Bass drop out at 75-55hz

[bpape]

Clint.

It is an interesting analogy. The difference the way I see it is that most of the people who buy Bose are uneducated about sound and/or simply want something small regardless of performance - function following form so to speak. Those in the recording industry who are designing these rooms are presumably very educated with regard to the need for treatment.

Gene.

I don't know what misinformation you're speaking of. If you'd read my posts on many forums, you'd know that I do absolutely push multiple subs and proper setup of speakers and seating. I also am a proponent of parametric EQ as part of an acoustical solution. I don't doubt the research and the theory, it's the real world implementation that makes it difficult.

The issue with EQ is that it must be done EXACTLY at the frequency in question in order to have a positive effect on time issues. The very fact that every seat in a room has different peaks and nulls even with 4 subs in the room makes broadband decay time control exclusively by EQ unrealistic. That doesn't mean I don't think EQ shouldn't be used - just that it isn't a total fix-all. By the way, how does one fix a null with EQ? Even with multiple subs and careful placement, there will still be some nulls - and some that can be very noticable.

As for building treatments in, absolutely. If someone is starting from scratch and has that option, by all means that's a much cleaner look. Unfortunately, that is MAYBE 5% of the people out there looking for better sound. Most of them have real rooms with openings into other spaces, large glass windows, coffee tables, fireplaces, etc. that they simply want to improve.

Bryan

 

[Savant]

The discussion of the use of passive absorption for low frequency control and "how-much-is-too-much" should be tempered with an understanding of what is and is not possible. Conventional "bass traps"* typically can only help down to the 60-80 Hz range. Some "traps" can get to 50 Hz, but their effectiveness is typically tapering off at that point relative to the "higher" low frequencies. If I'm not mistaken, the upper end of a subwoofers function is typically 80-100 Hz. So it should be stressed that "bass traps" are not actually going to help with much of the actual low frequency goings-on in a typical home theater. Understanding this should put Gene's argument into perspective. Using only conventional "bass traps" to treat LF problems in a room with strong modes in the 20-80 Hz range is likely to yield disappointing results. On the other hand, if all the strong modes fall into the 40-100 Hz range, as will be the case in many smaller rooms, then the value of the "bass trap" goes up relative to active control.

It should also be noted that the effective range of control for high frequency devices - e.g., acoustical wall panels - starts tapering off below 250-500 Hz depending on thickness. Thus, there is ample argument for using a goodly amount of "bass traps" to bridge the gap between active control below 60-80 Hz and the passive control above 250-500 Hz. I think Dr. Toole alludes to this in some of his presentations. The amount of "bass traps" for this type of application will be higher relative to the number of thinner acoustical panels used for reflective control. Alternatively, panels which incorporate both "upper bass" and mid/high frequency control could be used throughout. This, I believe, is the general approach being supported/promoted by companies like GIK and RealTraps.

Whether or not the manufacturers of low frequency control products "believe in" active control is moot. It works whether they are willing to believe it or not. In fact, (and IMO) it is in their best interests to help promote the use of forms of active control since they are the best available technology for addressing problems in many rooms that their products simply are not physically able to control.

*For the purposes of this discussion, I would like to clarify that by "conventional," I mean "bass traps" you can actually reasonably implement in a typical home theater room. There are passive devices on the market capable of addressing the 20-80 Hz range quite effectively. However, it is my opinion that these sorts of products are not a reasonable solution for the typical home theater since they are usually very heavy, bulky devices that are more conducive to building into a room rather than hanging on a wall or ceiling.

 

[gene]

I don't know what misinformation you're speaking of. If you'd read my posts on many forums, you'd know that I do absolutely push multiple subs and proper setup of speakers and seating. I also am a proponent of parametric EQ as part of an acoustical solution. I don't doubt the research and the theory, it's the real world implementation that makes it difficult.

The issue with EQ is that it must be done EXACTLY at the frequency in question in order to have a positive effect on time issues. The very fact that every seat in a room has different peaks and nulls even with 4 subs in the room makes broadband decay time control exclusively by EQ unrealistic. That doesn't mean I don't think EQ shouldn't be used - just that it isn't a total fix-all. By the way, how does one fix a null with EQ? Even with multiple subs and careful placement, there will still be some nulls - and some that can be very noticable.

The problem I have with the literature on your site as well as Ethan's is you both claim active equalization can NOT resolve modal ringing. This in NOT true. PEQ will resolve modal ringing issues if the correction is benefical in the frequency domain. In addition, using multiple subs will reduce the degree of peaks and nulls at all seats, reducing the need for passive treatments.

Actually there are far more sophisticated equalization systems on the market today that allow for multiple seat correction using fuzzy math to determine the best equalization for all seats. Its really not that difficult if you're an experienced installer who knows how to use an RTA.

Nobody said an EQ is a total fix all. Nothing truly is, which is why you use a combo of many methods prescribed here including multiple subs, EQ, some passive treatments.

 

[bpape]

Hi Gene.

I agree with pretty much everything you just said. My original post was in response to your statement that most rooms need no bass absorbtion.

EQ can certainly help with modal peaks - just not with nulls. As for the fuzzy math, sure, but it's an admitted compromise to come up with the best curve for all seats and not the best for any single seat. This contradicts somewhat the effects of EQ to minimize ringing since the EQ has to be centered and with the proper Q to exactly match a mode in order to have time domain benefits. Is it better than nothing? Absolutely. Can it do a good job? Absolutely. Can it deal with a situation where you have a peak at 50Hz at one seat and a dip at 50hz at another (very common)? No.

I'll state once again that I absolutely believe that anyone really wanting to deal with their room issues, in most cases, is best served by a combination of proper seating and speaker placement, PEQ, and passive treatments. Yes - multiple subs can help quite a bit too - but realistically, what % of the users have that? Many of them have a hard enough time getting 1 sub in an optimal position over the objections of the significant other. Most of the time, they're relegated to corner placement which only exacerbates the modal issues by exciting all of them to the maximum extent. How are they going to deal with 2 - much less 4? Now granted, getting treatments in a room can be just as hard a sell.

Bryan

 

[Savant]

Can it deal with a situation where you have a peak at 50Hz at one seat and a dip at 50hz at another (very common)? No.

Why not? It is agreed that PEQ is used to cut, not boost. (Goes without saying, I hope.) So, if (from your example) 50 Hz is cut in order to reduce the peak, the system will no longer be providing as much excitation at that frequency. Therefore, the 50 Hz null will also be affected (in a good way) by reciprocity, as it were. Take it a step further: If 50 Hz were completely removed from the system, there would be no room excitation at 50 Hz and no peak or null. (Of course, there would be a "null," but not in the sense of a cancellation - 50 Hz would simply be absent.) Cutting 50 Hz a little, but not completely, to smooth out the effects of both peak and null does seem to be a reasonable comprimise for the benefit of the entire listening area.

As you said, it's certainly better than nothing...

 

[bpape]

Interesting thought Jeff. Not sure - would have to try it and see. If that's the case, then are you proposing that we'd use an EQ to reduce a null that's present at all seats? So if I had the same 50Hz null at all seats instead of a peak at some of them - cutting the 50Hz band with EQ will reduce the null? If it works when you have both, it should work with only a null - right? Not thinking so.

This is where I feel passive treatments are more beneficial. They WILL work as you've just described. When absorbing the energy, you're minimizing one side of the wave front so it's can't constructively or destructively interfere. - and it's the same for every seat.

Bryan

 

[Audioholics]

So if I had the same 50Hz null at all seats instead of a peak at some of them - cutting the 50Hz band with EQ will reduce the null?

Unlikely, but you can also shift the seating forward or back, move the sub or tweak phase. Barring that you could hang 42 bass traps from the ceiling in the hopes that it helped - just kidding.

 

[audyssey]

Here are some of my thoughts based on our experience.

The thinking that a peak in one location and a dip at another can't be fixed simultaneously comes from the parametric EQ paradigm that operates on the magnitude of the response in the frequency domain only. With PEQ, Bryan is correct. This can't be done.

However, if you are allowed to use EQ that simultaneously operates on the time domain of the response then you are given more degrees of freedom. Taking multiple measurements and then combining them in a weighted way (not simple averaging that gives each the same weight) can actually achieve simulatenous peak and dip correction.

Note, that this has its limits and will depend on how deep the nulls are and how many normal modes are responsible for causing them.

Also, the notion that measuring multiple locations around the listening area compromises the "money" seat is flawed for the same reason. Time domain information captured around the listening area provides the EQ system with more control points to correct the sound arriving there including the money seat. You actually get better money seat performance by capturing points around it. Again, the single seat EQ notion comes from years of thinking about EQ as parametric bands based on measurements from a single point. There is research literature that shows the limitations of such methods.

Best regards,
Chris

 

[bpape]

Excellent response Chris - thanks. You bring up a good point about the severity of the peaks and nulls. If you've done everything you can via positioning and phase adjustments, and still have a swing of 15db to 20db or so from seat to seat at the same frequency center, that's too much for an EQ to overcome.

Also, one needs to consider the fact that even if the 'center' of the issue is at the same point, seat to seat variance due to overlapping tangential and oblique modes can yeild different Q's for different seating positions.

I would agree that in the case of a home theater, it's best to work toward getting 'no bad seats' as opposed to 1 optimal one.

Clint,

Agreed. One can shift seating forward and back and tweak other things to help. But there will still be some issues - especially in a home theater environment where you have seats side by side - potentially 4-6 wide.

A single row may have all the same length and height related modal issues (axial) but will all have different modal issues related to the width. This doesn't even consider all the differences in tangential and oblique modes.

Add in a second row of seats which again have the same different side to side issues but also all have similar (but different from the front row) issues than the front row.

Realistically, you're not going to get 8 seats positioned where none of them have significant FR issues. But now lets look at a different scenerio. Let's say we did get them all pretty flat simply via positioning and phase adjustments. How do we then deal with decay time? Having flat (relatively) response but leaving the decay time in the bottom end up in the 2 second range isn't desirable.

And no, I wouldn't ever think about putting 34 bass absorbers in a room unless it was absolutely huge. In a normal sized residential space for home theater usage, I generally find that it's more in the neighborhood of 50 - 60sq ft in some combination of treatments that will reach down into the subwoofer range. This would equate to 2' wide floor to ceiling absorbers straddling the front 2 corners and then adding additional control where required and feasible depending on room design and construction. Some rooms need more, some less.

One of the key factors is the construction of the room. Is it 16 or 24" on center studs? Is it single or double drywall? Are the cavities insulated? All of those things will alter how much the room itself is capable of absorbing and in what frequency bands.

A room with 16" on center steel studs and double drywall will require more additional bass control to come into the target range than a wood stud, 24" on center with single drywall for instance

Bryan

 

[gene]

Bryan;

I think your 2 sec decay times are a bit extreme and not typical in most home theater rooms. As chris has mentioned FIR base systems do frequency and time domain correction. Seat to seat variation goes down significantly when you use multiple subs. Imagine if you used a combo of passive treatments, multiple subs, and proper EQing what you could achieve?!?

I am working on an article that measures bass response at each listening seat before and after equalization. Stay tuned....

 

[bpape]

Agreed Gene. The combination of all of those things is absolutely the best way to go - don't think I've ever said any different. On the decay times, I think you might be surprised what it is at around 250Hz if you ever did the math on say a 14x17 room with single drywall, 2-3 seats, and a couple people.

When you do the article/measurements, please include decay times and not just frequency response.

Also, for more real world situations for many, it might be interesting to try it with only sub and in realistic positions that would be tolerable visually.

Bryan

 

[Glenn Kuras]

Of course you and Ethan have never been in rooms that don't need bass traps. You both making a living selling them exclusively

The truth is, if you use multiple subwoofers, proper speaker and seating placement, the need for bass trapping goes down significantly. As a prime example, I have no bass traps /absorption in my room (other than my riser) that functions below 100Hz and the bass response in my room is stunningly linear at all listening seats.

Bass traps are a tool to use (if needed) but NOT something that should be overly abused at the risk of acompromised esthetics, sacrificing amplifier power, and creating an anechoic type room response.


Dr. Toole discusses this in his CEDIA training courses and also has several white papers on the topic of modal control in small rooms using multiple subwoofers.

Your right I do sell bass traps exclusively and it was not something I got into because of wanting to get "rich", but working within my own room I found that room treatment made a HUGE improvement to the over all sound. I think every thing you said has truth to it so there really is no fight at all. If you feel as your room is at a point that it is at the best it can be then HEY it is your room and you should do with it what you think is best. Myself and Bryan are not out to cram room treatment down anyone’s throat or say "YOU MUST HAVE" bass traps, but merely pointing out the benefits of a well tuned room.

Glenn