Calculating with dBs

[OldSpook]

dBs are a unit of measurement which measures power.
They are named after Alexander Graham Bell. deci means "tenth" thus a Decibel is a tenth of a "Bell".
What does that mean?

Well from a ground truth perspective you don't care. What you want to know are what it measures and how to interpret those units in the real world. I am going to give you some simple rules you can use to figure out pretty much any problem involving dBs.

Rule 1: A 3 dB increase in power is the same as doubling power.
Rule 2: A 3 dB decrease in power is the same as halving power.
Rule 3: A 6 dB increase in power is the same as doubling range.
Rule 4: A 6 dB decrease in power is the same as halving range.

These rules apply at a standard temperature and pressure but you don't really need to worry about that in this instance.

What can you figure out using these rules and a microphone?

Suppose you use your cell phone and a sound meter app to read the noise level at your bench. Suppose it tells you that over a five minute period the average noise level is 58 dB. You want to know how far away someone can hear your rifle so you shoot a few shots over the phone and you find the average of those shots measures 81 dB. When you placed the phone out in front of the rifle you put it at 10 yards. You now have all the information you need to solve that problem.

But lets talk about "noise floor" for just a second. The noise floor is the level of "ambient" noise. The noise floor is the static on your radio. The noise floor is the "interference" which is always present when a signal propagates through space or down range. You generally can't hear things which are below the noise floor. To be honest, you generally can't hear things which are less than about 10 dBs above the noise floor but that is a different discussion. EDIT: see @nervoustrig 's post below

Now lets work that problem. The rifle is sounding of at 23 dB louder than the average noise floor at 10 yards from the muzzle. You know from rules 3&4 that doubling the range means halving reducing the power by 6 dB. So at 20 yards the "signal" from your rifle should be 23-6 (or 17) dB. We doubled the range and so could expect a 6 dB reduction in signal (noise volume). Well lets do that again then. At 40 yards (no not at 30 yards) the noise volume should be 17-6 (or 11) dB above the noise floor (58 remember?). We have the idea now so lets do that again a couple of times. 11-6 = 5dB @ 80 yards and 5-6=-1 at 160 yards. The sound of the rifle falls below the noise floor at about 160 yards.

That is the most common problem you are going to want to solve when working with moderators. And that solution is the best way to compare the performance of two different moderators because it converts dB into a number which means something to you on the ground.

decibel | Etymology, origin and meaning of decibel by etymonline

DECIBEL Meaning: "decibel," coined by telephone engineers to describe the efficiency of telephone circuits. It is a… See definitions of decibel.

www.etymonline.com

EDIT: Spelling

 

[nervoustrig]

Just wanted to add that our perception of loudness does not scale such that a doubling of power (3dB) is perceived as twice as loud. Rather, a difference of about 10dB is perceived to be twice as loud (or half as loud, depending on what direction you’re coming at it from). I think you may have called attention to this phenomenon in a different thread but it seems relevant to this layman’s summary as well.

 

[Healthservices]

deciBells are a unit of measurement which measures power.
They are named after Alexander Graham Bell. deci means "tenth" thus a deciBell is a tenth of a "Bell".
What does that mean?

Well from a ground truth perspective you don't care. What you want to know are what it measures and how to interpret those units in the real world. I am going to give you some simple rules you can use to figure out pretty much any problem involving deciBells.

Rule 1: A 3 dB increase in power is the same as doubling power.
Rule 2: A 3 dB decrease in power is the same as halving power.
Rule 3: A 6 dB increase in power is the same as doubling range.
Rule 4: A 6 dB decrease in power is the same as halving range.

These rules apply only in a vacuum BUT for our purposes are close enough.

Ok sorry but i only read up to here. I must have miss something.

When can you hear noise in a vacuum?

Allen

 

[nervoustrig]

 

[OldSpook]

Ok sorry but i only read up to here. I must have miss something.

When can you hear noise in a vacuum?

Allen

That was funny. I got a laugh out of it but for a moment lets assume you were actually serious. Those rules apply to RF. RF propagates in a vacuum. They describe "attenuation" due to the manner in which any wave propagates in any uniform medium. We can therefore apply them to sound propagating through air. In the specific case of sound waves, which as you rightly assert, can not travel in a vacuum, those rules apply in still air at a standard temperature and pressure. I'll go fix it in the post, so that you can get past that part and learn a new trick.

Thanks

 

[cavedweller]

Many people don’t know or forget how environmental factors affect sound waves. A low cloud overhead, for example, will intensify perceptible muzzle report, as the sound waves are bouncing off a close overhead and returns to the ears quicker. Nearby reflective surfaces also affect a sounds directional nature but not amplitude. Nearby soft or absorbent surfaces can deaden sound waves hitting those surfaces. I’m not a mathematical guy but as an end user, if I’m trying to be quiet, I always pay attention to the weather and immediate surroundings. So how does all this apply to the average shooters- that’s the big question! Great post by the way.

 

[dizzums]

theres a couple of other factors involved to .. assuming the attention span of the average pedestrian is floating somewhere in the vicinity of goldfish level, a single noise event rarely gets their attention, and then theres the visual cues that combined with a noise event would allow their karen senses to pinpoint a target to unleash the pent-up attitude at .. this involves a good defilade position, which combined with a lowered decibel level provided by an ldc, and timing shots out well, can avoid most of those nasty little altercations ..

 

[OldSpook]

There is another way to do it. Multiply the sensor distance times two raised to the power of the number of decibels above the noise floor divided by 6.

ln our original example that becomes 10 times 2 to the power 3.83.

That works out to about 142 yd.

Perhaps later today I'll put all of that on a graphic and attach it to this post if I get a chance.

 

[JungleShooter]

OldSpook,

THAT is an AWESOME thread!
Thank you for gathering this all toghether in one informative thread.

The final version of this post should go into our AGN library — the subforum Resources (well, after someone cleaned up that subforum since people put all kinds of posts in there, questions, anything whatsoever...).

Matthias

 

[badger5th]

My thoughts on this subject also take into account how a moderator changes the characteristic sound of a shot. Most good airgun moderators are in the 5-10 db reduction range. Compared to PB devices at 25-35 db reduction air gun moderators are "not that great" at pure decibel reduction.

However IMO the difference is that the airgun moderator changes the "crack" of a shot into a "woosh/thump" type of sound. This will be subjectively perceived as a quieter noise than it really is in terms of the actual science.

 

[MikeVV]

In reality, it's pretty difficult for the regular guy / girl to buy..."accurate"..."decibel" meters .
Coming out of Aerospace (retired), and having spent "some time" doing testing, in two different acoustic chambers during my time, in, having accurate meters is essential.

And I can tell you, accurate meters...are NOT...cheap ! $25, $50, $100, yeah...not do much ! Slightly better than toys.

Mike

 

[OldSpook]

In reality, it's pretty difficult for the regular guy / girl to buy..."accurate"..."decibel" meters .
Coming out of Aerospace (retired), and having spent "some time" doing testing, in two different acoustic chambers during my time, in, having accurate meters is essential.

And I can tell you, accurate meters...are NOT...cheap ! $25, $50, $100, yeah...not do much ! Slightly better than toys.

Mike

Well of course you're right about that. In order to get engineering data you need equipment designed to do engineering. I have indeed observed that my cell phone won't pick up high intensity short duration sounds like the crack of a supersonic pellet passing by it. I guess it is fortunate that is something a moderator can't moderate anyway, right?

However; you don't need equipment designed to do engineering to understand the math.

With all that experience you should be building moderators.

When somebody is testing moderators and tells me they don't have any equipment, I tell them they have ears. They think that's odd until I explain that we are trying to hide the muzzle blast from their ears not from a cell phone. That makes their ears the best sensor they can get.

Thanks for the spell check. Do you suppose the engineers who invented the term, see the link above, deliberately spelled Alexander Graham Bell's name wrong in their initial use of the term? What are the odds the original (as in engineering document that defines the term) spelling was deciBell, given the spelling of Mr Bells name?

Food for thought and future research.

 

[JackMan1]

Nice info, ty

 

[JungleShooter]

OK, I'd love to get back to the awesomenness of this thread and the very helpful way of how the OP explained some essential issues for reducing the loudness of a shot.


I am very interested in this topic, because Karen just moved in next door with an uncontrollable urge to feed pest animals, and Ken has been eyeing me suspiciously from his loft ever since he figured out that I like to eat a lot of red meat....

O yeah, currently I live and shoot in South America — Peru. So...., it's not Karen but Silvana who moved next door. And it's not the Ken's critical eye watching me, but Fernando's fastidious observations.....


Some of the issues I'm wondering about:

How does the tune effect the loudness of the shot? (HST, REG, valve dwell and valve lift, others)

How does an extended plenum help — o hurt — the loudness of the shot?

How can I determine theoretically how much muzzle energy in a .25cal will give me about the same loudness as a .22ca.


Those are all real questions that I'm up against.....

Matthias

 

[badger5th]

OK, I'd love to get back to the awesomenness of this thread and the very helpful way of how the OP explained some essential issues for reducing the loudness of a shot.


I am very interested in this topic, because Karen just moved in next door with an uncontrollable urge to feed pest animals, and Ken has been eyeing me suspiciously from his loft ever since he figured out that I like to eat a lot of red meat....

O yeah, currently I live and shoot in South America — Peru. So...., it's not Karen but Silvana who moved next door. And it's not the Ken's critical eye watching me, but Fernando's fastidious observations.....


Some of the issues I'm wondering about:

How does the tune effect the loudness of the shot? (HST, REG, valve dwell and valve lift, others)

How does an extended plenum help — o hurt — the loudness of the shot?

How can I determine theoretically how much muzzle energy in a .25cal will give me about the same loudness as a .22ca.


Those are all real questions that I'm up against.....

Matthias

Matthias, your second question is the only one that I can give you a generally correct answer. In general, all other moderator construction elements, materials, etc being the same, a larger volume is quieter compared with smaller. The extended plenum gives you the added volume.

Your first and third questions would be WAG speculation from me.. Regards, Badger

 

[OldSpook]

@Squack, you are the man. Thanks!

@JungleShooter
I really appreciate your attempt to put the thread back on track.

As for your questions about tuning that rifle, I can't help much but I can tell you some things which I have heard from others. First an efficient rifle is inherently quiet. I know this to be true as I've seen the difference it makes in my own guns. Also your rifle can be efficient with a number of different tunes. The trick is to get the hammer strike and the regulator tuned to deliver just the power you want with the ammo you are using and no more. Others have said wasted air makes muzzle blast worse. Clearly that must be true because muzzle blast is what we are trying to minimize.

A light hit on the hammer with a higher regulator setting will give you a shorter "pulse" of air. That can give you the same energy as a heavier hit on a lower regulator setting. That makes sense but what is not often said is that it is probably harder to setup and to keep set. Other shooters will be more likely to tell you how to accomplish that with your particular rifle.

Bigger calibers are more challenging. My HW110 just makes a "tick" sound when I pull the trigger. It is shockingly quiet. I believe you would not notice it going off at fifty feet. It is just that quiet. It has the integral silencer. I wish I could get a look at that but I am not going to touch it. A wise man does not mess with something which is that good out of the box.

Lets keep this thread going, shall we?

@badger5th
Thank you for trying to pull this thread back on line... It would be good to get some empirical evidence as far as how far away someone can hear rifles being fired. It would be even better if we also had some readings from dB meters, even if those were "cheap toys" or phone apps because we can cross corelate them given enough data. Science never stops for lack of instrumentation. It just forges ahead with what it can get and does the math.

 

[badger5th]

Thank you for trying to pull this thread back on line... It would be good to get some empirical evidence as far as how far away someone can hear rifles being fired. It would be even better if we also had some readings from dB meters, even if those were "cheap toys" or phone apps because we can cross corelate them given enough data. Science never stops for lack of instrumentation. It just forges ahead with what it can get and does the math.

To use a ham radio illustration and hope this is not confusing, (Oldspook, I know you are a ham...) We give signal strength reports to our radio contacts in terms of "S units" ranging from 1 to 9. Based on standards established in ham radio each S unit represents a 6 dB electrical increase (or decrease) in signal strength. A doubling of your transmitter output, say from 50 to 100 watts will translate to a 3 dB power change or 1/2 an S-unit. 50 watts to 200 watts is two doublings, or 6dB (1 S-unit) Unless you have really good hearing, without looking at the signal strength meter, many people can't really tell the difference between say an S7 and S8 but CAN for an S5 vs an S7.

As this relates to airgun moderators, I don't think a 3dB or 6dB drop will be noticeable to the ear unless the use of the moderator changes the characteristics and tone of the sound to one that is less sharp to the ear. This makes it "quieter". The distance to background noise test is a good idea. I suspect a less sharp noise from a suppressed airgun will camouflage with the background noise quicker.