Understanding low-frequency reproduction....room or speaker

Hi all, I have a question about low-frequencies and whether my speakers are doing it or its my room.  Allow me to explain...

I own BMC PureVOX speakers, and in our living room space approximately 16x17 with open hallways and a half-wall (not a fully closed square area) if I play the opening track of the "Titanic" soundtrack the low-frequency effects will shake my wife's trinkets off the wall if I'm not careful with the volume.   This is with the speakers out into the room, about 8 feet apart, not close to side walls and at least 2+ feet from the wall behind them.

Just for info, the PureVOX is a bipolar speaker, aluminum cabinet, sealed enclosure, with (2) 6.9" kevlar drivers in addition to the AMT tweeters.  BMC does not provide frequency specs, I assume this is because measuring frequency response is less relevant with bipolar designs.

We recently went to hear the new B&W 802D3, just out of curiosity, and when listening to that same track the 802D3 barely produced any of that low-frequency effect.   This surprised me, because just in terms of size (and price) the 802D3 is in a different league - it's much larger than the PureVOX.

In trying to understand, I pulled up one of those websites where you can listen to low-frequency tones to test audio system boundaries, and on the PureVOX the test tone becomes audible in between 20hz and 30hz.   

So my question is...does this mean the PureVOX actually goes that low, OR is this all just a function of my living room size and configuration, which also explains why my speaker did something the much-larger 802D3 couldn't do?   In my smaller office, I know my Wilson Benesch Arcs sound fuller when they're closer to the wall, so I at least superficially understand wall proximity and room reinforcement and want to understand if that's also what's happening with the PureVOX.

Up next, why do headphones which have relatively tiny diaphragms and no space as rooms do have such outstanding bass response?
Hardy, har, har. :-)

I believe it's mostly due to your room boundaries. My Clearwave Duet 6 monitors "only" go down to 42Hz and yet, with a RatShack SPL device I get into the mid 20s at 1 meter from the speaker. It's down some 14db but it's there. And it makes for some beautiful listening as I don't need to use my imagination to fill in what's missing. 

Sometimes one gets lucky with speakers and rooms.

All the best,Nonoise

First of all you need to look at the way speakers need to be supported correctly on the floor or if they are on stands.

Never spike a speaker (floor stander)or (stand mount) into a suspended floor, as you only make that floor a sound board for the low frequencies, and that will muddy up or ruin your bass. The speaker need to isolated from a suspended floor.

The only times a speaker (floor stander or stand mount) should be spiked into the floor, is if that floor is a cement slab, and not be able to act like a sound board. Start with this first, then positioning.

Cheers George  

How does one isolate the speakers from a "suspended" floor ? Thought that's what the spikes are for .
Thanks for the input, I think I was surprised given that the speakers don't have big cabinets - they're really just slightly oversized monitors - and they're out into the room away from the walls, and toed-in so that even the rear firing driver on one points off into a wide-open space.   They're not getting much help from nearby walls, or at least I didn't think they were but maybe that's not the case.

I don't have suspended floors, and the speakers are on slabs, not spiked.  But that's moot - I don't feel I have a solidity problem, or a positioning/toe-in problem.  I don't have muddy bass, I just have more than I expected given the size of the speaker, hence the question.

I can think of two things that may cause your wife's trinkets to shake in response to the low frequencies output by your speakers  

The first is room gain or boundary reinforcement as mentioned by "nonoise" ... below your room's transition point (Schroder frequency) the wave lengths of the frequencies' below the transition point will no longer fit within the room's dimension and these frequencies below the transition point will pressurize the room ... unfortunately this pressurization is not even through out the room with the four corners and the mid point of all walls having higher pressure than the rest of the room   

If your wife's trinkets are placed in or on a corner Hutch and placed in the corner of the room they will be experiencing higher pressures due to the wall's reinforcing and boosting of those low frequencies in the corners  ... this excessive build up in the corners during low frequencies passages may be just enough to shake the trinkets and is also the reason why you place your Resistive Bass Traps in the corners of the room to reduce the extra energy created by the corner walls  

Also you room no matter what the dimensions are will have higher pressurization at the mid points of all the walls including the ceiling and floor ... if your wife has her trinkets placed on a free standing book case and that book case is placed right at the mid point of the front/back or side walls they will also experience higher pressurization like in the corners and this can be the cause of the shaking during low frequency passages 

Placement of trinkets on a free standing book case or hutch which is placed at the mid point of any wall or in the corner may experience enough pressure from the wall's boundary reinforcement to cause them to shake when low frequencies below the room's transition point are present   

But I don't think that is what is causing your problems 

One of the biggest misconception in audio is that the speakers can produce enough vibrational energy that the cabinet's coupled to the floor can cause the walls ceiling and floors to vibrate 

If the speaker had that much energy to shake the room violently they would probably come apart at the seams and the stuffing would fall out into the room  

Here's the second reason I believe your wife's trinkets are being shook 

Your wall are excited and vibrate in sympathy with your speaker's low frequency response when the speaker produces a frequency that is equal to your wall's Primary Resonate Frequency 

Based on the length of the dimensions of the room each wall (length .. width .. and height )  has a primary resonate frequency dictated by it's actual length that will cause the wall to vibrate when that frequency is excited by the speakers low frequency response  

You can calculate this P/Resonate Frequency mode by dividing 562 ( the speed of sound divided by 2) by the walls length  

In your case 562/16 Width  = 35.125HZ's ... for the room's length 562/17 Length = 33.05HZ's  and for the 8 ft ceiling height (I'm guessing) it would be 562/8 Height or 70.25HZs  

When ever your speakers reproduce one of these low frequencies ... 33HZ .. 35HZ or 70HZ ... the wall whose primary resonate frequency (based on it's length) is close or a match to that frequency being produced by the speaker will vibrate in sympathy with the speaker which is pressurizing the room again below the rooms transition point 

You should be able to see that when your speakers produce these low frequencies that match the wall's Primary Resonate Frequencies' based on it's length  ... the wall or walls will vibrate violently shaking anything attached to them such as a shelf with trinkets on it or pictures hung on the wall or recessed lighting fixtures along with any Deer Head or Fish trophies attached to the wall  

Speaker produces low frequency that excites wall ... wall shakes in sympathy to speaker exciting it at it's P/R/Frequency and wife's trinkets on shelf attached to wall shake 

Also take  note  that because your room's dimension are very close (16x17) that the room's P/R/F points are very close also and will reinforce each other causing additional boost or gain  

You will also get additional reinforcement because your 8 foot ceiling height is a direct multiple of your 16 foot wall dimension in a 2 to 1 ratio ... Happy Birthday  

Most Home Theater low frequency punch is around 30 to 35HZ and seldom goes into the mid or low 20's .... this punch or slam is right at your room's dimensions (16x17) Primary Resonate Frequency excitation point ... so I feel the low frequency content in the 30 to 40HZ range is exciting your walls and shaking the wife's trinkets  

An easy way to check this out is to run a test disk ( like any of the Stereophile test disks)  with 1/3 octaves from 200HZ down  

As the 1/3 octaves play down to the lower frequencies the bass tones will be clear and distinct ... when you hit the octave on the test disk that matches your wall's Primary Resonate Frequency point ...   the walls and room will shake violently and it will be clearly audible ... once the test tones are out of the range that excites the wall ... the bass test tones will become clear and distinct again  

I'm just guessing by your room's dimension that your walls will vibrate audibly between 30 and 40HZ and also around 70 or 80HZ


Enter your text ...
davehrab, thanks for that excellent post.  This is where I get confused...I don't have a technical background so I while I enjoy music and audio equipment I don't understand the technicals as well as most of you do.  Nonoise's post made sense to me, that the speaker by itself isn't producing the low-frequencies, it's doing so in combination with the room and the walls.  What you seem to be saying is that the room and walls aren't creating the low frequency, they're simply responding to what the speaker is outputting and if the frequency of the speaker's output matches the wall's resonant frequency I get the LFE that I hear and feel.  Am I following you correctly?  

bcgator ...  you got it ... walls by their dimensions have a natural P/R/Freq, ... when your speaker produces this frequency the wall will resonate and vibrate in sympathy with the speaker

This is very hard to cure as it is based on your rooms dimensions that you can't  change ... you're more or less stuck with the walls vibrating at their P/R/Freq point

If there is any solace in this ... at least you know exactly what the problem is ... where it is occurring  and what is causing it ... this way you are not making yourself crazy trying to figure it out

The other issue not related to the walls vibrating is the peaks and nulls created by the rooms dimensions which can be very easily to deal with

Peaks and Nulls can effect the sound dramatically and need to be measured

If you can try the Test CD test of listening from 200hz down to 20hz ... you should clearly hear the walls vibrating when you hit the right frequencies    

davehrab, I want to clarify something - and I think George had the same idea - the bass response I'm getting, and the resonance, is absolutely not a problem.  It's not something I'm trying to cure, or eliminate.   The purpose of my question wasn't to complain about the low-frequency effects, or figure out how to remove them.  The purpose of my question was to understand how I was getting such great bass response from relatively small speakers (albeit sealed cabinets, and bipolar), in a room that I wouldn't consider small or closed-in, and with the speakers out in free space away from the walls.  I've had larger speakers in that same space, with bass response not close to this.  As I mentioned, BMC doesn't publish frequency response for these, and I figured from the size that if they got down to the 40hz level I'd be happy with them.  But it seems they're going lower.  I just didn't understand if it was the speakers themselves, or as Nonoise mentioned I was just getting lucky with the way this particular speaker was interacting with this particular room.  


How does one isolate the speakers from a "suspended" floor ? Thought that’s what the spikes are for .

Many get this confused, spikes couple the speaker or stand to the floor, this is fine for cement slab floors, and the best. 

But with any suspended floor the speaker or stand should be de-coupled from it to get the best bass response, as the manufacturer would have voiced it. As they would have been crazy to voice it spiked into a suspended floor acting as a sound board to any bass notes from the speaker.

To de-couple from a suspended floor you need to use an isolating puck or device like a sorbothane filled disc. Like these which you can still use your spikes with 

"Cone/Spike Decoupling Glider"  that are half way down the page.


Or you can substitute your spikes for these "Threaded Stud Glider " a bit further down the page. 

Cheers George


davehrab, I want to clarify something - and I think George had the same idea - the bass response I’m getting, and the resonance, is absolutely not a problem.  It’s not something I’m trying to cure, or eliminate.  

 I think you probably lucked out with a room standing wave problem that just happens to be just below the natural roll off of that 6.9" bass driver, giving you another octave lower perceived bass. Shh it’s free don’t tell anyone, and whatever you do don't move the speakers or you may loose it.

Cheers George 

Just thought I mention that one reason why bass response is incorrect or bass shy is that the manufacturer of the speaker simply puts way too much stuffing inside, choking them.  Throw out most or all of the stuffing for best results.  
So far I've been lucky with placement...I've had fun playing with the bipolar configuration of the speakers and trying zero toe-in, extreme toe-in, even toeing them outward and pointing the rear-firing drivers upwards towards each other which creates an amazing wall-of-sound effect.   But this hasn't changed or hurt the bass response...no matter where they're pointing, I get nice tight bass that seems to dip lower than I expected given the size of them.  Thanks to everyone for the input.