All things being equal?

[mikefraz]

Hypothetical here. I have a bolt action rifle and an AR15. Both of the same barrel length and suppressor on there. Both shooting the exact same round in the same exact conditions. The AR has an adjustable gas block and is set to "off". Would there be a difference in sound between the two?

 

[ryknoll3]

No, there shouldn't be.

 

[Bennettjh]

IMHO, they would sound the same.

 

[BehindBlueI's]

Sounds like a good justification to buy a Decibel meter to me.

(I have no idea, but sounds like an interesting experiment.)

 

[Indy_Guy_77]

Are you guaranteeing that the bolt on the AR isn't moving any?

If the bolt is completely stationary upon firing the AR - then my simple mind can't see a way for the explosion to not be the same.

Now - it may sound different to the ear depending on a muzzle device on the end of one/both - and where observers are standing in relation to the muzzle.

-J-

 

[mikefraz]

That's what I was thinking. A few buddies and I had this talk yesterday and in my mind they should be he same since there is the same amount of moving parts in the "gas off" mode. Thanks.

 

[Compuvette]

That's what I was thinking. A few buddies and I had this talk yesterday and in my mind they should be he same since there is the same amount of moving parts in the "gas off" mode. Thanks.

Yep. Bolt will still be locked to the barrel extension. I would think it would sound the same. Only thing I can think of is maybe gas leaking from the gas block itself.

 

[Davecz]

It should

 

[david890]

If the suppressed sound is 100db, the difference would need to be at least 10db - known as "Just noticeable difference". You should get a meter to determine if there's an actual vs. perceived difference (is your mind playing tricks?), and you should hand-load to ensure, as closely as possible, the "same" round used in each test. Since there isn't an industry-standard sound measurement test, you shouldn't worry too much about meter placement, air temp, etc. Just keep the variables as small as possible, and repeat the test a good 20 times or so for each to get a good average sound level.

If there is a 10dB or greater difference (and I'm sure the industry would kill to find a way to cut an additional 10dB from the report), I'd put it down to the ammo or barrel differences. You said the "same" barrel, but I suspect they are not identical, so there are barrel harmonics, cold-bore vs. hot, etc. to consider.

 

[2tonic]

If the suppressed sound is 100db, the difference would need to be at least 10db - known as "Just noticeable difference". You should get a meter to determine if there's an actual vs. perceived difference (is your mind playing tricks?), and you should hand-load to ensure, as closely as possible, the "same" round used in each test. Since there isn't an industry-standard sound measurement test, you shouldn't worry too much about meter placement, air temp, etc. Just keep the variables as small as possible, and repeat the test a good 20 times or so for each to get a good average sound level.

If there is a 10dB or greater difference (and I'm sure the industry would kill to find a way to cut an additional 10dB from the report), I'd put it down to the ammo or barrel differences. You said the "same" barrel, but I suspect they are not identical, so there are barrel harmonics, cold-bore vs. hot, etc. to consider.

OOOOH....close, but no cigar.....yet.
The part you have 100% absolutely right is the barrel harmonics, or more precisely, the pressure waves (known as sound) caused by the barrels and suppressors whipping back and forth after the shot (think of a ringing bell). Differences in barrel profile, attachment, and support will affect the perceived audible phenomenon.

Regarding that, when measuring SPL using either the "A" or "C" weighted scale, the minimum detectable difference, by the average person is 3dB. Some trained ears can recognize a 2dB change, but only relative to a previously established level (as in: tell me when the music is louder or softer as I change the volume, not: listen to these two sounds and tell me the SPL difference between them.) With me so far? Good, because I now officially sound the Nerd Alert!!! for what follows.
(Trust me, this all pertains to suppressor levels) First, some simple rules, an example, a couple caveats, and then the whole point.

- dB's (deciBel, or one tenth of a Bel) are used to measure both electric and acoustic energy.
- When measuring electrical energy (or gain) in audio signal paths the most common scales are dBa or dBm.
- When measuring acoustical energy (sound) we use dB SPL (sound pressure level). This number actually represents a ratio to our threshold of perception,or the noise floor, which is far above absolute quiet.
- Our aural sensitivity, acoustical energy, electric energy (as it pertains to sound) and frequency are all logarithmic in nature; so are the scales we use to measure this data and this makes it easier to understand and track their relationship to one another. This type of data set is routinely used to gauge the performance of a range of products from audio systems to industrial mufflers, including firearm suppressors.
Now this is the really important ****:
- To raise or lower a given SPL by just 3dB requires a doubling or halving of the current acoustic energy.
- To double the apparent level of a sound requires a tenfold increase in energy, this is represented as an increase of 10dB SPL.
- Every doubling of the distance from the sound source decreases the SPL by 6dB.

Here's an example to tie this all together and show it's usefulness : A FAQ about any audio system is "How loud will that get?"
Well, imagine I have a simple system consisting of a noise generator, amplifier, and loudspeaker. The amps output is rated at 400 watts/rms. The speakers sensitivity is rated at 100dB/1watt/1meter (this means that with 1 watt of input power, the speaker will produce a 100dB SPL as measured at 1 meter).
To get the smallest noticeable increase in volume (3dB) I must double the wattage to 2 watts for an SPL of 103, 4 watts for 106, 8 watts for 109, etc.
This works out to 10 watts (a tenfold increase in power) to gain 10dB SPL (an apparent doubling of volume) and reach 110dB SPL.
This extrapolates out to: 20 watts = 113dB, 40w = 116dB, 80w = 119dB, 100w = 120dB SPL (another tenfold increase in power and doubling of volume from 10w), 200w = 123, and 400w = 126dB SPL.
That's how loud it will get.
Remember, SPL dissipates -6dB for every doubling of the distance from the source, so 100w=120dBSPL@1 meter,114dB@2 meters,108dB@4 meters,
102dB@8 meters.etc. Obviously, the 1 meter change from 1 to 2 meters makes a lot more difference than the 1 meter from 16 to 17. Just as obvious is that distance counts more than power gain. The large print giveth, and the small print taketh away.

Okay, if you're still with me here's the two big caveats:
1) Most acoustical measurements are taken in an "anechoic chamber". That's a large room with very high ceilings (ex. 100'Lx85'Wx60'H) and various types of sound absorbing material on all it's surfaces, so that no sound reflects off these surfaces. This allows for the isolated measurement of just the test signal. It also makes for a room that's so scary quiet you can hear the blood rushing through your circulatory system. The test source is located in the center of this space (middle of room, 30'~40' off the floor) so sound can radiate in a spherical pattern, and thats where all these rules of acoustics work perfectly. However, we live in a real world where air movement, humidity, altitude ,compressibility,and even gravity affect these rules. Gravity??? Well yes, in that because of it we tend to rest on solid surfaces as opposed to floating in mid-air, ala the anechoic chamber,and therefore the sound producing devices we control tend to radiate in a hemi-spherical pattern (picture the ground as the equator on a globe), the sound goes sideways and up, but not down. the ground reflects it back. We are effectively radiating sound into a half-space, thereby skewing the energy equations. It gets worse when you add walls making it a quarter-space or smaller, but I digress. Basic point...in a half-space, energy builds faster, and dissipates slower, depending.

2) Our ears are more sensitive to some frequencies, or pitches, than others. There is a small band of upper-midrange tones to which we are extremely attuned.
This is due to the fact that most speech occurs in this range, and our ears and brain use these frequencies to detect direction of a sound. It's a survival instinct and it's in our DNA. In general, we are more sensitive to high pitches than low. A 100dB SPL tone at 1,000hz (cycles per second) seems MUCH louder than a 100dB SPL tone at 50hz, but a dB meter will read them as having the same energy.

How, then, do we make one can/barrel combo quieter than another, given the fact that the same potential energy is present in both?
Well, on paper we can accomplish this by just setting the measuring mic back slightly from the muzzle, or pointing it slightly off axis. This would be good for a 2~3dB reduction on paper, but it wouldn't hold up to real world scrutiny. What we have to do is somehow bleed off some of the energy in a "quieter" way.

-Heat...we can make the suppressor out of a material that converts pressure and vibration (acoustical energy) more readily to heat.
-Damping...we can make the suppressor larger or fill it with ablative material to slow the pressure pulse and release it over a longer period, thus more quietly.
-Rigidity...we can change the structural stiffness so the suppressor vibrates at a frequency to which our ears are less sensitive.
-Directionality...we can guide the pressure wave to release in a more spherical manner (louder for the shooter) or away from the shooter (louder downrange).

Ideally, we could influence all these parameters and still keep our can the smallest, lightest, toughest, coolest, quietist can out there. It's all in the compromises we're willing to make and how we measure them.

 

[Indy_Guy_77]

Speaking about no industry standard for measuring sound/suppressors....

I know a feller, an INGO'er to boot, who has, as a part of his employment, been working on a standard that NATO will/may use to test the suppressors that NATO forces use in the field...

If he sees this and wants to chime in, I'll let him. But I ain't gonna name him.

But - at least SOME kind of organization is working on a standard. A rather influential organization. And I'm sure that this standard just may very well trickle down...

 

[david890]

OOOOH....close, but no cigar.....yet.
The part you have 100% absolutely right is the barrel harmonics, or more precisely, the pressure waves (known as sound) caused by the barrels and suppressors whipping back and forth after the shot (think of a ringing bell). Differences in barrel profile, attachment, and support will affect the perceived audible phenomenon.

Regarding that, when measuring SPL using either the "A" or "C" weighted scale, the minimum detectable difference, by the average person is 3dB. Some trained ears can recognize a 2dB change, but only relative to a previously established level (as in: tell me when the music is louder or softer as I change the volume, not: listen to these two sounds and tell me the SPL difference between them.)

I was basing my statement on Weber's Law, which is the 10% I mentioned.

 

[2tonic]

First off, I didn't mean for my post to be taken as personal criticism, so apologies if it came out that way.

Weber's Law only deals with relative values, not absolutes. It merely states that, whichever parameter you are measuring, the amount required to change it the "minimum detectable difference", expressed as a percentage/ratio of original value, remains a constant (within a defined performance envelope); to wit, from your example: "Weber's Law, more simply stated, says that the size of the just noticeable difference (i.e., delta I) is a constant proportion of the original stimulus value."

In order to obtain the "just noticeable difference" measuring dB SPL then delta I = 3 dB, if measuring power delta I = 2x (or 100%). To continue changing the stimulus by this amount (JND) requires constant application of these values, within the range from "just above noise floor", about 60dB, to "atmospheric compression" at about 143dB SPL (also known as "threshold of pain").

I think the example cited in the link you posted above is what got you thinking in terms of 10%, but that was an "if", not an "is" value.