This is one of the most important articles that any audiophile could read, because it can help prevent hearing damage and tinnitus. Here at Audiophile Nirvana we want you to enjoy your music for a lifetime, and not become a victim of the technology that provides it.
Loudspeaker specs almost always include a graph illustrating the device’s frequency response from 20 Hz to 20 kHz. This appears to indicate that a flat frequency response is ideal, and that the perfect speaker system provides a virtually flat frequency response at the listening position. Using REW (Room Equalization Wizard) you can test your speaker system to determine how flat the response is at the listening position, and using a basic graphic equalizer flatten things out. Go ahead and give it a try right now.
Were you surprised to see the high frequency roll-off? What happened to the flat frequency response the sales brochure promised? Did fixing the problem using the equalizer improve the frequency response? Did you finally hear things the way they were intended?
No, your calibrated mic is fine, and REW is working properly. You just learned that having a flat frequency response at the listening position is undesirable. We’ll be going over this in another post that discusses House Curves.
Let’s try another experiment, because learning by doing is the most powerful teacher. Play a good recording such as Diana Krall’s Temptation from her album The Girl in the Other Room. Don’t apply any equalization. Set your system to flat, or tone controls off, if that’s an option.Start at a lower volume and continue raising it until it sounds as life-like as possible. (Warning: high amplitudes can harm your hearing!) You can measure the average amplitude with REW or a Radio Shack SPL meter. Your system probably sounded best somewhere between 85 db and 95 db. Now, lower the volume control so that the music is comfortable for background listening. The low and highest frequencies appeared to roll off. If you check the frequency response at both the preferred and background listening levels you will discover they are the same, but yet they don’t sound that way to your ears. It isn’t your imagination. It’s how your ears react to different sound pressure levels. The study of this phenomenon resulted in the Fletcher-Munson curves, or more accurately: the equal-loudness contour.
As we have explained in previous posts on Audiophile Nirvana, the accuracy of any good sound reproduction system is primarily a function of speaker/room interaction. When the “Purist Movement” demanded the removal of loudness contour and tone controls, they left audiophiles in danger of developing hearing problems. Without these controls the volume control becomes the only equalization, by allowing you to raise the amplitude to the point where the speakers sound best in your room. This is a direct function of the equal-loudness contour curve. The proliferation of high power amplifiers, and speakers capable of high amplitude reproduction, that began in the 1970’s, combined with the loss of equalization control demanded by audiophile purists was, and still is, a recipe for hearing damage.
The ISO Equal-loudness Contour graph illustrates ear sensitivity across the audible frequency spectrum as it relates to SPL (Sound Pressure Level) in decibels. Using this information audio equipment manufacturers were able to create loudness contour switches and variable loudness controls. This allowed listeners to enjoy perceived flat frequency responses (house curve applied) at lower listening levels.
It should now be apparent that the perceived tonal accuracy of your audio system is directly related to amplitude in the absence of some form of equalization. This leads us to the next diagram which illustrates safe listening levels. Earlier in this article we asked you to determine the amplitude at which your system sounded best. Compare that level to the ones in the chart. How long can you listen at that level before experiencing hearing damage, and most likely, some form of listener fatigue?
Using the Equal-loudness Contour and Safe Listening Level graphs, along with REW and a House Curve, we can begin to put together a basic required equalization graph for your listening room. This will allow you to listen for extended periods without suffering hearing damage or listener fatigue.
In a future article in our series Audiophile 101: Your First Audio System we will be discussing House Curves, and how to set a safe listening level.
Enjoy the music!
For more information on dangerous sound levels please visit Dangerous Decibels.