Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Forum for mic theory?
(This is an attempt to rejoin a discussion thread from this past
February. I hope that this new message will "attach itself" to the original thread.) Bob Cain wrote: The problem I see is in the low frequency polar diagram, and in the dual frequency plots (0 degrees and 180 degrees) that some makers show which maintains that there is good rear rejection all the way down. Theory tells me that is not possible. A pressure gradient response falls off at 6 dB/octave below the critical frequency, Ft, which is determined by the distance between the points involved in measuring the gradient. Bob's post concerned me because I translate and help edit most of Schoeps' English-language publications. And I agreed with Bob's viewpoint. But I had simply failed to notice that the polar graphs in Schoeps' product literature were being shown with no specific curves for frequencies below 1 or 2 kHz, and that disturbed me. So I wrote to the person at Schoeps with whom I work the most closely on publications, and asked him why we publish such misleading graphs (as I assumed them to be). His reply surprised me: He sent evidence in the form of response graphs showing that single-diaphragm cardioids really can, and do, maintain their directionality down to the lowest audio frequencies. In other words, what we publish isn't misleading at all. Single-diaphragm cardioids such as any of the Schoeps or certain of their competitors (e.g. the Neumann KM 140/KM 184) can really still be cardioid at 50 Hz and even lower. Thus the problem of widening response in the cardioid setting seems to be a special characteristic of dual-diaphragm microphones, not one of cardioids generally. How this squares with the notion of cardioids as a blend of omni with figure-8 response--and with the figure-8 rolling off at the lowest audio frequencies, leaving an ever-broader "cardioid" response--I simply don't know yet. When I think it through, however, it does seem right that a delay chamber ("acoustic labyrinth") behind the diaphragm of a pressure-gradient capsule would work on the basis of the absolute path length difference (i.e. the transit time difference), not the phase difference, between front- and rear-incident sound waves--so that at any audio frequency low enough to avoid being shadowed by the capsule, the rear-incident sound waves would "meet up with themselves" on both sides the diaphragm simultaneously and cancel each other out. --best regards |
#2
|
|||
|
|||
David Satz wrote: So I wrote to the person at Schoeps with whom I work the most closely on publications, and asked him why we publish such misleading graphs (as I assumed them to be). His reply surprised me: He sent evidence in the form of response graphs showing that single-diaphragm cardioids really can, and do, maintain their directionality down to the lowest audio frequencies. In other words, what we publish isn't misleading at all. Single-diaphragm cardioids such as any of the Schoeps or certain of their competitors (e.g. the Neumann KM 140/KM 184) can really still be cardioid at 50 Hz and even lower. I got your email, David, but will respond here in the spirit of a group hug. Here is a derivation of the 1st order mic that I've found (or maybe just came to understand) since I wrote you. It confirms exactly what you say: http://www.arcanemethods.com/single_diaphragm_paper.pdf In this paper, equation (14) shows that outside the range of the proximity effect both the omni and the gradient component are 1st order highpass filters with the same corner frequency. Thus the directional pattern should in fact remain cardiod all the way down even though the sensitivity is decreasing overall just as Schoeps told you. My misunderstanding came from thinking the omni component was flat all the way down rather than being in a lowpass with the same frequency dependancy as the gradient component. Inside the proximity effect region there is an additional proximity related low end boost which makes it more and more hypercardiod at the low ends as you approach the mic, eventually becoming a figure 8. The dual diaphragm is analyzed he http://www.arcanemethods.com/dual_diaphragm_paper.pdf It shows from equation (8) that there isn't the sensitivity rolloff at low frequencies in either the omni or gradient components and that the effect of proimity is similar to the single diaphragm mic. Thanks for bringing this back up. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
#3
|
|||
|
|||
David Satz wrote: So I wrote to the person at Schoeps with whom I work the most closely on publications, and asked him why we publish such misleading graphs (as I assumed them to be). His reply surprised me: He sent evidence in the form of response graphs showing that single-diaphragm cardioids really can, and do, maintain their directionality down to the lowest audio frequencies. In other words, what we publish isn't misleading at all. Single-diaphragm cardioids such as any of the Schoeps or certain of their competitors (e.g. the Neumann KM 140/KM 184) can really still be cardioid at 50 Hz and even lower. I got your email, David, but will respond here in the spirit of a group hug. Here is a derivation of the 1st order mic that I've found (or maybe just came to understand) since I wrote you. It confirms exactly what you say: http://www.arcanemethods.com/single_diaphragm_paper.pdf In this paper, equation (14) shows that outside the range of the proximity effect both the omni and the gradient component are 1st order highpass filters with the same corner frequency. Thus the directional pattern should in fact remain cardiod all the way down even though the sensitivity is decreasing overall just as Schoeps told you. My misunderstanding came from thinking the omni component was flat all the way down rather than being in a lowpass with the same frequency dependancy as the gradient component. Inside the proximity effect region there is an additional proximity related low end boost which makes it more and more hypercardiod at the low ends as you approach the mic, eventually becoming a figure 8. The dual diaphragm is analyzed he http://www.arcanemethods.com/dual_diaphragm_paper.pdf It shows from equation (8) that there isn't the sensitivity rolloff at low frequencies in either the omni or gradient components and that the effect of proimity is similar to the single diaphragm mic. Thanks for bringing this back up. Bob -- "Things should be described as simply as possible, but no simpler." A. Einstein |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Powerful Argument in Favor of Agnosticism and Athetism | Audio Opinions | |||
Announcement -- New Audio Discussion Forum | Marketplace | |||
Announcement -- New Audio Discussion Forum | Marketplace | |||
Announcement -- New Audio Discussion Forum | General | |||
Geekslutz forum - A forum for techie geekie nerds! | Pro Audio |