Low freq. decay time question

There's no single number, all rooms are different, and all rooms have longer decays at modal frequencies than at non-modal frequencies. But to answer your question anyway, I'd aim for 200 milliseconds or less in most domestic size rooms. Below is a waterfall plot for my living room, which has plenty of bass trapping, and the decay times are less than 200 milliseconds (to -30 dB) at all but the very lowest frequencies. This is the tightest sounding room you'll ever hear.

--Ethan

Scott that is not looking so bad. What is your room size, how are you set up (acoustics, speaker and setting) and what kind of treatment do you have now? 40hz to 80hz needs a little help.

Glenn

Remember that decay times are measured as RT60 values, Ethan. This is the time for sound to decay by 60 dB, not 30 dB. So if decay is 30 dB over 200 msecs, the RT60 is 400 msecs.

- Terry

What kind of eq are you using? Have you experimented with plugging the port (if any) on your speakers? Perhaps 40hz is the tuning freq. of your speakers and if you plug the port this effect might be lessened?

Wow for only 2 bass traps that really looks pretty nice. Most people, for a room your size I say 4 bass traps at min.
If you have a moment take a look at the following link of a case study done by Ryan Allen (AVS member) http://www.sbrjournal.net/journalsit.../Acoustics.htm it outlines 4 traps vs 8 traps and shows some nice before and after graphs.

As to your room, I would think the more bass traps you add the better the 40 to 80 hz area is going to look (or should I say sound). If you do decide to order say only 2 more, I would take a good hard look at the Tri Trap we have (if your looking at us that is). It really kicks some butt down past 50hz to 40hz very well.

Glenn

That is the problem with correcting rooms with eq. You can get the initial sound nearly flat, but it doesn't do anything for ringing. You might play with speaker position if you have not already (might not be much wiggle room in your room, but it can't hurt to experiment with speaker and listening position - by moving speakers you might excite that mode less, by moving the listener you may not hear it). It is going to take a lot of broadband absorbers to remove that 40 hz resonance. You might consider building tuned absorbers for that frequency.

Unless I misinterpreted the info on Gik's site, the 244 absorbs more effectively than the tri-trap. 16 sabins at 80hz, vs an extrapolated 11-12 Sabins for tri-trap??

I guess this would be more a question towards Glenn, perhaps he can shed some late on per-unit sabin measurements and interpretation of...?

The obvious solution is more treatments, especially ones that work in your trouble range (without killing upper frequencies too much - the ideal seems to be have all frequencies fall off equally in time, or at least smoothly).

You might also try not squashing the 43hz peak quite so much with eq. Sometimes it sounds more natural to have a peak in the long time response be present in the initial response as well. Anyway, it is something to try. Another thing to consider is door openings, if you happen to have a door near a corner the response might change if the door is open or closed (leaving it open might be to your advantage).

The next step would be something that also works in the time domain. Tact is one. There are others. This is an evolving field. . . But room treatment will likely always be the best thing to do first IMO.

Of course I know that - which is why I was careful to qualify it by saying 30.

--Ethan

I'd like to state explicitly what's been said implicitly: you can't do anything about the decay time using EQ. This is why EQ isn't all that useful. If the room has a mode at 41 Hz, then any sound at 41 Hz will excite that mode, and ring with whatever dB60 time of the room. Changing the EQ doesn't change the fall-off time at all, only the tonal balance.

If your living room "looks like Tweeter," can you buy nice-looking sound absorbers that hide whatever the unwanted stuff is? Kill two birds with one stone :-) Bass traps don't HAVE to live in the corners, it's just easier to do it there.

If you want really nice looking traps, try the SonoSuede from Auralex. Many hundreds per panel, though.

Not quite true. If the room mode is isolated from others, it is a simple 2nd order minimum phase resonance. This can be cancelled, both in frequency and time (decay) with a parametric equalizer if you match the frequency, bandwidth, and gain accurately. Since this forum's name begins with "Audio theory," at least some readers should know about such signal processing technicalities.

What you say is true for an overlap of modes, where 3 dimensionality comes into play, and the room is no longer strictly minimum phase. The modes have delays with respect to each other, and this creates a non-minimum phase condition. Increasing in frequency, as one approaches the thorougly overlapping modes of the reverberant sound field, you are pretty much helpless in effecting the decay time via EQ.

Matching the 3 parameters of an isolated room mode isn't easy. But trial and error will eventually pay off. I succeeded in improving the decay for exactly one room mode in the experiment with Ethan, as John Mulcahy's post-mortem analysis showed.

Regards,
Terry

Yes, I give you that. And my hat off to you for managing to actually do that in real life in one instance! It's not something I'd even try -- along with climbing really tall mountains, running a Marathon, etc. It CAN be done...

Have you looked into the room correction systems from Meridian and Lexicon? Both work the way Terry described, building filters to reduce long decay times rather than attempting to flatten out the frequency response.

Neither system corrects above 250Hz, because wavelengths become too small and room modes can't be isolated properly. For the rest of the frequency range, both companies recommend passive treatments.

Good explanation in this review:

http://www.hometheaterhifi.com/volum...4-12-2004.html

Sanjay

The Tri Trap shines below 80Hz in comparison to the 244.

In general, you'll need more square footage for broadband absorbtion.

In a room that size, while the decay times are not bad, they're not where they should be. You still have some issues below 80hz that are very long in decay time that need to be addressed. Have you also put up anything for reflection control? Front wall absorbtion?

Bryan

I realize now that I may be thoroughly behind the times in trying to do this correction manually, using frequency response flatness. I would try JohnPM's very excellent Room EQ Wizard for finding parametric filter settings. I invariably use it to collect impulse responses for further analysis, for which it does a superior job.

A minimum phase system has the wonderful property that any correction (valid inverse filter) applied in the frequency domain will also automatically correct the time domain -- and vice-versa. Parametric equalizers are composed of second order filter sections which are necessarily minimum phase.

After studying some of the literature on minimum vs. non-minimum phase responses of rooms, I have learned a lot. For one thing, there is a non-minimum phase portion of a room response, which has been found to be due to the introduction of zeros in the room transfer function. For non-geeks, zeros create resonant dips, whereas poles create resonant peaks.

So any attempt to smooth out resonant dips with a parametric equalizer will not address the non-minimum phase room component, and is probably destined to failure for time domain correction. Of course, any boost in a parametric equalizer is not a good idea, but now I better understand some of the mathematical complications. You must ignore the dips even when finding the inverse filter for the peaks, because any such atttempt can lead you down the unsuccessfu non-minimum phase correction path.

I have since done a bit of my own research using a signal processing transformation called the "complex cepstrum," which has the amazing ability to tease out minimum phase from non-minimum phase signal components. I was able to confirm earlier published research about non-minimum phase room conditions.

Sorry about the technicality of this discussion, but it may provide some useful insights for those who understand it.

Regards,
Terry

Terry, that sounds like fun :-)


Do I feel a business idea coming on? Note that myfipie (Glenn from GIK Acoustics) is selling panels with Guilford of Maine cloth on them. And I really think you should at least look at the SonoSuede panels. Auralex also has a diffuser made of wood, which would go well in any home interior. $600 per pair.

You ain't seen nothing yet!!!!!!



Glenn

Open the windows.

Well, the bass goes through even when the windows are closed so they are reflecting only HF. That makes it somewhat worse than a regular wall.


They can market it with more imagination.