Measured monopole and dipole room responses

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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
What IS the frequency in question?

I can follow the cycle scale, but it looks to me that any bass periods would not be shown.
Since this is a 'bass' measurement, a scale of 20 cycles at 50hz is far too course. Even in my small room the longest side is shorter than wavelength of 50hz, so what exactly is this really showing? Is it showing the point where the full cycle can form, i.e. Over 50hz, or what? Im not sure this shows quite what you think, or at least not a pure result.
 
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For what its worth, years ago I designed a woofer system called the CRAW (Controlled Radiation Active Woofer). The woofer could be operated in 3 modes: Monopole, Dipole and Cardioid. Equalization was applied to each mode to ensure that the free space, axial response was identical. I then measured the in-room response of each mode at various positions around the room, and with the woofer in various positions. Here is one screen shot of the measurements at one position (about 3 M from the source).

Dipole is red; Monopole is yellow; Cardioid is white. As can bee seen, below about 150 Hz the cardioid has the smoothes response. Below the room fundamental (around 28 Hz) the dipole falls off rapidly due to the inability to pressurize the room.
An externally hosted image should be here but it was not working when we last tested it.
 
Thanks John.

Did you also look at source locations very close to the listener?

Yes I did. As the source gets closer to the listener the response approached the near field response of the source, which is not unexpected. In such a case it makes sense to go with a monopole because gradient woofers require distance to form the radiation pattern. For example, if you place a dipole so that the front source is close to the listener what you hear and measure is dominated by the near field response of the front source. So you hear boosted bass. Of course if you position the dipole so that listener is 1/2 way between the sources you are still in the null of the response.

I did some room simulation (for a monopole woofer) of this as well:

Very close to the listen;

An externally hosted image should be here but it was not working when we last tested it.



Woofer positioned directly behind the listening position:

An externally hosted image should be here but it was not working when we last tested it.


Note that in both cases the direct sound is dominate over the modal response and the response is also minimum phase, given that the source is MP.

I have suggested placing subs close to the listener with level and delay adjusted for some time. At these low frequencies the delay doesn't seem too important. A steep slope LP filter must be used or else the midrange "leakage" gives the position of the woofer away. My listening experience and measurements were consistent with the sims.
 
Nice to see what I've stumbled on by trail and error born out scientifically by john k.
Even though a steep slope (4th order) will allow a higher XO frequency, I keep my transition down around 116hz. Almost all midrange content is masked at this range .. even if your ear is right in the driver.
 
Note that in both cases the direct sound is dominate over the modal response and the response is also minimum phase, given that the source is MP.

That's what I've found too but it's not necessarily minimum phase only. The problem with such a configuration is that the level quickly drops with distance. Here's a monopole directly behind the listening position (red) and 4 other points about 2' around it:

monopole.png


There are ways to improve the distance-dependency and the decay. I hope to have something to show soon.

I have suggested placing subs close to the listener with level and delay adjusted for some time. At these low frequencies the delay doesn't seem too important. A steep slope LP filter must be used or else the midrange "leakage" gives the position of the woofer away. My listening experience and measurements were consistent with the sims.

Very easy to achieve with any AVR. I' wondering why not more people use it. Any other method of reducing modal decay at low frequencies is more complex and expensive.
I believe that reducing modal decay is even more important than flat magnitude response although the latter is a good indicator of the former.
 
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That's what I've found too but it's not necessarily minimum phase only.

Agreed. It depends on how far the source is from the listening position. As the distance gets greater and the modal response becomes more dominate, the MP characteristic is lost. That loss of MP behavior typically starts at higher frequencies where the modal density is greater.

I also agree that with 4th order a cut off of around 80 to 100 Hz seems optimal.
 
Originally Posted by weltersys:
Subs equalized outdoors for the same target response through the passband of 40-120 Hz using a DBX Driverack PA, Smaart Magnitude response, one a small bass reflex, the other a bandpass box.
And then you put them in a room and listened A/B? Where did you place them in the room and where was the listening position (was it always exactly the same)? Did you also take in-room measurements at the listening position?
Markus,

As you wrote in post #115,

“Two systems with very different impulse responses can have exactly the same magnitude response. “

I spent quite some time getting the “Resopump” bandpass sub working well as far as magnitude response. It looked quite promising, but I was so disappointed in the sound quality when compared to any sealed or bass reflex cabinets that the cabinet was recycled and the raw speaker sold. It never made it in to any listening system.

The raw speaker in the Resopump was an Eminence HL-10C, I compared it directly to a ported Eminence Lab 12, and also to a ported Eminence 4015LF, and an old Acoustic Research sealed 10”.

Although the 10” bandpass sub had more sensitivity than all but the 4015LF, the slowly decaying impulse response made music sound like mush, not something I’d recommend to any listener unless they only listened to music with droning bass.

Art Welter
 
Art

I lost track what you did compare A/B and what not. You would need to describe the test more thoroughly. Let's revisit your original post:

In an A/B listening test with two systems equalized for identical response, a sub with poor transient response, as described above, will subjectively sound “tubby” or “slow”. Percussive notes tend to blend together, bass lines become less recognizable.

There really is no transient response at low frequencies in acoustically small rooms. What one hears is the room - the modal decay and the magnitude response aberrations. This makes bass sound “tubby” or “slow”, it's not the source. Of course there's always the chance that a sub didn't work as intended but this should have been obvious from free field measurements.
 
Art


There really is no transient response at low frequencies in acoustically small rooms. What one hears is the room - the modal decay and the magnitude response aberrations. This makes bass sound “tubby” or “slow”, it's not the source. Of course there's always the chance that a sub didn't work as intended but this should have been obvious from free field measurements.

I would certainly argue that point. Room or otherwise, if the source passes an impulse then what is measured or heard at the observation point is that impulse convolved with the transfer function of the room between the source and observation point. It is certainly a transient response by definition. That modal decay is the transient. for any pair of source and listening positions there is a unique transfer function which is (or can be) defined by either the impulse or frequency response observed at the observation point. You may be interested at looking at my full study on room transient response and woofer types.
 
Agreed but this is not how the wording "transient response" is used by most enthusiasts. I argued using the more "colloquial" definition and I'm sure Art did too.
Sorry if my comment wasn't clear enough. Let me rephrase. It's not the source that makes bass “tubby” or “slow”, it's how the source interacts with the room. Better?
 
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Agreed but this is not how the wording "transient response" is used by most enthusiasts. I used the more "colloquial" definition and I'm sure Art did too.
As I wrote in response to Earl in post #108:

“Transient response describes the behavior of a system following a sudden change in its input.
Transient response is not limited to frequencies "above 200 Hz".

My tests of the Resopump sub were outdoors, the sloppy transient response (or impulse response) it had would have only been made worse by room modes.

I agree that in acoustically small rooms, what one primarily hears is the room's contribution to modal decay and magnitude response aberrations, but starting with a sloppy speaker makes the LF sound even worse than it has to be.

Art
 
Agreed but this is not how the wording "transient response" is used by most enthusiasts. I argued using the more "colloquial" definition and I'm sure Art did too.
Sorry if my comment wasn't clear enough. Let me rephrase. It's not the source that makes bass “tubby” or “slow”, it's how the source interacts with the room. Better?

Sure! :)

I found it interesting in my sims that if I included only the axial modes the response was very good and remained minimum phase.
 
As I wrote in response to Earl in post #108:

“Transient response describes the behavior of a system following a sudden change in its input.
Transient response is not limited to frequencies "above 200 Hz".

My tests of the Resopump sub were outdoors, the sloppy transient response (or impulse response) it had would have only been made worse by room modes.

I agree that in acoustically small rooms, what one primarily hears is the room's contribution to modal decay and magnitude response aberrations, but starting with a sloppy speaker makes the LF sound even worse than it has to be.

Art

Agreed that in acoustically small rooms, room contribution will smear the time window, giving a poor transient bass reproduction. But this will not hold true for acoustically large/leaky room or even in near field listening, where the inherent transient response of the bass unit will reveal itself.

From subjective perceptions of many, dipoles sound the closest to musical instruments like kick drums. From all the previous comments about equalised woofers with different loading schemes , it seems that they still do not lead to the same percepetion of transient bass quality. So it seems that none of the modelling programs or even some RTA measurements are able to capture this difference of perception of transient bass , accurately. Although the graphs by the OP do show this difference...however I am not sure of the size of the acoustic environment used for those graphs.

Once again it seems to me that the stored energy of a cabinet (sealed or posted) and the backwave seeping out through the woofer are responsible for degrading the transient response. Whereas in a dipole the back wave just escapes into the room in an accountable manner and so there is minimal stored energy and no seepage through the woofer to smear the transient response. Going by the same logic, a cardioid with a delayed backwave will still have greater energy storage and leakage through the woofer than a pure undamped/stuffed dipole....although this will be less problematic than a sealed/ported woofer.

To prove this point to myself, yesterday I sealed the port of my 84" long Peerless 12"XXLS based bass only MLTL and compared it to my 15" alpha Hframes. I had to put a high pass filter at 40Hz on the now closed TL to compare the same BW. Surprisingly, the subjective transient response of the two loading schemes, namely the Hframe and a long sealed TL was remarkably close !

Any thoughts on the transient response of long closed bass only TLs v/s OB bass.
 
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Earl, did you follow the Fazenda papers in JAES?

Markus

I did find and read that paper, it was quite interesting (I had flagged it to read, but had not finished it). It does not contradict anything that I recommend. They did not test a multi-woofer magnitude response situation.

One thing I should point out in reading the comments here is that the "modal decay" depends entirely on the room and not on the subs or there positions. Even if the direct sound does overwhelm the modal contribution, the modal decay remains the same and will appear after the direct sound has passed.

Only an active system can affect the modal decay and this was clearly shown in the subject paper. Unless you can add-in a sound to cancel the decay (active absorption) it will always remain the same no matter what you do.

This is fundamental to why I have long recommended heavy room damping at LFs. Multiple subs helps the magnitude smoothness problem and the spatial smoothness, but it does not affect the modal decay. This can only be done with sound abosrption in the room - or active absorption. But multiple subs in a well damped room would achieve the ideal as highlighted in that paper.

As to your comment: “Two systems with very different impulse responses can have exactly the same magnitude response. “

This is true in theory, but requires a highly non-MP system to have any real practical effect, i.e. a significant difference in the impulse responses, especially at LFs. At frequencies below 200 Hz room acoustics are close enough to MP that the impulse responses of two systems EQ'd to be the same will also be the same.

Regarding "transient response" - this term is used in two different contexts here and they are NOT the same. All systems have a "transient response" the term comes from the differential equation solution, although it can often be insignificant in a forced vibration system. If the "damping" is high then the "transient" decays before any significant effect is noted and only the forced response is noted. Hence, according to the subject paper, the best bass is achieved when the "transient" is surpressed. When this term is used subjectively it does not have as well a defined meaning. What is a "good transient response"? - its one where there is no "transient at all". Is this what is meant subjectively? "Fast Bass" means no "transient"? Remeber that high damping causes a slower rise time, so "fast bass" would imply to me that the rise time is high and as such the system is underdamped.

Last point: dipoles and cardiods do not excite few modes, that is a falicy. All sources excit all modes to a greater and lessor degree. Dipoles and monopoles excite different modes and do so in different ways. Hence putting a dipole and a monopole at the same room location and comparing them does not prove a thing. Yes, they are different. So what? I did an AES paper some years back comparing dipoles, monopoles and cardioids - no one stood out as any better than another if each was placed optimally.
 
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Markus


One thing I should point out in reading the comments here is that the "modal decay" depends entirely on the room and not on the subs or there positions. Even if the direct sound does overwhelm the modal contribution, the modal decay remains the same and will appear after the direct sound has passed.

That is absolutely true, but it is a mater of relative levels. This in not unlike the critical distance at higher frequencies where the direct sound is louder than the reverberant sound. Example, in a highly reverberant room if you converse with a person who is well beyond the critical distance it is difficult to understand the conversation because what is heard is dominated by the reverberant field and the sound is diffuse and detail is lost. Move close to that person and it becomes clear what the conversation is about. Additionally, the speaker (person talking) can speak more softly and still be heard which further reduces the strength of the reverberant field. If close enough the strength of the reverberant field may drop below audibility.

The same applies at low frequency. As the source is moved closer to the listener the source strength can be reduced while still producing the same direct SPL at the listener's ears. The result is that the room modes are excited to a lower level. They are still present and still decay at the same rate (assuming the room represents a linear system) but their contribution to audible sound is greatly reduced resulting in cleaner bass response.

This situation is easy to demonstrate by making a near field measurement with a specified SPL at the mic and then slowly moving the mic away while maintaining the same nominal SPL. As the mic moves further away from the source the source level will need to be increased to maintain the same nominal SPL and the smooth response measured in the near field will continue to degenerate to one which increasingly shows the impact of the room modes.
 
Markus

Last point: dipoles and cardiods do not excite few modes, that is a falicy. All sources excit all modes to a greater and lessor degree. Dipoles and monopoles excite different modes and do so in different ways. Hence putting a dipole and a monopole at the same room location and comparing them does not prove a thing. Yes, they are different. So what? I did an AES paper some years back comparing dipoles, monopoles and cardioids - no one stood out as any better than another if each was placed optimally.

I would have to disagree with the first part of this statement, but would agree that when optimally placed, good bass can be obtained with any woofer system. The problem is with a full range speaker optimal placement for the bass is not necessarily optimal for the midrange. Not everyone has the luxury of having and being able to place multiple woofers around a room.
 
Markus


Regarding "transient response" - this term is used in two different contexts here and they are NOT the same. All systems have a "transient response" the term comes from the differential equation solution, although it can often be insignificant in a forced vibration system. If the "damping" is high then the "transient" decays before any significant effect is noted and only the forced response is noted. Hence, according to the subject paper, the best bass is achieved when the "transient" is surpressed. When this term is used subjectively it does not have as well a defined meaning. What is a "good transient response"? - its one where there is no "transient at all". Is this what is meant subjectively? "Fast Bass" means no "transient"? Remeber that high damping causes a slower rise time, so "fast bass" would imply to me that the rise time is high and as such the system is underdamped.

.

Have to address this to, though I think it's more interpretation.

I consider transient response to be the time dependent output of a system when subjected to a time dependent input. If the output is a scaled function of the input then the system has perfect transient response with gain. If the output does not duplicate the input by, for example, having a tail which decays after the input is reduced to zero, then the transient is not perfect. A typical sealed box woofer does add a tail to the response. But compared to the typical resonant behavior of bass instruments it is a pretty insignificant factor in reproduction. Room modes, on the other hand, can be high Q and can decay at a rate which is of the same order or longer that bass instruments creating mushy, ill-defined bass.
 
The problem is with a full range speaker optimal placement for the bass is not necessarily optimal for the midrange. Not everyone has the luxury of having and being able to place multiple woofers around a room.

Quite true on the first sentance and a good reason to seperate the bass from the rest. As to placement and numbers its a matter of priorities. Do you want good bass or not? I have three subs, only one is visible, but even then only if you know what you are looking at.

Have to address this to, though I think it's more interpretation.

I consider transient response to be the time dependent output of a system when subjected to a time dependent input. If the output is a scaled function of the input then the system has perfect transient response with gain. If the output does not duplicate the input by, for example, having a tail which decays after the input is reduced to zero, then the transient is not perfect. A typical sealed box woofer does add a tail to the response. But compared to the typical resonant behavior of bass instruments it is a pretty insignificant factor in reproduction. Room modes, on the other hand, can be high Q and can decay at a rate which is of the same order or longer that bass instruments creating mushy, ill-defined bass.

I think that this is precisely the point and I completely agree. The Q's of the room modes swamp out the Q's of the source and there are a lot more of them as well. The whole room situation swamps out any sub configuration considerations in the modal region. So why get so hung up on source type, alignments and all that other stuff. It's just blowing in the wind as far as I am concerned.

It's all the subjective handwaving explainations that I take issue with.

BTW - a closed box does not "ring" (add a tail) if its Q is < .7. And even if the Q is 1.0 the ringing is minimal.

I need to think about your other point, but I can see outright that it likely depends on the characteristics of the source material. If the source drops abruptly then so does the direct field, but the reverb field rings on. The level of the ringing depends on the source level, but not the rate of decay and hence not the duration of the sound. But yes, at some point this has to drop below audibility.
 
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