with these microphones would be pointless. I would like some clarification.
regards
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klaus

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Re: High frequency response of classic microphones
« Reply #1 on: November 12, 2016, 12:52:23 pm »

Hello Brian,
High frequency response is but a small, and I would say largely insignificant, aspect of a microphone's value as a recording tool for music.

What truly matters is how much detail-some call it resolution-of the original musical event gets passed along. Some of the most intimate, emotionally rich (what else matters?) recordings are and were made with ribbon mics whose high frequency response beyond 12kHz is usually very poor. The architecture of the mic's processor and its circuitry components play a huge role in the ultimate value of a mic as a recording device that can deliver truth or pleasure (what else matters?).

If you want to concentrate on what frequency response alone contributes to the value of a mic as a superior recording tool, it may be more rewarding to look at the ups and downs - where they happen and how large they are- within the response curve: certain boosts and attenuation throughout the curve, rather than ruler-flat response, make the difference between a very good and poor microphone.

Put another way: There are Chinese-made condenser mics retailing for less than $100 whose frequency response looks ruler flat, up to 20kHz, on paper. Are they any good? Doubtful.

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Piedpiper

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Re: High frequency response of classic microphones
« Reply #2 on: November 12, 2016, 01:56:53 pm »

And re: sampling rate, the relevance of high rates has more to do with the integrity of the signal, as well as processing of that signal, within the normal accepted hearing range than supersonic response.
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #3 on: November 12, 2016, 02:25:26 pm »

Thank you Klaus. I should have clarified my post by saying that I'm not an advocate for extreme sample rates. I'm happily recording at 24bit 48khz. I'm seeking confirmation that the microphones mentioned are not extended into the ultra sonic above 20khz and certainly not 48khz or 96khz.
It's always a pleasure to use such microphones, they are icons for good reason.
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #4 on: November 12, 2016, 02:30:09 pm »

And re: sampling rate, the relevance of high rates has more to do with the integrity of the signal, as well as processing of that signal, within the normal accepted hearing range than supersonic response.
Yes and that's another discussion (can of worms) :)
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Kai

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Re: High frequency response of classic microphones
« Reply #5 on: November 12, 2016, 07:32:16 pm »

I'm fairly certain my U67s roll off dramatically after 20khz. It seems to me recording at 96khz, 192khz with these microphones would be pointless.
There's a big difference in the way a mic rolls off (12-18 dB/octave) and an ADC/DAC cuts off the high frequency band (ca. 1000 dB/octave !).
The very steep cutoff a ADC working at 44.1 or 48 kHz needs to avoid aliasing is causing time domain problems in the upper octave of the usable band, known as "ringing".
Same applies to the reconstruction filter in a DAC, and I can asure you, decades of experience has show, each of those sound different.
An example: once I did a (very carefully set up) private listening comparison of 6 high class CD players.
I could clearly determin between all six.
Funny sidenote: the one that sounded the most brilliant and transparent had a measurable treble rolloff of 3 dB @ 20 kHz! The smoother filter resulted in (measurable) better time response, which by far overrided the HF energy loss in the audible result.
In fact it sounded as if it had more treble!
Other sidenote: when MP3 stuck it's ugly head out of the dirt it never should have left I wasn't very good in identifying it in listening tests.
I just did listen to spectral differences where higher rate MP3's don't show much difference to the original.
Today I don't have any problem identifying even 320 Mbit/s MP3's, partly even without the original as comparison. The artifacts often are so obvious, if you have a quality speaker or headphone system.
Probably it's what Neil Young had already discovered long time ago.
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Timjag

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Re: High frequency response of classic microphones
« Reply #6 on: November 12, 2016, 09:50:50 pm »

When I'm not recording I work in a music venue, and believe me MP3s are the bane of my life, the high end is so poor - even at the higher rates that it sounds like white noise - which of course it is.

Sorry to derail the thread.just had to get that off my chest
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #7 on: November 12, 2016, 10:46:12 pm »

It was not my intention to start a sampling rate discussion. Suffice to say that a U67 does not reproduce any significant signals in the ultra sonic range, say for the sake of argument above 25khz?
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Kai

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Re: High frequency response of classic microphones
« Reply #8 on: November 13, 2016, 07:09:55 am »

To come back to your primary question:
The U67 is not band limited and therefore has signal content above 20 kHz.
So if you want to keep everything the microphone catches you need to record at least at 96 kHz  sampling rate.
On the other side, if your recording will end up at 44.1 kHz it's questionable if not the final sample rate conversion will eat up this advantage.

But If you do intermediate processing this will benefit from the higher sample rate, specially equalizing sounds completely different in the higher bands.
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boz6906

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Re: High frequency response of classic microphones
« Reply #9 on: November 13, 2016, 08:53:14 am »

Recording at 96kHz or 192kHz samplimg rate doesn't mean you're recording audio at 96 kHz, just that the signal is being sampled at that rate.
IIRC, at 96kHz the UFX upper spec is -1dB at 45kHz.  A big advantage of higher sampling rates is moving the anti-aliasing filter's cutoff far above human hering response.

Regarding a mic's high freq. performance, I think smooth HF response is more important than extended HF output.  A good example is the RCA ribbons.  The 77DX is down about -10dB at 18kHz (no 'air' there!) but all agree they aound great.
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Jim Williams

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Re: High frequency response of classic microphones
« Reply #10 on: November 13, 2016, 01:29:06 pm »

Higher sample rate/bandwidth benefits include reduced phase shift of the filters, plus lower noise and better THD in some cases. Double the sample rate and you have double the "slicing" of the waveforms for easier "re-assembly".

Even with 20K hz bandwidth limited mic designs the added transparency is easily heard. You don't need 45k hz bandwidths for human voice but it will sound more natural. Use extended bandwidth 1/2" condenser mics on instruments like brass or percussion and you will hear/feel the closeness to the source. That stuff has large harmonics past 50k hz so any 22k hz bandwidth digital recording chain will low pass that sonic information away permanently at -100 db per octave. Even modified analog recorders can do 32k hz flat at 30 IPS, seems a shame to throw all that extra bandwidth away but that's what we have done for decades.
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #11 on: November 13, 2016, 06:00:29 pm »

To come back to your primary question:
The U67 is not band limited and therefore has signal content above 20 kHz.
So if you want to keep everything the microphone catches you need to record at least at 96 kHz  sampling rate.
On the other side, if your recording will end up at 44.1 kHz it's questionable if not the final sample rate conversion will eat up this advantage.

But If you do intermediate processing this will benefit from the higher sample rate, specially equalizing sounds completely different in the higher bands.
Has anyone here measured the top end response of a U67? What would the upper end extend to? Is this roll off a function of the capsule or the electronics?
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klaus

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Re: High frequency response of classic microphones
« Reply #12 on: November 13, 2016, 09:15:20 pm »

You can find the U67's frequency response on the Neumann website:
http://www.neumann.com/?lang=en&id=hist_microphones&cid=u67_publications

then look here:
Brochure U 67, 03/1966, English

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Klaus Heyne
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boz6906

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Re: High frequency response of classic microphones
« Reply #13 on: November 14, 2016, 09:32:59 am »

Thanks Klaus, freq response is 30 - 16kHz, looks to be around -6dB at 20kHz with a steep slope, no 'air' there.

The U67 HF response is similar to the RCA ribbons.

http://www.coutant.org/rca77dx/
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Kai

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Re: High frequency response of classic microphones
« Reply #14 on: November 14, 2016, 12:26:01 pm »

I would not exactly call this steep.
If you extrapolate the curve it looks like a 12 dB/octave slope.
This means 40 kHz is only 18 dB down, this could be brought back with an equalizer.
It's exactly what Schoeps does with their "CMC 6 Ug xt" -type mics.
If you want to see a really steep slope look at the brick wall antialiasing filter of any ADC.
It goes down to at least -96 dB or even -140 dB within the range of, e.g., >20 kHz to 22 kHz (~1/10th octave).
This is about 1000 dB/octave!
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #15 on: November 14, 2016, 10:04:31 pm »

I would not exactly call this steep.
If you extrapolate the curve it looks like a 12 dB/octave slope.
This means 40 kHz is only 18 dB down, this could be brought back with an equalizer.
It's exactly what Schoeps does with their "CMC 6 Ug xt" -type mics.
If you want to see a really steep slope look at the brick wall antialiasing filter of any ADC.
It goes down to at least -96 dB or even -140 dB within the range of, e.g., >20 kHz to 22 kHz (~1/10th octave).
This is about 1000 dB/octave!
My understanding is that modern delta sigma converters are oversampling in the Mhz range and allows for a much more gradual anti-aliasing filter roll-off.
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klaus

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Re: High frequency response of classic microphones
« Reply #16 on: November 15, 2016, 02:30:53 am »

Let's bring this discussion back down to a level that can be understood and appreciated by the majority of this forum's readers.

Please explain in simple terms what your previous post tries to convey.
KH
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Kai

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Re: High frequency response of classic microphones
« Reply #17 on: November 15, 2016, 11:14:01 am »

Please explain in simple terms what your previous post tries to convey.
KH
I'll do my very best to tell the story without using figures 8)
Frequency response and timing is strictly connected, no matter what technology is used.
Be it analog or digital.
There is no way around this. Even the famous "phase linear filters" do smear a signal the same amount as any other, they've just got a different shape that might even sound worse (pre-echo).
The steeper a filter works the more it shifts timing.
An AD-converter needs to remove every signal above one half of its sampling rate.
Every signal above will mirror at this border into the audio range, causing clearly audible artifacts.
No matter how the AD-converter is built, conventional+analog filter or with oversampling+digital filtering, finally the exact same very steep filter needs to be there.
This filter effects signal timing up to one octave down from its cut off frequency.
On a CD this means the complete highest octave off the audible range.

In contrary every microphone has a high frequency roll off with a comparably very soft steepness.
Therefore every acoustic analog source contains a significant amount of ultrasonic signals and the time response of a microphone is much better then the one of an AD-converter.

One needs to use higher sampling rates to avoid time smearing in the audible range, and loss of ultrasonic content.

 Mission accomplished, no figures used in this explanation 8)


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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #18 on: November 15, 2016, 12:40:04 pm »

Yes and the point of oversampling is that the anti aliasing filter on the analogue input can be pushed well out of the audible range. The result is then down sampled to the required sample rate. The decimation filter on the digital side is much easier to construct and does a much better job. As I said in my first post the sample rate discussions rage on. What prompted this discussion was a QC from a so called audiophile label that reject a mix on the basis that it had no ultra sonic content above 30khz and therefore cannot be a high resolution recording. What a load of rubbish!
I was certain that a U67 's frequency response in the 30khz range was very poor to non existent.
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Kai

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Re: High frequency response of classic microphones
« Reply #19 on: November 15, 2016, 01:54:15 pm »

... oversampling ... down sampled to the required sample rate. The decimation filter on the digital side is much easier to construct and does a much better job.
There is absolutely no difference, you cannot circumwent physics (read above).
You have a benefit if you do intermediate processing, but that's another story.
Sample rate conversion, on the other hand, is tricky if you want to have it well done.
I prefer working on the target sampling rate.

What prompted this discussion was a QC from a so called audiophile label that reject a mix on the basis that it had no ultra sonic content above 30khz and therefore cannot be a high resolution recording. What a load of rubbish!
I was certain that a U67 's frequency response in the 30khz range was very poor to non
It depends on how you define "no content".
In fact it's really hard not to have any (let alone the self noise of the recording chain) if your system has the bandwidth to record it.
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Brian Campbell

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Re: High frequency response of classic microphones
« Reply #20 on: November 15, 2016, 03:26:09 pm »

There is absolutely no difference, you cannot circumwent physics (read above).
You have a benefit if you do intermediate processing, but that's another story.
Sample rate conversion, on the other hand, is tricky if you want to have it well done.
I prefer working on the target sampling rate.
I believe that the ability to have a low order analogue filter before the ADC is considered an improvement.
Digital filters can have perfectly linear phase response and high order without significant noise problems. Anyway...

Has any one here actually measured the response of a U67 above 20khz. Just curious.
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Jim Williams

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Re: High frequency response of classic microphones
« Reply #21 on: November 16, 2016, 11:47:09 am »

Analog anti-aliasing filters do create phase shift, but the topology does affect results. Common "Butterworth" filters used in pro audio converters will offer maximally flat response, at a price. Run a square wave and you will see ripple on the top of the waveform.

Less common Bessel or Linear Phase filters have a less steep roll-off but offer a linear phase result. You won't see the ripple you see with the Butterworth slopes. The Butterworth slopes will also re-orient upper harmonics in time, placing them in an incorrect order creating another form of distortion.

When I did filter designs for Mytek I used a 65k hz Bessel 2 pole filter, that allowed the extra bandwidth of the higher sampling rates plus a benign filter without errors.
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Kai

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Re: High frequency response of classic microphones
« Reply #22 on: November 17, 2016, 06:52:10 pm »

Bessel ... filters ... offer a linear phase result.
... When I did filter designs for Mytek ...
Sorry, but this is simply not true. Bessel filters, like most other, do have a 90° phase shift at the crossover frequency, approximating 180° in the stop band.
Only the transition is softer.
This is very basic physics and there is no way around it, except if you compensate the phase shift for example with an all-pass-filter or in the digital domain.
Apogee used all-passes in their famous anti-aliasing-filters for 16 bit converters in the early ages of digital.
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Jim Williams

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Re: High frequency response of classic microphones
« Reply #23 on: November 18, 2016, 11:51:34 am »

I did not claim a Bessel filter has no phase shift, it does. It's a linear phase curve. 10 degrees at 1k will result in 20 degrees at 2k hz not 21 or 19 degrees. Read up on the late Dean Jensen's AES papers on the subject, it's good reading.  He showed that a linear phase curve is benign, non-linear phase will smear harmonics, hence the concept of the Jensen linear phase transformer designs. Many other filter topologies produce a non-linear phase curve. It's easily seen on a graphic analyzer like the AP. Square waves show the errors on a scope.

All recorded audio has phase shifts, they come with a reduced bandwidth. That is why my analog signal path has a 2 hz to 200k hz bandwidth minimum, it allows no phase shift in the 20~20K audio band. That includes the current feedback mic preamps at 30 mHz, modified microphones with the electronics at 300k hz,  the analog console with current feedback suming at 30 mHz bandwidth, the power amp at 500k hz bandwidth, current feedback headphone amps at 50 mhz etc. Phase shift comes from the filters in the ADC which is also a Bessel design.

All pass filters also create phase shift. It's the basis of many designs including old phaser guitar pedals that used cascaded all pass filters to create that modulated sound. GIC filters were also popular years ago as the audio path did not pass through them directly. These days the filters in the ADC chipsets do the work for you, but DAC's still require a filter unless they run DSD. My personal favorites were the 65k hz 2 pole Bessel Sallen-Key filters built with current-feedback opamps, 400 MHz bandwidth and no phase shift other than that linear 45 degrees produced by the 2 pole response. Those were done for a Mark Levinson high end DAC project that was based on current feedback, including the current to voltage stage, I still have one of those here, it's excellent.
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klaus

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Re: High frequency response of classic microphones
« Reply #24 on: November 18, 2016, 12:55:19 pm »

I understand how to reduce low-frequency phase shift in a mic amp- a pressure gradient capsule's natural low-frequency roll-off helps. But how do you avoid such phase shift in the high frequencies, especially in Braunmühl-Weber double backplate designs?

Take the U87: leave out the negative feedback that brings down the capsule's response to a -4dB level @16kHz. and you get an unusably bright mic. Even a gentler approach- shunting some of the high frequency energy to ground, still does not eliminate the phase aberrations altogether in the region above 8kHz.

I'd love to understand how that could be done in style, theoretically and practically.
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Klaus Heyne
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Kai

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Re: High frequency response of classic microphones
« Reply #25 on: November 18, 2016, 05:01:19 pm »

... U87: leave out the negative feedback that brings down the capsule's response to a -4dB level @16kHz. and you get an unusably bright mic. Even a gentler ... still does not eliminate the phase aberrations ..

I'd love to understand how that could be done in style, theoretically and practically.
The beauty is, no need to worry. Phase shift is strictly coupled to frequency response. If you compensate the frequency responses non-linearities you automatically compensate the phase response to linear.
And with linear I mean ruler flat.
Or, at least the more the frequency response is compensated, the more linear the phase response will be.
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J.J. Blair

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Re: High frequency response of classic microphones
« Reply #26 on: November 23, 2016, 11:58:17 pm »

I'd just like to remind everybody that fully extended frequency response is rarely enjoyable or musical.  As Klaus and I discussed years ago, there're a reason we don't record with measurement microphones. 
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Jim Williams

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Re: High frequency response of classic microphones
« Reply #27 on: November 24, 2016, 11:20:01 am »

That is a subjective belief as many high end classical and jazz recordings are done with higher sampling rates and greater than 20 k hz bandwidths. The other benefit is reduced phase shift. Reduced bandwidths, THD and colors may be applicable for pop/rock productions but an orchestra will sound truer to the source without those equipment limitations.
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klaus

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Re: High frequency response of classic microphones
« Reply #28 on: November 24, 2016, 01:13:20 pm »

That is a subjective belief as many high end classical and jazz recordings are done with higher sampling rates and greater than 20 khz bandwidths. (...) Reduced bandwidths, THD and colors may be applicable for pop/rock productions but an orchestra will sound truer to the source without those equipment limitations.

Another subjective belief that also cannot be scientifically verified. How many atrocious sounding classical recordings I have (correct that- I no longer have) which were "captured" with Danish 'measurement' mics... What some may call "truer to the source" I'd call "truer to a cold fish past its expiration date".

We agree on the sampling rate issue. A recent 'Golden Ears' listening symposium I participated in had some disagreements on some recording format issues, but one clear, unified outcome: all else equal, the higher the sampling rate the less annoying the sound.
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Klaus Heyne
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Jim Williams

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Re: High frequency response of classic microphones
« Reply #29 on: November 25, 2016, 12:22:52 pm »

B+K mics can produce emotionally pleasing recordings, if applied well. For doubting Thoms I suggest a spin of some of Todd Garfinkle's excellent CD's found on the MA label. Those used a matched pair of mics, a single set.
www.marecordings.com
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Timjag

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Re: High frequency response of classic microphones
« Reply #30 on: November 26, 2016, 11:13:09 am »

Ive have great results sticking a pair of B&K 4007 in front of a '67 ac30 running flat out - of course I stuck them through a narly old tube preamp just to confuse my intern. I guess  it's really down to who's doing the driving. If I had a quid for the amount of engineers I've seen fiddling about with SMAART in search of the elusive flat response I'd have, well... a lot of quids by now.

My good friend has a name for this type of engineer - which I'm sure you've all heard before - Engin-eye!
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Piedpiper

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Re: High frequency response of classic microphones
« Reply #31 on: November 28, 2016, 02:06:33 am »

B+K mics can produce emotionally pleasing recordings, if applied well. For doubting Thoms I suggest a spin of some of Todd Garfinkle's excellent CD's found on the MA label. Those used a matched pair of mics, a single set.
www.marecordings.com

I love Todd's work...
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J.J. Blair

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Re: High frequency response of classic microphones
« Reply #32 on: November 28, 2016, 04:23:33 pm »

I once bought a CD of Perlman doing Tchaikovsky's violin concerto, which is one of my favorite pieces of all time.  The premise seemed so promising.  It was a modern recording.  The sound of his violin was so shrill it hurt my ears to listen to with headphones.  I always wanted to know what mic was used. 
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klaus

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Re: High frequency response of classic microphones
« Reply #33 on: November 28, 2016, 05:07:56 pm »

Could be as trivial as placement*, or could be the mic, or could be sampling rate/compression/digital processing...
Nothing beats being there, in the control room during recording and listening to the direct, pre-tape (pre-conversion) sound.

*How man jazz recordings are out there, where the tone of a bowed or brass instrument is not allow to mellow-out, to letting the air molecules dampen the highs and overtones through distance, but where the mic-any mic- is placed a few inches from the source? Criminal.
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Klaus Heyne
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Kai

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Re: High frequency response of classic microphones
« Reply #34 on: November 29, 2016, 03:59:08 pm »

Aside from the more or less spectrally "normalized" modern pop productions, the sound of classical, jazz and others is all over the place.
This is especially true and more or less astonishing for classical, where the goal over the now about 10 decades of music recording always has been achieving a natural sound, and processing of any type was always sparse.

The reason for this cannot be simply connected to the use of a certain microphone type. There is a multitude of factors, in which the microphone plays a lesser role (although as a front end, it defines and sometimes even limits the quality of the outcome).

On the playback side one has to admit that there is no real universal reference loudspeaker (in a reference acoustic environment) or even less, reference headphones.
This is one of the biggest problems for every audio engineer: where and how will the recording be auditioned by the audience.

E.g., have a look at a typical modern (and expensive) television set: the tiny speakers are on the backside, radiating to the wall. Need I say more?
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leswatts

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Re: High frequency response of classic microphones
« Reply #35 on: December 14, 2016, 08:00:02 am »

A couple comments from a microphone designer after reading this thread...

Pressure gradient microphones need to be resistance controlled, but huge amounts of acoustic resistance lower
output. So there is a basic gain bandwidth tradeoff. At some point mass takes over and there is a decline in HF response.
LDC typically have a HF diffraction peak,SDC may have a peak from inertance in the rear delay chamber.
It's true (as was said here) that if you correct the amplitude peak with EQ the phase will tend to be corrected as well. Another way to say this is the microphone is largely minimum phase. (not completely)

Classic microphones were made considering the recording capabilities of the day. There was no point in sacrificing signal to noise to get HF response that the recording system couldn't pass.

BTW the same is true with other classical microphones like ribbons. Many classical models were made when audio
was a 5-10kHz world. But a ribbon can be made to go to 20 or 50 or 100kHz if one is willing to sacrifice output.

There is more about this from a historical perspective in a PSW article I wrote a while back:

http://www.prosoundweb.com/article/the_l_m_watts_technology_polyribbon_microphone_story/

Les
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