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Old 22nd July 2009, 08:30 AM   #71
Elias is offline Elias  Finland
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Hello Klaus,

I used the shaped tone burst as a real excitation signal fed to the speakers. It is true that the same results can be calculated from the impulse response (just make sure the impulse response is long enough to contain all the room reflections one likes to focus to).

I think the benefit of feeding the burst directly to the speakers is that one can listen the response at the same time while making the measurement! It is very revealing! One can detect all kind of phenomenas in addition to the normal amplitude level and room response, like resonances of the speaker cabinet or furnitures and also nonlinear distortion, if these are above hearing threshold. All this is unknown to the ear if one measures only the impulse response.

It is very informative to compare on the spot what the ear hears and what the measurement shows. I have found that this kind of burst test measurement correlates quite well what I can hear too.

The impulse response is mathematically correct (for a linear system) but the ear does not benefit from it. Only if the music would be an impulse.. But it's not

- Elias


Quote:
Originally posted by KSTR
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).
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Old 22nd July 2009, 08:47 AM   #72
Elias is offline Elias  Finland
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Hello,

The dipole line array I used for these tests is 1.5 m long vertical array of 8 pcs of 6.5" elements.

When comparing the measurements of the single dipole to the dipole line array it is seen that below about 100 Hz they behave almost identically. -> Thus one can conclude that dipole line array gives benefit to the room time response over the single dipole only when the length of the array is longer than about half of the wave length. In this case the 1.5 m corresponds about half wave length at 100 Hz.

Of course array of elements have other benefits like decreased nonlinear distortion. And array allows using 'cheaper' elements.

Well, propably those 18" elements do not play very high in frequency, maybe up to 100Hz-200Hz, so we are in the range where the array needs to be very long to give more directivity over the single dipole. But of course it will be slightly better than single driver anyway. If you make the array about 2-3 m long it will be very good around 100Hz.

For the bass array there should be no reason why horisontal would be different from vertical. I'm using vertical because it'll play over 1kHz.

- Elias


Quote:
Originally posted by CLS
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 22nd July 2009, 08:53 AM   #73
Elias is offline Elias  Finland
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We are still waiting your data to prove your claims about multible subs

Have you done room time response measurements of your system?

- Elias

Quote:
Originally posted by gedlee
...
multiple subs
...
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Old 22nd July 2009, 09:12 AM   #74
Elias is offline Elias  Finland
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Default Re: Re: Conclusions at this far

Hello Scott,

The results shown are almost purely due to the room effects.

See for example the measured monopole response picture:
http://www.diyaudio.com/forums/attac...amp=1248117969
The left hand side shows the input and measured envelope (blue). One can see that at first the signal tries to come but then first reflections cancel the signal away and it does not follow the input envelope but then later arriving reflections over emphasize the response. This is due the monopole having simple too many reflections to all directions because the lack of directivity.

If we have directivity we have less first reflections and the room response envelope follows more closely to the input envelope.

Important to note that not all the reflections are so bad. The first reflections are the worst! So reflections please do come, but not yet! Wait for a while so I can first hear the music recording, and after that I may be willing to listen to reflections too!

The key is to minimise the first reflections in a time frame compared to periods, since the ear integration time is not constant with frequency. And to be able to do this at low freqs the source needs to have directivity.

- Elias


Quote:
Originally posted by ScottG
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 22nd July 2009, 02:32 PM   #75
gedlee is online now gedlee  United States
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Quote:
Originally posted by Elias
We are still waiting your data to prove your claims about multible subs

Have you done room time response measurements of your system?

- Elias

I don't know where you have been, but multiple subs is accepting extremely wide acceptance.

I would question the validity of "time response" for LFs in small rooms. It has no relavence and this has been shown. What we hear at LF, where the periods of the sound are greater than our ears averaging time, is the steady state. We do not hear LF transients, or "time response".
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Old 22nd July 2009, 02:32 PM   #76
KSTR is offline KSTR  Germany
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Quote:
Originally posted by Elias
I used the shaped tone burst as a real excitation signal fed to the speakers. It is true that the same results can be calculated from the impulse response (just make sure the impulse response is long enough to contain all the room reflections one likes to focus to).
I use an impulse generated from a logsweep which is several minutes long. This captures everything. BTW I also listen to the impulse, slowed down to 1:5...1:50. Very revealing too, when it comes to judging the quality of the reverberant sound.

Quote:
I think the benefit of feeding the burst directly to the speakers is that one can listen the response at the same time while making the measurement! It is very revealing! One can detect all kind of phenomenas in addition to the normal amplitude level and room response, like resonances of the speaker cabinet or furnitures and also nonlinear distortion, if these are above hearing threshold. All this is unknown to the ear if one measures only the impulse response.

It is very informative to compare on the spot what the ear hears and what the measurement shows. I have found that this kind of burst test measurement correlates quite well what I can hear too.
Well, I'm using shaped tone burst sequences for about ten years now, for charactising LF responses. In fact I made a nice CD of that wich was optimized for practical use in many regards, both in home listeneing setups and for smaller live club usage. So I completely agree that listening to bursts is way more revealing than using any other signal, for the LF range. Especially as you are free to walk around and look what happens.

My signal shapes are a bit different, I use sequences of several cycles, like 20 or so (to have enough time to energize high-Q modes), with 2-3 cycles of raised cosine envelopes at beginning and end. This basic signal is repeated, with a continuously shrinking blank time between the busts. The blank time is either always a multiple of a cycle or an odd multiple of 1/2-cycle. This gives me two signal characteristics, one with continuous phase from burst to burst and the other with a polarity flip. From how the sound changes during these gaps I can learn about the character of the mode. I also use those two types in to L/R-polarity variants, one time simple mono the other with L=-R. This gives me four basic signal types which don't allow any modes to slip through (like those that cancel themselves when the speakers are in opposite polarity lobes of the mode).

Thes signals are available for download, but I have to write up an english documentation and have to check with the hoster if the traffic will pose a problem (as when I link to it from DIYaudio there sure will be a lot of traffic).

Quote:
The impulse response is mathematically correct (for a linear system) but the ear does not benefit from it.
With the Farina logsweep method you get the disortion (for the chosen signal level), too.

- Klaus
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Old 22nd July 2009, 05:12 PM   #77
Elias is offline Elias  Finland
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Very interesting Klaus!

Me too have the bursts on CD, it's very handy. I found that for home use for measuring speakers in the room the most suitable burst length is close to 5 periods. I've tried up to 10 period bursts but the temporal accuracy suffers because reflections mask it more.


Quote:
Originally posted by KSTR
BTW I also listen to the impulse, slowed down to 1:5...1:50. Very revealing too, when it comes to judging the quality of the reverberant sound.
I didn't catch how you listen to the impulse? You mean you first measure the impulse response and then play it back at slower speed? How you listen, with ear phones?


Quote:
Originally posted by KSTR
With the Farina logsweep method you get the disortion (for the chosen signal level), too.
Yes. If the sweep is very slow it's easy to hear the room modes and constructional resonances as well as distortion. But sweep is not fun to listen!

I have a hardware sweep generator, but I found it's pain to my ears to listen to it I do use it to locate the exact resonanses by manual tuning the amplitude and frequency, for that purpose it's very good.

- Elias
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Old 22nd July 2009, 06:39 PM   #78
KSTR is offline KSTR  Germany
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Elias,
Quote:
Originally posted by Elias
I didn't catch how you listen to the impulse? You mean you first measure the impulse response and then play it back at slower speed? How you listen, with ear phones?
Yes, I record a long logsweep to have a good S/N-ratio, derive the impulse from it (by convolution with the inverse), and then play it back, using either headphones or on my nearfield monitors (I hate listen to headphones). When slow enough you can literally hear the floor bounce and the following reflections from the various surfaces, especially if there are slap echoes from undamped tricorners and such. Occasionally I also apply a reverse log envelope gain to the signal, to make the reverb tail louder to more clearly hear what's going on. Something you just don't clearly see in the impulse waveform (let alone a FR mag plot) and you also don't quite catch it when listening to a real-time impulse, unless the effect are very pronounced and/or one has a lot of experience in hearig these subtleties in realtime.


Quote:
I have a hardware sweep generator, but I found it's pain to my ears to listen to it I do use it to locate the exact resonanses by manual tuning the amplitude and frequency, for that purpose it's very good.
Same with my test-signal CD (which also uses CD-text it the player can display that). The first signal is brown noise, the second a sweep (from 27.5Hz to 440Hz). When you find a problematic freq during the sweep then you just skip to about the track number corresponding to the current time of the sweep and can do more detailed analysis. The 97 burst tracks are staggered in semi-halftone steps (1/24 oct resolution).

The nice thing with an already recorded IR is that one can use any signal to covolve it with, so if I want to listen how your 5-period bursts sound like in my system I only need to convolve it and listen to it (not on the original system, of course), which I did. A problem I saw with such a short impulse is that at higher frequencies the round trip time is larger than the burst time, which means that you only get the the echo effect but not the effect of the mode buildup. With my signals you can clearly see the round trip time for the mode and also see the total time until the mode reaches equilibrium (something like 5 or more round trip times, for a typical higher Q mode). See attachment, for a higher order mode at 240Hz in my room, listen position at point where a boost results (Note: I've cut the central part of the wave to also show the decay. Bottom trace is stimulus). That's why I chose to have the bursts long enough to allow the mode to fully build up in the first place.

- Klaus
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Old 23rd July 2009, 02:09 PM   #79
Elias is offline Elias  Finland
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Hello,

This is a very good picture and tells many things. As I see the time axis the signal starts at 0.1 and steady state begins at 0.2. So there is a room response transient state duration of 100ms, that's about 20 periods at the measurement freq.

Obvious question considering this thread is have you done any tests comparing different directivity speakers to investigate if the transient state duration or slope is changing?

I don't know what was the speaker directivity for this test, but according to my measurements there should be a noticeable difference when using higher directivity source in this freq range of 240Hz.

Then one can think how this transient state is perceived. I've noticed in my listening tests that even if I change the envelope over the couple of periods only at the starting of the burst there is clearly perceived difference. In this room the transient lasts 20 periods, so that should be easily perceivable.

How do you hear it?


Another thing is of cource the transient decay, but there should be no questions about it's perception: It is easily perceivable - That's why they add artificial reverberation to studio recordings

- Elias


Quote:
Originally posted by KSTR
See attachment, for a higher order mode at 240Hz in my room, listen position at point where a boost results
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Old 23rd July 2009, 04:54 PM   #80
KSTR is offline KSTR  Germany
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Hi,
Quote:
Originally posted by Elias
Obvious question considering this thread is have you done any tests comparing different directivity speakers to investigate if the transient state duration or slope is changing?

I don't know what was the speaker directivity for this test, but according to my measurements there should be a noticeable difference when using higher directivity source in this freq range of 240Hz.
No, not with this setup. Those were standard monopole 3way-floorstanders, and 240Hz happen to be about the xover between 8" woofer and 4" midrange. But I have listening experience with dipoles and know what difference it makes, especially in the bass range.

Quote:
Then one can think how this transient state is perceived. I've noticed in my listening tests that even if I change the envelope over the couple of periods only at the starting of the burst there is clearly perceived difference. In this room the transient lasts 20 periods, so that should be easily perceivable.

How do you hear it?
Just as it looks like, the buildup/decay is clearly audible. But I have nastier freq spots than this, at 320Hz for example the buildup/decay is strange, sort of a hick-up. During buildup the hickup effect is pretty well masked, but on decay it really sounds funny, one can hear it.

Of course at the real listening spot and with my normal listening conditions (doors&windows open, mostly) it is not that bad.


What I learn from this that things happening in the modal range of a real room are nothing intuitive at all and very hard to predict and track down what really is happening. Best thing of course is to avoid the exitation in the first place with directional speakers located at the right places and ranging from dipole to cardioid and anything in between (hypercardioid looks very promising to me), for upper bass and midrange. For lower bass multiple subs (DBA included) looks easier to me to get best results.

- Klaus
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