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Help needed using ETF (fool with a tool)

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I've read everything I can find on using a parametric EQ to attenuate spikes in the in-room bass response and I recently purchased ETF and a Behringer 2496 DEQ to try my hand at it.
While I am familiar with reading freq response graphs and waterfall plots, I just picked up the ETF software, and frankly I'm not getting the results I expected.
Even without putting my subwoofer into the mix, bass in my close-to-full-range speakers sounds hooty, boomy and resonant at times. However, looking at the ETF graphs, I'm not seeing the large spikes in the overall room response I thought I would, given what I hear. The ETF help files talk about using the gate time and time slices to help find the resonances. I think I know what I'm looking for, but again the resonances don't appear to cause any large spikes in the overall freq response. I've tried reducing the broader humps in the freq response, but all I get doing that is lower bass levels that still sounds ill defined and resonant.
My room is 13 x 19 X 8.75 feet.
I hear it is good to take multiple measurements, but I've also read that using a PEQ only corrects the response at one position. Where in the vecinity of the listening position do I take the extra measurements?
Any advice or a point in the right direction would be appreciated.
Cheers,
- Tim

Follow Ups:

Re: Help needed using ETF (fool with a tool) - AVsponge 18:21:42 05/18/04 (3)
Ran the measurements again with the 5s sweep tone and got largely the same results as I did with the short tone. The good news is that the measurements are consistent and looking at them again, I was able to tame one of the resonances I hear with a -8db 1/7 octave filter at 80Hz, but there is still something going on in the 40-80Hz range that I am not able to get read on by looking at the measurements. To complicate matters, the two main speakers are located asymmetrically within the room, which gives them very different LF extension. I EQ'd each speaker seperately, but I wonder if this is the best approach. I wish I could move them, but they are in the only location that allows the rest of our furniture to fit without blocking a door way (one to the garage, one to the kitchen, and sliding glass to outside).
I'm going to pull my freakin' hair out trying to pinpoint the problem frequencies at this rate. I thought ETF would make the process easier, but I am truly a "fool with a tool" at this point. Every time I think I've made headway with my EQ settings, I play a different song and I hear different bass problems. Arghh!
- If you have a frequency sweep just sit down and listen to it (when in doubt use your ears) - Richard BassNut Greene 09:27:37 05/19/04 (2)
  The ultimate goal of a flat bass frequency response can be simulated by listening to a slow sinewave frequency sweep from about 20Hz. to 100Hz. If you do not hear any unusually weak or unusually loud bass frequencies during the sweep, then you are hearing a relatively flat bass frequency response.
  A more demanding test is rapidly repeating bass tone bursts,
  or at least a rapidly repeating kick drum.
  A good subjective bass test CD is from Linkwitz Labs (see hot link)
  You should be able to get the frequency response at one listening position (an average of two measurements made eight inches apart to represent your two ears) better than +/-6dB measured using a slow sinewave sweep.
  Most of the bass frequency response deviations after EQ will be caused by one or more frequency response troughs that can't be
  EQ'ed away.
  Fortunately your ears are MUCH more likely to notice LOUD peaks under 80Hz. that distract you from the other frequencies in the music ... while "missing music" caused by nulls is likely to be distracting only if you happen to be concentrating on the bass guitar.
  (Ethan frequently argues with me about this statement but he's biased because he & his business partner are musicians who obviously focus on the bass line and go berserk jumping up and down and waving their arms when a bass note appears to be missing due to a room null ... while most non-musician audiophiles don't even notice unless a bass note is much too loud.)
  You may have to move your ears up to one foot (sometimes more) to get your ears away from one or more deep nulls if you want to obtain a bass frequency response better than +/-6dB.
  Weak frequencies (at least some of them) will become louder if you move your ears further from nulls (1/4, 1/2 and 3/4 of the way between opposing walls are approximate locations for the very
  audible and deep axial room mode nulls).
  The loud portions of the frequency sweep can be reduced with an equalizer.
  Measuring sine wave tones is only necessary to set EQ controls -- listen to a sinewave sweep, bass tone bursts and music to test the results.
  - Linkwitz Labs bass test CD (Open in New Window)
  - Re: If you have a frequency sweep just sit down and listen to it (when in doubt use your ears) - AVsponge 11:48:49 05/19/04 (1)
    Hello Richard -
    I have listened to sine wave sweeps (Delos "Surround Spectacular") as part of my quest to achieve better in-room bass. Listening got me in the ballpark of the problem frequencies, but I needed a way to pinpoint and quantify them if I want to use narrow parametric filters to address the problems. In fact, I originally used a sine wave sweep and an SPL meter to plot the bass response in my room, but it is a laborious process, which was what lead me to purchase the ETF RTA program.
    You wrote:
    "Fortunately your ears are MUCH more likely to notice LOUD peaks under 80Hz. that distract you from the other frequencies in the music ... while "missing music" caused by nulls is likely to be distracting"
    I fully agree. This is exactly how I perceive low frequencies. Sure I can hear the missing music, but it doesn't call attention to its self like a peak.
    You wrote:
    "The loud portions of the frequency sweep can be reduced with an equalizer."
    Yes, of course, but this is where all of my confusion lies. When I look at the results of the measurements I've taken, other than a peak at 80Hz, the rest of the bass, while not smooth, is equal or lower in SPL than the midrange. Yet, even after notching out the 80Hz hump the bass still sounds boomy and ill defined at times.
    You wrote:
    "Measuring sine wave tones is only necessary to set EQ controls -- listen to a sine wave sweep, bass tone bursts and music to test the results."
    This comes back to my circular problem. I take the measurements, set my PEQ to reduce the visible peaks, then I listen to music and find that the bass doesn't sound right. Even if I do listen to a sine wave sweep and hear some "hot notes", how do I know with enough accuracy which frequencies are causing the problem, if I can't pinpoint the center frequency and width of peak with an RTA? I must be doing something wrong.
    Cheers,
    - Tim
    - There are lot's of bass problems besides peaks: Buzzing room objects and windows, resonating floors, harmonic distortion - Richard BassNut Greene 17:49:08 05/19/04 (0)
      + the wife bellowing T*U*R*N ...... I*T ...... D*O*W*N,
      the dog howling ... and Police sirens approaching after your neighbor called them about your excessive noise.
      You can't plot a bass frequency response with a sinewave sweep -- it's just a tool for listening ... and estimating total bass SPL deviations (+/-xdB) when you use a sound meter with fast response.
      I can get pretty close to locating the center frequencies of bass peaks using 1/6 octave spaced sinewave tones from my Stryke BassZone test CD. 1/12 octave is even better. Every Hz. for nerds who must have "perfection".
      I double check results with the ultra slow 5 minute sweep from
      1 to 100Hz. on that disc.
      My corrections are usually done with several 1/6 octave bands, sometimes 1/10 octave -- rarely 1/4 octave -- and never 1/3 octave (so far)
Re: Help needed using ETF (fool with a tool) - AVsponge 14:51:02 05/18/04 (0)
BTW - I meant to ask, what is my reference point supposed to be for balancing the LF with the rest of the spectrum? The average midrange level? The upper bass? I've read that the target room curve should be down 6dB in the treble and up 6dB in the bass, which makes sense since every measurement I've taken shows a declining bass to treble balance. How does that factor into smooting the bass response?
A bit off the subject, but still related to getting good bass, how important is it to install the spikes on my floorstanders? I temporarily have them on rug coasters so I can move the beasts (100lbs ea), but I've always wondered how much truth there is to getting better bass response from using floor spikes.
Re: Help needed using ETF (fool with a tool) - AVsponge 11:23:39 05/18/04 (1)
Thanks to all for info. Unfortunately, the graph I inserted in the orignal post isn't working (it worked last night). If it were, you would see that I am going off of the LF response graph (not the one that shows the reading in 1/2 to 1/12 octave increments). I've looked at the response with the parameters set to 260ms gate time with 20ms slices and with just one slice set to max gate time (360ms?). As far as I can tell, this graph is plotted in increments of 1-2Hz up to 200hz, so the resolution is there.
The neasurements were taken using the short sweep tone, though I have tried the longer one and didn't see much of difference in the results. I'll try it again to be sure.
Going by ear, when I play a recording that excites a particular bass note, I'll watch the input RTA on the behringer to determine the general vicinity of the offending note. Over the past few days, the trends seem to be at 40, 60, 80, and 100Hz, but the ETF graphs don't show problems here. At least not to my untrained eye.
This brings back to my original questions. How do I know I am taking the proper measurements and what am I looking for in the graphs? The overall Freq Response (long gate time, with single slice) or frequencies where the multiple time slices stack up (long decay)?
Rives Dude ;-P - Can I send you an ETF file to look at so you tell me how you would interpret the measurements?
- Re: Help needed using ETF (fool with a tool) - Rives 18:36:17 05/18/04 (0)
  If you purchased the software from us, we will do some basic analysis, just to insure you are getting reasonable measurements and a few tips on how it works and what you are looking at. If you purchased it from ETF, Doug Plumb at Acoustisoft is very helpful and I'm sure will help you out.
Re: Help needed using ETF (fool with a tool) - Rives 10:06:52 05/18/04 (0)
There are a couple of things you need to do with ETF to make it a little more user friendly. One is check your gate time. You would like a relatively long gate time, like 100 ms, for the best bass response. However, with such a long gate time you can have a noise problem, so this may not work in your environment and in some cases you have to really shorten the gate time. There are a few ways of determining this, but looking at the impulse response, you can see when the noise is a steady state, and your gate time should be inside (or close) to that. Be sure on all bass response measurements you use the long (5s) and sweep measurement. It's much better than the MLS method for low frequency--it's probably better for everything, but certainly for lf.
Second is interpretation, it's much more difficult than it looks some times. There is a plug in for a psychological response, which makes interpretation much easier. It actually automatically sets the gate times relative to frequency response (like the human ear functions). It gives you a 2 D graph that is much more easily interpreted than the waterfall, ETC, or steady state response.
As a small point of interest our company sells a similar product to ETF called BARE. This is for Bass and Room Evaluation. It is really very similar to ETF, and if you have ETF, BARE is probably overkill and not warranted, but it does include the psychological response filter. BARE is actually the software used by our dealers to calibrate the PARC. Anyway, I'm not trying to deliver a commercial--just trying to let people know about this if Bass response is what they are after.
Here's what subwoofer EQ can do for you - Richard BassNut Greene 09:37:04 05/18/04 (0)
I'm going to assume you're measuring steady state sinewave tones no more than 1/6 octave apart under 80Hz. (1/12 octave spacing is even better), and perhaps measuring every Hz. over 80Hz. ... and using a long enough time gate so that standing waves have time to develop.
After setting the EQ, I recommend listening to a very slow sinewave
sweep from 20 to 100Hz. as a frequency response check. Then listen to music with bass guitar or acoustic bass solos, if you have any.
I recommend averaging two measurements spaced about 8 inches apart to represent an average of the SPL that your right and left ears hear.
The difference between the two measurements has been up to 6dB
in my experience -- that's a lot (at very low frequencies +6dB seems subjectively twice as loud ... or more!).
Below about 80Hz. the most common bass problem in my experience is a bass boom or two or three caused by standing waves between opposing walls. Lots of bass traps are required to tame bass peaks under 80Hz. (especially for the 50Hz. and lower peaks caused by front-to-rear-wall standing waves that are the most commonly reported problem in my experience).
If you have a subwoofer for under 80Hz. (or 70Hz. or 60Hz.) bass,
an inexpensive dedicated digital subwoofer parametric EQ is the most efficient tool to reduce bass peaks. For full-range speakers high-end audiophiles may prefer an expensive analog parametric EQ like the one Rives sells. While bass traps are more effective (they address peaks and nulls at ALL listening positions), they're not efficient at very low frequencies so you'll need a large quantity of traps ... which means $$$ ... and they tend to make our wives go berserk.
Above about 80Hz. the most common bass problems (often not noticed, but they certainly do exist) are comb-filter nulls. The most efficient tool for these are bass traps. Because the bass peaks and nulls are so close together above 80Hz. it's very difficult to measure the peaks precisely to set an EQ correctly ... and the nulls would still remain so you would not have solved the most important problem using EQ.
Using EQ, you should be able to reduce bass peaks under 80Hz. measured at your listening position so that bass frequency response is +/-5dB or better. This may require you to move your ears up to one foot if you happen to be sitting in, or near, a standing-wave null because EQ can't fix that.
In general, anyone sitting close to you is likely to benefit from EQ
of your listening position, although not as much as you will ... while someone sitting far from you is at risk of hearing a frequency response worse than before EQ.
If you address a front to back wave standing wave peak, then other listeners sitting the same distance from the front wall will equally benefit.
If you address a floor to ceiling standing wave peak, then other listeners whose ears are a similar height off the ground, or lower, will benefit.
If you address a side wall to side wall standing wave peak, then
other listeners sitting the same distance from the side walls will benefit.
Re: Help needed using ETF (fool with a tool) - Ethan Winer 08:44:56 05/18/04 (0)
Tim,
> looking at the ETF graphs, I'm not seeing the large spikes in the overall room response I thought I would <
If you're looking at the 1/3 octave display then you are missing 90% of the detail. At the minimum, select 1/12 octave. In fact, select both 1/3 and 1/12 so you can see exactly how coarse 1/3 octave really is!
To obtain the true response you need to use the Low Frequency display. If you then set the gate time to maximum it resolves the LF response to better than 1 Hz. And that's what's needed to really see what you have.
--Ethan

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