From all I know "proper bass traps" mean structural changes in a room's walls. Virtually nobody does this. It's also nothing that can be done with porous absorbers.
I'd really like to see people post excess group delay along side frequency response when showing room bass measurements - especially when comparing discrete woofers against a good multi-sub setup, as I believe there is useful untapped information in the excess group delay to help identify whether a system is properly set up.
Why is it relevant ? Two reasons - the first is that it shows whether a rooms bass response is successfully equalizable, at least in a single position in space.
A notch due to boundary cancellation can't be successfully equalized even at a single point in space, partly because you will get a huge group delay spike near the frequency of the notch.
In my own room measurements I see a group delay peak as high as 80ms that is due to boundary cancellations - I have one at 73Hz in my room (no multi-subs here, alas)
This huge group delay peak means that the room is nowhere near minimum phase around that frequency at that listening point, so any EQ applied to try to fix the frequency response flatness is going to mess up the impulse response even further at that listening position let alone other locations in the room.
A high peak in excess group delay at bass frequencies is an indication that some delayed reflections in the room are greater in amplitude than the direct path at that frequency - probably because the direct path is being largely cancelled by a 1/2 wave delayed reflection. So its a good indicator of destructive interference, and a good indicator of which parts of the spectrum cannot be successfully EQ'ed.
(What's interesting too is in my room at least the peak of the excess group delay is near but does not coincide with a notch in the amplitude response - the amplitude response notches at 66 and 44Hz in a particular room measurement where the group delay spikes at 73Hz, and I'm sure that there is some significance to the fact that they don't exactly coincide in frequency, although I'm not sure what at the moment without having a larger array of measurements from different rooms and speaker/mic positions...)
Secondly I think its a good indicator of successful multi-sub placement, over and above frequency response flatness. One of the key aims of multi-sub should be to attempt to make the response as minimum phase as possible in as wide a range of listening positions as possible...so that any remaining variation in response can be successfully EQ'ed.
Any large peaks in excess group delay at certain desired listening locations are an indication that there are still significant problems with destructive interference that are likely to cause large shifts in frequency response as you move from that exact listening position. So even a measurement taken at one location can warn you that you're likely to see large shifts in response with a small shift in location, since cancellations tend to be highly specific in their location, whereas modes result in more gradual shifts.
Looking at both frequency response and excess group delay when setting up a multi-sub system should in theory lead to a better selection of sub locations with a bit less trial and error.
Why is it relevant ? Two reasons - the first is that it shows whether a rooms bass response is successfully equalizable, at least in a single position in space.
A notch due to boundary cancellation can't be successfully equalized even at a single point in space, partly because you will get a huge group delay spike near the frequency of the notch.
In my own room measurements I see a group delay peak as high as 80ms that is due to boundary cancellations - I have one at 73Hz in my room (no multi-subs here, alas)
This huge group delay peak means that the room is nowhere near minimum phase around that frequency at that listening point, so any EQ applied to try to fix the frequency response flatness is going to mess up the impulse response even further at that listening position let alone other locations in the room.
A high peak in excess group delay at bass frequencies is an indication that some delayed reflections in the room are greater in amplitude than the direct path at that frequency - probably because the direct path is being largely cancelled by a 1/2 wave delayed reflection. So its a good indicator of destructive interference, and a good indicator of which parts of the spectrum cannot be successfully EQ'ed.
(What's interesting too is in my room at least the peak of the excess group delay is near but does not coincide with a notch in the amplitude response - the amplitude response notches at 66 and 44Hz in a particular room measurement where the group delay spikes at 73Hz, and I'm sure that there is some significance to the fact that they don't exactly coincide in frequency, although I'm not sure what at the moment without having a larger array of measurements from different rooms and speaker/mic positions...)
Secondly I think its a good indicator of successful multi-sub placement, over and above frequency response flatness. One of the key aims of multi-sub should be to attempt to make the response as minimum phase as possible in as wide a range of listening positions as possible...so that any remaining variation in response can be successfully EQ'ed.
Any large peaks in excess group delay at certain desired listening locations are an indication that there are still significant problems with destructive interference that are likely to cause large shifts in frequency response as you move from that exact listening position. So even a measurement taken at one location can warn you that you're likely to see large shifts in response with a small shift in location, since cancellations tend to be highly specific in their location, whereas modes result in more gradual shifts.
Looking at both frequency response and excess group delay when setting up a multi-sub system should in theory lead to a better selection of sub locations with a bit less trial and error.
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I have done this, and yes it is rather dramatic both in the scale of what need to be done, and in sound quality. I have both panel absorbers and helmholz resonators.
Shorter decay at low frequencies is definitely audible, although I agree that long decay at bass frequencies is less disturbing than at higher frequencies.
A thing worth mentioning is that bass traps dont change the room's frequency response more than just a few dB at best. But they to shorten the decay, and that's why people want them.
The more recent speaker development programs extend the design of the drivers to include the behaviour of "the box" and use various techniques of absorbtion, diffusion, difraction, etc that don't rely on "guesstimates", and our common diy "rules of thumb"
It is unfortunate that the programs are neither cheap, simple, and require technical ability, etc, but the information and design concepts are readily available - just need a bit more 'outside the box' thinking for many of us, especially in regard to this "box colourations" thing! (we're not exactly talking about "Clio" on Mac here)
Markus, sorry, but what are "proper bass traps" in regard to domestric hifi situations and I assume that you meant that the world of "porous absorbers, etc" that is so well known in the "pro-audio" and acoustic engineering industries is virtually unknown in the "land of diy".
I'm really surprised that some of the ideas of Michael Green (Tuneland, etc) haven't been mentioned in regard to room acoustics, box resonance control, etc, etc - some really easily adapted techniques for diy.
We have to get our thinking outside the "traditional box" approach ....
It is unfortunate that the programs are neither cheap, simple, and require technical ability, etc, but the information and design concepts are readily available - just need a bit more 'outside the box' thinking for many of us, especially in regard to this "box colourations" thing! (we're not exactly talking about "Clio" on Mac here)
Markus, sorry, but what are "proper bass traps" in regard to domestric hifi situations and I assume that you meant that the world of "porous absorbers, etc" that is so well known in the "pro-audio" and acoustic engineering industries is virtually unknown in the "land of diy".
I'm really surprised that some of the ideas of Michael Green (Tuneland, etc) haven't been mentioned in regard to room acoustics, box resonance control, etc, etc - some really easily adapted techniques for diy.
We have to get our thinking outside the "traditional box" approach ....
Not sure what you're trying to say. I was talking about room treatments for frequencies below 100Hz.
Maybe nobody mentioned it because when you Google for it you just get a lot of esoteric verbiage instead of facts.
Sorry Markus - not sure what you meant by "Proper Bass Traps" that seems to be involved with structural changes - in pro-audio this is standard but practically unheard of in hifi, even with house remodelling.
The other bit about absorbers is just about a very frustrating attitude of many folks in the diy community that don't read/use the many alternative techniques of controlling bass problems in domestic rooms, is all - no problem with the standard references of Toole, Angus, Everest/Pohlmann, D'Antonio, etc and no criticism meant.
Yes, freq's generally about/below 100Hz
The other bit about absorbers is just about a very frustrating attitude of many folks in the diy community that don't read/use the many alternative techniques of controlling bass problems in domestic rooms, is all - no problem with the standard references of Toole, Angus, Everest/Pohlmann, D'Antonio, etc and no criticism meant.
Yes, freq's generally about/below 100Hz
Good question. One must wonder why none of the multiple subwoofer proponents are presenting such data.
Modulation transfer function has been used many decades in audio by researchers. Traditionally has been used mainly in analysis of speech perception, but lately been transposed to low frequency room analysis as well.
Regarding this specific study, the dipole was equalised only for 6dB/oct roll off, no other equalisation was used. Naturally monopole don't exhibit this roll off. It's fair to compare speakers with similar freq response, right ?
Edit: It would be interesting to provide more exhaustive research on this matter (including varied location), but at the moment I'm more involved in other frequency range than bass. Got to get those tweeters in control
- Elias
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There exists several academic articles on modulation transfer function MTF applied to speakers in rooms. Internet search engine is your friend.
Modulation transfer function as a measure of room low frequency performance
By BM Fazenda, KR Holland, PR Newell and SV Castro
CORE
Excess phase effects and modulation transfer function degradation in relation to loudspeakers and rooms intended for the quality control monitoring of music
By KR Holland, PR Newell, SV Castro and BM Fazenda
CORE
Interesting is that they conclude:
"... the application of equalisation has reduced the low-frequency information loss for the 1m distance but there is only marginal improvement at 4m distance."
It looks like equalisation is not the key thing to do to have low information loss at low frequencies with loudspeakers in rooms !
What you don't seem to grasp is the freq response, modal notches etc are symptoms the listener is fully being exposed to only under steady state signal conditions. Most music will not do that. Thus the perceptionally derived differences between, say monopole vs dipole, cannot be concluded from steady state measures.
I recall MTF ws introduced with MLSSA in the 80's (?), and while their definition of MTF differs from mine since I'm expanding the wavelet analysis for the task so I'm more into temporal aspects. But then there is TMTF (temporal MTF) which could find it's use here too. TMTF is in good use in the research field at the moment. Hope to see it's usage would spread.
- Elias
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If you read carefully what I wrote, I never suggested that equalisation alone was a solution to bass problems in rooms, quite the opposite in fact.
Equalisation by itself doesn't work for two reasons - one is that at a typical listening distance with 2 conventional full range speakers the room transfer function is not minimum phase, there are regions that are, but there are parts of the bass range that are very non-minimum phase, for example my room where the excess group delay peaks as high as 80ms at 73Hz, which is plenty high enough to be audible.
The reason why their tests show that the MTF improved with EQ at one metre but not at 4 metres is because one metre in front of a speaker in a typical room the bass response will be more or less minimum phase, with no non-minimum phase frequency ranges caused by room reflections.
This is easily verified empirically - if I measure bass response one metre in front of my speakers there are no large peaks in excess group delay in the bass, thus no significant excess phase components. If reflections are all lower in amplitude than the first arrival the result can only be minimum phase.
The fact that EQ works to improve the MTF at 1 metre where the response is minimum phase simply serves to show that EQ does work if the response that you're EQ'ing is minimum phase.
When you measure the same room response 4 metres from the speakers the response is now no longer minimum phase (some reflections at certain frequencies are stronger than the direct signal) so minimum phase EQ will not work. Minimum phase EQ can't fix non minimum phase response errors. No surprise there.
That's part of the reason why I suggested measuring the excess group delay at the listening position to see whether there are issues with non minimum phase response that would prevent equalisation from being effective.
The second reason EQ alone isn't effective is because even if the response is minimum phase at one listening point, any applied EQ will be wrong, usually worse than no EQ, at other locations.
Modal smoothing using multiple distributed subs addresses both issues - variations in response across a range of listening positions will be greatly reduced, thus making EQ to a single target possible, and secondly correct placement of multiple subs can potentially eliminate any non-minimum phase sections of the bass response at one location and perhaps even over a wide range of locations.
Once both those are dealt with, minimum phase EQ can "mop up" what is left of the response error to give the best possible time domain response and MTF.
I only skim read the articles but as far as I could see they did not test a multi-sub arrangement, so their article says nothing about whether such a system is effective, nor whether EQ is effective at improving MTF in a multi-sub arrangement.
None of that changes the fact that if the transfer function of the room is minimum phase, and we apply minimum phase EQ to achieve a flat frequency response, we must by definition fix the signal in the time domain as well, fixing all ringing and MTF deficiencies, at least at that one listening position.
Its because conventional arrangements of two main speakers at the far end of a room produce a room response that is non minimum phase that EQ fails to achieve this, even at one listening location, and because the transfer function changes so dramatically as you move location in the room that no one particular EQ target is viable even if it were minimum phase.
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and what about the impulse response of such systems? I haven't seen any mesures.. but I may be blind..
has any serious study compared all options in one go? single monopole/dipole/cardioid, multiple locations for each, line arrays for each? unequalized, obviously..
ok I am half blind..
http://www.diyaudio.com/forums/mult...x-vs-dipole-line-array-how-overcome-room.html
http://www.diyaudio.com/forums/mult...x-vs-dipole-line-array-how-overcome-room.html
HI .
This topic is interesting.
I built a Karlson using a driver that was originally inside a bass reflex case. I built it differently than the original by reusing a 12" woofer from an unused bass reflex container. Only the woofer without crossover to experiment on low frequencies, that's where Karlson gets interesting.
when I listen to music I hear a box effect and I struggle to understand if it is due to the room modes or the speaker enclosure.
I tried a software level EQ with pace and I have to say I hear differently.
however, since the container is made differently than the original, both chambers, rear and front, are smaller.
the rear chamber (K12) has a volume in liters which is approximately half of the original bass reflex container.
if the volume of the rear chamber must correspond to the VAS of the woofer then perhaps this is the cause, chamber too small.
what do you think ?
This topic is interesting.
I built a Karlson using a driver that was originally inside a bass reflex case. I built it differently than the original by reusing a 12" woofer from an unused bass reflex container. Only the woofer without crossover to experiment on low frequencies, that's where Karlson gets interesting.
when I listen to music I hear a box effect and I struggle to understand if it is due to the room modes or the speaker enclosure.
I tried a software level EQ with pace and I have to say I hear differently.
however, since the container is made differently than the original, both chambers, rear and front, are smaller.
the rear chamber (K12) has a volume in liters which is approximately half of the original bass reflex container.
if the volume of the rear chamber must correspond to the VAS of the woofer then perhaps this is the cause, chamber too small.
what do you think ?