[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 near-field monitors or other speakers when he/she 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 that this mixing engineer is using has been designed properly (including the choice/location/EQ of the monitoring speakers), the frequency response at his listening 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.”

Here’s the problem with this idea: 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. 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 CD 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. As we’ve thoroughly established, a house curve is compensation for the room, not the ear.


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:”

x curve.jpg

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 standard 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.


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