A few Subwoofer questions (1 Viewer)

[VepeR]

Hi to all.

I am quit new to home cinema. About a half a year ago i bought fronts center and surrounds, which all work great (JMLab Coblat series). Now i am deciding to buy a Sub. From the local shop i got one on test to se if it fits me. Testing it is bringing up quit a few questions which i am sure you can answer.

I am testing the JMLab SW 800 with Onkyo 600 receiver.

1. I made some test with AVIA. In the sweep test (100 hz to 20 hz downwards) and measuring SPL with the spl meter one meter from the sub i found out that sound level is first droping in the range from 100Hz to 75Hz (it drops for cca 5DB to 75DB). Than it starts rising in range betwen 75Hz and 65Hz( it rise for cca 8DB to 83DB). After 65Hz it starts droping again. At 45Hz it is already 13Db lower at 70Db and at 35Hz it is already at 65Db (i bearly hear it) and at 30Hz i can't hear it anymore. The sub specs says its frequency range goes down to 30 Hz (+-3Db) but as i said i can't hear it anymore. Is this spl variability normal? Should i be still able to hear it at 30 Hz?

2. When only the Sub is powerd up and everything else is off (including the receiver) and i put my ear next to the sub's speaker i can hear some noise. Something like an electricity interference noise if that is the right word. With other words, it is the similar sound you can hear from the main speakers when you pump up the volume on the receiver wihout any music playing. But i can hear this kind of the sound from the Sub even when the sub's volume control is on its minimum. Is this normal?

3. This sub is not brand new out of the box it was used for a short time in the shop. What should i check regarding the possible flaws? I would get a brand new one but this one is 30 % cheaper. And of course i get the full 5 year waranty with it.


I hope the post is not to long. I thank you for your answers in advance.

 

[EricSm]

I'm not the most qualified to answer your questions, but I'll take a stab:

1. Some variance is normal. The subwoofer interacts with the room to create the SPL levels at various frequencies you are hearing. Placement of the subwoofer is critical. Ideally, you want to find a spot where the sweep is the most flat without the huge variances. I highly recommend doing a search around this forum for "test tones" and downloading some to create a disk and that way you can very accurately plot out your frequency response. Even with the best spot, however, there will still be variance and that is totally normal. To further complicate things the measured frequency response changes depending on the listening position as well. Just play with what options you have and try to find the *best* one because you won't find a perfect one. Regarding how low the sub will go, if you can't hear anything below 30hz, I'd guess the sub simply doesn't go that low. Nothing you can really do about it, unless you want to buy a new one. Sub placement can help this issue to some degree. Also, if your sub has a phase dial you can play with it to see which position gives you the best measured frequency response(particularly in the higher bass frequencies).

2. The "noise" you are hearing might be a ground issue. It also might be that sub just has a buzz to it. I've owned a few in my life that did. Wasn't too much of an issue. If the sub has an "auto on" I'd recommend using that.


3. A lot of times a sub either works or it doesn't. If it works, the only thing you can really do is give a good visual inspection. If it doesn't work, well, then it should be obvious you have a problem...