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Natural Science Forum / Physics / Acoustics / July 2008Starter Pistol for Reverb Time Measurements
Hi, I've been looking into purchasing a starter pistol to use as a noise source for reverb time measurements. I've discovered that there are many different guns that shoot blanks on the market, and I'm not sure what would be appropriate for this application. Does anyone have any experience in this area and could provide a suggestion on what to use? Also, it would be desirable to find something that looked a little less like a handgun, so if anyone can think of other options for producing a loud impulse sound I would appreciate it. Matt >Hi, I've been looking into purchasing a starter pistol to use as a >noise source for reverb time measurements. I've discovered that there [quoted text clipped - 6 lines] > >Matt I guess loudness requirments might require a pistol ? How about the old spark gap or a pulled back lever hitting a block. greg >Hi, I've been looking into purchasing a starter pistol to use as a >noise source for reverb time measurements. I've discovered that there [quoted text clipped - 4 lines] >little less like a handgun, so if anyone can think of other options >for producing a loud impulse sound I would appreciate it. One traditional method is balloons. They add a party atmosphere to a project, but you need to pick up the remains. Another method is to make a "clapper." Get a couple of boards and hinge them together, like the old movie clappers. I guess if you're not thrilled with a pistol, cherry bombs (my personal favorite) are out. Ken > Hi, I've been looking into purchasing a starter pistol to use as a > noise source for reverb time measurements. I've discovered that there [quoted text clipped - 4 lines] > little less like a handgun, so if anyone can think of other options > for producing a loud impulse sound I would appreciate it. To clap two boards together is a good idea, also the starter pistol, or the bursting balloon as the noise source for reverb time measurements. There is one problem I always found: With this sources the reflected low frequencies are low and it is sometimes impossible to get the reverb time below 100 Hz. Cheers Jens > To clap two boards together is a good idea, also the starter pistol, > or the bursting balloon as the noise source for reverb time measurements. > There is one problem I always found: With this sources the reflected > low frequencies are low and it is sometimes impossible to get the reverb > time below 100 Hz. Never used a pistol myself but I used to work with someone who did. My recollection is that he used to buy special blanks designed to give more LF than a normal blank. Sorry my information is not better but as no-one else has yet come up with anything definite, I hope it might give you a lead.  SignatureTony W My e-mail address has no hyphen - but please don't use it, reply to the group. >> To clap two boards together is a good idea, also the starter pistol, >> or the bursting balloon as the noise source for reverb time measurements. [quoted text clipped - 6 lines] > than a normal blank. Sorry my information is not better but as no-one else > has yet come up with anything definite, I hope it might give you a lead. I have difficulty working out how a blank could have its spectrum modified. A child's cap pistol would do very nicely, with all the bits that actually make it look like a gun cut off. The less metalwork around the cap the better. d >I have difficulty working out how a blank could have its spectrum >modified. A child's cap pistol would do very nicely, with all the bits >that actually make it look like a gun cut off. The less metalwork around >the cap the better. The bigger the charge, the longer the duration of the pulse, and the more low frquency content. The level will also be higher, raising what there is at low frequencies. A cap deosn't have much oomph. I've used a .22 caliber starter's pistol. I remember there being at least two sizes of blanks being available for it, and I'd try to get the bigger ones. A colleague told me of using a shotgun with blank shells to do reverb measurements in a large industrial setting. Ken Plotkin >> I have difficulty working out how a blank could have its spectrum >> modified. A child's cap pistol would do very nicely, with all the bits [quoted text clipped - 14 lines] > Ken Plotkin > You do have to watch out for microphone overload. A pistol has a very high peak level. I've just looked up a reference to starters' pistols and peak readings of in excess of 180dB are common. There is no commonly available microphone which will even come close to dealing with that. When I did some impulse response testing of microphones, I used an old photo flash unit to generate very narrow spark impulses. They didn't sound at all loud, but I had to wind both preamp and channel gain back to minimum on the mixer. A shotgun sound like a good idea on the face of it, but the bang is being generated at one end of a highly resonant tube - not a wonderful idea for spectral purity. And of course reverberation measurements tend to fail at low frequencies because of room modes, which build up and collapse according to rules quite unrelated to RT60. d Don, > You do have to watch out for microphone overload. A pistol has a very high > peak level. Is that necessarily a problem? The main thing you're looking for is the slope of the decay, which is after the initial "bang" sounds. So even if the microphone or preamp clips for a quarter second, won't the decay itself still be valid? --Ethan > Don, > [quoted text clipped - 7 lines] > > --Ethan Only once it has dropped below the overload point. What that means is that there is nothing to be gained in producing a sound louder than about 135dB - the rest is wasted. and of course not all mics or their electronics drop out of overload cleanly, there can be a recovery period while the mechanical and electrical bias re-establishes itself. d Good points, thanks Don. > Good points, thanks Don. Matt, The appropriate source depends on how big the room is. You need sufficient energy to excite the room at the frequencies you are interested in. Personnally I use balloons for smallish rooms, anything up to a medium size college dining hall (around 100m3), and a pistol for bigger rooms or where it's particularly important to get meaningful results in 63 / 125 Hz oct bands. At the moment we use a Bruni PPK Police 8mm. We have used a bigger revolver, a 9mm Police Special I think but can't really remember, but it wasn't very reliable. If you need something for bigger rooms (auditoria) still you're probably looking at a dedicated omni loudspeaker (B&K, or Norsonic) and an interupted or sweep source method. Most often I use the interupted method for small medium spaces anyway, just becasue I tend to be doing internal SI measurements as well and have the loudspeaker with me and appropriate software on the SLM (usually a B&K 2260 though others are available!). I always carry a bag of balloons as a backup in case of power/battery failure! Also, make sure you find a reliable supplier of blanks before you decide what to buy. That has been an issue for us in the past. Hope this helps Adrian > > Good points, thanks Don. > [quoted text clipped - 23 lines] > > Adrian Sorry, I meant 300m2 in my last post... Thanks for all of the helpful responses. I ended up ordering a .22 caliber pistol and blanks and plan to test it out soon in a gymnasium. This choice was mainly motivated by cost and the concern that a more powerful gun might overload the sound meter. For smaller spaces I will probably try balloons. [snip] >idea for spectral purity. And of course reverberation measurements tend >to fail at low frequencies because of room modes, which build up and >collapse according to rules quite unrelated to RT60. Good reason why it's still best to analyze reverberation from level recorder plots, rather than take whatever number a fancy SLM declares T60 to be. Ken Plotkin > I have difficulty working out how a blank could have its spectrum > modified. Slower burning gun powder comes to mind. The method of enclosing the powder might also matter. > d Kind regards Peter Larsen > Hi, I've been looking into purchasing a starter pistol to use as a > noise source for reverb time measurements. I've discovered that there [quoted text clipped - 6 lines] > > Matt Years ago, I worked for a company that used a .32 pistol for T60 measurements. This was pre-9/11, so traveling was as simple as checking the pistol in at the baggage counter in the case that was clearly marked "Gun Case." I have no idea whether it's that easy anymore. The pistol approach does lack somewhat on the low-end, particularly as the venues get very large. More recently, I have took to using two other approaches. One is the balloon approach. They are inexpensive, lightweight, and never questioned when you throw a handful of them in your carry-on. For more robust jobs, I use the MLS, swept-sine, or TEF signals through large loudspeaker(s) approach. I either ship some borrowed loudspeakers, or pick them up from a local rep once I reached my destination. For this approach, it is very handy to be on good terms with A/V reps, both locally and nationally. They are often more than willing to let you borrow one or more large format loudspeakers and amplifiers to perform acoustical tests. (After all, it gives them an opportunity to scope out prospective customers...) You just have to be extra careful not to "let the smoke out." ;) For more discussion of the pros and cons of different room excitation methods, David Griesinger's paper is very good: http://world.std.com/~griesngr/sweep.pdf All the best, Savant |
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