Frequency responses

gordonf238

I had recently taken the grill off my bookshelf polks. I started up SoundForge and created a new sound file, then went on to insert a simple sin wave at 2Hz. Yes two hertz.

To my surprise, this actually played. From the specs of my polks, these speakers have a frequency response from 50Hz-26,000Hz yet they played a 2Hz tone. First of all, does anyone know why? Heck, even my amp's input line is rated from 20Hz on up, so how come a 2Hz tone plays? (I didn't actually hear two hertz, obviously that's below what human ear can pick-up, but seeing the woofers respond to the frequency was what bugs me, and creates further problems as you'll see below).

And yes, it was rather cool seeing the woofer cone on my speakers move in and out, slowly and gently like a breathing lung, going out twice per second. Well it was cool for a short while, until I overloaded my amp as I increased the volume at which point it shut down. (I'm hoping repeated amp overloads don't damage any of its circuitry. Shortly afterwards, the only sound my amp put out were the high frequencies, and mid-to-low level sounds came off sounding highly distorted. This went away after about 10 minutes)

My dilemma is that I've got my bookshelf polks, and a 12" powered subwoofer hooked up to a Denon amplifier This unit has a pre-amp out for the subwoofer (which has a built-in amp of its own). However, when watching movies, or other types of sound with Low Frequency Effects, my bookshelf speakers are trying to reproduce the low frequencies as well, alongside the subwoofer, which is often causing amp overloads and sound distortion. This isn't a 5.1 receiver, just a stereo receiver rated at 80w/channel @ 8 ohms (totally sufficient since my bookshelf polks are rated at 100w max @ 8 ohms). I have it hooked up via a stereo RCA connection to my DVD player.

I tried bringing down the bass on the equalizer, but doing so also impacts the bass level going to my subwoofer.

Is there any simple way to restrict the sound that my bookshelf polks play to be ABOVE, say 200Hz? And nothing below? (my sub is sensitive from 20Hz to 200Hz).

Before getting the sub, I normally assumed that my bookshelf polks (being that their spec sheet lists their frequency response from 50Hz-26,000Hz) would NOT play any sounds below the 50Hz range, but obviously I was wrong. These 5inch woofers on them it seems are trying to play and and every frequency (as I've previously tried with the 2Hz tone)

Maybe I'm just missing something here, or don't fully understand how all of this works? If anyone could offer any helpful info I would greatly appreciate it!

Ideally, I'd like my bookshelf polks to only play sounds above 100 or 200Hz, and let the sub handle all ranges below that.

gregure

Without getting a new receiver, your only other option is to get an external crossover device, such as those made by Paradigm. Check it out here.
You will need to run speaker cables out from the receiver to the crossover, then from the XO to the speakers. You can also run the sub line out from receiver to XO and then to the sub, to control what the sub receives. My advice, set the XO to 80 Hz. That is a standard XO, mostly due to the fact that bass below 80 Hz is less audible and non-directional, so that you don't "hear" the subwoofer, only feel it. Plus, setting the XO at 100Hz or above isn't really helping your speakers too much, or any more than XO at 80, since it's taking away the deep bass that helps to clean up the higher frequencies (the speakers don't have to work as hard).
Plus, even though a sub says it will handle 200 HZ and below, they really should be kept for the deepest bass, as most speakers handle mid-bass well.
As for your 2 Hz adventure, I can't really speak to that. I'm surprised your receiver could produce the sound at all, but obviously not with any real volume. It's too much for both the speaker and the amp, don't make a habit of it.
Good thing it's too much for the amp too. With enough volume, that frequency could explode your internal organs.

Mjr7531

Those specs are only +- 3 dB or +-10 dB (depending on the terms they use) a speaker can reproduce all frequencies.
Just not very well.
Or audibly, and for your question about longevity questions, it's not good for the amp, however, as long as you don't push the speaker beyond it's excursion limits, it's ok for the speaker.

jcaut

... And only a couple of Watts-- or less-- at 2Hz will push most any bookshelf speaker's woofer beyond its excursion limits. Messing around with low frequency sine waves and tone generators is a very good way to wreck your stereo equipment, if you don't know what you're doing.

The speakers have no high pass filter on the input, so they're going to attempt to reproduce whatever frequency the amp sends them. If you can't implement some kind of line level filter (which would be the best option), it would be possible to put a large value capacitor in series with them to block the low frequencies. The value would be somewhat trial-and-error, probably around 100-150uF for an 8 Ohm speaker.

Jason

PolkThug

You can get cheap "passive in-line crossovers" to filter low freq's from your bookshelves. Every car audio shop will have them.

gordonf238

Originally posted by jcaut
... And only a couple of Watts-- or less-- at 2Hz will push most any bookshelf speaker's woofer beyond its excursion limits. Messing around with low frequency sine waves and tone generators is a very good way to wreck your stereo equipment, if you don't know what you're doing.
Jason

This was the first and last time I've been bugging around like this. Although it is a nice visual seeing the woofer cone move like a respirator. This is probably more of a pain for the amp than the speaker.