Speaker Performance

Overview

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

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.

Off Axis Speaker 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 (i.e. ~300Hz) at the listening position.

In a small room, such as a home listening environment, we generally listen to speakers at a distance where the energy from reflected sounds is greater than the direct sound.

Due to this fact the speakers off axis performance is as important - if not more important - than the on axis performance in determining the frequency response at the listening position. The measured frequency response at the listening position is dominated by the spectrum of the early 'single bounce' reflections from the major boundaries in the room.

Whilst nearly all speakers will exhibit differences in frequency response when measured on and off axis, some are definitely worse than others. A typical issue is high tweeter directivity, where the frequency response of the tweeter is very different on axis from off axis. Other issues include poorly implemented crossovers where there is a difference in off axis frequency response at the crossover frequency between the midrange driver and the tweeter. Increasingly good speaker designers will not only optimize on axis frequency response but also choose a crossover point where the directivities of the two drivers can be matched such that there is no sudden change in off axis frequency response.

The graph below shows an example of an on and off axis frequency reponse measurement. Each line on the graph below represents a measurement at a different angle off axis. The chart shows on axis (0 degrees) and then off axis (30,60 and 90 degrees). The red line is an average of these.

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.