From ???@0x0000105F Wed Aug 14 14:44:12 2002 Path: pitt.edu!newsflash.concordia.ca!nntp.cs.ubc.ca!fu-berlin.de!uni-berlin.de!host217-39-173-33.in-addr.btopenworld.COM!not-for-mail From: Don Pearce Newsgroups: rec.audio.tech Subject: Re: LF limitations of Microphones Date: Wed, 14 Aug 2002 19:22:49 +0100 Lines: 72 Message-ID: <2k6llug6qcesp23hsje7nek7socin0q13i@4ax.com> References: NNTP-Posting-Host: host217-39-173-33.in-addr.btopenworld.com (217.39.173.33) Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: fu-berlin.de 1029349347 44747562 217.39.173.33 (16 [99781]) X-Newsreader: Forte Agent 1.91/32.564 Xref: pitt.edu rec.audio.tech:168987 Status: N On Wed, 14 Aug 2002 17:30:20 GMT, DPierce@TheWorld.com (Richard D Pierce) wrote: >In article , >Richard D Pierce wrote: >>In article , Todd H. wrote: >>>chris_wwww@yahoo.com (Chris Williams) writes: >>>> With particular regard to omni-directional moving-coil and capacitor >>>> microphones, am I correct in assuming that the low frequency cut-off >>>> point of these devices is limited or controlled primarily by the >>>> degree of sealing between the diaphragm and the cavity into which it >>>> operates? The learned input of some of you out there would be much >>>> appreciated. >>> >>>I don't know the answer you seek. But I do know that it's a lot >>>easier to find an omnidirectional mic with good bass reponse than a >>>cardioid, especially at the same price point! >>> >>>I'd be curious to know the physics/design specifics that make this >>>so. >> >>That's because the directionality is a direct result of rear >>cancellation on the diaphragm, and such cancellation requires a >>leak with a pretty short time constant. And that means a >>relatively high low frequency cutoff. Above a certain frequency, >>the path length difference between the front and rear of the >>diaphragm restores some of the gain. > >And one of the consequences of this design that I forgot to >mention is that the frequency response is dependent on the sound >pressure at the front of the diapgrapgm and the entrance to the >rear being equal. .If they are not, the frequency response >changes. And one common instance of when this happens is when >very-close mic'ing a singer. Pecause therte is now a real >difference in SPL between the two points, the frequency response >is perturbed, most often showing a significant boost in the >bass. This is the so-called proximity effect that cardiods >suffer from./ No, this is not why directional microphones suffer bass lift up close. The reason is that the velocity of an air particle in a spherical field is frequency-dependent, while the pressure isn't (OK, I know it is, but velocity is more so). So a pressure-sensitive omni mic will have a flat frequency response however close it is. Directional mics mix pressure and velocity responses to varying degrees. Figure eight are pure velocity, hypercardiods are mostly velocity, with some pressure. Cardiods are equal parts pressure and velocity. Because the velocity of particles in a spherical field rises with reducing frequency, in a directional mic, the velocity response in the bass region dominates. Two things happen as a result. One is that the response tips up in the bass, and the other is that close up a cardiod microphone has a hypercardioid response at low frequencies. If you want the maths of the frequency / velocity effect it is as follows: V = Po . Ro / (r . rho . c) . sqrt(1 + (c/(2 . pi . f . r) squared) Where Po and Ro are reference sound pressure and corresponding distance r is the actual distance, rho is the density of the air, c is the speed of sound and f is the frequency. V is the resulting particle velocity in the air. d _____________________________ Telecommunications consultant http://www.pearce.uk.com