# House curve: What it is, why you need it, how to do it!



## Wayne A. Pflughaupt

*House Curve: What it is, why you need it, how to do it
Part One 
Curve your subwoofer*

Part Two
Beyond subwoofer equalization: A full-range house curve

Part Three
Answering house curve critics​
Related articles:
Is the film industry's X curve suitable for home theater?
System Calibration vs. Program Compensation: Will a house curve deliver sonic bliss?



*Why you need a house curve*
It was a great day for me back in 1996 when I took delivery of the high-end AudioControl 1/3-octave equalizers I had been lusting after for a number of years. At long last I was moving up to the equalizer "big leagues!" At the time I had been installing pro-audio sound systems long enough to know that you don’t set these equalizers by ear, so delivered along with them was an AudioControl real time analyzer/pink noise generator.

It helps to be completely immune to pink noise if you aspire to fine tune your system with sophisticated equalizers (a dubious benefit of installing pro systems for any length of time). My poor wife, I think she ran out of the house screaming and pulling her hair out, but after an hour or so I had dialed in a nice, flat response curve. I popped in a CD and sat back to enjoy the wonders of a perfectly tuned system. But horror of horrors, it sounded awful! It was all thin and shrill-sounding with virtually no bass. Where had I gone wrong?

If I had bothered to consult the AudioControl manual before I fired up the pink noise, I would have known I was headed for disaster. Unfortunately, the manual offered no marital advice, but buried in the back pages I found this passage:

_"When you have flattened out the analyzer’s display curve as much as possible, you’re done with this portion of the adjustments. But you’re nowhere near finished. When you play music through your system at this point, you might not be completely pleased with the result. With the equalizer you have compensated for all sorts of factors, but you have also run up against an important psychoacoustic effect: Flat response doesn’t sound all that great to most people."_

Boy, no kidding, I was thinking. The manual went on:

_"Through long experience, acoustic experts have determined that a ‘curve’ which is tipped towards more low frequencies is much more desirable for music reproduction. It’s sometimes (although not totally accurately) referred to as a ‘house curve,’ a term that comes from the fact that adjusting music systems in individual theaters and auditoriums to achieve the most ‘listenable’ sound tends to produce curves like this, with a significant reduction in higher frequencies. Figure 14 shows a ‘room’ curve which can serve as a starting point for your secondary equalizer adjustments."_

Here’s the Figure 14 from the AudioControl manual, a 1/3-octave frequency response graph:










So there you have it: If you want your system to be "listenable" – i.e., sound balanced - you need a house curve.

Flash-forward a few years to 1999, when I got my first computer and started participating in the various home theater forums: At the time enthusiasts were just getting into using parametric equalization to flatten their subwoofer’s frequency response. I noticed people were saying they weren’t happy with the results, claiming their subs had gone from flabby and boomy to thin and insubstantial. Recalling my experiences from a few years earlier, I recommended that these folks try a shelved house curve instead of flat response, and most seemed happy with the results. The Internet was a big place even then, so I’m not sure I can lay claim to pioneering the concept of the home theater house curve. But at least one salty fellow blames yours truly for the whole “mania” (even if he isn’t clear on where I got the idea and is seriously confused about live sound).


*What exactly is a house curve?*
As you can see from the AudioControl manual’s picture and text, a house or room curve actually encompasses full-range response. However, on the various home theater forums the term has come to refer specifically to subwoofers and low frequency response, so that’s what we’ll focus on to start. In the second section we’ll discuss some theory and a full-range house curve.

I’ve looked at the on-line manuals for most of the pro-audio equalizers on the market, and the passage from the AudioControl manual is the best description of a house curve I’ve seen. Yet their definition is not as clear as it could be. Simply put, a house curve is *perceived* flat response as opposed to *measured* flat response. AudioControl also doesn’t tell us that a house curve is as critical in small rooms (where we use our systems) as it is in the theaters and auditoriums they mentioned. Perhaps even more so, because small rooms need a more aggressive slope than large rooms to sound balanced (we’ll discuss why that is in Part 2).

Since no two rooms are alike, the million-dollar question is: How do you determine how much of a slope you need for your room?

On various home theater forums you will find people recommending all sorts of methods for acheiving a room curve. One of those is what I call a "one size fits all" approach - a 12-dB rise in the subwoofer’s response, or some other rigid figure. Feel free to ignore such advice. Dialing in a house curve is not a "one size fits all" endeavor. The problem is that an ideal house curve varies from one room to the next. The rule of thumb is, the smaller the room, the steeper the slope needs to be.

Yes, that point has already been made. But it bears repeating, because even though all the rooms in your home are small compared to a movie theater or auditorium, the small room/large room rule applies here, too. A bedroom theater verses a system in a family room are a couple of typical small room/large room scenarios in the home. As such, a "set figure" approach won’t work unless you’re lucky enough to need precisely that slope.


*An easy way to determine the house curve you need*
If all this is sounding complicated, it really isn’t. It’s actually fairly easy to determine the house curve your room needs. Here’s a simple method that has worked well for me.

The first thing you need to do is use an equalizer, preferably a parametric model, to get your subwoofer's response reasonably flat. By that I mean eliminate as many peaks and low points as possible. It doesn’t matter if the response you end up with is more-or-less flat-lined or has something of a tilt. The idea is simply to get as smooth a response line as possible. Go for the approach that uses the fewest number of equalizer filters.

After your sub’s response is reasonably smooth, play a couple of sine wave test tones, one at 100 Hz (or whatever your sub’s crossover frequency is), the other at about 30 Hz. (Naturally, you don’t want either of these to be in a null - shift your test tone up or down if you have to.) With response smoothed, the 100 Hz tone will probably sound louder than the 30 Hz tone. If that’s what you find, your sub’s response needs to be adjusted so that both test tones _sound like they’re the same volume level._ Yes, that’s a highly subjective evaluation, but remember a house curve is _perceived_ flat response – that is, it sounds flat, not measures flat. Thus it _has_ to be subjective.

So, using the upper frequency (100 Hz in this example) for your baseline SPL reading, if you find you have to increase the sub’s volume say, 8 dB to where your lower frequency (30 Hz in this example) sounds as loud, then you need an 8 dB slope between 100 and 30 Hz.

As you can see, since you’re merely trying to get the higher and lower frequency test tones to sound the same, you don’t have to worry about any complicated formulas or calculations. Your room does it all for you automatically - great news for those of us who are mathematically challenged! 


*Dialing in your house curve*
If your baseline frequency response (i.e. before you smoothed it to determine the house curve) had exaggerated lows, you could simply re-equalize the sub to show a 8 dB boost between 100 and 30 Hz (keeping with the example above). However, if your base response is such that smoothing out the peaks and valleys gets you nearly flat response, the easiest way is probably to apply a so-called "shelving filter." A shelving filter is one that adjusts response for everything below the selected frequency. Fortunately, most of the digital processors folks use these days have shelving filters, as do many vintage analog and digital pro-audio equalizers that can typically be found at good prices on the used market.

Keep in mind that you will have to tweak both the gain and starting point of the shelving filter to suite your particular system and situation. The goal will be to the 100 and 30 Hz test tones to sound like they’re at the same volume. When you accomplish that, you’re probably golden. I say "probably" because since every room and system is different, you should let your ear be the final determining factor.

When you're satisfied with the way the 100/30 Hz test tones sound, re-check your equalization between those two points. To check your equalization, play some reference CD’s you’re familiar with and see how things sound. See if you can find some music where the bass line ranges from very low notes to very high. Ideally they should all sound the same from note to note – no hot notes, no weak notes (assuming the recording is good to begin with). Adjust your slope as you feel is needed. You may need to move the point where the curve starts to a higher or lower frequency. Whatever it takes to get those bass lines running smooth from top to bottom, that's what you want.


*Why shelve response?*
Earlier I mentioned shelving your house curve (to avoid confusion, that's different than the shelving filters discussed above). I personally prefer to shelve response at about 25-30 Hz rather than allowing the curve to continue rising. What that means is the rise stops at that point, and is flat from there down to the sub’s lowest limit. I initially kept the slope rising all the way to the bottom, but I found there was an overabundance of ultra-low energy. The bass in music sounded "heavy," and with movies things like car doors slamming had the "umph" of a distant explosion. Certainly did *not* sound natural. Shelving response at ~30 Hz solved that problem. Since every room and system is different, it doesn’t hurt to experiment, but if you find a continually rising curve sounds bad, try shelving it. Music should be greatly improved, as I’ve found that bass detail and resolution gets obscured if response continues to rise. There’s no penalty with movies, since they have greatly exaggerated extension that essentially overrides any shelving.

So that’s my "quick and easy" house curve method. I’ve been recommending this method to people for many years now and no one has ever come back and said, "It sucks, what’s your next bright idea?"  So, give it a try and see if works for you. I have found it to be very musical - balanced and accurate with lots of bass detail (assuming your sub is capable of rendering detail) - yet there’s enough impact with movies to vibrate the sofa. Who needs bass shakers? 


NOTE: More recently I’ve decided that I _greatly_ prefer the sound of a hard-knee house curve, with a sharp decline from the shelving frequency, rather than one derived from a simple shelving filter or re-aligning the crossover curve. Fellow Shack Moderator Ayreonaut’s experimentation led him to prefer to shelve his response lower than 30 Hz. Lately I’ve dropped my shelving down to ~28 Hz, and I like the way it sounds. Here’s Ayreonaut’s thread on the subject: House Curve Options


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## Wayne A. Pflughaupt

*Part Two
Beyond subwoofer equalization: A full-range house curve*​

In the first section we discussed a rise in a subwoofer’s bass response commonly called a house curve. In this section we’ll explore the reality that a house curve is actually a full range response curve, a fact many people are not aware of.

We saw in the first section that a house curve will be steeper for small rooms than for large rooms. In this section we’ll look at why that is. We’ll also discuss other aspects related to a house curve such as cabin gain, and the size and selection of the main speakers.


*It’s all about the room*
It’s difficult to determine where the term and practice of a house curve originated, but I imagine it came from the pro audio world, since (as far as I know) they were the first to extensively use room analysis and correction with sophisticated equalization. With touring acts that play in a different venue every night, the ability to tune a system to various auditoriums is a vital necessity. Therefore it’s no surprise that pro audio manufacturers like Rane, Ashly, Klark Teknik, and Carvin include references to a house curve in their equalizer manuals.

I mentioned in the first installment that AudioControl had the best definition of a house curve that I’d seen from an equalizer manufacturer. By contrast, Ashly’s manuals have the best explanation as to why smaller rooms need a steeper slope than larger rooms:

_"As sound travels through air, high frequencies are attenuated more than low frequencies. In general, large rooms benefit from some low frequency roll off [and] high frequency boost..."_

You have to keep in mind that when Ashly mentions "large rooms," they’re talking about convention centers, civic auditoriums, arenas, or other concert venues. In those places, most of the audience is a considerable distance from the speakers. Due to the loss (or attenuation) of the upper frequencies over distance, a sound system in a large room, when properly tuned, will compensate for that loss by boosting the upper frequencies. As a result, these large rooms typically require a fairly flat response curve to 12 kHz or so (very few sound reinforcement speakers extend all the way out to 20 kHz).

At home we have exactly the reverse situation. Primarily, we’re sitting fairly close to our speakers – usually only 10-15 feet. As anyone who’s spent any amount of time around audio knows, speakers sound brighter the closer you get to them. So if your main speakers sound shrill and harsh, you may need to compensate with a tilted response curve that reduces the highs and gives more emphasis to the lows. In other words, a full-range house curve, from the highest to the lowest frequencies.

The truth is, any audio system that demonstrates balanced octave-to-octave performance has an ideal full-range house curve, and it would be verified with broadband frequency spectrum readings. It may or may not have been an intentional goal when the system was set up. But things like speaker selection and positioning, subwoofer level, and utilizing a receiver’s tone controls, all play a part in fine-tuning the room curve that’s required in order to have a "listenable" system, as AudioControl describes it.

Let’s take another look at the graphs from the AudioControl manual we saw in our first installment:










We didn’t explore it before, but you can plainly see that the sloping room curve extends from the highest frequencies to the lowest. I ultimately found a full-range curve like this to be a necessity when I dialed in my own system after acquiring my AudioContol EQs and RTA. Here’s the room curve I ended up with:










As you can see, the curve I settled on was 16dB higher at 32Hz than at 20kHz. From an arbitrary 0 dB reference, upper-frequency response dropped 8 dB from 160 Hz to 10 kHz (a 6-octave span), while bass response rose 8 dB from 160 to 32 Hz (3 octaves). Note that bass response ramps up much "quicker" than high freq response drops; this is why people pay so much attention to their subwoofer’s curve. Also note that between my usual reference points for a subwoofer house curve, 100 and 32 Hz (see Part 1 for details), the rise was only 6 dB, a relatively shallow slope. This is because the room was very large – over 6000 cubic ft.


*How does cabin gain figure in?*
Cabin gain is an acoustical phenomenon whereby a speaker’s bass response is affected by the size of a room. I’ll be perfectly honest here and admit I’m a little murky on this. I understand how cabin gain works, but I’m not so certain why. I have a couple of half-baked theories that I won’t bother to explore here (a shameless attempt to spare myself extreme embarrassment.  ). We’ll just stick with the well-established fact that a speaker will exhibit increasingly greater bass performance the smaller the room gets.

For example, I can’t tell you how many times I’ve seen someone post a cry for help on one of the Forums that goes something like this: "I just set up my speakers in my new house and all of a sudden I have no bass!" Or this: "I moved my system into a spare bedroom so I could have a dedicated listening room, and holy cow, the bass is overwhelming! I haven’t changed any tone controls on the receiver. What happened?"

Welcome to the effects of cabin gain!

What happened to these two hapless individuals? In the first query, where there is no mention of relative room sizes, I’ll typically respond back with, "Is your new living room larger than your old one?" That’s mainly for his benefit, because I know what he’s going to say: "Yes, it is."

The situation is that this person’s speakers were a great match for his old living room. They sounded perfectly balanced and he was a happy camper.  But in his new, larger place the effect of cabin gain is working against him. To restore the missing low end he will need to increase the bass output of his speakers via the tone controls, or perhaps trade up to some larger speakers.

The second person has the opposite problem. He doesn’t specifically say so, but if he’s moving his existing system to a bedroom it’s a safe bet that he had it set up in a larger room before - probably a family or living room. If his speakers sounded great there, you can bet the bass will be killing him in the smaller room – and it is.

So, you can expect changes like this anytime you move your speakers to a larger or smaller room.


*Obtaining a house curve via "natural selection"*
It’s a fairly common problem when shopping for speakers that when you get your new babies home, they sound drastically different from what you heard in the showroom. You can minimize this phenomenon by being aware of how cabin gain works in small vs. large rooms. Specifically, when you audition speakers at a dealer, the size of their demo room, relative to the size of your room at home, is a vastly important factor that you should take into account.

Basically, if your room is smaller than the dealer’s, you’ll want speakers that sound thin (i.e., weak bass) in the showroom. If your room is larger than the dealer’s, you’re looking for speakers that sound bass-heavy. Just how thin or how bass-heavy depends on how much larger or smaller your room is compared to the dealer’s – a little or a lot. Is that clear as mud? 

What you are doing, in effect, is dialing in a full-range house curve by the selection of your speakers.

Even if you’re shopping for a sat/sub system instead of full-range speakers, cabin gain will come into play. The satellite speakers must to have enough bass output in your room to blend with the subwoofer. Satellites with little 5-1/4" woofers will probably do fine in a bedroom system; try using them in cavernous living room that’s open to the upstairs game room and you can expect to have a big hole in their mid-to-lower bass region, between the point where their lower response sags and the sub takes over.

Polk Audio and Axiom Audio are a couple of speaker manufactures that "get it" when it comes to matching speakers to the size of a room. Or at least they once did. Polk used to break down their home theater systems for small, medium and large rooms, for on-wall satellites, bookshelf and floor-standing speakers respectively. Axiom was even more specific, offering the following advice from their website:

_"Look at a bookshelf system if you are considering a home theater for apartment-sized living rooms and bedrooms, as well as dens. Look at a tower system if you are considering using a home theater in larger rooms or "great rooms" with vaulted ceilings, which require the higher volume output capability and bass extension of floor standing main speakers and large subwoofers..."_

(Note: Axiom also used to carry room-size designations for all their speakers, recommending appropriate models for small, medium or large rooms. Unfortunately they have generally discontinued this practice for the more generic recommendations mentioned above. Too bad.)

Simply put, the larger your room is, the larger your speakers need to be, with larger (or more) bass drivers in order to sound balanced. It bears repeating, what you’re doing is dialing in a house curve for your room by selecting the right speakers. This is certainly more desirable than "forcing" your house curve electronically via equalization – at least for the mains.

In Part 3 we’ll explore the idea that house curves are built into program material and therefore unnecessary.

_NOTE: Fellow Shack member Spridle tried a somewhat different approach that he was very pleased with, keeping response fairly flat down to the point where the sub takes over. Here's his thread on the subject:_
Spridle's house curve


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## Wayne A. Pflughaupt

*Part Three
Answering house curve critics*​
Every theory in audio has its proponents and detractors. However, as we’ll see, room curve detractors typically don’t have a good understanding of what it is, nor of its function.


*A house curve built into recordings?*
One thought critical of house curves is based on a rather idealistic concept of the recording studio environment and exactly how it affects our program material. The following is from a house curve discussion thread a few years ago on another Forum:

_”Consider a properly set up mixing booth in a studio. The mixing engineer is listening to either nearfield monitors or other speakers when he is laying out the final 'sound' of the music that is being mixed down to two (or more) channels. If the physical space and hardware has been designed properly (including the choice/location/EQ of the monitoring speakers), the frequency response at the mixing position should be flat from 20 Hz to 20 kHz. Neither the booth nor the mixing electronics should add or subtract from the sound on the master tape. If you wish for your home system to be accurate, it needs to be as flat as the one they used in the studio.”_

However, this fellow offered no frequency response graphs from any studio, famous or not, to support his theory. He’s merely supposing.

But here’s the problem with this idea: A room is a room is a room. If the mixing engineer’s monitoring system is as flat as all that, _it will sound as bad to him as it does in your living room!_ There’s no way around it; a properly-tuned studio monitoring system will also have an appropriate house curve for the room it’s in. It has to. If not, the engineer is simply going to compensate with equalization of the program material. Good engineers know that their room and speakers of choice will affect their final mix. That’s why they usually demo it in different environments before they finalize it.

Another thought critical of house curves acknowledges and indeed embraces the fact that they come into play in the production stage. It holds that a room curve at home isn’t necessary because it’s built into the product’s final mix. Therefore – once again - we should set up our systems with response as flat as possible, otherwise nothing will sound right.

Nice idea, but as we’ve established, very few people actually think flat in-room response sounds good on their playback system.

Furthermore, the question needs to be asked: which house curve are they using in that mixing studio – small room, large room, or something in between? As we saw in Part Two, room size matters tremendously, and once again this can’t be understated. After all, the engineer can reasonably expect that his music will be played back in cars, dorm rooms and living rooms. What about the DJ spinning tunes in a huge hotel ballroom or at an outdoor event? What a dilemma: Which house curve is our hapless engineer supposed to “build in?” Obviously something fundamental is missing from the built-in curve theory: an industry standard.


*A Fletcher-Munson house curve?*
Other room curve skeptics base their doubts on the Fletcher-Munson curves, which show deficiencies in human hearing at the lowest and highest frequencies. They claim that studio engineers, being human themselves with the same auditory deficiencies, compensate for that in the recording process. The following was also presented on a discussion thread a few years ago:

_“The engineer who mixed the material you are listening to has the same hearing response you do, that is less sensitive to lower bass frequencies. Do you think he would increase them to compensate? Of course he will.

“Now, enter a concept like a house curve to this equation. If your playback system has a curve attempting to compensate for a frequency response associated with human hearing, what do you think the results will be when you play back material that was mixed and prepared on a flat system? 

“Well, you'll be re-compensating, adding again the same compensation curve that the engineer added.”_

This critic’s disapproval is fundamentally flawed because it’s based on the notion that a house curve is synonymous with the Fletcher-Munson curves. It isn’t. The Fletcher-Munson curves show how our perception of bass and treble frequencies _change with variations in volume levels_. That is a wholly separate phenomenon that has virtually nothing do with a house curve, other than the fact that it's best to calibrate your system at the volume level you use most. As we’ve thoroughly established, a house curve is compensation for the _room_, not the ear.

Along the same lines we have this complaint:

_The problem with a "house curve" is that it's a static solution which only works at one playback volume. It may be inadequate at lower volumes, and it may be overly intrusive at higher playback volumes._

I have not found this to be the case at all. Calibrate your system and house curve for the level you normally listen and you’ll find that it will be adequate for most listening from that point, except perhaps for extreme level variations. With extreme changes from your normal settings you aren’t doing critical listening anyway – e.g. ultra-low levels for background music, or ultra high when you’re “showing off” the system's capabilities. In these instances, all that’s needed (if anything) is a simple level adjustment of the sub, not a total re-curving.


*The movie industry settles the issue*
We can look to the movie industry to see the inherent problem with these lines of thinking. Unlike the music industry, the movie industry actually _does_ has a standardized house curve, called an “X curve:”










As you can see from this chart, the X curve has flat response from 63 Hz to 2 kHz. Response below 63 Hz drops 3 dB/octave, as does response above 2 kHz. At 10 kHz, the slope increases to -6 dB/octave.

As industry pro Tomlinson Holman explains here, the X curve is used in both theaters and dubbing soundstages. He readily acknowledges the problem most of us are aware of with home movie releases, that “when heard over a modern flat loudspeaker in a small room, program material balanced on an X curve monitor sounds overly bright.” Mr. Holman adds, “This is not too important because, so long as everyone [in the industry] agrees to use the same curve, then the response sounds the same to the mixer on the dubbing stage as to the audience member in any auditorium. Interchangeability of X curve material with home video can be handled with a simple re-equalization.”

With all due respect to Mr. Holman, the problem I have with the X curve is that it fits the problematic “one size fits all” scenario we discussed in our first installment. There is no industry standard for the physical size of a movie theater or a dubbing stage, yet they apply the same house curve to all of them.

And indeed we can see the discrepancies it generates in the DVDs we buy. Sound editors are forced to compensate both for a curve that might be inappropriate for their dubbing stage, and the inherit shortcomings of the X curve, such as the roll-out of the low bass frequencies. Thus it’s not hard to see why the frequency balance in DVDs varies from one to the next (just like it does with CDs) and why so many movies (especially action fare) have such extreme and exaggerated low frequency content (not that we’re actually complaining about that! ).

Shortcomings aside, there is something very important that we can glean from the movie industry’s X curve standard that should settle any house curve debate, the fact that it is applied to both production facilities and theaters. In other words, program material that’s prepared with a house curve _must also be played back with a house curve_ in order to sound correct.

If it goes for movies, the same is true for music as well.


Related articles:
Is the film industry's X curve suitable for home theater?
System Calibration vs. Program Compensation: Will a house curve deliver sonic bliss?


Thanks to:
brucek for his excellent Excel program, from which we created our charts.
Donna Pflughaupt, for creating the Excel charts.


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## Wayne A. Pflughaupt

Since this is an informational thread, it has been locked. For questions, comments or other discussion, please open a new thread here on the Equalizers or REW Forum.


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