Topic: Rode NT55 with cardioid capsule

[Organfreak]

The cardioid capsule of the Rode NT55 shows an off-standard response. See attached frequency graph. To me the normal response is with the 75Hz high pass filter switched on. Normal position of the switch seems to give a bass-boost.
Maybe a trick to get a cardioid microphone with better bass response?

[aaronji]

At what distance was the frequency response spec'd? Maybe close enough to have substantial proximity effect...

[heathen]

Obligatory "don't put too much stock in the graphs published by most manufacturers" response.   

[DSatz]

of course this is an "idealized" graph, but there are three overlaid curves--with the filters switched off, there's a flat gray line from 90 Hz on down to 20 Hz at -4 dB, which is somewhat hard to see but it's there. Are you saying that you hear a rising response below 90 Hz, or ... ?

And yes, of course, every frequency response graph of a directional microphone is drawn w/r/t some particular measurement distance, contrary to what the IEC standard specifies ("plane wave" conditions, i.e. no proximity effect). Most of the top-rank manufacturers have adjusted their measurements for many decades to describe the behavior at 1 meter as a compromise. For more distant recording, the microphones will have less actual low-frequency output than what the graph indicates. The big exception is DPA, who adjust their measurements for, I believe, either 50 or 30 cm, I forget which at the moment. That throws off any comparison of their curves with anyone else's, making their directional microphones look on paper as if they have fuller, more extended low-frequency response by comparison.

Again, this doesn't refer to their omnidirectional microphones, since proximity effect isn't a factor there; also I'm not going to deny that DPA makes truly excellent microphones; it's only their published response curves that I'm grousing about.

That said, even if you were tipped off that manufacturer X adjusts to 1 meter while manufacturer Y adjusts to 15 or 30 or 16.2 cm, what correction should you apply to put their graphs on an equal footing, or to show the expected performance of their microphones in your actual application (e.g. more distant recording)? There's no formula; it depends on the acoustic design of each microphone (mainly on the transit time for sound waves between the center of the front and the center of the rear of the diaphragm, which is largely but not exclusively a function of the capsule diameter).

In other words there's no substitute for actually measuring under appropriate, highly similar conditions and reporting honestly. But if anything, the industry has been moving for years in the opposite direction from that.

And by the way, the farther you go along the spectrum of directional patterns from omni through cardioid, supercardioid and hypercardioid to figure-8, the more difference the measurement distance makes; proximity effect is considerably greater for a figure-8 than for an otherwise comparable cardioid. The attached graph is from an old brochure for the Neumann U 67 large-diaphragm, three-pattern microphone (released 1960).

[Gutbucket]

^Excellent and informative description.

Wondering if there might arise some independent 3rd party testing outfit, that buys, rents or borrows microphones and makes truly comparable response and polar measurements, at a number of set distances, including the 1m standard but also closer and farther as typically used (and atypically too) .

At what distance was the frequency response spec'd? Maybe close enough to have substantial proximity effect...

Don't mean to nit-pick, but actually the question posed, if true, would imply the opposite.  Or rather, imply that Organfreak might be using the microphone at a closer distance than the measurement was made which indicates no bass-boost.

[Organfreak]

Thanks for all these excellent replies!

I typically use my microphones for pipe organ recordings in large churches with lots of acoustics. Distance to the organs will be about 7 meters, so no proximity effect anymore?
The Rode NT55 with the omni capsule has quite a bit more (fat) low than the DPA4090. So what I was thinking - based on the graph for the cardioid capsule - that the NT55 mic preamp provided a boost for the very low frequencies.

[GLouie]

I'm confused!
According to the graphs from DPA and Rode, the omni bass response on both is ruler flat, with no presence peak on DPA and about 9dB HF boost on the Rode 45. As DSatz notes, take all such manufacturer's specs with a grain of salt.

This is from the downloaded Rode owner's manual (the Rode data sheet has a major error, showing the graph for the cardioid capsule and labeling it as omni), where the color line may help. The Rode cardioid has a shelving bass reduction that DSatz also notes, down 4dB at 90Hz. My guess is that they built this into the capsule construction to tame proximity effect, but at 7 meters you should have minimal effect, but are stuck with the shelf.

The Rode NT45 omni would probably be best used at distance due to the HF peak. Good bass for organ is usually done with an omni, and you usually want the room acoustics, too.

[Organfreak]

There is a test in the German "Professional Audio Magazin" (December 2006) showing some actual measurements. As per the magazine using an Audio-Analyser Audio Precision Sys 2722 and a ceramic, heavy damped tube.

The NT55 omni emphasizes the lower end while the 4090 has a bit rise in the upper end. Maybe this is the reason for an overall difference in bass response?
In addition, the DPA being a pure omni, will pick up more acoustic-high form its sides and back than the Rode. The overall balance in the sound will be different.

[aaronji]

At what distance was the frequency response spec'd? Maybe close enough to have substantial proximity effect...

Don't mean to nit-pick, but actually the question posed, if true, would imply the opposite.  Or rather, imply that Organfreak might be using the microphone at a closer distance than the measurement was made which indicates no bass-boost.

The way I read it is that Organfreak was surprised to see that, with no roll-off, the mic was basically flat down low, whereas the 75 Hz roll-off was what he would expect to see from a cardioid. Which is what you would see if you had a card closer to the source, with proximity effect making it seem as if there was more bass. As DSatz put it, "making their directional microphones look on paper as if they have fuller, more extended low-frequency response by comparison."

[Gutbucket]

^

Normal position of the switch seems to give a bass-boost.

I now understand your interpretation and mostly likely Organfreaks intended meaning.  The disconnect was what the boost is relative to.  My assumption was relative to the flat response at a typical measurement distance, where proximity effect is in play.. thus creating a response boosted above flat, rather than an analysis of the response graphs relative to a generally expected cardioid response when uses at a greater distance, bringing the response back to flat.

Nit formally retracted.

[Gutbucket]

As GLouie says, I'd not expect much proximity effect at 7 meters.

[Gutbucket]

I'm pretty ignorant on microphone response measurement best practice.  But I do know that measuring anything other than a true pressure omni complicates things significantly, so not sure how comparable measurements between them and those with a pathway to the back of the diaphragm can be.  Perhaps DSatz will comment on this.

[aaronji]

I now understand your interpretation and mostly likely Organfreaks intended meaning. 

When I read the original post, I immediately thought of the graphs below (for DPA4015s and 4011s) and his post made sense in that light. Posted previously, but maybe useful given the discussion...

[Organfreak]

The way I read it is that Organfreak was surprised to see that, with no roll-off, the mic was basically flat down low, whereas the 75 Hz roll-off was what he would expect to see from a cardioid. Which is what you would see if you had a card closer to the source, with proximity effect making it seem as if there was more bass. As DSatz put it, "making their directional microphones look on paper as if they have fuller, more extended low-frequency response by comparison."
[/quote]

This is exactly what I tried to make clear!
What I understand from the replies is that the response of the NT5 below 100Hz is related to the proximity effect rather than a kind of correction in the capsule or preamp.