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-   -   Some basic FR measurements (http://www.diyaudio.com/forums/multi-way/228399-some-basic-fr-measurements.html)

Alex M 22nd January 2013 12:52 PM

Some basic FR measurements
 
Last year I finished the first round of tweaking a new passive crossover for my three-way speakers (Audax HM210Z0 and HM100Z0 and Scan-Speak D2905-9300). The speakers sound very nice and very easy to listen to, but are perhaps not as transparent or immediate as they might be with my push-pull 300B monoblocks, and have what I perceive to be a dark tonal balance.

I did some electrical measurements of the crossovers, and after some substantial work on impedance correction, they are now an excellent match to 4th-order Linkwitz-Riley, as intended.

So to get an idea whether the warm bias of the sound was an issue with the crossovers themselves or a room problem, I thought I would do some simple acoustic measurements. I played 20 seconds of a white noise track from a test CD, and then used a Panasonic EM60 microphone into an EMU0202 interface to feed this into Audacity, which I used to calculate a power spectrum. The mic is on axis about 50cm from the respective midrange driver, but for the moment I'm not being obsessive about position.

The room is about 4-5 metres square, with a couple of metres square cut out of one corner for the doorway; the room is carpeted, with soft furnishings and Venetian blinds on the windows. The speakers are about a third and two-thirds of the way along a long wall, and are about 20cm from the wall with a slight toe-in.

Here are the responses of the two speakers, first the left then the right channel:

http://www.southampton.ac.uk/~apm3/d..._response.tiff
http://www.southampton.ac.uk/~apm3/d..._response.tiff

The responses are quite consistent between channels, which is reassuring, and also perhaps surprising, since the room is quite asymmetrically laid out.

I note the following:

- A raised response between 1.5kHz and 2kHz;
- Some peaking between 3kHz and 5kHz;
- The relative magnitude of the peaks at 2kHz and 4kHz is different in the two speakers;
- A rolloff above 6kHz. I wouldn't expect this from the Scan-Speak tweeters, as they have a very flat response up to 15kHz or more (though I am measuring slightly off the tweeter axis).

So - first of all, should I expect to get results from this technique which are at all useful? In other words, are the three features I mention above likely to be real? Secondly, using white noise at 50cm, how much would you expect room effects to influence the measured response?

The regular peaks at the HF end (separated by about 800Hz) look suspicious to me. Are they likely to be a measurement artefact?

Any other comments would be welcome!

Thanks,

Alex

bear 22nd January 2013 02:51 PM

use manage attachments to include images...

Alex M 22nd January 2013 03:16 PM

2 Attachment(s)
In case my original plots were not visible, I have now uploaded them to the site.

Left channel:
http://www.diyaudio.com/forums/attac...1&d=1358871176

Right channel:
http://www.diyaudio.com/forums/attac...1&d=1358871176

Alex

speaker dave 22nd January 2013 03:50 PM

Hi Alex,

I think your measurements show some merit and should correlate to some degree with what you hear.

With regard to the periodic peaks above 2k, those are typical of having a strong early reflection. You noticed they have a regular spacing of 800 Hz and that confirms they are from a delayed reflection of about 1.25mSec delay. That would typically come from a reflecting point (say a cabinet edge or cabinet top that the tweeter "sees") that is about a foot or so distant from the tweeter. If you can track that down you should be able to find a way of treating it with damping foam or fiberglass.

Your low frequency curves look very smooth, devoid of any typical room effects. Is this some type of windowed or gating measurement system?

David S.

Alex M 22nd January 2013 04:08 PM

Hi Dave,

Yes, I wondered about reflections too, but there is certainly nothing that close to the speaker or to the microphone. I'll have another look at my setup this evening and think about it.

I am using the plain vanilla default settings in Audacity: I have a feeling that some windowing is selected, but digital signal processing is something I am relatively unfamiliar with. I will check this out later too and report back.

Alex

speaker dave 22nd January 2013 04:16 PM

Your software window has a box marked "size" "512". See if there are any options with many more points. That should give you more low frequency resolution.

David S.

weltersys 22nd January 2013 04:22 PM

Quote:

Originally Posted by Alex M (Post 3337262)
The speakers sound very nice and very easy to listen to, but are perhaps not as transparent or immediate as they might be with my push-pull 300B monoblocks, and have what I perceive to be a dark tonal balance.

The mic is on axis about 50cm from the respective midrange driver, but for the moment I'm not being obsessive about position.

- A raised response between 1.5kHz and 2kHz;
- Some peaking between 3kHz and 5kHz;
- The relative magnitude of the peaks at 2kHz and 4kHz is different in the two speakers;
- A rolloff above 6kHz. I wouldn't expect this from the Scan-Speak tweeters, as they have a very flat response up to 15kHz or more (though I am measuring slightly off the tweeter axis).

So - first of all, should I expect to get results from this technique which are at all useful? In other words, are the three features I mention above likely to be real? Secondly, using white noise at 50cm, how much would you expect room effects to influence the measured response?

Alex,
The response is representative of what the microphone is picking up at the location you placed it at, and the microphone's frequency response.
Without testing the microphone against a known "flat" mic you can't be 100% sure what you are seeing is what the actual response is.

The high frequency roll off probably looks to be worse than actual because the mic is off axis from the tweeter. Placing it midway between the midrange and HF would give a better impression of the crossover region and HF rolloff. Testing outdoors at quite some distance from the ground will determine if the early reflections are cabinet or room related.

Multiple mic locations in room will also give you a better idea of what the cabinet response compared to room response is.

10 dB variations equate to sounding half as loud or twice as loud referenced to 1000 Hz.

You say you used white noise (which sounds like the hissing you hear between radio stations) as a source, but the response looks like pink noise (equal volume per octave) was used.

speaker dave 22nd January 2013 04:55 PM

Looks like an FFT like approach. White noise would be correct for constant bandwidth analysis, as opposed to constant percentage bandwidth (RTA) approaches.

David

bear 23rd January 2013 02:09 AM

Suggest removing the tweeter connection, test, then remove the woofer connection, test and you can see what the drivers are doing separately. Assuming parallel xover.

If you don't see the same ripple separately, then you have interference going on...

Also it is good to look at just the impulse. Well, I like to.

_-_bear

ctrlx 23rd January 2013 07:58 AM

Theres quite a few of dedicated f/r software, try to switch to one of these.
As pointed out, you need to ensure your mic/preamp is calibrated.
Then you can measure each drivers output to ensure that your xover & driver combination results in the desired acoustic LR4 output.


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