Fact or fiction re: low frequency sound waves (1 Viewer)

[Keith Barnett]

Fact or Fiction on low frequency sound waves?

In a 22' X 14' listening room playing a frequency at 20 Hz will you hear a true 20 Hz frequency.

OK here is the scenario. This room is acoustically tight and there are no openings in the room for sound to escape. Given the fact that the tighter the sound-wave the higher the frequency. Does the wave compress in this room? If the wave does compress then would it be safe to say that the frequency is then modified and what you really hear is say beefed up 35 Hz or so? This has nothing to do with any ones sub-woofer, or specs, or independent reviews, etc.. I would like an opinion based solely on physics of sound.

 

[Tom Vodhanel]

The wave doesn't compress, it reflects...the 20hz tone stays true. If we needed a soundwave to unfold completely to hear its frequency...headphones would be useless(at least under what...500hz or so?).

TV

 

[Keith Barnett]

"The wave doesn't compress, it reflects...the 20hz tone stays true"

But now are'nt we talking standing wave? Which is not what we want. In a listening room that is acoustically set up correctlly you would not want low frequency waves reflecting off of the walls, which is the case in my theater room.

 

[Dustin B]

Room dimensions dictate which frequencies will form standing waves. There should only be three and their fundimentals. There should be one for length, one for height and one for width. Without some very elaborate room treatments they are impossible to avoid.

The easiest and cheapest way to compensate is to try and place your sub so that you only have reasonances that cause peaks rather than nulls in output. Then add an eq to tame those peaks.

Once frequencies start getting longer than your room dimensions you get into full room pressurization. I don't understand this concept to well. Hopefully Tom will come back and elaborate a bit more on the subject.

 

[Keith Barnett]

Dustin, Tom

Thanks for the input.

 

[Chris PC]

Thats really good advice. Simple and to the point. Now if only it were that simple to fix I often think about getting an EQ. Technically, it would ad a fraction of headroom to the bass also, would it not? Either way, I guess EQ'ing that way is the most logical way to get the best, flattest bass.
How should I approach trying to do this with a test tone CD? What is the least expensive EQ that is worth while to use for this?

 

[Dustin B]

I haven't done any of it yet, but my reading would have me firstly looking at the Behringer Feedback Destroyer ($140USD). Then a trial version of some audio software that allows you to quickly get a graph of the frequency response would make it easier and quicker to adjust. Which one I don't know yet.

 

[Keith Barnett]

"Then add an eq to tame those peaks. "

I have heard that there are trade offs using an EQ. However on this board I see that the practice is often used. What are the trade offs if any when using an EQ to tame peaks. I can think of many if using the EQ to increase bass at a certain freq. I don't need an external EQ because my pre/pro allows you to setup a notch filter to do just what you are speaking of.

 

[Patrick Sun]

It's not a good idea to use an EQ to try and fill in the nulls, it just overdrives the amp, and you still don't get the boost in the nulls.

 

[Manuel Delaflor]

Keith,

Are you "MM" on the Klipsch Forum?

In case you are not, here is a copy of my two replies to a similar thread in that forum:

_______________________________________

"Perhaps this image can help. Imagine that we are talking about pools. In your view, if a wave is so big that, to accomplish its cicle, it would require several pools, you are not experimenting the wave. Right?

But we are talking here about the pressure a wave can make. And of course, no matter how big the wave is, its movement will MOVE WATER, it doesn't matter if their cycle is not completed."

________________________________________

"Another example, why do you think that a small sub like the Sunfire's can put almost as much deep bass as a "giant" (by comparision) SVS 20?

Because the pressure is there. We are talking here about the continuous application of a concrete force. If the force is transmited the sound will be there, it doesn't matter the size than the wave need to "complete" a cycle.

The wave (or our ears) don't care *at all* about this (conceptual) "completeness"."

 

[Keith Barnett]

Patrick,

The notch filter on my pre/pro will only allow you to remove the peaks. It will not increase in db only decrease. Using EQ to fill in the nulls is one of the trade offs I was thinking about due to putting higher demand on the subs amp to increase a certain frequency.

 

[Keith Barnett]

manuel,
yep, he is me. Im straight on the soundwave topic. That conversation was giving me a brain cramp. Its really not that important as long as your system sounds good to you, but you have to admit it was a interesting conversation. I have learned that folks get pumped when talking about the infamously sought after low frequencies.

 

[Manuel Delaflor]

Keith
I understand you
Yes, it was interesting. In fact, I am very interested in the physical aspects of high quality audio. Im reading a lot about speaker measurements, what is exactly objective data, why is relevant and such topics.
But I find relatively little interest in those matters, most people seems more interested in have something to presume to others (my speakers are better than yours), than to understand what is really happening when they hear something they feel is "accurate".

 

[Richard Greene]

To Chris PC who wrote:

"Room dimensions dictate which frequencies will form standing waves. There should only be three and their fundamentals. There should be one for length, one for height and one for width. Without some very elaborate room treatments they are impossible to avoid. The easiest and cheapest way to compensate is to try and place your sub so that you only have reasonances that cause peaks rather than nulls in output. Then add an eq to tame those peaks. "

My comments:

There are hundreds of standing waves in a room, not three.

The axial (wall-to-wall or floor-to-ceiling)

room modes are the most audible room modes at bass frequencies under about 80Hz. Room modes at these low frequencies are too few and too far apart for our ears to smooth the frequency response (as they do above 100Hz.)

Under 80Hz. there will usually be three first-order axial room modes and two second-order (one octave higher) axial room modes for a total of five. Of the five, the only ones of interest are those room modes that cause a large frequency response peak (or trough) at your listening position. That is usually one or two room modes of the five.

All bass resonances cause peaks and troughs.

That's the problem -- uneven distribution of bass energy. Troughs can't be fixed by equalization -- you can boost the frequency but the result is little or no increase of SPL.

The SPLs of peaks heard at a single listening position

(or close to that position) can be reduced with equalization.

Bass traps affect the SPL of both frequency response troughs and peaks (not only peaks)

throughout a room (not only at one listening position)

by reducing the bass reflections that cause peaks and troughs.

Equalizers are used to treat one "symptom" of standing waves (frequency response peaks) at one listening position. Since peaks are far more audible than troughs,

equalizers can significantly reduce bass frequency response deviations at a single listening position (and sometimes at two side by side seats). Equalization can't improve frequency response throughout a room as bass traps can.

 

[Dustin B]

ChrisPC didn't say that, he was quoting my response up higher.

Unless I have my terminology mixed up. I intended three and their fundamentals (these are your 2nd order and higher modes not?) to refer to the 5 that usually occur. Reason you are saying there are only two 2nd order, is the last one is far enough above 80hz it don't matter any more?

I'm a little unclear as to how there can be hundreds of standing waves in a room. You only have 3 sets of walls that can form standing waves. The correct math term escapes me right now for what refers to the frequencies that will be multiples of those distances. There will be quite a few, but hundreds?

Other than that, I aggree with everything else you've said. Thanks for taking the time to elaborate a little more on the topic.

Have you considered writing a little article for the newbie faq thread on equalization?

 

[Chris PC]

Yep, I'm aware of the software and BFD EQ. What about the ART 351 or whatever its called from SVS? Is that ok too? I was just wondering if I could do my own freqeuncy response myself and try to EQ the peaks out on my own without software.

 

[Roger Kint]

You can guesstimate the freq room nodes using this simple formula:

562.664 / room length in feet = Freq(Hz).

Of course actual measurements will vary.

Hundreds? Ya, if you count all the harmonics.

 

[Mark Seaton]

Hi guys,
There are in fact a large quantity of room modes. In a rectangular room there are in fact only 3 axial modes, yet there are also tangential and oblique modes which ideally would be considered. In the grand scheme of things, the axial modes are the most significant in that the large parallel surfaces of the opposing walls allow for a greater buildup of energy relative to tangential and oblique modes. These other modes also require reflection off more surfaces than the axial modes, which incurrs more loss at each reflection (hopefully).

 

[Chris PC]

I don't know if this has ever been said before, but I find bass quite amusing because it is SO dependant on the room and placement of the subwoofer or speaker. I imagine there must be sweet spots in a cathedral or concert hall where you get the best bass