Where has all my bass gone?

[RugbyAl]

Right guys, bit of a strange one here…
I have been satisfactorily using a MJ Pro50 for 5-6 years. Ok it doesn’t do real bass like a ‘proper’ sub but with nothing to compare it to, I have been quite happy with it. The opening seen from LOTR had all the impact I needed at the time.

I moved house 14 months ago, putting my sub and surround/centre speakers away until I had re-decorated my new lounge. On putting the system back together for the Christmas hols something was definitely amiss! My god - no real bass at all in comparison with what I had before.

I have tried moving the sub into differing locations within the room but with no real difference being heard. To investigate this further I bought an analogue Radioshack SPL meter (If anybody reading this hasn’t got one – get one NOW!) and downloaded some test tones off t’internet.

I have been running the sub 3-4 db ‘hot’ when setting up for a film using the AV amp’s test tones. All speakers are set to small and no -10 db settings activated….

For testing, I have connected my MP3 player directly to the sub’s LFE input (filter bypassed). With C weighting, slow response selected I set the gain to approx 88 db at 70hz (for comparison with other test results – see below) and measured the response at head height at the seating position. The sub was located in the only corner* (to maximise room gain) available to me and got the following response:

My measured response / Free air response**
20 Hz ----- / 65 db
25 Hz 60 db / 74 db
30 Hz 68 db / 77 db
35 Hz 71 db / 80 db
40 Hz 79 db / 82 db
45 Hz 87 db / 84 db
50 Hz 87 db / 85 db
55 Hz 89 db / 86 db
60 Hz 88 db / 87 db
65 Hz 91 db / 88 db
70 Hz 87 db / 88 db
75 Hz 79 db / 87 db
80 Hz 77 db / 87 db
85 Hz 81 db / 87 db

*Other positions tried, but this position also gave the smoothest response
**Results obtained when another Pro 50 was tested in free air by another web site…

It looks like I’ve got negative room gain! with my sub producing less than one located in free space at <40 Hz. The only positive being a remarkably smooth response from 45 to 75 Hz (although the 5 Hz spacing’s may hide something….).
The room measures 12ft by 21ft by 7.5ft high and has 2 regular doorways (negating the use of 2 corners) a set of large patio doors at one end and a set of large windows at the other end. A tv also uses the only corner bounded by 2 solid walls. For info my old lounge measured 13ft by 15 ft with a large opening into a small 9ft by 9ft dining area So overall volumes are similar. Walls are regular solid construction.
So whats wrong - is it room set up or is my sub knacked?

As it's my birthday soon (life begins and all that) the wife has offered to buy me a sub (xls300/pr?) but I am loathe to spend any more money until I have got to the bottom of this.....

Any ideas please

Ps. I dont think the RS meter will be that far out at lower frequencies - is this correct?
PPs. REW downloaded but no soundcard in my PC yet.

[Ianfromnotts]

If you have downloaded REW have you not also downloaded the meter calibration files, from memory the meter can be out a good 12db at 20hz.

If you want to have some serious fun and you can get the meter/pc near the sitting position I would get an external soundblaster card - you can have some seriously sado fun testing the sub in all different places and it great for upsetting the neighbours too

As for all the base has gone this does sound a little more dramatic than just fine tuning - I would stick the sub tight up in a corner and double check all the amp/speaker settings and wiring as a starting point.

Then again it might be that your old room was creating lots of base that you have now lost - my room 7m x 3m gives a huge boost to everything from about 40hz down - I mean like 20db+

[russ.will]

I wouldn't worry too much about the relative levels as regards the playing field tests. For one they're measured at 2m which may be far closer than you are and therefore effectively louder. It's more relevant to compare to the relative levels within your room/system.

Secondly, room gain is affected by the size of the room yes, but it only comes into affect under the lowest room mode, taking off at a rate of 3-6dB/octave from this point. In your room, the 21ft dimension governs that and would equate to 26Hz, which I might suggest is lower than the usefully audible output of the Pro 50 - extra on top of not a lot, is still not a lot. Your previous rooms equivalent point was a more useful 37Hz for instance and the extra gain would have perhaps boosted things down into the high 20Hz and low 30Hz region at least.

Your old room might have been a similar volume, but it was a collection of shorter dimensions and whilst it's tricky to generalize (especially where anything other than an entirely regular room shape is present), but I'd put money on this being the root of your situation.

What is good is that you've found a position with a relatively smooth response and future subs will likely perform well somewhere around this point too. In the mean time there are two solutions which are move the sub closer to you so it's louder for the same settings or two, take the wife up on the offer of a sub more capable of driving the room.

Russell

[cribeiro]

Originally Posted by Ianfromnotts

you can have some seriously sado fun testing the sub in all different places and it great for upsetting the neighbours too

So that's sadomasochist, then, right
BTW, I also qualify, I think I spent about 10 hours in total running sweep tones And I will spend even more!

[RugbyAl]

Thanks for all your replies guys. Yes I have been having lots of fun with the test tones, fortunately the neighbours are not a problem - unfortunately the other half is!

Russ, I have just been on the mcsquared website looking at the room mode calculator, makes a lot more sense now following your post - thanks for that. Does seem to explain the problem I am experiencing...

I will try the sub closer to where I sit though I doubt if this is practical.

I am pretty set on getting a new sub now, at least I think I should get a nice even response where my sub is currently located. I have narrowed ithe choice down to the xls200df, xls300/pr and the svs sb12 - dont really want to go any bigger than those really. From what I have read the xls200 works well with a bit of room gain (which I dont seem to have), and the svs looks to be of similar performance so it will probably be the xls300! I also hook up a high level connection so the Bk seems a pretty logical choice.

I will post some results when I get the new sub (I will also adjust for the spl meter with the correction values). I will also get a soundcard so I can also play around with REW and see how well my main speakers integrate, although I hope not to have as much 'fun' as Criibeiro!

Thanks again, Alan