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Discussion Starter · #1 ·
I was under the impression that a little low-end boost was needed with the THT to get to 20 Hz, althought I know that any boost is a dangerous game and it's best to tame the high than raise the lows.
It’s a tired myth that seems unwilling to die. See here for more detail.

Regards,
Wayne
This response is just a little naive. It presumes that the peaks and dips you see on a response curve can be treated alike. Nothing could be further from the truth. The fact is that a room response is a simplification of a complex system of speaker and room dynamics interacting over space, time, and frequency. Some of those dips can be the result of resonance processes that are far more powerful than your sound equipment, and they are best left un-excited. Now this fact is truer, the larger and more complex your room becomes. For a small listening room, peaks and dips may be little different, and raising a peak may actually work. However, as the OP first said, this is a dangerous game, although it is only dangerous to sound quality, and most of the time it won't damage anything.

So how do you know if you can boost the dips? Well if you have a dip, try to raise it 3 dB by boosting the filter 3 dB. If it responds, then fine, you are OK with that. If it doesn't respond, that is, if the dip moves, say only 1 dB to your 3 dB adjustment, then put the filter back, and let that one alone.

Wayne did warn against one kind of boost you shouldn't tackle- boosting a ported woofer below its port cutoff. This can damage the speaker because its not supported by any air coupling below port resonance. Raising that dip will damage your speaker. OK, now imagine your entire room is a port, coupling into heating ducts, or some other resonant column. Trying to raise that dip will start the walls of the ducts rattling, and do nothing for your sound quality, although it may shake out some dust.

Every peak and dip has a reason for being on your plot. You would do well to try to find the source of the peaks and dips before you try to correct them. Lacking this dedication, make small corrections and be sure your room is responding to your corrections before you add any more. Pathological problems can't be fixed with equalization, and should be understood so a workable fix can be addressed.
 

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Re: 1124p or miniDSP?

Good points. Except for one thing:

Boosting a ported woofer below its port cutoff can damage the speaker because its not supported by any air coupling below port resonance. Raising that dip will damage your speaker.

Say you increase response by 3 dB at 18 Hz on a speaker tuned to 25 Hz. The speaker has no way of knowing the difference between that simply raising the overall volume by 3 dB on a track with 18 Hz content. So Wayne has a valid point. You can only boost until you run out of head-room, whether that occurs from raising the overall volume or a single frequency via EQ.
 

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Admittedly, the post in the link is incomplete. I should have discussed a third myth, that nulls can’t be boosted. While that is technically true, it’s also true that the great majority of depressions in response are not nulls. Nulls are easy to identify, as they are usually deep and narrow and won’t respond to equalization. Depressions or troughs, on the other hand, can be equalized as headroom permits.

I’ll add more text to the post to further clarify.

EDIT: Text added covering depressions and nulls in response.

Regards,
Wayne
 

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Discussion Starter · #4 ·
Re: 1124p or miniDSP?

Good points. Except for one thing:

Boosting a ported woofer below its port cutoff can damage the speaker because its not supported by any air coupling below port resonance. Raising that dip will damage your speaker.

Say you increase response by 3 dB at 18 Hz on a speaker tuned to 25 Hz. The speaker has no way of knowing the difference between that simply raising the overall volume by 3 dB on a track with 18 Hz content. So Wayne has a valid point. You can only boost until you run out of head-room, whether that occurs from raising the overall volume or a single frequency via EQ.
Yes, yes, your last point is correct, but you are missing my point: When the dips are caused by certain phenomena, one runs out of headroom before one can raise the dip.

These cases are not linear. We work under the notion that our rooms are linear, so that we can add power at any frequency, and that power becomes immediately seen as an improvement in our frequency response. Fortunately this is usually true, but certain types of dips in the response do not follow those rules.

A ported speaker below cutoff runs out of excursion almost immediately if it gets power below cutoff frequency range. That is why it is folly to try to raise that dip. In fact, one should use a filter on the amplifier to cut the power below port cutoff to prevent damage to the driver.

A duct resonance in an HVAC system removes sound power from the room, just like it does inside a speaker cabinet. Unfortunately, we are on the wrong side of the cabinet, and we get a bass dip instead of a bass boost. Adding power to that frequency only sends more power out of the room into wherever the duct terminates. I remember a high school auditorium in Point Pleasant N. J. that had a duct resonance so strong that I could not raise the dip; even when I added 8 dB of power to it, it came up only about 2 dB. I remember putting 1/3 octave noise at the center of the dip to try to find the problem, and there was dust coming out of the auditorium ceiling. A trip up to the catwalk running the length of the auditorium showed me the offending duct. Its walls were vibrating along with the resonance inside the duct. The more power this duct received at resonance, the more its walls vibrated and just converted my sound power to heat. The only equalization fix was to ignore this dip in the frequency response. Any flattening of that dip would require an acoustical fix to the duct resonance.

HVAC resonances are depressingly common in large auditoriums, because these rooms need ductwork long enough to resonate, and wide enough to become very powerful. In a small listening room you may not have an HVAC resonance, or one of the other strange problems I encountered "tuning rooms". You also generally have far more amplifier power per cubic foot of room volume to cover up bad architecture than in a large venue. This is where the rule of not trying to raise a dip came from. In large venues there usually isn't enough headroom in the speaker-amplifier chain to cover up those kinds of problems. In a small listening room, you may be able to over-power some of these problems, because they may also be much weaker. You don't have HVAC ducts large enough to walk through!
 

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Re: 1124p or miniDSP?

No problem with all that. I think we all agree.

One possible exception for home HT and boosting below tuning frequency. Say I have a Tuba HT in my home and I never plan on using the top 10 dB of headroom because it gets just too loud, then there's not much wrong with EQing the bottom end a bit to get a few more Hz out of it. I just need to remind myself that while there's lot of headroom left at 30 Hz, there isn't at 20 Hz.
 

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Discussion Starter · #6 ·
As long as you are so aware of the slippery slope you are on while eq'ing a ported speaker below resonance. Be sure you listen for a driver slap on the loudest LFE you plan to use. It would be really sad to damage an expensive driver for a few more Hertz.

A more elegant solution you may consider is to reduce the diameter of the port. This will extend the bass by pushing the port resonance down. There is no danger to the driver, although this method does reduce headroom because port chuffing becomes a problem at high levels.
 
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