Tech Talk
Ported Enclosures – Why Do We Use Them?
Mark Glazer | Senior Principal Engineer, Revel Speakers
What are ported box loudspeakers?
Ported, also called Vented, box loudspeakers, are a common staple among Luxury Audio free standing speaker systems. To understand how a ported box performs, let’s use a drinking bottle as an example (Figure 1). Have you ever blown across the neck opening? If so, you know a tone generated known as the resonant frequency, Fs. The narrow neck of the bottle represents the vent diameter and length, and the volume of the bottle below represents the enclosure. The ratio of the neck cross sectional area and length, plus volume parameters, determine the resonant frequency tone. This device is called a Helmholtz resonator. This is why filling up the bottle partially would produce a higher frequency tone.
Now let’s apply the ‘water bottle’ principal to the loudspeaker enclosure.
Once the enclosure volume is determined, the maximum output level and low frequency range of the system is tuned by changing the length and diameter of the port.
Now lets examine the seperate output responses of the port and woofer, plus the total system output, using the Revel F126Be 2-way Bookshelf for an example. (Figure 3)
As the frequency reduces, the woofer output and excursion begin to drop below 90 Hz due to the air stiffness of the enclosure. During the same period, the port output begins to increase as it approaches the Helmholtz resonance range of frequencies. The overall sum of woofer and vent has higher low frequency output than just a sealed box. Also, a vented system has lower distortion, plus the dynamic range is increased due to the port being more linear than the woofer at these frequencies.
Below the port resonance frequencies, the summed output tends to roll off more rapidly, therefore ports are not used for certain subwoofer applications. Also, for very small enclosures a port may not be physically practical.
Conclusion:
This article explains why ports are used, not only in Luxury Audio loudspeakers but widespread in the loudspeaker industry.
Luxury Audio Engineering further refined port technology improving dynamic range and lowering distortion by contouring port shapes. One example is the patented Constant Pressure Gradient Port implemented in some Luxury Audio loudspeakers (Figure 4).
