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Old 20th July 2009, 08:16 PM   #61
Elias is offline Elias  Finland
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Hello,

I thought some explanations might be helpful what is actually going on. So this is the shape of the excitation signal I'm using for these measurements. It's a short burst of 5 cycles of sinusoid modified by a shaped envelope. The picture shows the signal and it's envelope in linear and dB scale.

This signal is copied from the idea from Linkwitz.

The burst is short enough to give quite good resolution in time domain, and the shaped envelope helps to keep the spectral regrowth in minimum and the bandwidth in acceptable limit.

- Elias
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File Type: png input_burst.png (35.4 KB, 524 views)
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Old 20th July 2009, 08:26 PM   #62
Elias is offline Elias  Finland
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Here's the method I use to calculate the envelope peak magnitude and the envelope peak delay.

The left hand side shows the input signal envelope and the blue is the measured room response envelope at a particular frequency (56 Hz in this case). The peak point of the measured room response envelope defines the peak magnitude and peak delay as shown in the middle and right hand side pictures.

To get the freq responses as shown in the picture the input burst frequency is stepped in 1/12 octave intervals and the above is repeated.

Hope this helps

- Elias
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File Type: png envelope_peak_mag&delay.png (46.3 KB, 521 views)
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Old 20th July 2009, 09:30 PM   #63
Elias is offline Elias  Finland
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Default Conclusions at this far

Ok, some revolutionary conclusions are in order based on the measurement results so far

I divide the conclusions in three parts according the selected freq ranges.





* 20 Hz - 35 Hz:

1 a ) Monopole bass and dipole bass show identical room time response in all locations A - D.
In all cases the envelope closely follows the input signal envelope.





* 35 Hz - 100 Hz:

2 a ) Dipole shows clearly better room time response than monopole in all locations A - D.

2 b ) Monopole room time response is the most sensitive to placement A - D.

2 c ) Dipole room time response is less sensitive to placement A - D than monopole, but more sensitive than dipole line array.

2 d ) Dipole line array room time response is the least sensitive to placement A - D.

2 e ) Dipole line array room time response is almost identical to dipole room time response in all locations A - D. This is due the limited hight (1.5m) of the array and thus the array directivity in this freq range is not enough to improve the response over the dipole.





* 100 Hz - 1 kHz:

3 a ) Monopole has worse room time response than dipole or dipole line array.

3 b ) Dipole has better room time response than monopole but worse than dipole line array.

3 c ) Dipole line array has the best room time response over monopole and dipole.

3 d ) Monopole room time response is the most sensitive to placement A - D.

3 e ) Dipole room time response is less sensitive to placement A - D than monopole, but more sensitive than dipole line array.

3 f ) Dipole line array room time response is the least sensitive to placement A - D.

3 g ) When approaching 1 kHz monopole and dipole room time responses are getting similar. This can be due to increased directivity of the monopole and decreased directivity of the dipole because of cone and baffle size.

3 h ) At 1 kHz dipole line array still has the best room time response.




More conclusions to follow when they appear to my mind


- Elias
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Old 20th July 2009, 09:54 PM   #64
Elias is offline Elias  Finland
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Default Re: OB line array vs. sealed

Hello,

Quote:
Originally posted by sendler
Fascinating work with an excellent test signal/ analysis method. Were you surprised by the substantial superiority of the dipole line array? I had often thought to try that as it would seem a good solution to the increased volume displacement demands of open baffle.
Thanks! YES, when the first time I switched the signal to the dipole line arrays I was totally blown away I'm still looking for my hat! What can I say.. It sounds very detailed starting from low frequencies already. Many systems can sound detailed in high freqs, say above 1kHz, but lack the accuracy at lower freqs due to the room.



Quote:
Originally posted by brianpowers27
THank you for posting your test results. I would like to raise a concern I have regarding the comparison between the various tests. I am concerned that by using different drivers and baffle widths you are ignoring the baffle's role in the equation.

I would not consider the comparison between dipole and dipole line array to be complete until you are using the same drivers on the same shape baffle.
You are welcome. Well, I selected smallish baffle for the dipole because it is the only way to extend the constant dipole directivity freq range all the way to 1kHz. If you make the baffle too big then the driver is starting to act like a monopole which makes things only worse.



Quote:
Originally posted by sendler
Yes, the advantage of the line array could be even bigger with better drivers.
I think 'better' drivers may not be needed for an array. Cone excursion is small and nonlinearities usually not a problem. Selecting a driver with low stored energy is of course an advantage. Fortunately also some cheap drivers have low stored energy.



Quote:
Originally posted by sendler
Now that I see the OB line array results look so good, it would be intersting to throw a box over the back to see the same set up running sealed.
Hmm.. We can pretty much guess what happends for the room time response in this case if we have monopole line array: From the measurements of dipole and dipole line array (of 1.5m height) we can conclude they behave similarly at low frequencies 100Hz and below. So the case would be similar for a monopole and monopole line array, they would behave similarly at 100Hz and below. Then we also see from the measurements that monopole is much worse than dipole at 100Hz and below. So all in all monopole line array does not pay unless used at higher frequencies only, say above 200-300 Hz, and crossed over to dipole below that. But what's the point of using monopole at so high freqs when dipole will be better anyway?

If you find interest to build a monopole line array, let's compare the room time response measurements then

- Elias
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Old 20th July 2009, 11:32 PM   #65
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Elias,

Great posts! Thank you for sharing.

Regards,
Bill
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Old 20th July 2009, 11:46 PM   #66
KSTR is offline KSTR  Germany
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Great thread, Elias.

Did you measure these burst sequences directly or did you convolve the bursts with a measured impulse reponse? For convenience I now do mostly the latter after I had confirmed that it really gives identical measurements (unless I run into severe distortion issues).

- Klaus
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Old 21st July 2009, 02:18 AM   #67
CLS is offline CLS  Taiwan
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Elias, thanks a lot for the thorough explanation

Did you define line array? (or did I miss it?) Does it mean vertical array strictly? How about horzontal? I have 4 18" woofers on OB, all very close to floor and unevenly spaced. Does this count?
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Old 21st July 2009, 03:00 AM   #68
gedlee is offline gedlee  United States
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I really don't want to throw rain on your parade, but I really don't see the relavance of this. First, above about 300-500 Hz the perception issues are far more complex than you can show with your technique. Below 150 Hz no one who knows how to setup a sound system in a room is going to use a single sub, monopole, dipole or line array and multiple subs are going to work completely different than what you show. Between 150 Hz and say 300 Hz, I really don't know what you are showing means, but I really doubt that it is as simple as you seem to believe that it is. Perception issues are just not that simple.
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Old 21st July 2009, 05:46 AM   #69
ScottG is offline ScottG  United States
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Default Re: Conclusions at this far

Quote:
Originally posted by Elias
Ok, some revolutionary conclusions are in order based on the measurement results so far

I divide the conclusions in three parts according the selected freq ranges.


* 20 Hz - 35 Hz:

1 a ) Monopole bass and dipole bass show identical room time response in all locations A - D.
In all cases the envelope closely follows the input signal envelope.


* 35 Hz - 100 Hz:

2 a ) Dipole shows clearly better room time response than monopole in all locations A - D.

2 b ) Monopole room time response is the most sensitive to placement A - D.

2 c ) Dipole room time response is less sensitive to placement A - D than monopole, but more sensitive than dipole line array.

2 d ) Dipole line array room time response is the least sensitive to placement A - D.

2 e ) Dipole line array room time response is almost identical to dipole room time response in all locations A - D. This is due the limited hight (1.5m) of the array and thus the array directivity in this freq range is not enough to improve the response over the dipole.


* 100 Hz - 1 kHz:

3 a ) Monopole has worse room time response than dipole or dipole line array.

3 b ) Dipole has better room time response than monopole but worse than dipole line array.

3 c ) Dipole line array has the best room time response over monopole and dipole.

3 d ) Monopole room time response is the most sensitive to placement A - D.

3 e ) Dipole room time response is less sensitive to placement A - D than monopole, but more sensitive than dipole line array.

3 f ) Dipole line array room time response is the least sensitive to placement A - D.

3 g ) When approaching 1 kHz monopole and dipole room time responses are getting similar. This can be due to increased directivity of the monopole and decreased directivity of the dipole because of cone and baffle size.

3 h ) At 1 kHz dipole line array still has the best room time response.



- Elias

Couldn't much of this have to do with energy storage from the differing formats rather than totally from interaction with the room?

In other words - what would a pressure-based dipole act like vs. a pressure-based monopole?

Also - how much of it is the interaction with the enclosure (or lack thereof)?
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Old 21st July 2009, 06:14 AM   #70
ScottG is offline ScottG  United States
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Quote:
Originally posted by gedlee
I really don't want to throw rain on your parade, but I really don't see the relavance of this.

First, above about 300-500 Hz the perception issues are far more complex than you can show with your technique.

Below 150 Hz no one who knows how to setup a sound system in a room is going to use a single sub, monopole, dipole or line array and multiple subs are going to work completely different than what you show.

Between 150 Hz and say 300 Hz, I really don't know what you are showing means, but I really doubt that it is as simple as you seem to believe that it is. Perception issues are just not that simple.

I think the relevance is rather obvious - to show if there is a significant difference other than simple deviations in amplitude. In this case higher levels of linear decay "hanging" in the room.

Considering the issues of "masking" at low freq.s - any lingering high amplitude signal is (IMO) bound to impact the perception of clarity at those freq.s and perhaps perceptually across the entire bandwidth.

While the issues of perception may be very complex (for any pass-band), this doesn't mean that what is shown doesn't have relevance. It may not be "relevant enough", but it's certainly a subject for thought - and at some point "relevance" may be more deeply investigated BECAUSE of this work.

People "who know how to set up a sound system" use a single monopole sub all the time. Not likely because they want to, but rather because that's the limitation they are stuck with. Note that for a single listener - a single monopole or dipole sub *immediately* behind this listener (with appropriate phase adjustment) can work exceptionally well for that one listener (even at higher freq.s approaching 100 + Hz).

I also don't think that at any time Elias is "definitively" stating conclusions. It doesn't appear to be a statement of: "this is what it is", rather more like: "this seems likely with what I have so far".
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