Topic: A question of polar patterns and SPLs

[echo1434]

I've been assuming something for quite some time, but it just dawned on me that I'm not completely sure about this...

With a directional mic, such as a cardioid, is the sound that is rejected (based on the polar pattern) filtered out before it enters the mic capsule?  In other words, does the directionality increase the tolerance for high SPLs?

Let's just say you have a cardioid rated for a max SPL of 120 dB and that you're at an extremely loud concert that is peaking at 130 dB or more. However, since much of this sound is bass and hall boom coming from the back of the arena, some of it will be rejected. Thus, perhaps only 115 dB of the original sound will actually show up on the recording.

Does this mean that a cardioid mic rated with a max SPL of 120 dB can actually produce good results at a concert louder than this as long as enough sound is rejected beforehand due to the polar pattern?

I hope my question was clear. Thanks.


P.S.  Please note that this has nothing to do with bass-roll off from a battery box or something like that. I realize this has no effect on SPL tolerance.

[Church-Audio]

The basic principle of a cardioid mic is this. There is the front of the diaphragm that is going to produce ( if noting is inverted ) a positive output with a backward motion of the diaphragm. The back of the cardioid diaphragm is exposed via the back vent that is theoreticly 180 degrees out of phase with the front of the diaphragm. This difference in phase causes phase cancellation that results in a more directional polar pattern and you can mess with something called the front to back ratio and control the directionality of the microphone. This is how we get some of the different polar patterns we use today.

Your question is rather complicated because distortion is measured at least for microphones many different ways by different manufacturers. I use a ultra low distortion speaker and do a direct coupled distortion measurement not in free air. In most cases when we are talking about distortion we are talking about a reference frequency of 1k. This high frequency is very directional, as such the back vents play very little role in determining distortion as a specification most specs are at 0 degrees to the source. Vent distortion does happen on mics because vents in them selves introduce a none linear response in relation to the front of the mic capsule.

The reflected energy and how loud that reflected energy is depends on frequency and absorption coefficient of the back wall. Also the inverse square law. For every doubling of distance you lose approximately 6 db of level now by the time it hits the back wall and comes back taking into consideration that the sound has to travel though bodies and the absorption coefficient of the wall you might have a signal that starts out at 130 but by the time it gets to you the reflective energy might be as low as 90 db. The real threat for overloading is from the direct field of the speaker array. Not from reflections from a surface. So no reflected energy is not a factor in mic overloading simply because the direct energy will always be greater in the front of the mic capsule * provided that the capsule ( front ) is pointed at the source One thing to note as the frequency gets lower the directionality of most sound systems becomes more omni directional.


When a manufacture says his mic is good to 120 db in most cases that's a best case scenario...So you might find problems at much lower db levels at lower frequencies. Because its hard to measure acoustic distortion at low frequency from a microphone simply because its hard to find a low frequency driver with low distortion and not have any sympatico resignation that could effect any distortion measurement in the first place.

Chris

[DSatz]

echo, the capsule of a condenser microphone can usually handle sounds that would deafen you, but the circuitry overloads at some lower SPL. Thus the maximum SPL figure for a condenser microphone generally describes the limits of that microphone's circuitry, not of its capsule. Typically the maximum SPL figure doesn't take the capsule into account at all.

Instead, the typical measurement method is that the capsule is removed and a fixed capacitor of equal value is put in its place. Then an audio frequency tone is sent through that capacitor, and its voltage is gradually increased while the microphone's output is continually monitored for distortion. When a certain percentage THD is reached--generally 0.5% these days--the output voltage of the microphone at that point is noted.

In effect, that is the microphone's maximum output voltage. Since the microphone's sensitivity is known, its maximum SPL can then be determined. For example, if a microphone has 12 mV/Pa sensitivity and reaches 0.5% THD when its amplifier puts out 750 mV, then its maximum SPL would be about 130 dB SPL. 1 Pa is a physical constant roughly equal to 94 dB SPL; the ratio of 750 mV to 12 mV is nearly 36 dB, so the microphone can go 36 dB beyond 94 dB SPL, or 130 dB SPL.

As you can see, directionality really has nothing to do with that specification at all. Nearly all microphone specifications, other than polar pattern, are (at least conceptually) anechoic, on-axis measurements.

--best regards