Impulse Source, Mechanical Device, Clapper

[bowl_actually]

I started a thread in the subwoofer forum asking about an appropriate subwoofer for a portable room analyzer kit.

The more I searched, I am now interested in possibly using something like a clapper device to generate sound to be captured with REW for room analysis.


Here's an investigation of using such a device.

A simple impulse sound source for measurements in room acoustics​

A simple impulse sound source for measurements in room acoustics
www.academia.edu

There is a company called Larson Davis that makes a commercial one, the BAS006. But I can't find where I can buy one of those. I want to make a few prototypes, clap them and use REW to measure the response I get. Can probably do that outside so I can get reasonable results. If one of these things will generate a good response, then it might be as good or better than a sub. Certainly more portable!


Then I found this one, which looks a bit better.

https://euracoustics.org/documents/11/AiP-2020-01_February_2020.pdf

In addition, I also posted an old thread about building Limp Mass Absorbers, which is very useful. I'll leave that here as well in case it helps anyone else.

Tim's Limp Mass Bass Absorbers - Page 34 - Gearspace

Quote: Originally Posted by Tim Farrant ➡️ Over the years I have had great success fixing smallish room bass problems with limp mass absorbers. As there are so many threads here about this very problem, I have posted my design here for you to use. It's not that flash, but they do work if you...

gearspace.com

 

[Sonnie Parker]

So, do you use the clapper at the listening locations?

 

[bowl_actually]

Not really as that would be more appropriate for the mic. However, you could, since what' you're trying to do with the clapper is discover all the room modes and problems regardless of where they are. The clapper is an almost perfect omni source, so it's going to send sound every direction.

It's fairly common in acoustics to actually measure in the corners and other places in the room. what you're looking for are places of increased pressure, which are the same as increased loudness. Room trapping needs to be done at the places where there are maximums. Nulls are just the opposite part of the wave, actually a null can predict where a hot spot is. If there is a null at a certain frequency, you can determine where the peak will be in the room by moving 1/4 wavelength. And that's where you want to put the trap.

Of course you're going to find those at the corners and boundaries for the most part. Even if you find them mid-wall, chances are they are part of a wave that also exists at a boundary. So you find the frequencies that are increased at the boundary points and place traps that reduce the volume of that frequency at that place.

Indeed, the best instrument for exciting the room are special dodcahedron speakers like this one.

Dodecahedron Speaker Kit I NTi Audio

Omni sources that excite the room evenly so you can measure at all the right places.

 

[bowl_actually]

UPDATE: I made a clapper based on the round one in the above link. It's about 16" in diameter. I just made it out of scrap pieces I had in my shop.

It works! and sometimes it works well. I get sloping response from down around 10hz to about 200hz. With the 10hz end being 10db or so above the 200hz.

The problem I have is that it's very difficult to make them consistent. Using ARTA, I can record multiple hits and average the responses. That helps, but it's certainly not perfect.

I will use it some over time, but at this point I'm having a hard time trusting the results because they are so inconsistent. For that reason I may be back to trying to find/build a Subwoofer that can be made to work.

From doing some research on drivers, there are a couple of 8" Dayton audio drivers that are in the ~$100 range that could work. Add a few bucks for the plywood and connectors. With a proper box design I can get a pretty flat response down to around 20hz. Not at huge SPL levels, but I think I can get around 85db, which is enough for measurements.

 

[Sonnie Parker]

No doubt the DIY sub would have more consistent results for you.

 

[bowl_actually]

No doubt the DIY sub would have more consistent results for you.

True, but I also need to qualify consistency as there are several components for this experiment.

Consistently the same frequency response, even if that is a curved, not flat response is the key. Consistency in volume level is not as much of a problem. We're just trying to understand how the room changes the frequency of the signal. So if it can produce the same frequency response, even if it's at varying levels, it's very useful.

But consistency in volume level would also be useful for other reasons, especially when you are working to resolve a problem. So even though that is not a primary piece of this particular exercise, overall it is still important.

In order to understand the base frequency response of this "gadget," I did some testing out in an open field. I have the benefit of living in a rural area and have an open field behind my house. So I can go out there and measure things in a semi-anechoic environment to establish a base line frequency response.

I want to put together a video of what I've done. If nothing else, it's been a fun science/physics experiment and helps to learn and understand more about acoustics in general.

For what I want to use it for, I think I'll wind up with a Sub. But it's still been a very fun thing to figure out and is something I could take to a room along with a portable recorder and do a quick/dirty initial test.