JBL 560GTI, 660GTI manual TWEETERS, Waveguides and Optimal Frequency Response, Listeners

TWEETERS, WAVEGUIDES AND OPTIMAL FREQUENCY RESPONSE

The dispersion pattern of the sound produced by a speaker is different at low frequencies than at high frequencies. At frequencies with wavelengths greater than the circumference of the speaker’s cone – the speaker’s piston range – the sound is radiated in all directions. At higher frequencies, the dispersion pattern narrows. At frequencies for which the circumference of the speaker is about five times the sound’s wavelength, the coverage area narrows sharply and the off-axis sound contains far less high-frequency content. This phenomenon occurs for every speaker. For multi-way speaker systems, designers have to contend with this condition for each driver in the system. What makes matters more difficult is that in the crossover region between a large low-frequency driver and a small high-frequency driver, the speakers’ behaviors are opposite – the dispersion pattern of the woofer is narrow and the dispersion pattern of the tweeter is wide (see Figures 2 and 3). A multi-way speaker that is designed using only measure- ments of the on-axis response can sound terrible. An analysis of the frequency response of many speakers designed this way reveals a big hole in the off-axis response at the crossover frequency, where the woofer’s output is focused into the forward angles (see Figure 4).

									On Axis
110 dBSPL									Off Axis
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20Hz	50	100	200	500	1K	2K	5K	10K	20K

Figure 4. On- and off-axis frequency response of a conventional component system.

When we listen to speakers, we hear a combination of the sound that comes directly from the speaker (the on-axis response) and the off-axis response, which arrives at our ears after being reflected by boundaries and other objects in the room. In large rooms, the walls and other objects are often located several feet from the speaker enclosure, so the intensity of the reflected sound is lower than that of the direct sound. In rooms, the direct sound in the “listening window” dominates the sound we hear. The off-axis sound is a secondary but important part of the overall sound of the speaker (see Figure 5).

In cars, the boundaries are much closer to the speaker and to the listener, so the intensity of the reflections is more similar to the intensity of the direct sound. Additionally, the reflective surfaces in a car are so close that our brains can’t distinguish between the direct sound and much of the reflected sound. Reflections contribute greatly to the quality of the audio experience in cars. Since it isn’t practical to change the overall shape or size of the car to minimize the effects

of the reflections, closely matching the on- and off-axis responses of the speaker system can help to ensure a smooth and balanced sound (see Figure 6).

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270° 90°

Figure 2. Woofer dispersion pattern in the crossover region.

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Figure 3. Tweeter dispersion pattern in the crossover region.

Sound from Speakers

Reflected Sound

Figure 5. Reflected sounds are attenuated at the listening position because the boundaries are far from the speakers and the listener.

Reflected Sound

Sound From Speakers

ENGLISH

Figure 6. The intensity of reflected sounds in a car closely match that
of the direct sound, because the boundaries are close to the speakers
and the listeners.	7