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Frequency Response Measurement - First Attempt

BmWr75

BmWr75

Top octave? pfffffftttttt
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Decided it is time I learned what "flat frequency response" sounds like. So, took the poor man's approach and used a RS SPL meter and the Bink Audio Test CD.

Placed the SPL meter on a speaker stand in front of my listening position. Ran through the test tones, recorded the dB readings, plugged them into Excel and normalized the data to 80 dB.

Did this on two sets of speakers being driven by two different preamps/amps (so this is probably not a fair direct comparison). The first speakers are Tyler Acoustics Reference Monitors with a pair of subs from the same manufacturer. The second speakers are Zilch designed speakers based on 1960s vintage Heath cabinets.

What conclusions should I draw from these results?
For the Tyler set-up, any adjustments needed to the sub crossovers?
For the biamped Zilch/Heath set-up, any adjustments needed to the Behringer active crossover?
 
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Note the RS meter does not have a flat response. There's a guide packed with them to tell you how much to add and subtract from certain frequencies in order to make their readings usable.
 
Thanks Celt. Found the RS SPL meter correction table. Here's the corrected frequency response chart.
 
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Something is up. Why do they both drop like a brick at 4000 on top? Do SPL meters register bass and treble the same way? Confused. As indicated these speakers would sound very dull and lifeless.
 
kfa888 said:
Why do they both drop like a brick at 4000 on top?
Click to expand...

Might my measurement position have caused this? The SPL meter was placed in front of my listening position. Which put it at the bottom of the Zilch/Heath woofers and between the Tyler Acoustics mains and subwoofers.

Think I'll rerun the measurements with the SPL meter at 1.5 feet in front of the centerpoint of the woofers.
 
Measuring frequency response of speakers is difficult to do, since actually you are measuring the combined responses of the speaker and whatever environment the speaker is in. And you are only getting it at a specific distance, so you are getting the specific response that results from interdriver interference at exactly the point where the mike it, and the treble response is affected by whether you are exactly on axis for the tweeter. Various other factors also matter.
I'd try a couple of runs at different distances and on different axises.
 
I have a behringer 8024 with noise generator and RTA functions plus the proper measuring mic, when I test speakers it usually shows relatively flat between 40-16000/18000 just like you would think it should from the listening position.
 
All boundaries, hallways, etc. will be seen by the meter. It's not perfect, but moving the speakers outside and eliminating many of those boundaries will help. Doing nearfield measurements is a must IMO when judging speaker response. Unless of course, when trying to equalize to the room.
 
Try measuring on axis with the tweeter. Set your RS meter so it's mic is looking right at the center of the tweeter. Many tweets get beamy at the upper end of the audio range, and that may be what you are seeing.
 
Here's a chart that compares the measurements taken at near my listening position and then at 1.5 feet directly in front of the woofer, slightly above its centerline. The blue line is for the 1.5 feet measuring location.

Measuring location certainly made a difference in the frequency response curve.
 
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I believe the measuring mics have a narrow field for eliminating some reflections etc. I bet SPL meters are much more broad path.
 
I've been interested in doing something similar myself, so I am enthusiastically watching your progress. Keep us posted!
 
Based on the measurements taken at 1.5 feet, am wondering if the chart indicates I should do anything with respect to changing the crossover point (currently at 1.6 kHz) or the tweeter gain settings??? Both of these are controllable on the Behringer active crossover.
 
make sure the mic is on axis of the tweeter, make sure you'ce selected "C wieghting" on the RS meter.

Other than that, measuring continuous wave near boundery reflections is very error prone. The more near-field you can get, the better.

If you're measuring for flat from your listening position, the result won't be appealing. It'll be mish-mosh of direct energy and reflected energy.
 
Pretty neato, huh? I've never done it with the R/S meter, though I did measure its accuracy once. I should do that again, since more members are now joining the ranks of "Curve Junkies." ;)

I offer these observations, consistent with what others have already advised:

1) I beleive you want to be measuring on the dBC scale, as dBA is attenuated at both ends of the spectrum, as I recall. The two curves may be found online.

2) The R/S SPL meter is not accurate, and corrections must be factored in at various frequencies, particularly in the region of ~7 kHz, where it significantly overstates the response. This has been studied by others extensively, and results from various researchers are also online. It's a statistical exercise; there is unit-to-unit and model-to-model variability, but with that knowledge, it's possible to generate reasonably accurate response curves. Even with inaccurate ones, so long as they are repeatable, you can still certainly see the results of changes.

3) Loudspeaker measurements are made under standardized conditions in order to obtain meaningful and comparable results. Generally, that's on-axis with the tweeter, at a distance of 1M. The speaker should be at least 1M away from all boundaries and other objects to minimize reflections, and the meter on a stand. A tripod will do; there's a threaded attachment on the bottom of the R/S, usually. The height should also be at least 1M from the floor and/or ceiling.

4) Results are usually reported with a logarithmic horizontal frequency scale, because that's how we hear, basically. Not a deal-breaker, but if Excel has that capability, use it.

5) There is information on measuring loudspeakers in Dickason to study. The "Bible" is D'appolito's "Measuring Loudspeakers," a difficult read, but well worth the effort for the definitive scoop on this stuff.

Welcome to the fascinating realm of loudspeaker science.... :yes:
 
Thanks Zilch, see my responses to your questions/comments:

1) Yes, am using the "C" scale.

2) Yes, am now using the correction factors found at this link.

3) Already thought about the tripod and plan to get one. Will try measuring next at the tweeter as you recommend.

4) Yes, the Excel charts are using a logarithmic horizontal axis. Correction, the Excel charts are not logarithmic on the horizontal axis. Can see how to make the vertical axis a log scale, but not the horizontal axis in Excel.
5) Read a book.... :scratch2:
 
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BmWr75 said:
Measuring location certainly made a difference in the frequency response curve.
Click to expand...
Yes, you're getting the hang of this, now. !.5 feet is a bit close; you need some distance for the individual drivers' responses to integrate. In large spaces, 2M is preferred, but the further back you get, the more reflections from the floor and ceiling come into the picture. That's why 1M is used; it's a compromise.

The exception is low frequency response measurements. Those are done extreme nearfield, 1/4" - 1/2" from the woofer dome. Try that also; slam-dunk accurate bass response for closed boxes up to ~200 Hz. See Paratima's experiments with that in my KLH-17 thread. He illustrates his method with a R/S meter there.

kfa888 said:
I believe the measuring mics have a narrow field for eliminating some reflections etc. I bet SPL meters are much more broad path.
Click to expand...
Measuring mics are omnidirectional, actually. The R/S mic has a shroud; I don't know the effect of that, but it's probably in the lit.

Fire up your 8024; let's see some experimental data here! :banana:

BmWr75 said:
Based on the measurements taken at 1.5 feet, am wondering if the chart indicates I should do anything with respect to changing the crossover point (currently at 1.6 kHz) or the tweeter gain settings??? Both of these are controllable on the Behringer active crossover.
Click to expand...
Change some settings and see how it affects the measurement results. You'll soon develop a correllation between what you hear and what the measurements say.... :thmbsp:
 
BmWr75 said:
3) Already thought about the tripod and plan to get one.
Click to expand...
If you're going to buy something, get a mic stand and mini-boom from Guitar Center or equal, ~$35. That gets the stand reflections out of the readings. You'll need a right-angle adapter for the 1/4" tripod mount on the R/S meter, which should be available.... :yes:
 
Download Room EQ Wizard. It's freeware and can use the mic output of the RS meters.

Makes everything you do pretty much automatic. You can specify sound card correction files and microphone correction files and it automatically applies them to the measurements.

It'll do a full range frequency sweep in about 5 seconds and plot it. Plus, it has all kinds of other measurements/calculations it'll perform.

Widely spaced measurements like that don't look too bad or with 1/3 octave, or more smoothing. But when one begins to look at the FR in 1/12 octave or more or with no smoothing, you really begin to see the bad effects of combing and room modes.

Here are graphs of exactly the same measurement. One has 1/3 octave smoothing, the other is no smoothing.

smoothing_s.jpg


wosmoothing_s.jpg
 
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