understanding speaker specs

[northhillca]

I have Polk Audio RM6900 with Yamaha RX-V559. I am trying to set crossover frequency. My problem is I dont understand speakers capability properly.

In manual it says, for front, center and rear speakers,
Overall freq. response: 80 Hz to 24 kHz
-3dB Limits: 100 Hz to 20 kHz

for subwoofer
Overall freq. response: 35 Hz to 125 Hz
-3dB Limits: 28 Hz to 180 kHz

Whats the difference between overall freq. and -3dB limits? Whats good crossover point for this set? Thanks.

 

[MDS]

The -3 dB point (aka F3) is the frequency range the speaker can reproduce without losing more than 3 dB from whatever was input. The spec means that over the frequency range of 100 Hz - 24 kHz, the output from the speaker will (should) never be more than 3 dB below the amplitude of the input signal. Normally the point in the range where the response will be down 3 dB is at the extremes (100 Hz and 24 kHz in this case) BUT that doesn't mean that those are the only points where it might dip. The frequency response of a speaker is never perfectly flat at all points in the range.

The 'overall' frequency response is a little nebulous because they don't qualify it. Usually it is the range of frequencies where the output will differ from the input by no more than 10 dB. So the specs say that the speaker can reproduce 80 Hz - 20 kHz with variance no more than 10 dB.

The speaker will still try to reproduce frequencies lower than the stated lower limit but they will be rolled off quickly and will be substantially lower in amplitude than what was input.

For the xover, the rule of thumb is 1/2 to 1 octave above the lower F3 point but that 'rule' only really works for speakers that go as low as 40-60 Hz, so you pick a xover near the F3 point. You could choose 80 Hz as that is the lowest number in the overall frequency response but realistically 100 Hz would be better as that is the stated F3 point (usually you set the xover based on the F3 point).

 

[mr-ben]

This is basically talking about the loudness (I'll call it simply 'the volume' from here on). Speakers will output different volumes than the source specifies depending on the pitch, due to the limits of physics. So if you have a high pitch sound at say 10Khz and another sound at 2Khz, the CD may say that they should be the same volume, the amplifier puts out the same volume, but the speaker plays one louder than the other. How well the speaker is able to output all pitches at the same volume is a major part of what makes a good speaker.

Your main speakers are capable of outputing something from 80 Hz to 24 kHz. It's loudest in the middle, and quieter at the ends. So if you start in the middle, by the time you go down to 100Hz, it's half as loud. By 79Hz you can't hear it anymore. Same thing as you go up - at 20Khz it's half the volume and by 24Khz it's not making sound at all (you can't hear that high anyway).

The subwoofer has its own curve too, and since the top end of the subwoofer overlaps with the bottom end of the other speakers, you get a relatively flat volume from 35Hz to 20Khz. Put your crossover wherever it sounds the best in the 100Hz to 180Hz range. 140Hz is right in the middle if you just want a number.

 

[MDS]

Your main speakers are capable of outputing something from 80 Hz to 24 kHz. It's loudest in the middle, and quieter at the ends. So if you start in the middle, by the time you go down to 100Hz, it's half as loud. By 79Hz you can't hear it anymore. Same thing as you go up - at 20Khz it's half the volume and by 24Khz it's not making sound at all (you can't hear that high anyway).

No, that is not how a speaker behaves.

 

[jonnythan]

No, that is not how a speaker behaves.

Eh, close enough for these purposes.

The poster does imply that it's a steady dropoff from the "middle" to the F3 point, which is obviously not the case. 100Hz frequencies will be half as loud as the middle part of the range, and 80Hz frequencies will be barely, if at all, audible.

 

[no. 5]

100Hz frequencies will be half as loud as the middle part of the range, and 80Hz frequencies will be barely, if at all, audible.

Except that -3dB isn’t half a loud, -10dB is half as loud.

 

[jonnythan]

Except that -3dB isn’t half a loud, -10dB is half as loud.

You're totally right. My bad.

3dB requires twice the power. I mixed those up.

 

[no. 5]

It happens.

 

[skizzerflake]

Regardless of how we interpret speaker specs, remember to take them all with a grain of salt. They may help as a general frame of reference but unless you know exactly how the numbers were measured and duplicate those conditions in your listening environment, if you did similar measurements you would get different numbers. Bass numbers are especially sensitive since bass response varies widely depending on how close you are to walls and corners.

Since speaker makers like to cite favorable numbers, at least you can assume that the performance will not generally exceed what is published. From what you cite, your satellite speakers will probably fit in fairly well with the subwoofer. The sub will probably produce most music (except for extreme pipe organs) pretty well but it might be a little light on explosions during movies. I would suggest that you get a test CD that has test tones at various frequencies and attempt to manipulate the controls on your sub so that the loudness of the lows sounds the same until the sub rolls off.

 

[mr-ben]

Except that -3dB isn’t half a loud, -10dB is half as loud.

Oops. I mixed up power/volume too. Thanks for the correction. I think I got the general concept ok though. I know there are loudness variances throughout the frequency range, but my goal was simply to give northhillca the reason behind my 140Hz suggestion as a starting point. If we really wanted to get technical, we could talk about the problem of the subwoofer becoming directional at the higher frequencies, but I don't want to type that much.