newbie question about HPF on mains

A

Art Hays

Freshman
Nov 10, 2014
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Gaithersburg, MD
Suppose I am setting up a sound system with no subs (say board, amp, two-way speakers). It will be used for a band (instruments and vocals). The speaker's -10db point is 48Hz. Is it necessary to have a high pass filter so frequencies below 48Hz are not sent to the amp?

Thanks.
 
Re: newbie question about HPF on mains

Even if the speakers will not reproduce LF content, sending LF audio to the power amp will add to the rest of signal and make it clip sooner. While you can not hear the LF info you will hear the rest of the signals riding on top of the LF getting scraped off when the LF clips.

Filtering out LF will improve effective amp head room.

JR
 
Re: newbie question about HPF on mains

Typical HPF built into economical mixers might be unknown order and phase. Could I mess the sound up more by using one of these than sending the inaudible frequencies through the amp?
 
Re: newbie question about HPF on mains

Art Hays said:
Typical HPF built into economical mixers might be unknown order and phase. Could I mess the sound up more by using one of these than sending the inaudible frequencies through the amp?
Click to expand...

What does it sound like?

If your speakers don't have an internal high-pass filter (few do), then sending a signal with very low frequency content could potentially damage the speaker, if you're trying to ask too much out of it.

A channel HPF is a different tool than a loud speaker processing filter. The channel HPF should be used on any channel that shouldn't have low LF content - vocals, acoustic guitars, cymbals, etc, to keep your mix from being muddy. You should also have a speaker processor that has EQ, including a HPF filter set for speaker protection and sound quality.
 
Re: newbie question about HPF on mains

No. High pass filters are your friends. Use them liberally. There are limited ways to implement high pass filters economically and a second order Butterworth is pretty widespread. A second order Butterworth on say a vocal mic on the input strip of the console in series with an assortment of third octave filters on the mix graphic in series with a second order high pass on the graphic EQ main output is going to have far less detrimental impact than the entire mix to the woofer modulated by out of band low frequencies or worse yet that 15 hz thud when the bass player pulls the plug from his instrument with the DI output hot to the PA system with a lot of 40 hz boosted on the channel EQ.
 
Re: newbie question about HPF on mains

HPF filters are your friends, as someone mentioned above. I'd also say that they are very good friends for your amps and speakers too. There's a lot to gain by avoiding that amps and speakers waste their energy on frequencies they are not designed to be able to reproduce. Few amps and speaker-cabs have built-in filters to remove frequencies it isn't able to handle well, and significant energy may be lost trying to accomplish the impossible. This becomes apparent if you have a modular rig where you use different bits for different purposes. For example will a two-way 12" or 15" active cabinet that works well as a full-range cab with no HPF for small events appear much louder and capable of significantly bigger tasks when it is paired with a woofer that relieves it from any duties below 120 or 150Hz.
 
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