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-   -   Measured monopole and dipole room responses (http://www.diyaudio.com/forums/multi-way/145876-measured-monopole-dipole-responses.html)

 Elias 14th June 2009 06:17 PM

Measured monopole and dipole room responses

2 Attachment(s)
Hello,

I did some measurements with monopole and dipole bass in a room.

Particularly I try to pay attention to temporal behaviour since as I believe steady state measurements in this case are pretty useless when considering human perception. Also I try to match the measurement excitation signal to represent the final situation, that is music.

Music is no impulse nor steady state sinusoid, thus I'm using short tone bursts with shaped envelope. It will give quite good visuality to what is happening in the room due the reflections.

I plotted some spectrograms. Note the horisontal axis is the time in periods. Vertical axis is frequency.

Also note that each frequency the maximum value of the burst envelope is normalised to 0dB. In this way it is possible to compare the room responses in perceptual sense since each frequency the amplitude is the same.

To start here's the ideal case for a reference, response of a laptop soundcard:

- Elias

 Elias 14th June 2009 06:22 PM

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Here is the measured response of a H frame dipole. It has 2 12" elements. Measurement distance is about 1m and hight 1m.

Some room reflections are present:

- Elias

 Elias 14th June 2009 06:24 PM

2 Attachment(s)
And here is the measured response of the monopole. It has one 12" element in 100 litres closed enclosure.

The room response looks pretty messy:

- Elias

 Elias 14th June 2009 06:42 PM

From these measurements it can be said that in the case of the dipole the time the envelope maximum occurs follows quite well the original envelope, but in the case of the monopole the envelope maximum occurs in almost a randomly manner. With monopole at some freqs the envelope maximum and thus the energy that reaches the listener is delayed - it has a poor energy delay (if one not like to use the term group delay).

Is this a reason for a perceived 'slow bass'? In this sense the dipole is 'faster' - the energy delay is smaller.

The beauty of using this excitation signal is you can listen how the room sounds like while doing the measurement. I can state that comparing dipole and monopole in this room one can hear big difference. I think in this case the visualisation yields the same result as the ear hears: In the case of the dipole the bass notes are clearly separated and it is easy to follw the bass line, whereas in the case of the monopole the bass sounds like it's never going to die out and the bass line is mudded.

- Elias

 David_Web 14th June 2009 07:07 PM

Great to see some measurements.

However -10dB is not that interesting as "direct sound" is what you mostly get. -40dB is a lot more useful. -60dB even more so. RT60 and all.

10ms is only 3,4m so you can't really tell me that you are showing any "in room" response.

 Elias 14th June 2009 07:37 PM

Hello,

Quote:
 Originally posted by David_Web However -10dB is not that interesting as "direct sound" is what you mostly get. -40dB is a lot more useful. -60dB even more so. RT60 and all. 10ms is only 3,4m so you can't really tell me that you are showing any "in room" response.
Take a another look at equal loudness contours at bass frequencies. 10dB is a huge perceived difference. 60dB dynamic range at bass in a small room is a wishfull thinking only - the signal has to go above pain level to achieve that.

And I think RT60 is not much of a use in small rooms.

Note again that the time axis is in periods, not linear time!

- Elias

 David_Web 14th June 2009 07:58 PM

Sorry. I must really be slipping. I reread your post before and didn't see it. Got it now.

Would it be possible to see the graphs in ms?
I find it hard to translate to cycles as time matters to me.

 Elias 14th June 2009 09:01 PM

Quote:
 Originally posted by David_Web I find it hard to translate to cycles as time matters to me.

That's just the key! :) One needs to get rid of the traditional idea of having looking the room in linear time. For a human linear time have little perceived importance at different frequencies.

For example, take a 10ms reflection: At 20Hz it would be very hard to detect by ear as one period of 20Hz is already 50ms. But at 20kHz (one period is only 50us) 10ms reflection should be easily detectable by the ear.

Thus it is better, in my opinion, to scale the time axis to the frequency in question one is making the observation.

Keep the end user in mind! :)

- Elias

 jeepee 17th June 2009 11:25 PM

Hi Elias,

I like your approach described here. Could you post a graphic of the special waveform you are using?

 jason_watkins 18th June 2009 05:35 AM

I also would love to hear more about how these measurements were done.

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