Sweep test questions

[Jonnyp]

Hi there.

I'm doing some "do it yourself" style sweep test to check how bad my room is and what acoustical treatments I need. I downloaded a tone generator program and set up a full sweep from 20 - 20000Hz. Then I recorded that with a condenser mic set in the listening position. Already I can see the problems visually from the wave form. My quetion is about the mic. Since the microphone itself has a certain frequency response how much would this affect my results? This didn't come up in anything I've read on the topic so far. The microphone is a CAD GXL3000. (great mic considering my budget)

Also the Monitors I have are not as strong in the low end as they should be. Not only have I found this to be consistantly the same with the monitors set up in differnt rooms, but other people with the same monitors have also said the same thing. They have good range, they go down to 30 -40Hz, but its the bass LEVEL that seems low. So should I not use an EQ to try and fix this problem first before doing test on the room? I thought maybe I could set up one Monitor with a mic directly infront then cover the whole thig with heavy blankets then try and tune the monitor itself. The Monitors are Hosa QZS 8Ps

And back to the microphone quetion. Should I maybe use a Decibel meter instead. (down side being that I wouldn't get a nice visual representation like recording with the mic)

[snellular5]

I think what your lookin for is a measurement microphone
correct me if I'm wrong.

[Jonnyp]

Is that what people always use for his kind of test?

[mshilarious]

Your measured response is subject to the frequency response of the mic, so you have to adjust for that. I wouldn't use it for RTA EQ adjustments on a PA, but to validate bass trapping it should work fine--the likely peaks and nulls in the bass response will be waaay larger than any effect of the mic's frequency response; the goal is to get them smaller.

[Ethan Winer]

Originally Posted by Jonnyp

Since the microphone itself has a certain frequency response how much would this affect my results?

See this:

Comparison of Ten Measuring Microphones

--Ethan

[Jonnyp]

Thanks for the info guys well appreciated.

Any more thoughts about trying to flatten the speakers themselves before hand? I know that isolating the speaker from the room isn't gonna be easy. I have a bunch of acoustic pannels I've already made. If say I took all these and surrounded the speaker and mic similar to what is shown in the reference microphone test, except go all the way around, would this be worth while?

Or should I not bother and use my best guestamation and boost the bass a little with the EQ (probably pull the rest of the frequencies instead of boosting)

[jhbrandt]

Jonnyp,

I don't suggest that you try to flatten your speakers, especially with EQ. You could add a sub. Maybe the very low freq resp that you quoted for the speakers is the cut-off frequency.

If you want to test your speakers response without coloration, for a poor man's anechoic chamber you must test them outside on a quiet day, surrounded by acoustic panels. Then using one of the mics suggested by Ethan (the behringer works good), pink noise - mic 3 feet away from the front of the speaker (1 meter aprox).. and this will give you a pretty good idea of what you are getting from the speaker... until your neighbors call the cops.

Back to your original question. If the coloration you noted is in the low frequencies (20Hz to about 300Hz), you have modal issues - corrected by trapping. If it's in the mid range from 400Hz to 3kHz, you are getting too many early reflections. These can be corrected with alternating patches of absorbent on either the walls or ceiling - creating a reflection free zone and adding diffusion.

Please post your room dimensions and we can calculate your modes for you.

[Jonnyp]

I did read that an EQ would colour the sound, I suppose this is the main reason for not doing this. I will look into some kind of sub.

I'm failry new to this stuff, I'm pretty sure I have a good understanding what modes are. But I don't really understant how you calculate them and then know what absorbtion is need from those calculations.

My room is L shaped with an 8ft. celing. I can't do anything on the roof because theres some crazy Italian stucko roof design. Some mud worker spent alot of time on that years ago.

I'll post a rough drawing of the Dimentions

[Jonnyp]

"You are only allowed to post URLs to other sites after you have made 5 posts or more."

I really like the forum but this seems a bit silly.

[Jonnyp]

http://i155.photobucket.com/albums/s...n/untitled.jpg

[Ethan Winer]

Originally Posted by Jonnyp

I'm pretty sure I have a good understanding what modes are. But I don't really understant how you calculate them and then know what absorbtion is need from those calculations.

Start here:

Graphical Mode Calculator

Mode calculators are not useful for assessing room treatment. They're meant for picking good dimensions in a room you plan to build or when you can change walls etc. As for treatment strategies, here's the short version which will get you 99 percent of the way there. All rooms need:

* Broadband (not tuned) bass traps straddling as many corners as you can manage, including the wall-ceiling corners. More bass traps on the rear wall behind helps even further. You simply cannot have too much bass trapping. Real bass trapping, that is - thin foam and thin fiberglass don't work to a low enough frequency.

* Mid/high frequency absorption at the first reflection points on the side walls and ceiling.

* Some additional amount of mid/high absorption and/or diffusion on any large areas of bare parallel surfaces, such as opposing walls or the ceiling if the floor is reflective. Diffusion on the rear wall behind you is also useful in larger rooms.

For the complete story see my Acoustics FAQ.

There's a lot of additional non-sales technical information on my company's web site - articles, videos, test tones and other downloads, and much more.

--Ethan

[Jonnyp]

Thanks Ethan, I'll be sure to read up on this somemore and try and get a better understanding of it.

Here are some of the pannels I've already made.

A friend managed to get four huge garbage bags of left over roxul insulation from a construction site were he works. They were just gonna throw it out so I got it for free! Its all just small 1ft. chunks but thats ok.