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initial Frequency Response - Now what?

post #1 of 28
Thread Starter 
I am starting to do some room measurements for the first time in my dedicated home theater. Here is my initial frequency chart below. From what I have read I want the graph to be as flat as possible. I have a little bit of stage dampening to do yet but this is what I have so far.



What I don't know is what to do next. I plan to add some acoustic panels at first reflection. Then finish up with some bass traps behind my AT screen.

What does this initial chart tell you and what do I do next to get a flatter response?
post #2 of 28
Quote:
Originally Posted by edmove View Post

I am starting to do some room measurements for the first time in my dedicated home theater. Here is my initial frequency chart below. From what I have read I want the graph to be as flat as possible. I have a little bit of stage dampening to do yet but this is what I have so far.



What I don't know is what to do next. I plan to add some acoustic panels at first reflection. Then finish up with some bass traps behind my AT screen.

What does this initial chart tell you and what do I do next to get a flatter response?

You do not need any HF absorption. If you are going to add panels, make them thick and with a reflective skin. Same for the bass traps.
post #3 of 28
+1. That's quite a bit of HF roll-off. What are the room dimensions, and what sort of furnishings/coverings/rugs now?
post #4 of 28
Does your receiver have eq? There is a tad to much 50 hz, but your low end is really good actually. Turn up the treble a LOT. You're about 20 db down at 20khz. What kind of speakers? It'll be hard to bring up that much high frequency.
post #5 of 28
Oh and how did you measure? It's possible your mic is off above 10 kHz making it look worse than it is.

And toe in your speakers if you can.
post #6 of 28
If the mic is pointed straight up, aim it at the speaker you are measuring. Measure each speaker independently. Measure the sub by itself, and then measure the blend at the crossover.
post #7 of 28
Quote:
Originally Posted by edmove View Post

Here is my initial frequency chart below. From what I have read I want the graph to be as flat as possible.

Yes, but the graph you showed is misleading. For assessing low frequency response you do not want to use third-octave averaging. That just hides all the detail. Below is a graph showing the exact same measurement with 1/3 versus 1/12 octave smoothing. This article explains the best way to display the various graph types (and many related topics):

Room Measuring Primer

--Ethan

post #8 of 28
Thread Starter 
My speakers are Polk RT55 for left center and right. For surrounds and rear I only have some in-wall Monoprice speakers which are okay for now. Subs I have two M&K v75 IIs. Entire front is behind xD screen. Room size is about 12' wide and 20' long. First row that measured at is 11' away.

My mic was slightly up but not straight up. My Yamaha XV-1065 only has treble abc bass adjustment and they at flat right now. I'll adjust those and measure again. I will also post at 1/12 smoothing as suggested.

I'll adjust and get a new graph up this afternoon.
post #9 of 28
Quote:
Originally Posted by edmove View Post

My mic was slightly up but not straight up.

That may explain the roll off of high frequency. Try pointing the mic straight towards the tweeter and see how it measures assuming your speaker height is set so that the tweeter is at ear level when sitting down. Also try different toe in angle of speaker to see how it changes the high frequency response.
post #10 of 28
Thread Starter 
Okay, mic pointing directly at front stage. I am resting it on the cushion of the theater couch. So right at ideal seated position. Before had it wedged between center seating which gave a slightly elevated position. Other adjustments was setting the treble up to +8 and leaving Bass at +0. In addition I also ran the setup on my receiver and selected the "Flat" setting. I think before I had a "Natural" setting selected.

What do you think now?



I assume that if I picked up the Behringer DSP1124P Pro I could make the adjustment at my 30Hz dip? Would bass traps help that specific dip?

What option do you recommend for bumping up the HF 2-4 db? Or do I need to do some flattening first at the HF?

I did not mention my furniture yet. But you can see my build link in my signature. There are theater seats in a back level. For now I have a traditional chair and a leather couch in the front of the room. 3 reclining theater seats are planned to replace those.

Here is the low end zoomed in from 0 to 300Hz

post #11 of 28
Thread Starter 
Quote:
Originally Posted by bcruiser View Post

That may explain the roll off of high frequency. Try pointing the mic straight towards the tweeter and see how it measures assuming your speaker height is set so that the tweeter is at ear level when sitting down. Also try different toe in angle of speaker to see how it changes the high frequency response.

I will try to flip the speakers upside down. This would essentially bring the tweeters down about 18 inches. We will see what it does to the HF. Here is an early photo of my front stage build. Only thing is I now have another RT55 as the center and not the speaker in the photo. All carpet is in now as well.

post #12 of 28
To me that looks pretty good except for the 40 Hz null. That null is most likely a room mode, and bringing it up will be difficult. Lots of bass traps will help, though there's fairly low. It may be easier to move the furniture to get out of the null.

Peaks and nulls are caused by sound waves bouncing around in the room. A peak is when they add, and a null is where they subtract. Because of other factors, like vibrations in the walls and ceilings, peaks rarely exceed 10 - 20 dB and can usually be brought down by EQ. Nulls are harder to handle since the signals can cancel quite well, making a very deep (and thus difficult to correct) valley.

If you can't move the listening position, then you could bring the subs out so you are near-field. Note that at 40 Hz a wavelength is about 28 feet; you may need to move a couple of feet to see a big difference. For starters, measure at spots perhaps 2' forward and back of your first set and see what happens.

You should also follow up with Ethan about bass traps -- you may be able to make a significant improvement by investing in (or building) some good bass traps.

HTH - Don
post #13 of 28
Quote:
Originally Posted by edmove View Post

Okay, mic pointing directly at front stage. I am resting it on the cushion of the theater couch. So right at ideal seated position. Before had it wedged between center seating which gave a slightly elevated position. Other adjustments was setting the treble up to +8 and leaving Bass at +0.

You really don't want to do that (it'll sound unnaturally bright) and don't want a flat reverberant field.

The problem here is that your ears are looking at the direct sound, similar reflections that are coming later, and combining their spectrums with some sort of time-based weighting to determine timbre. They expect some roll-off in the reflections at high frequencies because those are more readily absorbed in natural surroundings and have higher losses with distance (listen to what happens to highway noises as you get away from one - you hear more low frequencies when you get distance and things like trees between it and you).

The microphone is just adding all the reflections together equally (apart from its pickup pattern, which we'll ignore to simplify the discussion).

Quote:
What option do you recommend for bumping up the HF 2-4 db? Or do I need to do some flattening first at the HF?

Leave the high frequencies alone unless you have room problems you can't address acoustically (speakers in a big room with lots of sliding glass doors, tile floors, etc. A cut helps but doesn't work as well as fixing the problem). Don't make fine grained corrections.

Low frequencies are different. Your ears need a cycle to pickup the tone and are picking up reflections before they're done with a full wave of direct sound. Perception seems to have more to do with the steady state measured response.

You just want to consider what's going on when you want multiple "good" seats, where the interactions of room modes can be radically different (even 6" away) and aim for the best average corrections.
post #14 of 28
Thread Starter 
DonH50 and Drew thanks. Both your comments, like the others, are helping me greatly. I got to thinking how the impact of my sub crossover point might impact the reading. Here is what I found. Pretty interesting stuff.

This first one sub is set at 125.



Now set to 100



Now set to 75



It is very interesting to see the impact of the crossover point has on nulls in the low end. But what I find even more interesting is the fact that my null at 40Hz is pretty consistent across all of them. Does this mean it is probably a result of something in my room?
100 really seems to be the best option for my current setup.
post #15 of 28
Thread Starter 
Two final charts for the day. I had my air compressor in the room so I removed it. That give me a little lift on my 40Hz null. Here is the reading after the compressor removed.

Two subs in front of room.


Now I decided to put one of the subs in the back(right) of the room.


There is some improvement to moving the sub at 40Hz. Do you think it is enough to warrant the move? I really do like having everything concealed on my front stage.
post #16 of 28
Thread Starter 
This might be better to view. Yellow line is one sub front and one sub rear. The green is two subs up front.



I am guessing not a lot of different to warrant the move of one sub. Like I mentioned hiding them in the front stage is aesthetically a better option.

Knowing that I have a pretty decent curve here I will finish up my last remaining acoustic treatments to my stage, add some bass traps to the corners and then start taking measurements again.

Thanks to everyone for the tips and advice. It really helped me understand what I am looking at.
post #17 of 28
Quote:
Originally Posted by edmove View Post
Do you think it is enough to warrant the move? I really do like having everything concealed on my front stage.
Try putting one sub on top of the other at the front and see what happens. Try both left and right location for this and pick the flattest bass response. Also, I would turn up the sub just a little.
post #18 of 28
post waterfall plots (0-300hz)

flat freq response is only one side of the story --- the time domain is equally important, as it will show modal ringing.

also, you may not need "HF absorption", but you would still benefit from absorption at the first order/early reflection points. side walls + ceiling.
post #19 of 28
Thread Starter 
Quote:
Originally Posted by bcruiser View Post

Try putting one sub on top of the other at the front and see what happens. Try both left and right location for this and pick the flattest bass response. Also, I would turn up the sub just a little.

I am at +3 on last results. I'll try higher and see what it does to the curve.
post #20 of 28
Thread Starter 
Quote:
Originally Posted by localhost127 View Post

post waterfall plots (0-300hz)

flat freq response is only one side of the story --- the time domain is equally important, as it will show modal ringing.

also, you may not need "HF absorption", but you would still benefit from absorption at the first order/early reflection points. side walls + ceiling.

If I can figure out how to do the waterfall in FuzzMeasure I'll post them tonight.
post #21 of 28
That looks better now. I think the sub level looks good, but bass level can be a little personal. I don't like my bass cooked.

Try putting the sub xo back to 75 or 80 and change the subwoofer phase until the xo null is gone. Then you get the advantage of a lower xo and keep it flat. 100 works because it's in phase is all.
post #22 of 28
Thread Starter 
Quote:
Originally Posted by bcruiser View Post

Try putting one sub on top of the other at the front and see what happens. Try both left and right location for this and pick the flattest bass response. Also, I would turn up the sub just a little.

Do I need anything between the subs If I stack them? Never heard of stacking them before. I'll give it a try.
post #23 of 28
Thread Starter 
Quote:
Originally Posted by tuxedocivic View Post

That looks better now. I think the sub level looks good, but bass level can be a little personal. I don't like my bass cooked.

Try putting the sub xo back to 75 or 80 and change the subwoofer phase until the xo null is gone. Then you get the advantage of a lower xo and keep it flat. 100 works because it's in phase is all.

Do I need to keep each sub in the same phase?
post #24 of 28
Quote:
Originally Posted by edmove View Post

Do I need anything between the subs If I stack them? Never heard of stacking them before. I'll give it a try.

You can put pieces of rubber or felt pads at 3 or 4 spots. If it's only temp, just use a sheet of thick fabric or rug. Stacking is to use one best spot for room response while maintaining the output of 2 subs.

Quote:
Originally Posted by edmove View Post

Do I need to keep each sub in the same phase?

Yes, if they are both at the same location such as stacking.
post #25 of 28
Thread Starter 
Unfortunately I don't have a phase dial on my subs. Just a switch to reverse the phase. I plan on Tuesday to make sure my mains and center are in phase with the subs.
post #26 of 28
Use the Distance setting for the sub in your receiver. It's a better tool for setting the relative delays between the speakers and sub than a phase control.

Craig
post #27 of 28
Thread Starter 
Quote:
Originally Posted by craig john View Post

Use the Distance setting for the sub in your receiver. It's a better tool for setting the relative delays between the speakers and sub than a phase control.

Craig

I have only run the auto setup on my receiver. I assume there is a manual option as well. I'll check that out. Thanks.
post #28 of 28
I like having a phase (or distance) knob on the sub for these situations. If you have one sub output from your AVR, or just one sub processor even if there are two outputs, you need some way to adjust the phase of each sub independently if they are not co-located, or at least equidistant from the listener.
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