volume vs. distance in mic

[selurusey]

I am wondering if someone can explain how the relation between mic's volume and the distance from the mic can be expressed.

I am looking for a tiny mic with a good separating capability according to the distance. In other words, I would like only the closer sounds to be picked up by the mic but the sounds in the distance very weak. I understand that mics like Shure 58 is particularly useful because it has this distance-based isolation ability as well as useful cardioid pattern.

Do condenser mics in general have less distance differentiating ability? What would be the right category of mics for differentiating distance and which one would be the smallest mic there?

Thanks for all your help!

James

 

[dgatwood]

I am wondering if someone can explain how the relation between mic's volume and the distance from the mic can be expressed.

The intensity for an omnidirectional source falls off with the distance squared. Period.

No change in mics will make distant sounds seem more distant, though choosing a narrow pickup pattern can cause emphasis in lower frequencies (proximity effect) which may make some very sound sources (within a couple of feet or so) seem closer. This difference falls off very rapidly with distance, though, so this only applies to close miking.

http://en.wikipedia.org/wiki/Proximity_effect_(audio)

 

[Alden_Sloe]

I can't explain the physics but noticed a huge difference between the two mics bought as part of the MXL Aniversary set. The 603 was able to pick up a vocal track in a noisey room (piano + hussle and bussle of xmas dinner preparation) whereas the 2001 seemed to pick up every conversation around the room. Funny thing is if you believe the manufactures hype the behavior should be opposite but most reivews of these mics I've read seem to support the same experience we had.

Wouldn't recording at a lower level (less gain from the preamp) have the effect of isolating the sounds closer to the mic?

We have only one (very cheap dynamic mic. It requires a lot more sound pressure to achieve the same input level as the condenser mics. If you have to scream into the mic doesn't that have the effect of making background noise seem farther away

 

[mixsit]

Do condenser mics in general have less distance differentiating ability? What would be the right category of mics for differentiating distance and which one would be the smallest mic there?

Let's do three things here. 1st is mics can be directional, reject sides and/or back, but not distinguish close/far.

...Wouldn't recording at a lower level (less gain from the preamp) have the effect of isolating the sounds closer to the mic?

No. This is an acoustical 'signal-to-'noise' issue. 'Signal in this case being the desired sound, and 'noise' being the other sounds around that you are not trying to pick up.

...If you have to scream into the mic doesn't that have the effect of making background noise seem farther away

This on the other hand is pretty close to exactly right. As long as the distance to the mic hasesn't changed, in raising your voice - (signal) - you have effectively increased the volume difference between it and the background noise.

I understand that mics like Shure 58 is particularly useful because it has this distance-based isolation ability as well as useful cardioid pattern.

So, no. Some mics just have tighter patterns and reject off-axsis better.

 

[Alden_Sloe]

This is an acoustical 'signal-to-'noise' issue. 'Signal in this case being the desired sound, and 'noise' being the other sounds around that you are not trying to pick up.

Could you elaborate a little here. Maybe I'm reading the original question wrong. It seems that a certain level of input gain is required to get background/noise to record at an audible level. So if it's set below this minimum you'll all but eliminate the "noise". The source close to the mic on the other hand will be above this threshold and therefore produce an audible recording albeit at a lower level than if you adjusted to almost "peak" the meter.


Is the deal that when you amplify the recorded signal the background noise which "seemed" to disappear will be restored right along with the source closer to the mic?

 

[mixsit]

...Is the deal that when you amplify the recorded signal the background noise which "seemed" to disappear will be restored right along with the source closer to the mic?

If I understand your question right -yes. It's the relative difference between loud and soft things (for what ever reason- because they're close/far or just loud and soft at the same distances) that remains the same regardless of your gain, or sensitivity of the mic, or whether you turn it up now' ...or later.

 

[Robert D]

Paging Dr. Gerst.......


This is a tuff one for me to think of an intelligent answer to. In theory, the fundimentals at play are sensitivity, polar response, gain, and the decay in energy that sound waves lose in free air over distance. But anyone who's used a bunch of different mics, and to a lesser extent different preamps, will tell you that there is a variability in the degree to which different mic/preamp combinations will pick up a mouse fart from 40 feet away while recording an acoustic guitar.
I"m guessing the hidden variable here is in the transfer function of different types of microphones, with a non linear response at low pressure gradients, but I've never really pondered this.

 

[mshilarious]

You have to understand that people react to mics according to their response. Not too many people eat LDCs when they sing, but everybody (well not literally, but you get the idea) eats a 58. This is partially because the sensitivity of the 58 is low, and maybe people don't have enough preamp gain to use it from 12" away. Many non-pro singers don't sing very loud or project well, so the only option with a 58 is to eat it.

Then they get their first condenser, and they think hey, I don't have to eat this, it's so sensitive. Then they start thinking hey what is that dog barking?

But it's there in the 58 track too, just 16dB further below the signal--due to source-mic distance, primarily.

Try a simple test--get a test tone; the A440 on a tuner works pretty well. Set up two mics the same distance (and direction) from the tuner (be sure to get the distance to the capsules calibrated, not just the front of the mic!), record a few seconds of test tone and a few seconds of "silence". Normalize each track to the test tone, and compare the noise signals.

I just did that test with a Shure 55SH and KSM141 (set on cardioid), with the PC on high fan speed and the kids upstairs. There is little difference in the amplitude of the background noise signals.

I can't fathom any reason that preamps would cause a material change in this phenomenon. Any such preamp would be extremely colored, to say the least.

 

[selurusey]

Thanks for all your replies.
It looks like this issue is divided between the theoretically minded and the empirically seasoned.
I am thinking that the opposite of compression could be possible at the input device, i.e. the mic. That is, the higher the input intensity the higher the output volume, the lower the intensity (as in the distant noise) the lower the output volume. This would kind of have the effect of separating out the distant background noise. The physical law of the sound wave's intensity being proportionate to the distance will not change, except for minor variations due to temperature, humidity, etc.

 

[Almy]

Is this the kind of thing shotgun mics are designed for?

 

[60's guy]

Paging Dr. Gerst.......

I had tucked this interchange bewtween Harvey and Alan away for future reference. I guess the future finally got here.


Harvey Gerst wrote:

Okay, remember that volume drops off with the square of the distance for small "point sources" (whether it's a singer or a P.A. system). It's called the "inverse square" law. Let's see how it works out in actual use:

Let's say the singer is 2" from the mic. If the singer moves back to 4", the output from the mic will drop to 1/2 of what it was (-6dB). If the singer moves back to 8", the sound will drop to 1/4 (-12dB) of what it was at 2", and 1/8 (-18dB) of the original level if the singer moves back to 16". So a 7" swing either way can make a big difference in the recording level.

But let's move the singer back 2' from the mic. In order to double or halve the signal from that distance, the singer would hafta move in to the 1' mark or out to the 4' mark - a lot harder to do. Moving 7" back and forth at 2 or 3' will only result in a few dB or so difference in level.

In close, you need compression to keep the levels consistent; far less compression (or no compression) when the singer is further away. It also helps with the singer's head-turning problems.

But, you might ask, won't moving the singer that far back from the mic change the sound drastically?

You bet it will, and that's why I said I've learned over the years how different mics work at different distances. Figure 8 and hypercardioid mics have greater amounts of proximity effect than cardioids and wide cardioids, so I'll use those patterns if I need proximity effect at a greater than usual distance.

If there's too much room sound at that distance, I'll use a couple of sound deadening panels, set up in a "V" to reduce the effect of the room.

Alan Hyatt wrote:

I have had many issues with singers who have jumping beans in their pants. I usually can get a pretty good even recording by using a FET compressor, but I really prefer nailing their feet to the floor.

What I like about FET compressors is that they are very fast.
VCA compressors while quick, are not as fast as most FET's, and Optical is just a bit too slow for this kind of job, but a very fast attack with a ratio of about 2:1 to 5:1 has pretty much done the job for me in keeping the singer level when his beans start heating up, but each vocalist is different. So, even if the vocalist moves a couple of inches I am pretty well taken care of in either bringing up the level, or keeping it down. Since the ratio is light, you really don't hear much of the compressor at all. From there, I work with it in the mix if I need to.

Harvey is correct that proximity can be better on certain mics in different patterns, but you all have to have the experience of knowing which ones they are, and how the tend to alter the sound of what your trying to capture. Trying to keep the vocalists concentration on what they are doing is the best way, but for those who can't...FET, FET, FET!

Harvey Gerst wrote:

The singer faces into the open part of the V, like this:

o (assuming the 'o' is the singer)
V (the 'V' is the two sound deadening baffles)

At Village Recorders in L.A., they had a 3-sided 8'x8'x7' tall room, made with PVC pipes, with cool tie-dyed or paisley material around the back, both sides, and over the top. A nice Persian rug was on the floor.

I think it was set up for Christine McVey (Fleetwood Mac), since her piano was there as well. She could sit or stand in this little room (inside the studio) and do vocals without distractions, and the material was heavy enough to block some of the room reflections. It could be taken down or put up in about 15 minutes.

Alan is absolutely correct about having a fast compressor available when you can't get the distance, for whatever reasons. It doesn't solve the problem of the singer turning their head (and you hear all the high end in their voice disappear), but at least it keeps the levels steady.

It's a big problem with inexperienced singers and there's no real cure, except to point it out to them, and hope they learn.

 

[Robert D]

Good piece of reading there 60's guy, thanks for bringing it out of mothballs. And good point, mshilarious, about the way we use different mic types. I think there is, amoungst discussions of similar mic types, and even preamps, plenty of references to "mic A (and even preamp A) brought out the room more", or "mic B would be better on a bad room since it picks up the source and not the room". In the case of mics, this is often a matter of polar response differences, assuming differences in sensitivity were negated by gain structure. But still..... I can't help but feel there is a little more to this, and I think it probably has to do with some of the differences between the behavior of an SDC vs an LDC, let alone a dynamic and a condensor. The things that make one mic more detailed than another it seems to me would also make them more or less likely to pick up the "details" of unwanted distant sounds.

 

[Harvey Gerst]

Yes, and no. Let's forget about recording a source and just talk about room noise for a minute. Then we'll move on to a source, and then we'll talk about both happening at the same time.

Noise is all around us. An omni polar pattern will pick up noise from every direction, while a tighter polar pattern will only pickup noise from a specific area (usually directly in front and somewhat to the sides). This holds true for dynamics, small condenser mics, and large condenser mics.

The gain of the microphone (how loud the output is) determines how much room noise you hear, assuming the volume control is untouched, but remember that the output of a large diaphragm condenser mic is about 100 times louder than a the output of a dynamic mic. The noise will be 100 times louder (which is about 20 dB more).

To get the noise levels even, you must turn down the condenser mic by 20 dB, so now, the only difference is where the noise is being heard from - a function of the polar pattern.

But if you turn the condenser down by 20 dB, doesn't that defeat the sensitivity to soft signals that you get with a condenser? Yup! That extra "sensitivity" is both a blessing and a curse. Left alone, you'll pick up those soft signals with a condenser mic, but you'll hafta balance that against the extra room noise you'll get.

The trick is to find the right distance, level, and polar pattern that will give you the signal you want, while using as little gain as possible to keep the room noise acceptable.

If you get in close to the mic and keep the level down, you'll drop the room noise, but then you'll have proximity effect to deal with. That may be good or bad, depending on the sound you're trying to record, but each case is different.

But the bottom line is pretty simple:

If there's too much room noise, either make the desired source louder (or mic it closer), or do a better job of eliminating the room noises.

 

[60's guy]

Good piece of reading there 60's guy, thanks for bringing it out of mothballs.

Thanks.

I did realize that it was only partially pertinent to the discussion at hand but I still thought it worth posting for the benefit of newer members who haven't read some of the archived info.

 

[ofajen]

I am wondering if someone can explain how the relation between mic's volume and the distance from the mic can be expressed.

It's true that in the free field sound falls off 6 dB/doubling of distance. However, in an enclosed space, as you get farther away from the source, the reverberant sound will eventually equal the direct sound. It does so at the "critical distance". At the critical distance, the reverberent sound keeps the level about 3 dB above what the falloff rate would indicate and of course, as you go farther away the sound gradually converges to the reverberent level and goes no lower. Also, in a hemispherical space (with a floor!) your falloff will be more like 4 dB/doubling of distance once you are far enough away from the source that the distance from source to mike is comparable to the distance to the floor (or greater).

Cheers,

Otto