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Is Frequency Response the End-All Measurement of Sound Quality?

Is anything more important than a flat frequency response in your room? For studio engineers or HiFi enthusiasts, the answer is quite clear. ASC founder, president and TubeTrap inventor, Art Noxon, PE Acoustical shares his knowledge with us this week on this topic. Enjoy!

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Sonic clarity & flat frequency response

“Contrary to popular belief the big problem with bass in HiFi is not lumpy bass, standing waves, room modes, hot spots and suckouts. We need to ask ourselves: Why is sonic clarity more important than a flat frequency response in your room? The big problem is sound masking.

That’s right, sound masking is what happens when an unwanted sound overpowers our ability to hear and discern the fine details of a wanted sound. We’ve found over our 25 years of bringing acoustic upgrades to the audiophile world that by and large, customers prefer clarity in their sound over having a smooth frequency response in their room.

Let’s take this idea to the extreme to get a better grasp on what is going on here. A thought experiment: Let’s move your HiFi system, carpet and listening chair from your house to a local racquetball court.

Here the walls and ceiling are 7 layers thick and the floor is solid concrete, and they are rigid and unmovable. In this big room we pop a balloon and listen to the noise slowly decay. It takes maybe 7 seconds for the noise of the pop to completely disappear. The room has an RT60 (reverb time) of 7 seconds. Next we turn on the stereo and sit back to listen to a good record.

As the speakers play music, the sound we hear is a combination of the direct sound, the one that travels from the speaker to our ears, plus a cacophony built up out of the previous 7 seconds of music we have been playing. If we are sitting, say, 8’ back from the speakers, the built-up cacophony of diffuse reverberance we hear is at least 10 to 15 dB louder than the desired direct sound we wanted to hear. Under these circumstances, we cannot begin to hear the direct sound because the diffuse reverberance overpowers and drowns out the direct sound. This is an example of serious sound masking.

Next thing to do is whip out our spectrum analyzer or download one of the many free ones on the internet. We take a look at the frequency response of the music and it looks pretty flat. If we play a sound-test track, pink noise, such as on Stereophile Test CD 2, Track 15 and look at what we get on the RTA, real time spectrum analyzer, we see a pretty flat frequency response. Well, isn’t that what we think the goal of audio room tuning is all about? Getting a flat frequency response? Well, one thing is for sure, play audio in a reverb chamber and you get a flat frequency response.”

“If we magically left the racquetball court, on our flying carpet, and landed somewhere outside on a grassy field and played the same music. All we would hear would be is the direct sound, there being no reflections or diffusion, hence no possibility of a sound masking reverb buildup. Now we hear perfectly clear sound. The spectrum analyzer response curve jumps around with each note or musical moment, reflecting the tonal character of each of the rapidly passing by train of sonic events, called music. Then we play Track 15 of the test CD again and we again measure a flat frequency response.

Playing our system outside, we measure it to have a flat frequency response, with no EQ adjustments. Playing our system inside the reverb chamber we measure it to have a flat frequency response, with no EQ adjustments. Outside, where the sound is 100% clear, we like a flat frequency response. Inside the reverb chamber, where the sound is 100% not clear, we also have a flat frequency response. But we don’t care about having a flat frequency response because we care more about having sonic clarity.

There. We’ve said it. We care more about sonic clarity than having a flat frequency response.

As we leave these two extremes, listening in an open field and listening in a reverb chamber, and move back into real rooms we discover that in real rooms we do not have a flat frequency response nor do we have crystal clear clarity. Next we have the opportunity to upgrade our audio playback room. Do we upgrade with an eye for improving the frequency response towards being more flat? Or, do we upgrade with an ear for improving the clarity of sonic events in the room. Given the choice, most choose to improve clarity.

We know how to test, listen to and measure a flatter frequency response. We use Track 15, Stereophile Test CD 2. When we EQ for a flatter frequency response we only change how loud certain bandwidths are. Notice that we cannot change the decay time of a bandwidth by using EQ, only acoustics can change reverb times.

Can we test, listen to and measure improved clarity? Yes, we use Track 19, Stereophile Test CD 2 to determine if we have any blurred bandwidths that are loud. If so, we use EQ to lower the volume of the slurred frequency ranges down a little, about 5 dB, just enough so they are not both loud and slurred at the same time (no one likes a loud drunk). Similar to reverb, EQ does not change the Clarity of sound in the system, it only changes how loud it is or isn’t.”