How To Choose An Enclosure for Your Woofer
Using the Enclosure Design Sheets 1
Power Ratings and T&S Parameters These are thermal power handling numbers. These should be used as a guide in choosing an amplifier. – A 500 W RMS amplifier is 4x the power handling rating of the amplifier—too much power. – Depending on the box and the customer, you may be able to use an amplifier that provides 2x or 3x the RMS power handling rating.
Power Handling and Xmax These are thermal power handling numbers. These should be used as a guide in choosing an amplifier. – A 500 W RMS amplifier is 4x the power handling rating of the amplifier—too much power. – Depending on the box and the customer, you may be able to use an amplifier that provides 2x or 3x the RMS power handling rating.
Sealed Box Design This is the suggested sealed box volume. – We suggest stuffing the box with Dacron or fiberglass insulation for best performance. – We don’t provide the dimensions of the box, because you may want to design a box of a different shape. We provide two frequency response curves so you’ll be able to compare this woofer with others and also so you’ll know what this box will sound like in the car.
The Excursion Graph in Detail (Sealed Box) This is the amount of RMS power applied This is the distance the cone will move This is the frequency scale This graph shows how far the cone will move at the frequencies the box will play when a certain amount of power is applied – In a sealed box, excursion is greatest at low frequencies. – This graph shows that in a sealed box, 125 W RMS will ALMOST drive the woofer to Xmax. The graph shows about 5.
Vented Box Design This is the suggested vented box volume. These are the suggested dimensions of the vent and the frequency at which the box will be tuned. – We recommend round ports—they sound better. In-car and anechoic frequency response curves – Notice that the vented box plays louder in the car than the sealed box. Excursion graph for the vented box – Notice that the shape of the excursion graph is different than that of a sealed box.
The Excursion Graph in Detail (Vented Box) At 125 Watts, excursion exceeds Xmax, but only at the very lowest frequencies – A subsonic filter should be used and should be set to a frequency a little lower than Fb (you can find Fb on the previous page). 30Hz is a good point to use., – The subsonic filter will reduce the power at the lowest frequencies and REDUCE the excursion. At frequencies above Fb, 125 Watts only drives the woofer to about 4mm excursion.
Bandpass Box Design Box volumes and port dimensions – We recommend round ports—they sound better. Bandpass boxes perform like sealed boxes with a low pass filter. – The port only allows low frequencies to pass through it.
The Excursion Graph in Detail (Bandpass Box) At 125 Watts, the bandpass box is similar to the sealed box at low frequencies Cone excursion is reduced at Fb, but that reduction is as useful as it is in a vented box because the frequency is MUCH higher. Bandpass boxes handle about the same amount of power as a sealed box. A vented box will play LOUDER than a bandpass box and It will handle More Power. – A vented box is a MUCH better choice than a bandpass box.
Comparing the Woofer’s Excursion in the Three Boxes The sealed box requires the same excursion at all useful frequencies The vented box reduces excursion at the frequency where the box is tuned (Fb). Adding a subsonic filter (included in most JBL amplifiers) minimizes excursion below Fb Once the subsonic filter has been set, excursion in the vented box is LOWER than in the sealed box with the same amount of power.
Comparing the Sound of the Three Boxes The sealed box has a flatter response in the car The vented box plays almost 6dB louder with the same power as the sealed box! That’s equivalent to four times the amplifier power. More amplifier power can be applied to the vented box than the sealed or bandpass box! Double the RMS power rating is often OK. That’s 9dB louder in the car! The bandpass box plays louder than the vented box, but only at one frequency.
Conclusions Sealed boxes are small, but they don’t play very loudly. Sealed boxes require woofers with high Xmax ratings in order to handle lots of power. Vented boxes make much more bass and even more power can be applied. Vented boxes are bigger than sealed boxes. Woofers designed specifically for vented boxes often have low Xmax ratings. Bandpass boxes have one good feature—a big peak in the response of the car, but they’re bigger than the other boxes and more difficult to build.
