Explain speaker cut off frequency

[Dan Lazzara]

If i set my main speakers for 100 hz will that cutoff frequency below 100 hz or above 100 hz, please explain how it works.

 

[shadyJ]

If you set the crossover to 100 Hz, it will redirect the frequencies below 100 Hz to the subwoofer, so the speaker itself should not be playing those frequencies anymore.

 

[TLS Guy]

If you set the crossover to 100 Hz, it will redirect the frequencies below 100 Hz to the subwoofer, so the speaker itself should not be playing those frequencies anymore.

That is not totally correct. If you set the crossover to 100 Hz, then the frequencies will start to roll off below 100 Hz. Generally receivers have Butterworth crossovers, which means the roll off will be gentle down to around 80 Hz and then roll off 12 db per octave. There will still be significant signal to the driver down to between 50 and 25 Hz. Below 25 Hz there will be no significant output from the crossover.

Now speakers all have a natural roll off, sealed boxes second, 12 db per octave at a frequency where the driver becomes inefficient. Ported boxes roll off fourth 24 db per octave below the point where the port assists the driver. The electric crossover and speaker roll offs are additive. So what happens is the mathematical addition of the two roll offs.

The sub on the other hand will roll off 24 db per octave at the set crossover frequency. In general the driver would not roll off until a frequency significantly above the selected frequency.

This system was actually devised to be used with sealed or TL type speakers that roll off second order. As the electrical and speaker roll offs in theory sum to fourth order so the slopes are symmetrical either side of the crossover. This can never occur with ported speakers. However since the ear is not sensitive in this region, or at least not highly so, you get away with it, at least for most.

You would never get away with this if you bought separate tweeter and woofers, and used a generic crossover in a receiver!

 

[3db]

That is not totally correct. If you set the crossover to 100 Hz, then the frequencies will start to roll off below 100 Hz. Generally receivers have Butterworth crossovers, which means the roll off will be gentle down to around 80 Hz and then roll off 12 db per octave. There will still be significant signal to the driver down to between 50 and 25 Hz. Below 25 Hz there will be no significant output from the crossover.

Are you saying that the digital crossovers on AVRs mimmic a Butterworth crossover? Would it be a 3rd order crossover?

 

[TLS Guy]

Are you saying that the digital crossovers on AVRs mimmic a Butterworth crossover? Would it be a 3rd order crossover?

No. As I understand it the high pass filters are second order and the low pass fourth order, with the Q 0.707 range, which would make them Butterworth. Even order crossovers can be LR, Butterworth, Bessel or Chebychev.

The standard of second order high pass and fourth order low pass was specified by Lucas/THX years ago. I think that has been pretty much adhered to. I can't remember if Q was specified, but as I understand on AV receivers etc, are constant power Butterworth pretty much.

We had a post the other day, about someones measurements. Although these were home in room, the curves looked like Butterworth to me most likely, they were not LR for sure. Could have been Chebychef, but that high Q 1 filter is not used very often.

 

[3db]

No. As I understand it the high pass filters are second order and the low pass fourth order, with the Q 0.707 range, which would make them Butterworth. Even order crossovers can be LR, Butterworth, Bessel or Chebychev.

The standard of second order high pass and fourth order low pass was specified by Lucas/THX years ago. I think that has been pretty much adhered to. I can't remember if Q was specified, but as I understand on AV receivers etc, are constant power Butterworth pretty much.

We had a post the other day, about someones measurements. Although these were home in room, the curves looked like Butterworth to me most likely, they were not LR for sure. Could have been Chebychef, but that high Q 1 filter is not used very often.

Thanks TLS

 

[TLS Guy]

While on this topic, I'm going to expand in the actual reason for this LFE crossover in AV.

I will have one more go at this. It keeps being erroneously stated that the primary reason is to offload the receivers amps. This is not true. There is very little acoustic power demand below 80 Hz, and the use of a sub does virtually nothing to off load a receiver's amps.

It happens that a lot of subs are small and can not produce deep LF efficiently, and so need lots of power.

If you have a large efficient and sensitive LF system, the LF amps just coast and use very little power indeed, despite producing thunderous sound effects.

The real problem that is being solved is drivers being off loaded at low frequency.

This is particularly true of ported speakers, where the cone is effectively in free air below the port cutoff frequency. So at that point large useless cone excursions occur and exceed the mechanical limits of the transducer. This can result in mechanical damage to drivers. This is the main reason for the bass management set up we have come to know.

In fact if speakers can handle the LF without distress the best sound is generally produced by running speakers full range and supplementing with the subs.

In fact I run all my speakers full range, with two drivers handling LFE below 60 Hz, and two drivers also handling the LFE + BSC. This produces by far the best step/transient response by measurement, and listening test confirm that it produces an uncannily realistic and integrated bass response.

 

[herbu]

If i set my main speakers for 100 hz will that cutoff frequency below 100 hz or above 100 hz, please explain how it works.

Dan, wow! You asked a very simple question and got a quite technical and detailed answer. Don't be discouraged. I'll bet MOST people could not follow or understand all that.

shadyJ gave you a good answer.
First, you say, "set my main speakers to 100hz". By that, I'm guessing you refer to "Crossover". That is a setting you can control in most AVRs. It is for people that have a separate subwoofer, (or 2, 3, 4. )

A subwoofer is a speaker designed to produce ONLY the low frequencies. These are the low frequencies of thunder, explosions, monster foot stomps, and very low bass in some music. No "bookshelf" speakers can reproduce those frequencies, and many "tower" speakers can't either. You have to get into pretty expensive tower speakers, (several thousand $), to accurately give you those lows. And even those likely can't do the thunder and monsters very well.

Also, movies have a separate channel just for the lowest of the lows. It's called the LFE channel, (Low Frequency Effects). The LFE channel is the ".1" in a 5.1 or 7.1 soundtrack. It doesn't exist in music, only movies & TV. So a lot of people get subwoofers.

But now you have a speaker, (subwoofer), designed only for very low frequencies, and you have to tell your receiver where to split the signal between your regular speakers and subwoofer... thus the "crossover". It is the point you're telling your receiver to send signals above that freq to your regular speakers, and below that freq to your subwoofer(s). Depending on the capability of both your speakers and your sub(s), you can set this crossover at the point that best matches your equipment. Usually people set it around 80Hz.

You got some detailed explanations about how the crossover is not a sharp cutoff, but a rolloff. True, but from the nature of your question I'm guessing it's something you don't really need to worry about.

If you have more questions, please feel free to ask. Don't worry. Sometimes we get carried away w/ tech talk, but we'll always come back and answer your question in plain English.

 

[JohnnieB]

The tough part is having an intimation of what TLS is talking about, reading it two or three times, then having to research it anyway. Sometimes it's like going to college. As my great grandfather used to say, "good stuff".