Turtlerock Forum

Hey!

So I was lying around this morning, thinking about microphones as I'm sure we all do. And it struck me, some microphones respond to the movement of air particles, their velocity, while others can detect a change in pressure even though the particles might be moving less, right?

So does that mean that some microphones will have a greatly degraded response when placed near a large solid boundary like a wall? That their performance is affected by the acoustic impedence of the space they're in?

I got thinking about this because I was wondering about the "back" and "front" of a ribbon microphone. Especially in regard to their response to reflections. Am I right in thinking that a ribbon microphone would, with an increase in pressure on the front side of the mic produce a positive voltage, and with an increase in pressure on the rear side of the mic produce a negative voltage? OR would that only occur if the wave length was smaller than the thickness of the ribbon material? In which case it would be outside our range of hearing?

So - if a microphone with a figure 8 pattern has one side pointed toward say, a snare drum, and the other is facing a wall, when a snare is struck, if it's a pressure gradient microphone, would the reflection off the rear wall have the same "phase" as the direct sound? Ie, an increase in pressure at the capsule = increase in voltage.
And would that mean that a velocity based microphone would then produce a negative voltage for an increase in pressure from the reflected wave?

Sorry this post is such a mess! I can never understand a concept until I can imagine the inner workings of every component, each particle, each electron, the energy shifting between objects that mightn't even be moving, argh!

Well, all microphones respond to changes in air pressure where the air particles are moving back and forth. The speed at which they move back and forth is the frequency and distance they move is amplitude of the wave.

If they're placed near a wall, they may reduce their performance due to waves that bounce off the wall bouncing back into the diaphragm. The reflection from the wall will be slightly delayed compared to the direct sound and dependent on the material the wall is made from the rebounding wave will have greatly dropped in amplitude. This means that you will probably get phasing issues but it's not going to cancel the sound completely if that's what you're thinking.

Just think of Phase = Time.
Look at the space you are recording in - and persue long reflection delays into the back of the ribbon - rather than short confusing delays. (when you are talking walls)
Its an approach I think about every time I record with a ribbon.

The workings of a ribbon mic are quite different to a dynamic or condenser. With a ribbon the sound wave actually has to travel past the ribbon for it to work, hence why "almost" all ribbons are figure 8. The way a ribbon works is that as a pressure wave approches, the air in front of the ribbon has greater pressure than that at the rear so the ribbon will move towards the area of least pressure, the rear. As the pressure wave moves past the ribbon the greater pressure is at the rear and the ribbon will move towards the front. This is why the ribbon is said to react to the pressure gradient, change of pressure, rather than the sound pressure directly, as a dynamic or condenser diaphram does.

If the back of the ribbon is closed and the pressure wave cannot move past the ribbon the only movement will come from the'" diaphram" type movement, similar to a condenser mic, but this will be much less than the normal action described above and will severly limit the frequency response.

I said above that "almost" all ribbons are figure *. Well in fact they ALL are BUT some are arranged so that they appear to be cardioid, such as the RCA 77. This is done by having two ribbons, a pressure operated one and a pressure gradient operated one connected in series and an acoustic labyrinth to produce the cardioid pattern.

Another approach, used by Western Electric, is to combine a ribbon with an omni dynamic, the two patterns added together create a cardioid.

With regard to placing any mic near a wall, this will always cause frequency notches as certain frequencies will reflect in phase and others 180 degrees out of phase (and of course, everything in between), causing reinforcement or cancellation respectively. Whilst this may be less noticeable with a cardioid mic than a figure 8, it will still occur to some degree. Hence the hours spent by engineers to get the right room placement for mics, and their ability to achieve specific responses by moving a mic to a particular position.

Thanks Bob!

I should point out, I only brought up the subject of differing phase depending on location and direction of the wave in regards to stereo ribbon mic placement. Sorry to confuse things!

The question in regards to walls was about particle velocity. In the sense that sound waves can exist with varying degrees of particle movement/displacement but the same change in pressure. This, from what I understand, is why with acoustic treatment we move velocity absorbers such as panels of exposed fibreglass away from walls, because while there is a pressure change, particle velocity at a boundary is essentially nil, but at 1/4 wavelength from the wall is the particle velocity maxima, whereas on the boundary is pressure maxima.

Also the means by which horns work in loudspeaker design to create a change in acoustic impedance.

I guess it's a moot point, because no ones going to put anything except piezoelectric mics on a solid object like a wall, but I thought it a curious idea.

I guess it's a moot point, because no ones going to put anything except piezoelectric mics on a solid object like a wall, but I thought it a curious idea.

I wouldn't go that far Anthony. One of the best ways of utilising PZM's ( Pressure Zone Mics) is to adhere them to a large flat surface, such as a wall, studio window, underside of the lid on a grand. This improves their LF response and minimises phasing problems.

Greg Hooke

I thought PZM's were pretty exclusively built with piezoelectric transducers?

I thought PZM's were pretty exclusively built with piezoelectric transducers?

No, they use a very small omni electret condenser capsule.

Greg Hooke

+1 on using PZMs under the lid of a grand piano. I used one on the bass end and another at the treble and with the lid open at about 15 degrees with excellent results. BTW Blue Tack works well to hold them in place without marking the borrowed Steinway...............

obutcher wrote:Well, all microphones respond to changes in air pressure where the air particles are moving back and forth.

Pressure and particle velocity have an inverse relationship. If the particles are moving at their maximum velocity, there is no pressure. If there's no movement in the particles the pressure is at its greatest (the surface of a wall for example). It is not the movement of particles that a microphone responds to, it's the pressure, they're very different things as I'm sure you know, just wanted to clarify.

Cheers

Eeep, my bad! Wrong choice of words will get you every time.

the other game in town to worry about is distance
the further you get from a sound source the more the high freq delays start adding up

so if you put a microphone in anything but omni
way back from the source to capture the sound of the instrument in the room
it quite often doesnt sound the same as your ears tell you it should
and even then as i sure you know most good omnis are essentially "flat " devices
it doesnt help that your ears are not essentially "flat devices " so you always "guessing " about the room sound
some omnis are made bright because of this and they sound wierd unless they are way back
( or great on acoustic guitar (but i digress )

one trick is to add a sphere to the surrounding "flat sounding " omni element which acts as a passive equaliser and boosts some of the high freq that are lost ( delayed) over distance acoustically
which gives you a much more natural room/ instrument sound much more like what we hear
the the "bright omnis" they are all over the place

neumann did this acoustic trick with the m50 and the newer m150 to amazing effect

what i have is the el cheapo neumann plastic sphere they sell for the km 83 / km 183 kk 83 element
it tops up the eq passively in a (typically scientific neumann figured out way )
and hey presto you can put the omni where your ears tell you to
and it sounds normal
think of it as the poor mans m50

i hated omnis until i got this little set of plastic balls - i think dpa make them as well
you could make one easily enough as well
in fact later down the track i will be turning them out of wood for other omni mics i have .
I also have some early beyer dynamic omnis which are essentially the same idea ( sphere shaped )
i cannot wait to see if they work as well

so the whole microphone thing used to fry my brain until i figured out perhaps i was hearing something different to the microphone... !
once i learnt that my understanding of microphones become much more practical

i am still left wondering often enough - dont fret

To save a bit of google-ing, these are the spheres from Neumann http://www.neumann.com/?lang=en&id=current_accessories&cid=db_current_accessories&dbid=139 and Schoeps http://www.schoeps.de/en/products/ka40