@Wasnex posted:
IMHO an HPF is beneficial for many reasons. The most important is when you are using a ported speaker. In order to understand why it would be helpful if you had some idea how ported cabs work.
With a ported speaker at some low frequency point, the driver will begin to roll off. The port is tuned to start becoming active so that it reinforces and smoothly extends the low frequency response below this point. A benefit here is, as the port becomes more resonant, it suppresses the drivers cone excursion. This continues until the port tuning frequency (Fb). Below Fb, output drops of very quickly and driver excursion increases dramatically. So there really is very little benefit in running a ported cab much below Fb, because it does not efficiently produce sound and, if pushed hard, the driver will exceed its excursion limits and incur damage. This is related to the mechanical power handling limits of the design which is determined by the drivers T/S parameters, the volume of the cab, and the dimensions of the port(s). Often a designs mechanical limits below 100hz are less than the driver's RMS power rating.
If you look at the various charts in the attached documents you should be able see what I am talking about.
Notice the Deltalite 2512 has a 250W RMS rating and Xmas is 4.9mm
Take a look at the first design in the cabinet design doc. The notes say: "Displacement Limited to 125 Watts; F3 of 64 Hz. Use a steep high pass filter set to 35 Hz to protect your woofer."
If you look at the graph on p. 2 labeled "Cone Displacement (mm/Hz) with 125 watts," you should be able to understand why the note makes sense. Port resonance (Fb) for this design is 44hz. Note that driver excursion decreases as the frequency approaches 44 hz and then quickly increases to Xmax of 4.9mm at ~35hz, which is the recommended HPF frequency. Keep in mind the HPF will be 3dB down at 35hz. Without an HPF, the driver's excursion will be almost 8mm if 30hz is applied at 125W.
One reason you can usually get away with using a ported speaker safely with bass guitar is because the instrument does not produce a strong fundamental to begin with. To understand, take a look at this thread: URL='
Bass']Bass frequency/waterfall plots: what they mean to rigs frequency/waterfall plots: what they mean to rigs[/URL] So if you don't artificially pump up the lows an excessive amount, the electric bass has sort of a natural HPF built in. But you can still get some low frequency crud that is not harmonically related to the notes you are playing. Filtering this crud out can significant improve the sound.
In my experience an HPF is most beneficial when you speakers are trying to reproduce frequencies below their pass band. Preventing the speakers from trying to reproduce frequencies they are not designed to handle allows them to work better in their intended pass band.
Although less critical, an HPF can also tighten up your sound with speakers that are capable of extended low frequency response. Here the benefit is partly related to the type of sound you want and also partly related to limiting the range of the bass to what is necessary to sound good, so it does not cause other problems.
The next part of this post will get into sort of a larger world-view of the benefit of HPFs:
Low frequencies tend to build up and create a reverberant field that causes the low end to sound boomy and undefined. If the decay time is long enough, the resultant sound field will tend to mask the direct sound of low frequency instruments. In other words the audience will hear more of the incoherent reverberant field than direct sound coming from amps and PA speakers. This problem is compounded because there are usually multiple sound sources, so the sound from each sound source arrives at the listeners position at a slightly different time causing phase and other timing issues. One solution to masking is to put less energy into problem frequencies.
Typically frequency slotting is used in mixing, so for example a choice may be made to reduce the extreme low end of the base guitar to make room for the bass drum. An HPF can be applied to both the bass drum and bass guitar, but of course the knee will be at different frequencies for each instrument. Basically the HPF is rolled up until it start thinning to sound to an undesirable degree, and then you back it down a bit from there.
When I mix, I tend to apply a variable HPF to every channel. If the vocal mics are allowed to run full range, they will pick up and amplify low frequencies. Keep in mind that both the low frequency sound sources and microphones are in different locations, so any low frequencies picked up by the mics will further compound the masking problem by introducing low frequencies with different phase angles and other timing issues. Usually I can run the HPF on a male vocal mic up around 150-180hz, and the HPF on female mics can usually be run even higher. I often run the HPF on guitars up around 180hz as well. I usually start with a 100hz HPF on all monitor wedges.
