Can Someone Help Me Interpret My Graph?

[Kenryan]

I think I managed to get an actual reading from my SPL...finally! I ended up using my cheap on-board soundcard (SoundMax). Anyway, can someone help me interpret the resulting graph? From what I can guess, this is telling me that at my listening position, sound up to 180 Hz is not detected/heard well. Starting at around 180 Hz, there are various peaks (up to a max of around +10 dB) and valleys/nulls sometimes as far down as -25 dB (around 300 500 and 600 dB). Am I reading this right?

Thanks!

Ken

[brucek]

can someone help me interpret the resulting graph

Change the graph axis to the standard we use here at the Shack for easier comparisons.

Use Vertical = 45dB-105dB and Horizontal = 15Hz to 200Hz.

brucek

[Kenryan]

Brucek,

Is this what you meant?

Ken

[Kenryan]

Here is another one starting at a different point to see some key frequencies.

If this is right, I am surprised that a bedroom studio (high ceilinged and devoide of 90-degree ceiling joins as it is) displays LESS bass rather than more. Does it jibe with what you've seen?

Thanks again.

Ken

[brucek]

Is this what you meant?

You have the graphs set to Linear instead of Logarithmic.....................

[Kenryan]

OK. How about now?

Ken

[brucek]

Yep, looking good, except for one thing. When you present frequencies that are above a subwoofer range, you should turn on Smoothing to remove comb filtering. Smoothing is used for full range measurements, where reflections can cause comb filtering that makes it difficult to see the underlying trend of the response. It is rarely used for low frequency measurements as it would obscure the true shape of the response and so not allow accurate correction filters to be determined.

You'll find this feature in the left hand panels of REW (hidden stuff). Usually 1/3 filtering is best.

Obviously these are your mains only. They certainly appear to drop off rapidy below ~150Hz. Is this what you expected?

Anyway, let's see that smoothed response.........


brucek

[Kenryan]

brucek,

OK, here it is with 1/3 octave smoothing. And to answer your question about my expectation....I don't think it IS what I expected. On my other forum (Tweak's Home Studio stuff), they all but guaranteed that in a bedroom listening situation (a recording control room), I would expect to see resonances in the bass frequencies. So I expected to see more of the stuff in the 80-200 Hz range be ABOVE the 75 Hz line. I am confused as to why wo much if it is below....and some of it WAY below that line. Any thoughts? Does this look right for a rectangular room that is about 12x14x10 feet (10 feet high)?

Ken

[brucek]

First let me say that you're being kind to yourself by choosing a vertical axis of about 20dB-130dB, instead of my recommended 45dB-105dB. It makes the plot look better than it really is. I know I'm being a nuisance about this, but you'll get more out of a graph if you keep consistant with a standard set of axis which reveal a proper resolution. For your mains it's fine to use a horizontal of 20Hz-1000Hz because you're looking at the room effects. I would always use a vertical of 45dB to 105dB, since the target is 75dB.

But that aside, I think we're looking more at a set of mains that is rolling off quite a bit starting about 150Hz, and I don't feel it's the room causing it at all. It looks like a natural rolloff to me. What is the spec on your speakers (i.e. 50Hz-22KHz +/- 3dB). They have very poor bottom end. You might want to eliminate the room somewhat and do a nearfield response. Place one speaker in the middle of a large room (or outside if that works for you) and place the mic about a foot away (depending on their size) and do a response of a single speaker.

Your friends at Tweaks are quite right when they say you should expect to see resonances in the bass frequencies. They mean from 10Hz to 80Hz. Resonances are caused by room modes which are a result of the size of a room. There are lots of programs to graphically show a room resonances of the axial, tangential and oblique modes. The axial dominates and so most programs will show those frequencies. So, if you had a room that was 18ft(L) x 12ft(W) x 8ft(H), there would be resonances at ~31Hz, 47Hz and 70Hz respectively. You have one around 31Hz in the graph you posted, so I suspect the room has a 12ft dimension in it. (oops, just looked at your post and one dimension is 12ft........)

So, you can see these resonances will be situated (by nature of standard room sizes) between ~20Hz to ~80Hz. Between 80Hz and 500Hz the room still has an effect, but more from reflections. After ~500Hz the speaker itself dominates.
brucek

[Kenryan]

Doh! I didn't realize that I was zooming and affecting those settings. I want to thank you SO much for all your patience in helping me understand all this.

BTW, my speakers are Alesis Monitor Ones from about 9 years ago. The specs say: Frequency Response: 45 Hz - 18 kHz, ±3dB. I don't know why it is showing such a bad response.

Anyway, thanks again for all your help.

Ken