Axiom Home Page Forums General Discussion Home Theater EP500 Response

Hey Folks, given the rainy day today I tried measuring the Freq respond of the EP500 using some WAV files of test tones [sine wave] I generated on my PC and burnt to CDRW. I'm using a Rat Shack SPL meter. Values have been adjusted based on the corrections for the Radio Shack meter given on the SVS site. Initially I played a 50 Hz tone and adjusted the volume to 90 db then took the measurements. As of yet I haven't seriously played with placement. This is what I got:


As I don't have a long enough cable yet to move the sub to the listening position I haven't tried crawling around to find the position that sounds best.

After having done this little experiment I'm wondering if I would be better off to do these types of measurements to try and find the position that offers the most linear response instead of just listening to some bass on my hands and knees and decide what sounds better.

Would taking this approach offer better or worse results do you think?

Given the EP500 is front firing in which direction should I face the sub when it is positioned at the listening position while determining optimal placement? Given LFE is non directional does it matter?

Any thoughts on how I can flatten this pic out...?

Cheers, Jag

Room dimensions are 129"W X 199"L X 83"H should this matter.

It looks like you've found a pretty good spot for it right where it is. There are no big nulls to deal with. They are the most difficult thing to fix.

You have a big drop off after 100Hz. Did you have your main speakers connected when you did these measurements? If so, then your sub level appears to be a bit too high.

Hi Michael_A thanks for the help!

No the mains where disconnected and I pushed the recievers crossover up to 150Hz. Is this OK?

I really have no idea what a good or poor in room freq response would be... Is it reasonable to hope for something flatter?

Is this measurement approach better than the listening and crawling approach? I was thinking of placing the sub in the listening position and then taking measurements like these at different places in the room to find the placement that would give the flatest response.

An thoughts on which direction the sub should face if I were to do these tests with the sub at the listening? Face forward or backward?

Or, should I just be happy with the way it is now?

Thanks a bunch, Jag

I think this is probably pretty good. They have huge anechoic chambers for a reason, you know

First guess would be that you have a pretty large room and the sub is near one or more walls. You could probably flatten it out by pulling it out into the room a bit if you want to experiment with something

Michael, you didn't describe the position of the 500 when you measured, but after doing a room mode calculation from the dimensions it looks like the usual suggestion(except for square rooms)to put it in the tighter corner(farther from a doorway or other large opening)behind the speakers should work well.

A consideration which is separate from the room modes is the boundary reinforcement which a sub driver gets when placed within a certain distance from the three nearest room surfaces(i.e. the floor and two nearest walls). There isn't too much available online discussing this, but here's a pretty good description in a slightly awkward translation from the Swedish. The basic point is that contrary to the advisability of keeping a full range speaker out of the corner and at different distances from the three room surfaces, for a sub the exact opposite applies. The orientation of the driver isn't of any real significance from the directionality standpoint, since those very low frequencies are omni-directional, but it should be positioned if possible so that the driver cone is less than about 22" from the floor and both walls. This results in the strongest and smoothest reinforcement from the three boundaries.

Thanks JohnK, this is of great help.

The position was at the front right hand side about two feet from the corner.

Last night I went ahead and took a bunch of measurements and got some very interesting results! Confusing but Interesting. Its going to take some time to sort through the numbers and to confirm things but I'll be posting them soon and would love your input.

At first assesment it would appear the sub freq response becomes incredibly linear ( even down to 12 Hz ! )when set to play at about 10db louder. ie Rather than initially setting the 50 Hz test tone to 90db as my begining point I set it to 100 Hz. This doesn't make sense to me yet but I'm going to try some more measurements and then see if I can figure out whats going on.

I did try the corner position and indeed noticed at least a 10 db gain in volume but no change in linearity of the response. In fact I couldn't find any position that changed much in the linearity.


I'll be getting back to you folks with more measurements in the next little while and would love all your input.

Thanks a bunch, Jag

You have the beginning of a great house curve. Tame the peaks at 40 and 100hz and you will have an awesome response curve.

OK Folk's here's some more thorough measurements. I think these are begining to show just what the EP500 can really do!
The SPL meter was mounted on a tripod and placed on the couch in the listening position. The sub was moved to the different positions indicated on the plot for testing. At each position the 50Hz test tone was played first and the volume adjusted to 90dbs for the first graph and either 90db or 100db for the second. Once the 50hz tone was adjusted, measurements were taken. Values again were compensated for the Rat Shack non-linearity given by the SVS site. Note: the Hertz are plotted from 125 to 10 (opposite to the above posted plot)



As can be seen all the positions in the room (original position= position in previous graph above) yield pretty well the same freq response with the exception of the placement in the Middle Front and in the Centre of the Room. The Centre of the room just plane sucks; clearly there's little benefit from boundary reinforcement. Other than an obvious increase in loudness due to boundary reinforcement, placement in either corner yeilded no improvement in Freq Response. The Middle Front seems very impressive to me particularily in the low end. There's one difference with this placement though; I really had to increase the overall volume of the sub (just like for the centre placement) in order to get a 90db reading with the 50Hz test tone prior to testing.

After getting this better curve with placement in the Middle centre I wondered if increasing the actual output from the sub had any influence on the Freq Response (even though the EFFECTIVE volume at the listening position was the same as the other placements)

Consequently, I decided to put the sub back into my "Original Postion" with out adjusting the volume output. This resulted in the initial 50Hz test tone being 100db (instead of 90db). In so doing I managed to maintain the smoother profile even in the different position!??!



Now I'm actually getting higher output down at 12 and 10 Hz than at 100! And the response has smoothed out quite a bit.

Questions:

1) It seems the Freq Response gets flatter when the sub is made to exert more. I figure that this is either due to the sub itself or some phenomena from the room. Does any of this make sense to you Experts? Would this be a characteristic of the sub or the room. Louder gets smoother...? Given the DSP on the EP500 I'd assume the sub should be equally linear at any volume???

2) Now as nice as I think the Freq Response is at this volume the problem is I can't imagine listening to it this loud and placement in the middle front position is not an option for me since the TV and equipment cabinet containing the DVD, and reciever sit just behind there. I actually saw the TV picture shake in this position. So any ideas how I can get around this? (other than ear muffs) ???

3) Any critique of my measurement method that may offer some more insight on how I get this affect. Louder=Smoother???

I very much appreciate all your input.

Cheers, Jag

The louder=smoother thing is not what I would expect. Might be something in the DSP programming, I guess...

Jag: to answer 1 and 3...:

Actually, that is exactly what I would expect given the type of measurement system he is using. What is going on is that the amplifier or the woofer is distorting, and thus raising up the total SPL. The only REAL way to test the true output when the amp is being stressed like that is to get an RTA and have that tell you on an individual test tone what frequency bands produce how much energy. You cannot isolate a frequency with a simple SPL meter.

Edit: Essentially you are doing a distortion test. The logic behind this is that the lower frequencies require more driver and more amplifier power to produce. When you up the volume by a few dB on the higher notes, the amp will not be clipping and the woofer will not do any distortion and so the volume will go up linearly. When you either reach amp clipping or woofer distortion, the frequencies at which it distort appear to be much louder due to the harmonics being played at a much higher level than usual. This keeps on rising exponentially until the amp either is too distorted and sounds like crap, the woofer bottoms out, or the sub's internal limiting circuitry decides that it cannot produce a louder tone.

I thought that I should clarify a bit on the first part of what I said... The THD will be higher in the lower frequency region because it requires more power for the same dB level. Thus, when pushed hard, it will distort the lowest frequencies first, making them appear to get a boost in amplitude. What is really going on is that they (the fundamental frequencies) do not actually go up, but rather their harmonics increase substantially.

DJ_Stunna thanks for the replies.

I think I understand what you're saying and it definitely fits what I'm observing. I'm not convinced though, the sub is actually being over driven or is clipping as I'm not seeing/hearing anything to give me the sense that this is occuring during the measurements.

I realize this is a bit of an unknown beast given the new technology in the sub and according to the Axiom literature the sub nor its amp can be over driven...

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Its (EP500) special algorithm commands the subwoofer to deliver peak performance without ever being overdriven. And its maximum undistorted output of 120 dB is louder than a full orchestra in a concert hall.

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And there's just no way I'm even approximating the limit of 120db anechoic in my little room.

I'm also very confused as to why I see no improvement/variation in smoothness from one position to another in my room as virtually everything I've read on the subject of sub placement suggests should happen. I definitely notice it louder in the corners but that's the only difference.

Well I'm still gonna work on this a bit more to see if I can figure out the "optimal" placement and it may be time to send a few queries to the Axiom experts.

I'll keep you all posted.

Thanks very much again, Jag

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I'm also very confused as to why I see no improvement/variation in smoothness from one position to another in my room as virtually everything I've read on the subject of sub placement suggests should happen.

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It's funny you say that. I have been talking to Tom from SVS over the last few day's yapping about placement etc. One of the things that came out of our conversations was the fact that by moving my sub around, I won't necessarily make it tighter, less boomy etc, all I'm doing is creating more db the closer I get to the listening position. I have been moving my sub all over my basement over the last few day's and it sounds the same no matter the location. I thought I was doing something wrong until Tom broke it down for me.

Sub placement seems to be something I always thought was an issue but I have now learned that as far as my room goes, I can drop it almost anywhere and it works more or less like the spot I had it in before. What got me on this path was the crazy thought of adding another SVS PC-Ultra to the back of my room. Here is a direct cut on a few of the things Tom had to say:

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The room size and the distance between the subwoofer and the listening positions will largely determine how loudly the subwoofer will be able to play. In a larger room like yours, with the subwoofer about 5m from the key seating positions…the single PC_U will be capable of approx 109-115dB of clean output.(at the seating positions). But in a smaller room…say 20x14x8…with the PC_U 2-3m away from the seating positions…you could be looking at 115-121dB of output.(or, the amount of clean bass it would take DUAL PC_Ultras to produce in your room).

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Jag:
I haven't had much time for these boards lately, but I DID want to mention that your plots certainly looked better than the ones I came up with for my JBL sub a couple of weeks ago (I haven't had a chance to do my new Axiom sub yet).

I had HUGE rises and dips, not all of which could be attributed to the room (for instance, I had a sharp dip of about 18db at 60Hz no matter what the position)!

Michael, while I agree with Will as to the general effect of subwoofer distortion as he clarified in his second post, upper harmonics added by possible distortion wouldn't explain the increase shown in the 10-20Hz octave. Also, the 100dB level, while quite high, should be well within the capabilities of the 500, as you point out. I find it hard to believe that the increase shown is "real", but just as hard to come up with a solid explanation. I wonder if the "correction" to the RS meter, which is actually an attempt to undo the C-weighted curve compensation for the Fletcher-Munson effect, is invalid at the higher level and that the correction should in fact be much less in the 10-20Hz octave.

Viewing the curves shown, one thought that struck me was that(if you aren't totally exhausted at this point from lugging the 500 all over the room)another position to try was the left front corner, but with the 500 turned around so that the driver cone faced diagonally into the corner and was about 1'-1 1/2' from each adjacent wall.

Woooowwww! This has exactly been my experience. I tried crawling around alot to see if I could hear a difference and just couldn't. I was thinking I just didn't have a good enough "ear" and this has been the reason I have gone to some effort to try and map out the In Room Freq Response. So perhaps I"m not so crazy after all . Well on second thought...

Seriously this is extremely helpful. I bet alot of what people hear as an improvement from one location to another by crawling around is simply differences in loudness. As is often pointed out here on the forum a speaker sounding just a little louder will sound "better". Perhaps this is what's going on when you crawl around?

Now that I've started though I"m still going to pursue the placement issue a bit more but it's certainly reassuring knowing that I may already be there.

Thanks alot NeverHappy, Jag

Hey JohnK, I'm definitely up for it! I agree with you I also have a hard time believing the higher output in the lower end and the compensation point is certainly possible. One thing to note though compensation aside the lower end was definitely higher when it was playing louder. Ultimately I'm just looking for the smoothest response and playing this loud certainly gets me there, but I just don't know how to use it. I was thinking something similar to your idea of facing the sub towards the wall thinking perhaps the sub would be driven a little harder but the loudness at the listening position would be more comfortable giving me the best of both worlds.

I'll give this a try in the next little while and let you know how it goes.

Thanks for the help, Jag

Hey Mark thanks, as I've never had a HT before I'm really in the dark of what a reasonable expectation for an In Room Freq response should be.

If and when you do have a chance to take your measurements it would be cool to also see them.

Thanks, Jag

Hey JohnK, I was thinking about your suggestion of the compensation issue and it certainly makes the point that the measurements could be in question with regards to compensation or the even worse the SPL meter itself.

I'm wondering is there any way I can cross reference the accuracy of the meter itself. One thought I had is to exchange it for a new one. At least that way I would feel more confident the meter was behaving properly.

Thanks, Jag

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think I understand what you're saying and it definitely fits what I'm observing. I'm not convinced though, the sub is actually being over driven or is clipping as I'm not seeing/hearing anything to give me the sense that this is occuring during the measurements.

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Yea - you won't actually hear the distortion until it reaches very high levels. This is typically 1-10% on higher frequencies but can be MUCH higher on bass notes (especially ones that are below audible threshold (under 20 Hz).

Also, just to point out... the 120 dB anechoic max is for one frequency - its peak frequency. There is no possible way that a subwoofer with an amp of only 500W, with only a 12" driver, and with a box that small will put out 120 dB C anechoic at 16 Hz. It would be a VERY impressive feat if it could do 120 dB anechoic at 22 Hz without distortion, but I imagine this peak output frequency will be more along the lines of 25-30 Hz. I suggest you use a decent quality microphone and download some RTA software and look at the different frequencies outputted when you run a specific test tone. I know there are a few websites online that offer 1/3 octave RTA software provided you have a decent mic. Alternatively, you can use your Rat Shack SPL meter AS a microphone (or so I hear), but I have yet to try with mine.

Thanks DJ-Stunna point taken it could be difficult to actually hear the distortion. What I was trying to get after though was the onboard DSP is supposed to prevent the sub from distoring. I'm way out of my depth in understanding how I'm getting these readings but it could be attributable to the DPS in some way. Possibly the DSP limits specifically only those frequencies that are disorting. Hence at a certain level only the lower freq are limited. As the volume is increased so are higher and higher frequencies are limited by the DSP and the curve then starts to look more and more flat????? At any rate I'll search around for this RTA software you mention and see if I can get anywhere with that.

Thanks a bunch, Jag

What I got from reading the specs was that it would prevent the amplifier from damaging the driver... I would imagine that with the 600W amplifier in the EP600, it would be more of a factor than in the 500 if that were the case.

I've got a question/comment about the sub: I am very surprised Axiom didn't implement a subsonic filter on the subwoofer. In the Audioholics preview, I believe that Ian mentioned that one would be implemented, but I guess that never happeend? (At least it doesn't appear from your readings that such a filter is in place.) It really isn't good to waste precious power on frequencies that cannot even be heard. Is there an option to set it at like 16 or 18 Hz or something around there and you defeated that or did they just not include that feature?

In short no. At least not an adjustable subsonic filter on the back panel. It does have a Boost adjustment for boosting frequencies in around 33Hz and is supposed to be used for small to medium rooms. I haven't played with this yet to see what affect if any it would have for me. In all my measurements I left the knob set to Flat were there is apparently no Boosting of any frequencies. My one complaint if I have one is you get very little info with the Axiom speakers on specs, set up, warranty ect. However, if you go to the EP500 page they give some general description of the DSP functionings. You'll note it says:

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XLF won't stand for peaks and dips! It controls every frequency from 16 - 100 Hz, maintaining a smooth, flat response within 1.5 dB across the entire range. That means you'll hear undistorted, profoundly deep rumbles all the way to 15 Hz.

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I would speculate its possible the DSP functions in part as a subsonic filter, but whether or not it does is entirely unkown to me.

Cheers, Jag

PS if you have a site/link on hand for this RTA software could you send it my way? I've found some Demo's but they're quite limited and don't allow you to save anything.
Thanks!

I think this or this would work, but sadly they are only time-limited trial versions. They probably offer more than enough time to test out that monster sub though...

Cheers, DJ_Stunna. MUCH appreciated. I'll take a look. Hopefully I'll beable to explain those low down numbers...

I just did some further tests as to John's recommendation of trying a diagonal corner location. I've included Uncompensated and Compensated plots. No real difference with these positions either. Yet again if I turn it up a bit to normalize the 50Hz tone to 100db I get more in the lower end ???





It does seem regardless of where I place it I don't significantly change the response.

So I guess I'll be happy with my original position given that its the most convenient location.

Thanks all, Jag