First Response...doesn't look right

**ngiachino** · **07-07-09, 10:37 PM**

I read over the help documentation, and I'm pretty confident I've calibrated properly, but these response graphs look a little crazy past 180hz.

I'm using a behringer emc8000 mic, behringer 802 mixer, and my onboard soundcard. Measurements were taking from my listening position.

Here is an example of the graph I got...(remove *....couldn't post link since I'm new here)

img21.imagesh*ck.us/img21/5048/nikg.jpg

**brucek** · **07-07-09, 11:05 PM**

Quote:

couldn't post link since I'm new here

Go to the Post padding thread and make 5 dummy posts and then post your pic.

brucek

**ngiachino** · **07-07-09, 11:08 PM**

Got it...

**ngiachino** · **07-07-09, 11:11 PM**

Pic of bookshelf speaker set in built-in bookshelves.

[IMG]

[/IMG]

**robbo266317** · **07-07-09, 11:35 PM**

That's not a bad response for a bookshelf.
For full range you may want to use 1/3 octave filtering.

**ngiachino** · **07-07-09, 11:38 PM**

The speakers are a diy build I did with the help of some forum members. They use usher 8945p mid-range and peerless hds tweeters in a .5cuft enclosure. This is my first xover...so I'll probably look to doing some tweaking once I know for sure I'm generating these measurements correctly.

**wackii** · **07-08-09, 02:43 AM**

Quote:

ngiachino wrote:

Pic of bookshelf speaker set in built-in bookshelves.

[IMG]

[/IMG]

I wish my response graph looks like this at listening position.

Al,

**brucek** · **07-08-09, 07:49 AM**

Quote:

I wish my response graph looks like this at listening position

You're being overly kind to yourself with respect to your graphing axis values.

You're using a vertical axis of -60 to +130dB (190dB spread). This would make most speakers look great. You should switch to the standard 60dB spread we use here with a vertical axis of 45dB-105dB. This centers the 75dB target around a plus and minus 30dB spread.

For full range measurements you need to turn on smoothing. This feature removes the comb filtering and reveals the underlying trend of the signal for easier evaluation.

brucek

**ngiachino** · **07-08-09, 09:49 AM**

Changed the axis and put on 1/3 octive smoothing...

**brucek** · **07-08-09, 09:54 AM**

Yeah, that's good.

Quite a dip at ~200Hz to examine..

I guess now you can do some near-fields or outside measures to try and remove external influences, so you can see how the speakers are performing.....

brucek

**ngiachino** · **07-08-09, 11:00 AM**

Bruce -

Is there anything specific I need to do to perform near field measurements? Is there any general rules regarding how far to measure from the speaker, do I point the mic directly at the speaker, settings in Romm EQ Wizard that would change?

Thanks!

Nik

**brucek** · **07-08-09, 01:11 PM**

Depending on the number of drivers in your speaker, will determine how far away you do the measurement. A single driver would be in the 1"-3", so quite close.

But, if you have 2 drivers, you would need to determine where a proper mix/integration was realistic. Certainly not more than 12". Best to move the speaker as far away as possible from any boundaries.

The mic is pointed directly at the speaker (horizontal axis) and at 0 degrees. You can play with taking measures of horizontal off axis (30 deg.) too, to see how the off-axis distribution looks.

REW remains the same for taking the measurement, but once taken, then you can work with the impulse response to set an appropriate gate time depending on the frequencies you're measuring. Simply open the IR Windows pop-up and select a gate that you want and watch the impulse response to ensure it's appropriate (with respect to reflections), then select Apply Windows and your response graph will adjust to that gate time response plot. I've used a 1msec left window and a 5 msec right window quite successfully for 500-20000Hz measures. The popup will show the frequency resolution for you, so you don't need to do the math. You're trying to exclude reflections from any surfaces nearby. It could be as simple as the mic stand, but predominantly hard surfaces such as the floor.

So, a gate time of 5 msec would give you a reflection free time of about 0.86 meters. Remember the sound has to hit the boundary and bounce back. So, distance=(time*speed)/2 = (5msec*344m/sec)/2. Remember that a short gate time limits the lowest frequency you can get useful response information for. So 5 msec limits your lowest frequency to about 200Hz (freq=1/period).

So, for reflection free measurements where your closest boundary is 1 meter away, you could use a 5 or 6 msec gate and check your response fairly accurately without smoothing for a measure from maybe 500Hz-20000Hz. Then for the bandwidth below 500Hz, you could measure from 0-500Hz and extend your gate out.

If you wanted an overall full spectrum measure, you could use a wide gate and use smoothing. A bit less accurate, but suitable to take a wide general look.

brucek

**ngiachino** · **07-08-09, 03:39 PM**

Thanks Bruce. I'll probably try the process you outlined tonight or tomorrow and post the results or any pending questions I have.

**Jonomega** · **07-12-09, 01:22 PM**

I don't post here often, unless I am calibrating my system

Your dip at 200hz could also be due to the null from the front wall. This depends on how far you place the speaker from the front wall. The closer the speaker is to the front wall, the higher in frequency the null will be. For example, if your speaker is right up against the front wall such that the drivers are about 1.4' away from the wall, you will get a null around 200hz.

I generally, try to keep my speakers 4' or more from the front wall to minimize this effect.

Of course, i am unfamiliar with your speakers, so the problem could be something else. However, an easy test is to move the speaker further from the front wall for the purpose of testing. If the null moves down in frequency, then you know what is the main cause of the problem.

As per your tweeter's drop off, this might be due to the polar response of your tweeter. You might try toe-ing the speaker in towards the listening position to get closer to an on-axis FR.

Of course, an on axis near field measurement with the speaker away from boundaries could show if any problems might be due to the speaker design.

**ngiachino** · **07-12-09, 01:36 PM**

Thanks for the response! I ended up being busier than expected this week, so I didn't have much time to play around, but I plan on doing on-axis near-field measurements next and will post my results.

These are literally right against the backwall, in a bookshelf, but were designed with very little bsc for such a purpose. I'll continue to post my updates as I take new measurements.

Nik

**Jonomega** · **07-12-09, 01:48 PM**

Yeah, I don't know anything about boundary compensation. I do know that 1120±10 ft/sec (speed of sound) divided by 200hz multiplied by 0.25 (quarter wave length for null) corresponds to how you have to speakers set up.

It could be many things, but to my mind, the main culprit is because 200hz wave going rearwards is canceling the source 200hz wave due to the reflection from the wall behind the speaker.

**ngiachino** · **07-12-09, 02:17 PM**

Agreed - the other experiment I want to try with with some defuser panels behind the speakers. I need to do a little bit of research on what type of treatment would best help in my configuration. Anyone advice? I'm sure there is plenty of info out there, just haven't gone fact finding yet.

**Nuance** · **07-18-09, 03:10 PM**

You've got quite the roll off of high frequencies after 8KHz. How does that sound in your room?

Advertisement