When I started in audio, speaker design was a black art. A few engineers made listenable speakers. Most failed. Their cut-and-try approach mostly yielded bad design. But progress came quickly. In the 1970’s Richard Small and A.N. Thiele’s papers pointed the way to repeatable bass response. In the 1980’s, computer measurement and computer aided crossover design became invaluable and affordable tools.
During this same time, Floyd Toole, then of the National Research Council in Canada and later at Harman Inc., developed frequency response criteria for speaker systems that have become industry standards. My simple understanding of Toole’s work: Design for smooth on-axis frequency response and be sure the energy response of the system–the sum of all direct and indirect sound–mimics that response.
Today, across the industry, designers and engineers use computer tools to adhere to Toole’s criteria. Bad speakers have disappeared.
So, with nearly 50 years of psychoacoustic science reflected in today’s speaker offerings, you can purchase and install a perfect (or near perfect) speaker system. It has flat frequency response, consistent energy response, low distortion and high spousal acceptance. Unfortunately, the sound from your perfect speaker is still not perfect. Why not?
Here’s the easy answer: Go into your parlor and sit down at your grand piano. Don’t have a parlor or a concert grand? No problem. Go into your mental parlor and sit down at your imaginary piano. The lowest note there, the low “A” on your left is 27.5 Hz. Unless you’re listening to Saint Saens’ Organ Symphony, that note is lower than any note in any band or orchestra.
The highest note on your right, that very high “C”, is pitched at 4,196 Hz. That’s even above the top note on the piccolo–the highest pitched instrument in the orchestra! With the entire musical spectrum now arrayed before you, reach out with your right index finger and play the note in the center:
Middle “C”. (Pitch for Middle “C” is 262 Hz). Every note played back on your perfect speaker system located to the left of your index finger is marred by your room! Fully one-half of the musical spectrum suffers from destructive interference caused by your room! Why?
Let’s take your perfect speaker system and rename it. Henceforth it will no longer be a loudspeaker system, it will be a “Wavelength Generator”. It works by pushing air in one direction and by pulling it (so to speak) back. It generates a pressure wave length that impinges on the ear drum.
That movement generates electrical impulses our brains convert into sound. The frequency of the pressurization and rarefication determines the pitch of the note. At normal temperature and pressure the wavelength of a 27.5 Hz note (the lowest on your piano) is 41.45 feet. With the dimensions of the average listening room closer to 9 feet x 12 feet x 8 feet, we begin to understand the problem. One cannot properly propagate the 41-foot pressure wave in a normal room. The wavelength will rebound from the walls, floor and ceiling back onto itself causing destructive and additive interference.
As you move slowly, foot by foot, around the room you will find spots were the sound level is overwhelming and spots, quite nearby, where the tone nearly disappears. These disruptions in the steady tone are caused by the room. For another take on this destructive interference, here’s an interview from the ISE 2020 show (click here). Beginning at 3:08, Anthony Grimani of Grimani systems discuss the problem:
Or listen to the backing track from Lorde’s song Royals. Move around your room and hear the hard-hitting bass drum/synthesizer change depending only on your position in the room.
If Lorde is not to your liking please try Karin Allyson and bassist Ed Howard’s duet on “Round Midnight.”
Or pianist Glen Gould playing Bach’s “Goldberg Variations”:
To further our understanding of the room interference problem, imagine the listening room as two separate spaces: a resonant space and a reverberant space. In the resonant space, sound is disrupted. In the reverberant space, sound is enhanced. The frequency at which the transition from one to the other occurs is often called the Schroeder frequency after the scientist who first described the phenomenon. We’ve called it 262 Hz which is probably close enough for most typical home listening rooms. You can use this equation to discover the actual transition frequency for your own listening space, but in my opinion, 262 Hz is close enough.
- Use electronics that offer effective built-in room correction programs. I have extensive experience with both Dirac Live® and the Trinnov Optimizer. Both offer programs that measure and correct frequency and phase errors. I also hear good things about Anthem’s ARC Genesis and Lyngdorf’s Room Perfect, but have no personal experience with either solution.
- Use third party hardware and software to measure and correct resonant peaks. For hardware I recommend the purchase of the UMIK-1 calibrated USB microphone from MiniDSP.
When used with REW (Room EQ Wizard) software on a laptop or PC you can measure the actual system frequency response in your room and construct PEQ (Parametric Equalizer) filters to tame the resonant peaks. Mini DSP also offers their 2×4 HD DAC/DSP processor for the same purpose.
These solutions work. At T.H.E. Show in 2015, several exhibitors used the same speaker system for their demonstrations. Only one corrected the room resonance/destructive interference problem. (We used Dirac Live®). An attending journalist wrote: “I think this speaker was actually in more than one room, but I liked this one the best. The XXX sounds huge for such a compact speaker. Have to have it. Achingly gorgeous sound.”
The Bottom Line:
Your speakers and your room don’t get along. One-half or more of the musical spectrum, from the lowest notes up to at least Middle C, is contaminated by room resonance. Without specific equalization to counteract the room’s deleterious effects, your perfect speakers cannot and will not deliver the accurate and emotionally satisfying sound intended by their designers and desired by you.