SPL meter cal looks way off...am I wrong?

[Nuance]

I've attached a picture of my calibrated soundcard (blue line) with the trace of my calibration for my SPL meter (black line). I noticed that with the .cal file loaded the response drops off sharply after 80Hz. I assume it shouldn't do that?

Why does it look like that and how can I fix it? Perhaps it is suppose to look like that?

I know when I clear the SPL meter .cal file and check levels it looks normal, but when I load the .cal file it drops again. I attached an image of each to compare.

Thanks much!

No .cal file loaded:


With SPL meter .cal file loaded:

[brucek]

am I wrong?

Only in your interpretation of the graphs.

See the information I've added below.

Note the different scaling used and how it can show a different perspective.

Your files are perfect - get on with your measuring.

When you post a regular graph once you've taken a measurement - use a scale of vertical = 45dB-105dB and a horizonal of 15Hz-200Hz.





brucek

[Nuance]

Thanks, Bruce. As always, you are the man!

I figured everything was okay and went on to measuring anyway last night.

Well, I might as well post my before and after plots to see what you think. This is the first time I've done this, so I think it's pretty respectable.

These plots are with my mains from 10-200Hz. I did the subwoofer first, then added the mains.

Without any EQ:



With BFD EQ'd:



Thanks for everything, Bruce.

[brucek]

to see what you think

The after looks quite good.

I'm wondering about where the sharp dip went from the sub around 65Hz. If it went away by adding the mains, that's great. If it went away because you added a narrow boost filter to remove it, that's bad.

brucek

[Nuance]

brucek wrote:

The after looks quite good.

I'm wondering about where the sharp dip went from the sub around 65Hz. If it went away by adding the mains, that's great. If it went away because you added a narrow boost filter to remove it, that's bad.

brucek

I bumped the 63Hz dip a little. Why is that bad?

[brucek]

I bumped the 63Hz dip a little. Why is that bad?

The dip at 63Hz exhibits the look of a classic phase cancellation. Adding gain to that type of sharp cancellation uses headroom with little advantage.

A phase cancellation is where the sound reaching the measurement point is a combination of an original direct soundwave and a reflected soundwave that is 180 degrees out of phase at the dip (one half wavelength). When you add a gain filter at that dip frequency, not only does the direct sound increase by the number of dB of the filter, but unfortunately the 180 degree out of phase signal also applies an equal and opposite signal to counteract. The result is that your dip is still there and you have wasted the gain you've thrown at it.

Now consider what creates a peak. The direct signal arriving at the measurement point is combining with a reflected signal that is in phase at the peak. When we apply a cut filter, not only does the direct signal drop, so does the reflected signal drop at the same time. The peak is easily reduced.

Gain bad - Cut good.

brucek

[Nuance]

Okay. Understood. So if I have a DIY subwoofer with no phase control, how do I fix this issue?

I am using a TC2000 15" in a 20" ^3 enclosure with an internal volume of around 3.3 cu. ft. It is being powered by a Behringer EP2500 in bridged mono mode.

Thanks.

[brucek]

if I have a DIY subwoofer with no gain control, how do I fix this issue?

Well, your gain is controlled by the sub output trim of the receiver if the sub amp has no gain control.

brucek

[Nuance]

Oops, I meant phase control. Either way, I reversed polarity and that dip at 63Hz is now gone.

Here is the new graph, albeit not quite as good, but with no boosting:

[brucek]

I meant phase control.

When a phase control isn't present, you use the sub distance control in your receiver.

It's good to try (as you say) do a 180 reversal and see if that works first, but if not, the distance control works the same.

brucek