How are they measuring the noise level?

[SunnySlopes]

For a 12 guage shotgun, these are the dB levels of the gunshot:

12 Gauge 28" barrel 151.50dB. 26" barrel 156.10dB. 18" barrel 161.50dB.

Two questions. 1) Are they measuring the noise level at the muzzle, or where the shooter's head would be? 2) If they are measuring at the muzzle, is there any reduction in dB at the shooter's ears?

 

[Telekinesis]

Typically the sound levels are taken one meter to the left of the muzzle. There shouldn't be too much difference in sound level (if any) from the muzzle to the shooter's ear, but there may be a little depending of things like if the barrel is ported (or if a rifle has a muzzle break).

If you want to see what it looks like when someone's sound metering a gun, look on youtube for suppressor sound level testing. Sound levels from unsuppressed weapons are measured exactly the same way.

 

[SunnySlopes]

Thanx.

 

[MachIVshooter]

As TK said, usually ~3' @ 90* from the muzzle.

dB at the shooter's ears can vary substantially from this level, though, depending on muzzle crown shape or type of muzzle device. Some muzzle devices of the compensator or break category direct blast backward at an angle, while other flash hiders and backpressure increasing cone type units (like the Troy Claymore or Noveske KX3) direct more of the blast forward.

I have a Shark brake on my AR-10, and it substantially increases perceived blast on the back end (but reduces recoil & muzzle climb). I have a KX3 on my 7.5" AR pistol, and it is far less offensive to the shooter than my YHM phantom-equipped 9.2" PLR-16.

Also remember that dB is only one aspect of noise; pressure, duration, frequency all matter. Two noises with the same dB level can have dramatically different effects on the ear, both in terms of perception and damage.

 

[SunnySlopes]

pressure, duration, frequency all matter

I understand duration. When we're talking about gunshots, any noise above 140 dB will damage hearing, and all my guns, except my 22 long rifle guns, generate around 155 dB, give or take a few dB. The 22 long rifle is 130 dB according to other internet sources. Even though the 22 LR is rated below that "magic" number, I still wear hearing protection.

As far as frequency, handgun shots produce two frequencies of noise. Ear plugs reduce one frequency; muffs reduce the other. But long guns produce only one frequency.

Pressure? Are you referring to bone conduction of concusive sound waves?

 

[Dave P]

I dont know who THEY are, but i am very impressed that they can measure accurately down to 1/100 of a dB. I wonder how repeatable those measurements are???

12 Gauge 28" barrel 151.50dB. 26" barrel 156.10dB. 18" barrel 161.50dB.

 

[MachIVshooter]

I understand duration. When we're talking about gunshots, any noise above 140 dB will damage hearing, and all my guns, except my 22 long rifle guns, generate around 155 dB, give or take a few dB. The 22 long rifle is 130 dB according to other internet sources. Even though the 22 LR is rated below that "magic" number, I still wear hearing protection.

Duration is what can cause lower dB noises to do damage due to longer exposure. It is accepted that anything over 140 dB does instant damage, but 130, 120, etc. can do damage with increased duration (exposure).

More specific to firearms, some of the larger cartridges may have similar dB to a smaller one, but they are perceived as being louder due to the duration.

As far as frequency, handgun shots produce two frequencies of noise. Ear plugs reduce one frequency; muffs reduce the other. But long guns produce only one frequency.

It's not that simple, and I wasn't referring specifically to gunshots. There are ear damaging frequencies that are not painful (sometimes not even detectable).

The human hearing range for a young, healthy person is roughly 15 Hz to 20 kHz; That doesn't mean that noise at 10 Hz or 50 kHz can't hurt you. Frequency is completely independent of dB (intensity). You may not hear a 200 dB noise at 50 kHz, but you will be deafened by it.

Pressure? Are you referring to bone conduction of concusive sound waves?

I'm referring to the direct concussion caused by the pressure. A .458 mag and a .50 BMG are very close on the dB scale (about 175), but the concussive blast of the .50 burning 3 times as much powder is much more offensive. The .50 displaces more air, has a longer duration and a different frequency.

When suppressors are designed, one has to consider all of these aspects of sound to make an effective supressor.

 

[benEzra]

Frequency is completely independent of dB (intensity).

Correct me if I'm wrong, but I am pretty sure the commonly used dBA scale (used for most gun measurements I've seen) is indeed frequency-weighted to match the frequency response of the human ear. dB is a measurement of *relative* intensity, and on most scales 0 dB is set to approximate the threshold of human hearing at that frequency. Hence, a dB measurement for a sound at 50 kHz would probably be meaningless on the dBA scale, but might mean something in dB SPL or somesuch.

 

[Owen]

yes, dBA is weighted to attempt to match human ear sensitivity, but I don't think it's appropriate for impulse noises like muazzle-blast which are essentially a broadband signal.

 

[MachIVshooter]

Correct me if I'm wrong, but I am pretty sure the commonly used dBA scale (used for most gun measurements I've seen) is indeed frequency-weighted to match the frequency response of the human ear. dB is a measurement of *relative* intensity, and on most scales 0 dB is set to approximate the threshold of human hearing at that frequency. Hence, a dB measurement for a sound at 50 kHz would probably be meaningless on the dBA scale, but might mean something in dB SPL or somesuch.

I'm not an expert on audiology, but you are correct on the basis of the dB scale; When testing other animals, negative dB are often be used.

I should clarify that, while dB and Hz are independent of each other, perception and potential for damage does require a combination of the two. For instance, when testing dogs, some breeds were found to perceive frequencies as high as 65 kHz, but only when the intensity was set at 60-some dB. On the same note, those dogs were able to hear noises that a human wouldn't, even though the frequencies were within our range; the negative dB levels were an intensity lower than humans can detect.

An interesting tidbit: Though we usually think of dogs as having the best hearing of common pet animals, house cats are actually able to perceive a wider frequency range and lower intensities (they can also locate sound source within a few degrees).

Also, I believe it is whales that can hear the highest frequencies of any mammal.

 

[kludge]

We get this magazine where I work, or used to, and a while back they had a really good article, Here's a pretty good article.

http://www.sandv.com/downloads/0908rasm.pdf

 

[ajc3rd]

A dB (deciBel) is only a ratio and is meaningless without a reference. When used with regard to how loud some sound is we are talking about dB SPL. SPL is sound pressure level. The assumed reference is 20 micro Pascals. That is not all there is though. There are weighing filters (A, C, flat generally) and integration time (slow, fast, peak, impulse generally). A complete description would be something like 123 dB A slow.

The A weighing scale roughly aligns with ear sensitivity at normal conversation type SPL levels, basically around 65-70 dB SPL A slow. Our frequency sensitivity varies with level. The C weighting scale lines up with "loud music" and has less weighting in the lower frequencies. A 10 dB variation between A and C weighing is not uncommon for moderately loud music. A 6 dB variation between fast and slow is not uncommon for moderately loud music.