[Wayne A. Pflughaupt]

Hi Mike,

Glad you liked the article! I’ll try to address your concerns as best as I can.

The rolled-off house curve has a good basis in psychoacoustics, because a soundfield originating at a distance is "expected" to be more rolled-off than one originating nearby. It is a little like optical illusions in vision that show, despite occupying the same area on the retina, pictures look bigger on a larger screen, even when a small screen is closer and takes up the same horizontal and vertical angles. As it turns out, both spectrum and level are affected by the perception of the size of space you are in, and "getting it to match" perfectly from large to small room in physical sound pressure level and response does not result in sounding the same

The issue we are dealing with in a home setting does not appear to be based on the same cause or does it?

He’s mainly addressing how sound is perceived in a large theater vs. a small one, primarily related to the viewer’s distance from the screen. So I really don’t think much of that statement applies to us. Real theaters, even small ones, are huge compared to even the largest home theaters. We typically are 10-15 from the speakers (and screen), no matter what the size of our room is, and the size of a large home theater vs. a small one (i.e. total cubic ft.) is inconsequential compared to the difference in size between a small vs. a large movie theater. So we don’t have to worry about the physoacoustic effect Mr. Holman mentions.

"Another development of the X curve is how it should vary with room volume. Although a variation in the response with room volume was written into the original standard, further work shows that the response should be "hinged" at 2 kHz, and turned up at high frequencies in smaller rooms. Curves that extend the range out to higher frequencies before breaking away from flat do not seem to interchange as well."

What do you make of this given your theories on the subject?

What he’s saying (in this quote and in the one above) is that in smaller theaters, the highs need to be increased compared to larger ones, which is essentially counter to my personal experience. It also contradicts what I related from the Ashly equalizer manual, that it’s the larger rooms where highs need to be boosted, to compensate for the natural distance-related high frequency loss.

I certainly don’t pretend to be more knowledgeable on this topic than Mr. Holman, but this contradiction is something I haven’t been able to fully reconcile. It doesn’t help that he really doesn’t explore why a smaller theater requires more high frequency output than a large one, except to say “if you’re closer to the speakers, you expect them to sound brighter.”

The best I can come up with is that it’s somehow specific to theaters. For one thing, the effects of cabin gain in a smaller theater vs. a large may be increasing perceived bass response, so increasing the treble may be an adjustment for that. Or perhaps they intentionally endure reduced highs in the larger rooms, expecting that that’s what people expect to hear, even if it’s less than accurate.

"The largest variations among different practitioners are in the use of microphones. The problem is that the soundfield seen by a microphone in a large room is a mixture of direct sound, early reflections, and reverberation. Standard measurement 1/2-inch microphones demonstrate very different high-frequency response when measured anechoically on axis and with a diffuse field. Differences are on the order of 6 dB in the top octave between the two, and response in rooms is highly affected by the differences between these two. Only by the use of small, low-diffraction microphones, such as 1/4-inch or smaller diaphragm mics, are the differences kept small."

My SPL meter has a 1/2" mic as I suspect the majority of meters out there do. I wonder if a smaller mic would actually be more acurate for measuring SPL for home theater? I wonder if the key is the size of the room?

I understand Mr. Tomlinson's suggestion to measure "grazing incidence across the diaphragm rather than perpendicular to the soundfield" do to the diffuse nature of the surround channels but I don't really understand the need to use a smaller mic.

I don’t know much about the science of microphones, but from what he says, smaller mic diaphragms have more uniform response off-axis than larger ones. In other words, their off-axis response is much closer to their on-axis response.

Once again, this doesn’t pertain to us in the home environment. What Mr. Holman didn’t mention there is the fact that high frequency response is what is lost when the sound is picked up off-axis. Our SPL readings are typically maximum readings taken with C-weighting. In that situation, what the meter registers is the hottest bass frequency – and bass, as you probably know, is omni-directional, so on- or off-axis is inconsequential. Sure, there are some caveats, like the fact that the meter’s response starts rolling out below 40 Hz, but basically that’s what the meter’s giving you – the hottest bass frequency your system (and room) is generating. (To verify this, take a reading with your sub on, then another with it off. By the way, if SPL registers lower with the sub off [and it probably will], guess what? You have a house curve!!)

If you’re taking upper frequency readings, then certainly, make sure your SPL meter measurements are taken on-axis.

Interesting about the quote from Robert Margouleff. If he plagiarized Thomas Holman, maybe that’s why the link I originally used was taken down. But thanks for providing me with a new one I can reference in house curve the article.

Regards,
Wayne