Lsi15 Frequency Response

jrlouie

All,
Can someone maybe put this in easy to understand terms?
I emailed Ken asking for a frequency response graph of the Lsi15 and received one from him. After noticing that it drastically rolled off after about 60Hz, I asked him for an for any information without spending too much time explaining it to me as I'm sure he has tons to do. He pointed me to the Speaker Specialist #11. After reading that, it seems as though there are tons of variables when plotting those graphs (lab conditions, positioning, etc.).
I guess what I still don't understand is how Polk's manual states it goes to 30Hz at the -3dB limit yet the graph from Polk shows it rolling off drastically before that.
In case this helps, here's the specifications for the graph...
LSi15 tower speaker “Full-band” magnitude response; derived from ground plane measurement over the range f <= 400Hz and free-air on-axis magnitude response @1m 2.83Vrms drive level over the range f >= 400Hz. Smoothed to 0.17th octave.
There's probably an easy explanation, I'm just lacking the knowledge and figured you all might have an excellent explanation. Plus I don't have to keep nagging Ken

Thanks in advance for your responses.

Dr. Spec

Basically the speaker was measured outside.

For frequencies below 400 Hz, the mic was placed on the ground (hence the term ground plane). Above 400 Hz, it was placed on axis.

I have no idea how Polk rates bass extension. Considering the roll-off starting below 60 Hz, I doubt Polk's numbers are anechoic chamber or ground plane (2M GP = 1M anechoic).

It wouldn't hurt to ask, but maybe the -3 dB at 30 Hz is corner loaded in a typical room?

If a speaker or subwoofer can truly get down to 25 Hz (or deeper) cleanly ground plane @ 2 meters, it has outstanding deep extension.

jrlouie

Oh, got it. I incorrectly assumed the frequency response graph was equivalently tested with respect to the +-3dB specifications given in the manual since they were from the same company.
Thanks for the info!

Dr. Spec

Yes, that is odd. One would assume if Polk tested the LSi-15 ground plane for deep extension, it would also rate it that way on the specifications page.

The disparity is evidence enough that the rated spec is not ground plane, but something else (maybe corner loaded?). I suggest asking Ken Swauger for more info.

polkatese

I am reviving an archive thread here, but I am curious to know if a LSi15 can handle a true realistic "full range" FR for two channels use. If it is indeed it is roughly rolled off at 60 Hz, then it seems that one needs to add a separate subwoofer to reach lower FR.

Since I am not familiar with the SDA series (at all), is the FR of any better (specifically on the lower FR)?

Btw, where is Dr. Spec (Ed)? Haven't seen him around too much lately..

TIA

mantis

Depending on why you wanna know, the Lsi15's have usuable bass response in room to about 32hz. After that it's useless.

I tested mine and found that to hold true in 2 different rooms in 2 different houses.

For theater a sub is a must. Even for music playpack a nice low hitting sub will help them.

Dan

polkatese

Dan,

I am going to move my Storm to pair with the 15 for two channels use. I was just trying to gauge feedback before I do it, so that I know what to expect. Thanks for your input. I'll let you know how it goes.

mantis

polkatese,

I would set the Storm ( My baby) somewhere around 40hz. The 15's start really falling off there. This is going to make for a really good experience.

Let me know how you make out.

Dan

polkatese

Thanks, Dan! I'll try out your suggestions, that and perhaps 60Hz. I'll let you know.