A good overview article on Impulse Responses

[Phil O'Keefe]

Ever wondered about impulse responses? Here is a really good article that was posted over on Prosoundweb (it's an excerpt from a white paper on the subject) that does a really good job of explaining the basics.

 

http://www.prosoundweb.com/article/what_is_an_impulse_response/P1/

 

[MrKnobs]

Impulse testing has become pretty sophisticated and is now used for a wide range of noise engineering applications as well.

 

Recently, my research team was tasked with designing the world's largest absorptive highway barrier. Impulse testing was a critical part of that because a neighborhood near Interstate 30 in Dallas believed they were experiencing a significant noise impact reflected from a barrier across the freeway from their homes. You can read about it here in Chapter 6 of this report if anyone is interested in the details:

 

Typically, impulse testing is performed by popping a balloon or creating some other sharp peak noise and then recording the reverberation as described in the article Phil referenced. But that conventional method relies on the impulse noise being significantly louder than the ambient noise - not a problem in an empty concert hall or a recording studio, but how do you impulse test a large urban freeway? No one had ever done that before.

 

Probably a propane cannon (which I happen to have) or other explosive noise might be loud enough, but my requests to the DOT to close the busy freeway even for five minutes were denied for understandable reasons. No way was I going to fire off a cannon with traffic happening; it might cause an accident or summon an anti-terror unit and either result would not be good for a humble researcher from Austin.

 

But this had to be done somehow as it would be the only way to measure reflections from the barrier before and after application of absorptive material, i.e. the as-built effectiveness of the noise solution.

 

What we ended up doing was a state of the art impulse test that uses computer generated pseudo random noise at a much lower peak level than the traffic noise. It would not be heard, but because the sequence is known, specialized software can pick out the noise and it's multi-path reflections from the much louder traffic noise. The time differences between the various reflections on the recordings could then be matched to the geometry of the highway bridges and retaining walls to determine how much reflected energy was coming from each structure.

 

It worked very well. Since Texas, like many other southern states, uses permeable friction course pavements to reduce noise, our first test had to be redone to account for absorption as the reflection wave traveled across the permeable roadway surface. I think we'll be seeing a lot more use of this less intrusive test method on other applications in the future.

 

Terry D.

[Anderton]

Since you wouldn't let Dan Gonzalez from Cakewalk borrow your propane cannon to do his impulses, he ended up using a different process. Those who want to create their own impulses can find out here. Or you can ask Terry to borrow his propane cannon.

 

And Terry, you have clearly figured out a new way to...hit the road.

[Phil O'Keefe]

There's a castle in Ireland that once belonged to my family that I'd love to have an IR of... but all the easy ways are pretty much out of the question. They're pretty strict about guns over there, so even a starter pistol is out of the question. Same for explosives and large fireworks (so no M-80's) and I don't think even a very large balloon would cut it, and the Guarda would probably get downright apoplectic if we used something like Terry's propane cannon. That pretty much leaves sweeps as the only viable alternative. I think that in a lot of situations (although certainly not what Terry was dealing with) they're probably the best way to go.