What are the sonic DISadvantages to EQ'ing bass ( & only bass)? Please be specific?

[FrantzM]

Where on earth do people get the idea that they are conserving headroom by not applying any boost filters? It just isn’t so.

Let’s say your room generates an 8 dB peak at 60 Hz from your sub. Since that peak is the determining factor of what you hear from the sub, your sub / mains blend is based on that peak. What happens when you apply a cutting filter to eliminate it? Your sub level is now too low, so you have to increase it. Well – say “goodbye” to any headroom you thought you saved, because now you are pushing your sub harder than you were before.

People don’t consider what happens to electrical response when applying cutting filters, but with a little modeling using REW we can see that they leave peaks in response between the cuts, not to mention (at least in the case of the graph below) a big honkin’ boosted shelf on the back end.

Bottom line, there is no free lunch: The simple truth is, virtually any equalization taxes amplifier (and driver) headroom, so you have to have enough to spare going in.

There is also a lot of misinformation that floats around about whether or not nulls can be corrected with equalization. The short answer is “they can’t,” but people often confuse nulls with a mere trough or depression.

A true null is typically narrow and deep, as the graph below shows. Nulls can’t be corrected with equalization.

Nulls before equalization

Nulls after equalization​

A trough is usually a rather broad depression in response and can typically be corrected with equalization, as the graphs below show.

Trough before equalization

Trough after equalization​

Note that what appeared to be a possible null just north of 45 Hz smoothed out nicely with equalization. So it wasn’t a null at all, just a low spot situated between a couple of peaks.

Many misinformed people would apply a slew of cutting filters to bring everything down to the level of the 30 Hz trough, in the interest of “saving headroom.” However, it should be evident that’s merely going to lower the overall sub level considerably, requiring a significant boost in gain on the back end – again, no conservation of headroom in that. Not to mention, using the equalizer as a de facto volume control is poor form. A series of boost and cut filters would be the appropriate action.

Regards,
Wayne A. Pflughaupt

The best answer I've seen about whether one can effectively EQ a response dip is in the Room EQ Wizard manual. There's this idea of "minimum phase" in system theory that relates to whether the inverse of a system transfer function is causal and stable. If both are true, the system is minimum-phase. Given an amplitude response, there's a formula for computing the (unique) phase response of a minimum-phase system having that amplitude response. In the strict sense of the word, a system is either minimum-phase or it is not. But there's also this relaxed notion of "minimum-phase over a finite band of frequencies". In this view, if the phase response is numerically close to the minimum-phase response computed from the amplitude response, the system is said to be minimum-phase over that finite band of frequencies. If one takes such a system and equalizes it only over the frequency range for which it is minimum-phase using minimum-phase equalizers such as parametric EQ IIR filters, then fixing up the magnitude response will also fix up the time domain response caused by the non-flat response in the minimum-phase frequency region(s).

There's a fly in the ointment though. Because of delays due to physical distance in acoustical systems and the speed of sound, no such system is minimum-phase, even over a finite band of frequencies. But a constant delay due to the speed of sound and the distance the wave travels does not affect the ability of minimum-phase EQ to fix up the time- and frequency-domain responses of a system that would have been minimum-phase over a frequency band were it not for the propagation delay of the wave.

The Room EQ Wizard manual describes in ingenious solution to this problem involving the computation of the so-called "excess group delay" from the magnitude and phase response measurement. Here is the section of the manual that shows how to compute the excess group delay, which in turn can be used to determine whether it's "safe" to EQ a given band of frequencies.

Excellent ,useful posts.. Great information

THanks guys, keep these coming!!

 

[caesar]

Gentlemen,

Outstanding thread!!!!

A couple of more basic knowledge questions:
- In reality, isn't the frequency response of a room dynamic, unless you are in an anechoic chamber? Do the EQ settings assume a steady state or average frequency of your room? Or is the EQ measuring the frequency response each second, half second, etc., and is adjusted dynamically?
- Aren't most amplifiers in subs are fairly large, usually class D amps that have power anywhere from 600-1,000 W? Are you guys saying that when dealing with a huge dip like Wayne is showing at 38.3 HZ the amp is working so hard that it may jeopardize performance and headroom so much, the system may sound anemic?
- If in your setup your speakers are fully driven by your amp, and the sub is only augmenting, isn't there even less power draw on sub's amp, and thus even less chance to jeopardize the performance?
- Also, what are the general causes (in nature/ room) of a basic trough vs. a very deep null?
- Generally speaking, are nulls the "death knell" of a room, or can nulls be dealt with by moving speakers and subs around?

Thanks!!!

 

[JackD201]

Hi Caesar,

Let's assume for now that you are to use two subs, which I think we all recommend. A null happens when the output of one will be met with the output of the other at 1/2 wavelength and opposite phase. IMO headroom isn't the issue because even if you had tons to spare, the boost in one will be met with the boost of the other, dip remains. Positioning can surely help. With subs there's the issue of not having for example equal distances from the subs to you and other points. Stick 'em in the corners to address this is a rule of thumb that still holds. Main speakers away from walls preferably in a rear/side ratio that isn't 1:1 also helps in this regard. What a lot of people who don't really want to obsess about this is that the quickest way to deal with a null is to not sit in one. Okay, that's a band aid true.

Given your speakers I'm betting you will be crossing over very low. Maybe as low as 50 or even 40Hz. That makes things easier to. Quite a different scenario from speakers down 6dB in the high 40's for example where one might have to come in as high as 100Hz.