Loudspeaker Waveguides – Why Do We Use Them?
By Mark Glazer, Principal Engineer, Revel Speakers
The performance of a loudspeaker system is largely influenced by the dimensions and boundaries of its surrounding environment.
In a typical 2-channel listening room set up (Figure 1, below), the first sound that arrives at the listener’s ears, the direct sound, is just one portion. Multiple first reflections, arriving after a few milliseconds, have a large effect on the sound balance. Therefore the ‘directivity’ of a loudspeaker will have a major impact.
What is Directivity?
Directivity is the term used to describe the way a sound source frequency response changes off axis. A wide directivity sound maintains amplitude (sound pressure level) consistency between the on and off axis sound, such as the popping sound of a balloon. A narrow directivity sound is where the off axis amplitude is substantially lower than the on axis sound, such as from a bullhorn.
A loudspeaker’s directivity depends on the diameter of the radiating driver and the frequency it is reproducing. Lower frequencies have a wide directivity and higher frequencies have a narrow directivity for a fixed size diameter driver.
How would this create a problem in loudspeaker systems?
Let’s use the popular 6.5-inch 2-way bookshelf loudspeaker for an example. As the frequencies transition from the woofer to the tweeter which is controlled by the crossover, there is a large discrepancy in the radiation of sound due to the large diameter 6.5-inch woofer and the much smaller 1-inch tweeter.
Figure 2 (Left): 6.5-inch 2-way bookshelf radiation without waveguide.
How does it affect the loudspeaker sound?
Referring back to figure 1, the discontinuity of the radiation pattern in the crossover transition region, 800 Hz to 3000 Hz, produces a sound imbalance in the 1st reflections.
What is the solution?
A tweeter waveguide is used to control the directivity or coverage of the tweeter so that it more closely resembles the directivity of the woofer or midrange. This is critical to achieve a more balanced output from the important 1st reflection angles.
Figure 3: F328Be Waveguide
The flare angle, depth, and overall dimensions of the waveguide alter the directivity of the tweeter to better match the directivity of the woofer. As a bonus, the waveguide flare geometry and acoustic lens add additional output to the tweeter – up to 2.5 dB in the overall sound pressure level – improving dynamic range and reducing power compression.
What is the Overall Benefit in a Loudspeaker System?
Figure 4 illustrates the radiation pattern (800 Hz to 4000 Hz) utilizing a properly designed waveguide. Note that the sound field is now distributed evenly into the enviroment, making the 1st reflections more uniform and producing a much more balanced natural sound.
Figure 4: 6.5 inch, 2 way bookshelf radiation with waveguide
Utilizing waveguides on tweeters has been a standard feature of Revel for years. The added cost is negligible since a tweeter faceplate is required anyway, and the benefits are huge. However, engineering a well designed waveguide is not trivial, and Revel has refined it to the 6th generation version in the F328Be, evolving the performance to a higher level of natural sound balance and improved imaging.