Improving frequency "dips"

[Richard Daigle]

When doing an impulse response in a 11x13x8 room which has broad band absorption, a frequency dip at 286 hz appears, I also have other dips at 323,308,297,290 hertz when measuring at 5 other locations in the room.
If someone gives me their email, I can send impulse response which can viewed with Room Eq Wizard.

Questions:

How do I remove or improve these dips?
I'm assuming that the peaks can be improved by making an absorber for the peak frequency,
since dips may be caused by the same frequency which is out of phase in the room, what would happen if I made an absorber at the same frequency as the dips?

Richard

[bpape]

In a room that size, most likley a tuned absorber is not your best bet. It's simply not big enough to be an effective solution IMO. If your dips are way up toward 300Hz, I'd also want to know how severe they are and specifics of which positions in relation to the room each occurs. Which speakers are you using to take these measurements - just the front 2? All 3? All 5?

Bryan

[Sonnie]

Hey Richard... you can save those REW graphs and upload them to our Image Gallery then link to them in your post. Everyone can see them then.

[Ethan Winer]

Richard Daigle wrote:

How do I remove or improve these dips?

Bass traps. In a room that size and shape you'll need lots of them. :raped:

I'm assuming that the peaks can be improved by making an absorber for the peak frequency

Broadband absorption is better in a room like yours.

--Ethan

[Richard Daigle]

By the way Ethan, thanks to your comment, I resolved my other problem of "the shift in frequency which occured when I pressed the low cut". My aux input was causing a feedback problem between the input and output when using RAD while playing white noise, when I used the Mic input, my problem was resolved. I tried to reply a few times in the thread, but for some reason it didn't append to the message. I don't know if there is a limit to the amount of threads in a thread.

In reply to Bryan, my speakers are rated 25 hz to 20 khz, one large speaker (I assume it's a subwoofer)
and 2 small speakers.
I placed the speakers around 1 1/2 to 2 feet from the mike, the reason the mike was so close to the speaker, is because I wanted to simulate the position of a guitar, and find the best frequency response in the room. I measured dips at spots (1)286 hz,(2)323 hz,(3)308 hz,(4)294 hz,(5)290hz,(6)294 hz. The six spots measured in relation to one another in the room are front (spot 1 and 2) middle (3 and 4) back (5 and 6) evenly spaced. The mike is not a calibrated mike, but the speaker is compensated for 1 ms (compensation before reverb of the room). White Noise is played for 1.365 seconds when creating impulse.
I have since learned how to use REW and have put the jpeg image spot1 under Richard Daigle in image gallery. Thank you for your reply.

Have a good day,
Richard

[woodworld soundz]

Are you calibrating your front stereo speakers to 83 dB? That is first left with uncorrelated pink noise at -20dB DBFS to 68.2dB by using your amp. Then the right. Theoretically in ideal situation when summed together it would give you 83dB. Never is though. Usually when summed would give you 72/ 73 dB. Why 83 dB? This is the optimal SPL level for the ear that gives a flat frequency response across the audible frequencies. Further info on this and how to set up a 5.1 surround system can be found at digital village.com by the Audio Master Bob Katz. Really easy to understand. As far as the dips around the 250 Hz mark (1/3 octave band) this is a frequency that the ear has very large difficulties in differentiating with. This is due to the physical construction of the ear. Further information on this can be found in F. Alton Everests seminal work on Acoustics. In a room this size and at the problem frequencies that you are having, this is a result from the crossover frequency from the direct influence of room size dimensions to that of it's room modes. This is always a difficult one. If the peaks and troughs in the troublesome 1/3 octave bands are greater than 2dB then try first by rearranging the furniture to see if there is any difference. If not, then there are lots of sites showing you how to build simple, but effecient bass traps.

learning by our mistakes, allows us to go on and make bigger ones.

[bpape]

The only place(s) that matter in terms of frequency response is where you'll be sitting and listening. I can move a mic to a position in ANY room, treated or not, and get nasties. But, it doesn't matter. It only matters what it sounds like where you listen.

That's the trick - to get the seat in the right place, the speakers set accordingly to match with that, the sub placed and integrated properly, THEN take the measurements at your seat and see what it is that we have to deal with and what's causing it.

Bryan

[woodworld soundz]

Totally agree with you on that one. In otherwords, just trust your ears and have fun....

[Richard Daigle]

Thank you for your replies,

I'm now building my third panel absorber, all 3 of my panel absorbers will absorb between 270 to 310 hz, I used 1/8" plywood, over 1" to 1 1/4" frames with 1/4" MDF for bottom, I've made them portable, so I can move them around the room, if the sound doesn't improve I can always remove them. One day I'm
planning to build a bigger studio, and what I don't use in the small studio, I'll use in the bigger studio.
When I'm done, I'm going to do a recording with a martin guitar and post it on a web site, for evaluation as to whether or not the quality is good enough for radio.

Have a good day,
Richard

[bpape]

What did you use for the absorbtion inside then since the cavity is so shallow?

Bryan

[Richard Daigle]

I used rigid insulation 1/2" to 3/4" wide (cut out of 2" rigid insulation) and I then used caulking to hold the ridgid insulation in place. The panel absorber are 2' X 6'.
The biggest problem I have is the 1/8" plywood which has a tendency to buckle inwards, this changes the frequency of absorption, any ideas?

Richard