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Compensate for LF roll off on cardioid mics

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Compensate for LF roll off on cardioid mics

Postby twotoedsloth » Wed Apr 23, 2014 5:11 pm

Does anyone use an EQ boost to compensate for the low frequency attenuation inherent to cardioid mics? For example, I frequently record classical music with pairs of Neumann KM184s and KM183s, using the KM183s primarily for the bass, and the KM184s in ORTF. Would it be feasible to just use the KM184s with an EQ bump in the low register?
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Re: Compensate for LF roll off on cardioid mics

Postby Hugh Robjohns » Wed Apr 23, 2014 5:18 pm

Well you can if the mic is used at a reasonable distance from the source... the KM184 is about 6dB down at 40Hz, so a gentle LF bell can help, but you have to be careful not to boost the VLF too much as that may well reveal problems with unwanted rumbles (hence using a bell, not a shelf).

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Re: Compensate for LF roll off on cardioid mics

Postby twotoedsloth » Wed Apr 23, 2014 5:25 pm

Thanks.

I'll give this a try tonight. I'll still set up the omnis in AB, but hopefully I won't need them.

Do you recommend a bump at 40hz, then a highpass filter at 30hz? Tonight I am recording a vocal recital, but tomorrow it will be string quartet, and they are notoriously unforgiving compared to singers who are usually pleased just to hear themselves.

Again, many thanks.
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Re: Compensate for LF roll off on cardioid mics

Postby Sam Inglis » Wed Apr 23, 2014 5:44 pm

The lowest open string on a cello has a fundamental frequency of 65Hz, so it's unlikely there would be much wanted signal to boost at 40Hz in a string quartet.
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Re: Compensate for LF roll off on cardioid mics

Postby twotoedsloth » Wed Apr 23, 2014 5:56 pm

This is a bit perplexing. If the low frequencies go missing at 40hz, but the lowest note of the cello is 65hz, the cardioid mics should be just fine. Except, there is a very audible difference when I add in the omnis.
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Re: Compensate for LF roll off on cardioid mics

Postby John Willett » Wed Apr 23, 2014 6:31 pm

twotoedsloth wrote:Does anyone use an EQ boost to compensate for the low frequency attenuation inherent to cardioid mics? For example, I frequently record classical music with pairs of Neumann KM184s and KM183s, using the KM183s primarily for the bass, and the KM184s in ORTF. Would it be feasible to just use the KM184s with an EQ bump in the low register?

Well - you could use the KM184s as an ORTF pair and have the pair of KM183 as omni outriggers to bring up the bass end.

Cardioid microphones such as the Sennheiser MKH 40 and 8040 have a very much better lower end than normal cardioid mics as they are RF condensers and are flat down to a much lower frequency.

Personally, I don't like to add artificial bass to a mic., but get the bass end by choice of microphone and positioning.
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Re: Compensate for LF roll off on cardioid mics

Postby twotoedsloth » Wed Apr 23, 2014 7:14 pm

Yes, this is what I'm already doing, ORTF in the center, and a wide AB pair of omnis.

I guess that there's no way around this?
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Re: Compensate for LF roll off on cardioid mics

Postby Sam Inglis » Wed Apr 23, 2014 7:45 pm

twotoedsloth wrote:This is a bit perplexing. If the low frequencies go missing at 40hz, but the lowest note of the cello is 65hz, the cardioid mics should be just fine. Except, there is a very audible difference when I add in the omnis.

Well, the roll-off begins higher up than 65Hz, so it's not surprising they sound different. But if you want to use EQ boost to compensate for it, in this case it might be better to boost a little higher up than 40Hz.
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Re: Compensate for LF roll off on cardioid mics

Postby Hugh Robjohns » Wed Apr 23, 2014 9:34 pm

As Sam says, the roll-off starts quite high -- maybe around 200Hz or so -- but its a very gentle slope. So if you want to use eq to bolster the low end you'll need to experiment with a wide bell centred around 80 Hz maybe, and adding only a couple of dB boost.

Part of the difference you're hearing between the 183 and 184 is that bottom end roll-off, but there's also the phase response differences to consider too, which gives a different character.

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Re: Compensate for LF roll off on cardioid mics

Postby Guy Johnson » Wed Apr 23, 2014 10:32 pm

In a good acoustic with good instruments, don't worry. Add too many mics and you'll end up with a muddy bass. bear in mind that a well recorded piece will be good on tiny speakers, and that most speakers will roll off (with a nasty hi-bass hump) at 50 hz or so…

But if you feel tha lowest 1/2 octave is not quite there (64 foot stopped organ pipes?) … what I've found works well is an omni with the ORTF (or whatever) rig, just to add sub 30 hz to 10 Hz stuff.

But.
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Re: Compensate for LF roll off on cardioid mics

Postby Bob Bickerton » Thu Apr 24, 2014 12:30 pm

I nearly always supplement an ORTF pair (of MKH8040s) or near co-incident MKH8050s with a spaced pair of omnis (8020s or KM183s) for chamber ensembles, choirs and orchestras (well actually more outriggers in that case).

If I take out the omnis in post, I pretty much always feel there's something missing, and it's not just bottom end.

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Re: Compensate for LF roll off on cardioid mics

Postby Jeraldo » Thu Apr 24, 2014 7:29 pm

John Willett wrote:

Cardioid microphones such as the Sennheiser MKH 40 and 8040 have a very much better lower end than normal cardioid mics as they are RF condensers and are flat down to a much lower frequency.


There is nothing intrinsic about RF mic's that give them better low end performance. In Sennheiser's case, the mic's are very heavily inversely equalized to give them the low end (and the upper end, IIRC) that they have.

The Sennheiser MKH EQ is part of a multi faceted design gestalt. I use and like the mic's.
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Re: Compensate for LF roll off on cardioid mics

Postby John Willett » Thu Apr 24, 2014 9:33 pm

Jeraldo wrote:
John Willett wrote:

Cardioid microphones such as the Sennheiser MKH 40 and 8040 have a very much better lower end than normal cardioid mics as they are RF condensers and are flat down to a much lower frequency.

There is nothing intrinsic about RF mic's that give them better low end performance. In Sennheiser's case, the mic's are very heavily inversely equalized to give them the low end (and the upper end, IIRC) that they have.

The Sennheiser MKH EQ is part of a multi faceted design gestalt. I use and like the mic's.

Yes and no - you can do things with an RF condenser that you can't with an AF.

The only mic. that went down to 0.1Hz was an RF condenser.

But, yes, the extended frequency response in the MKH 40 and 8040 is due to the capsule being lightly damped with a smooth response and the converse of the that response being in the electronics.

Unlike an AF condenser, the capsule of an RF condenser is a tuning capacitor - an explanation of this all is here.
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Re: Compensate for LF roll off on cardioid mics

Postby Jeraldo » Sat Apr 26, 2014 9:57 pm

Obfuscation aside:

So that the amount of EQ is appreciated, with the MKH40 - for example - there is a little more than 10dB boost going on at 50Hz, and there is 15dB going on at 20kHz. Presumably, the MKH30 would require significantly more on the bottom end. (One of my favourites.)

Nothing unique to RF design.

Non selectable EQ has been/is on board with well established AF microphone makers as well. For example, to extend the response of an already existing mic system to 40k. Had they wished, they could have easily EQ'd bottom end as well....


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Re: Compensate for LF roll off on cardioid mics

Postby Hugh Robjohns » Sun Apr 27, 2014 10:57 am

Jeraldo wrote:Nothing unique to RF design.


I'm not sure that's fair or true. There are many unique benefits to using the RF approach, and some of those do relate to the application of corrective equalisation.

Manfred Hibbing of Sennhesier has this to say on the matter:

Manfred Hibbing wrote:The output impedance of the RF circuit is low and independent of the audio frequency. Therefore the inherent noise is low and nearly white, almost without flicker noise. These properties enable frequency response correction (equalising) without causing annoying noise. For instance the response of pressure-gradient microphones can be extended at low frequencies. Thus the bass response of small diaphragm capsules can be improved to an extent comparable with much larger capsules. But small capsules feature better directional characteristics at high frequencies as the onset of the pressure build-up effect is shifted towards higher frequencies.

Frequency response corrections are also feasible at high frequencies, thus acoustic resonators – as used in many AF condenser microphones – can be avoided. This improves the impulse response and prevents tonal colourations. The phase response is not affected by this means because linearisation of the frequency response also corrects the phase response. This is valid for both electrical and acoustical minimum phase networks. So MKH microphones not only have a flat frequency response, they also have a linear phase response.

Electrical linearization of the frequency response was utilised from the very start to improve the properties of the MKH microphones, and it was also beneficial for improving the noise characteristics. The theory was as follows: Each microphone capsule incorporates acoustic resistances for forming the frequency response and the directional characteristics. Equalising the frequency response by pure acoustical means requires quite large resistances that cause additional noise like electrical resistances. So, by reducing the acoustic resistances, the noise floor can also be reduced. This also improves the matching of the transducer to the sound field and increases its sensitivity.

Furthermore, due to an appropriate acoustic design, the transducer sensitivity can be increased, especially between 2 kHz and 8 kHz where human hearing is most sensitive to noise. The higher capsule output also reduces the contribution of the amplifier noise. These effects support each other so that this ‘low impedance design’ improves the noise performance of the microphone considerably. The frequency response caused by this ‘physiological’ optimisation is no longer flat but can easily be corrected electronically. Due to this design, even the first MKH microphones had an extraordinary low inherent noise performance. An added bonus in this design is that it enables
the designer to achieve a polar pattern closer to the theoretical with less off-axis anomalies. The directional performance can be designed nearly independently of the frequency response because the latter can be corrected electronically.


So, in brief, it is a direct benefit of the RF technique that, by operating the capsule in a low-Z environment as part of a phase-modulated RF system, the system self-noise is considerably lower than is possible in a traditional AF design. That is what allows electronic equalisation of a magnitude which could never be tolerated in simple AF designs and gives a much more extended and flatter overall response than is achievable in conventional small diaphragm mics.

Jeraldo wrote:Non selectable EQ has been/is on board with well established AF microphone makers as well. For example, to extend the response of an already existing mic system to 40k. Had they wished, they could have easily EQ'd bottom end as well....


The application of corrective EQ in AF capacitor mics is usually of a relatively small magnitude specifically because of the inherent noise problems it brings with it.

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Re: Compensate for LF roll off on cardioid mics

Postby twotoedsloth » Sun Apr 27, 2014 5:45 pm

Greetings again,

Unfortunately, I was unable to find an EQ setting that sounded even in the same ballpark as the pair of "outrigger" A-B omni mics (KM183s in this case). I guess I'm stuck hauling extra gear for the time being.

Thanks for all of your suggestions.
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Re: Compensate for LF roll off on cardioid mics

Postby Hugh Robjohns » Sun Apr 27, 2014 5:53 pm

As was said earlier, the sonic differences are about much more than just EQ.

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