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The optimal acoustic design for any given room should take into account the specific speakers that are being used in that room. Whilst it is possible to create a design that works reasonably well for every possible speaker type an optimized acoustic design will take into account the speaker directivity / off axis response and low frequency extension.
Directivity / Off-Axis Response
It is our philosophy that loudspeakers designed to reproduce a sonic event should have an essentially flat frequency response when measured at the listening position. Most of us understand the importance of a flat on axis frequency response. Fewer of us, however, are aware of the importance of a speaker's off axis performance in determining the frequency response above the transition frequency at the listening position.
Measurements published by Toole show that the frequency response measured at the listening position above the transition frequency is dominated by the spectral characteristics of the sound energy returned to the listener from the major reflection points in the room i.e. the first reflections from the floor, ceiling, front wall, side walls and rear wall. Below the transition frequency room modes become the dominant factor in what we hear. This has two important implications:
The off-axis response of the loudspeaker is critically important in determining the frequency response at the listening position. For example the ceiling reflection is typically at angles of between 40 and 60 degrees vertically off axis.
The acoustic characteristics of the major reflection points significantly influences the frequency response at the listening position. The characteristic of the boundaries is therefore ‘imprinted’ on the spectrum of the sound reflected back from the major reflection points.
The optimal acoustic design for any given room should therefore take into account the specific speakers being used in that room. There are some generic radiation patterns and therefore acoustic treatment strategies that we can discuss:
Conventional, forward radiating box speakers, are characterized by an on-axis and off-axis response that varies significantly as the angle off-axis increases. This variance in off axis energy requires more aggressive and muscular acoustical control at lateral and vertical first reflection points. Absorption of the lower mid and midrange frequencies (100Hz-2KHz) is typically a must to experience a dynamic and clean response at the listening position.
CD type speakers or Constant Directivity (also called termed controlled dispersion) are characterized by an off-axis response that varies minimally from the on-axis response. It is not uncommon for the off-axis response up to as wide as 40 degrees in the horizontal plane to have essentially the same frequency response. It is also typical that the vertical dispersion is more tightly controlled.
Dipole speakers have cancellation nodes perpendicular to the baffle axis. Due to this they do not excite room modes along this axis and have reduced interaction with the side walls.
Low Frequency Extension
The lowest fundamentals of pianos are just below 30Hz, with bass guitar and bass drums just over 40Hz. With this in mind it is our opinion that for reproduction of a broad range of music it is critical that the speaker system have a high level of output down to below 30Hz. Many speakers, even large floorstanders, do not go this low and it is often easier and better to supplement them with a high quality subwoofer.
Acoustic Frontiers Products and Services
Acoustic Frontiers offers acoustic treatment products from many companies to help balance the spectrum of on and off axis sound. Our end-to-end Acoustic Consulting service will help identify the exact issues of your room / speaker combination and deliver an Acoustic Treatment Design to address them. We also integrate subwoofers from JL Audio and Velodyne to help your system achieve proper low frequency extension.
Nyal Mellor, Founder, Acoustic Frontiers