A Guide to Bass Management (Part II)

[Big Daddy]

PORT CHUFFING

Port Chuffing is a term used to describe the sound created by the port/vent during periods of high excursion of a bass driver. This can happen to any ported speaker system, not just subwoofers. Chuffing is created by large quantity of air moving at high speed within a speaker port tube.

Air moving below 10 meters/sec in a port generally will have no problems with compression, turbulence and noise. As the velocity of air is increased beyond this point (e.g., 20 meters/sec), turbulence occurs as air exiting the port is forced to slow down quickly as it encounters the surrounding still air. Chuffing usually occurs at or below the tuning frequency of the port.

The solution is to slow down the speed at which the air is moving. This can be done by one of the following methods:

1. Modify Port Dimensions: Increase the length of the port. If there is no room, the port can be bent
2. Flared Ports: Another solution is to flare the ends of the port tube to slow down the movement of the air. Flared ports cause the airflow to expand and loose speed, allowing higher speeds without turbulence. This method targets the air close to the walls of the port.
3. Passive Radiator: A passive radiator is basically a driver without the magnet or coil assembly. A passive radiator can be used instead of a port to avoid problems associated with port design and port chuffing. To prevent over-excursion of the passive radiator, it is recommended that you have twice the displacement of the driver. For example, a 10-inch driver would require a pair of 10-inch passive radiators with the same excursion, or a 12-inch driver with higher excursion.
4. Polyfill: Add polyfill to the enclosure. Polyfill has the indirect effect of making the subwoofer appear to be bigger by slowing down the movement of air and may help reduce port chuffing.

As you can see, the first three options are only available to DIY subwoofers/speakers builders. The fourth option can be used by all.

The only other solution that does not require modifying the sub enclosure has to do with lowering the level on the back of the subwoofer and/or the receiver. Port chuffing is generally a sign of a subwoofer/speaker being over-driven. When a subwoofer is placed in a a poor location in the room or your listening sofa/seat is placed in a poor location, the bass will be weak. As a result, one tends to overcompensate this by turning up the volume on the subwoofer. This causes the subwoofer to work much harder than it should, causing it to chuff and possibly damage the driver or the amplifier.

1. Placement: Good placement can help. For most rooms, corner placement is a possible solution. A subwoofer in a corner excites all room modes and play louder without needing additional amplifier power. The best part is that it is for free. Alternatively, you can use the Subwoofer Crawl Technique to find the best location for your subwoofer.
2. Multiple Subwoofers: Use multiple (2 or 4) subwoofers. Multiple subwoofers will give you additional output and allow you to turn down the level of each subwoofer. Moreiver, multiple subwoofer can even out the bass in the room and get rid off the peaks and valleys associated with standing waves and room modes.
3. Subsonic filter: It is highly recommended that you use a subsonic filter for all ported subwoofers. Some subwoofer amplifiers have this option. The Velodyne SMS-1 subwoofer equalizer/analyzer offers this option. Depending on the capabilities of your subwoofer, you should set the subsonic filter to 25Hz, 20Hz, 18Hz, or 15Hz to cut out the lower frequencies.
4. Use a Limiter Amplifier. A Llimiter Amplifier is an amplifier which uses a protection circuit to limit its output power to a predetermined level, regardless of input.

Subwoofers with large slotted ports should not have any port noise. There is a misconception that ported subwoofers are "slower" that sealed subwoofers. That is a myth. A properly designed ported subwoofer will offer greater efficiency and higher output when compared to a sealed subwoofer. Nonlinearity and distortion are more functions of the quality of the driver, not the cabinet.

Remember that a subwoofer driver can unload and lose control below the port frequency of the enclosure. Most normal human beings cannot hear frequencies below 25H-30Hz and only feel them. If you think you are hearing these lower frequencies, you are not. You are hearing the upper harmonics of these frequencies in your room. Go to an anechoic chamber and see if you can still hear 15Hz. If you want to feel these lower frequencies, Bass Shakers/Buttkickers/Tactile Transducers are much cheaper and do a much better job of shaking your butt.

POWER RATINGS FOR POWERED SPEAKERS/SUBWOOFERS

Amplifier ratings for powered speakers and subwoofers are meaningless. They only have a meaning for the designers. They should not even be published.

The amplifier power that you need for a particular powered speaker/subwoofer depends on two major factors:

1. Sensitivity of the drviers.
2. Design and size of the cabinet (smaller and/or sealed subwoofers need more power).

On some very small subwoofers such as the Sunfire True Subwoofers, the amplifiers are extremely powerful to compensate for the small size of the sealed cabinets. For example the amplifier's power output on the True Subwoofer EQ 12 Signature - TS-EQ12 is rated 2,700 Watts, RMS and the size of the cabinet is only 13.5" x 13.5" x 13.5". You cannot simply look at the power rating of this amplifier and conclude that this subwoofer is better than a comparable subwoofer from another manufacturer.

These are some other examples of subwoofers that are small sealed or with passive radiators that require very high output amplifiers.

http://www.definitivetech.com/Produc...s/default.aspx

Definitive Technology SuperCube® Reference:
List Price: $1,799
16-3/4” W x 16-3/4” D x 16-15/16” H
Amplifier Power :1800 watts

Definitive Technology SuperCube® I:
$1,199
14-1/4” W x 14-1/4” D x 14-1/10” H
Amplifier Power :1500 watts

Definitive Technology SuperCube® II:
List Price: $899
12” W x 12” D x 12-1/2” H
Amplifier Power :1250 watts

Definitive Technology SuperCube® III:
10-1/4” W x 10-1/4” D x 10-1/4” H
List Price: $699
Amplifier Power :650 watts


http://www.sunfire.com/products.asp

Sunfire True Subwoofer TS-EQ12:
13.5” W x 13.5” D x 13.5” H
Amplifier Power :2700 watts

Sunfire True Subwoofer TS-EQ10:
11.5” W x 11.5” D x 11.5” H
Amplifier Power :2700 watts

Sunfire True Subwoofer TS-SJ8:
9” W x 9” D x 9” H
Amplifier Power :1500 watts


Here is a good article from Harman International.

http://www.harmanaudio.com/all_about...werratings.asp

Most separate subwoofers, on the other hand, do incorporate their own amplifiers. When a speaker does include amplification, the manufacturer usually will give a power rating for that amplifier–which you should simply ignore. If power is so important a specification for separate amplifiers and receivers, why isn't it for a powered speaker or subwoofer? Actually, it is important, but only to the designer. He's the one who has to figure out what combination of speaker sensitivity and amplifier power will yield the desired sound output capability, which is what really matters. What you need to know is how loud the speaker or subwoofer can play over what frequency range and with how much distortion. In other words, you need to know what's coming out of the speaker, not what's going into it. It is entirely possible, for example, that a subwoofer with a 100-watt amplifier could outperform one with a 200-watt amp in every respect. For a consumer, the power rating of an amplifier built into a speaker is an absolutely useless number. In addition, there is the matter of the accuracy of such ratings. Because amplifiers built into speakers or subwoofers are inaccessible, it is rare that anybody checks how much power they can really deliver. Power numbers sell, which creates a temptation to–how shall we put this delicately–exaggerate that some companies find irresistable. Just one more reason to ignore them.

To help you understand this, let me give you the example of the horn-loaded speakers with full-range drivers that I built a while ago. Let's start by looking at the sensitivity of the drivers.

Fostex FE206E 8" Full Range Driver:

Frequency response: 39Hz to 20kHz
Sensitivity: 96dB
Impedance: 8 ohms

FostexFrequency.jpg

Fostex Fostex T90A Horn Super Tweeter:

Frequency response: 5kHz to 35kHz
Sensitivity: 106 dB

FostexHornTweeterFreqResponse.jpg

The sensitivities are way above average. This means that with only a few watts, you can make the speakers very loud.

Now, let's look at the cabinets. The cabinets are medium size horn-loaded. Horn-loaded cabinets are very efficient.

Folded Horn Design
Folded horn design enables the enclosure to contain a longer sound path which amplifies the bass frequencies while keeping the cabinet small in size. These horns, with the rear of the driver loaded by a bass horn, help a full range driver produce more bass with less work.

I use these speakers in my two-channel room with an old school two-channel preamp and a Carver two-channel amplifier rated at 375 watts RMS per channel. When I listen to these speakers, the preamp volume control is turned up about 10% and the sound is still extremely loud. If I were to make these speakers powered, I would add a single mono amp to each that is rated about 20 watts to a maximum of 50 watts. If you compare these speakers with another set of speakers that are made by other manufactures and have a standard bass reflex cabinet with sensitivity of 87dB, you would probably need about 200 watts to make them sound as loud and still have enough reserve power. Although the horn-loaded speakers may sound louder and better with 20-watt amplifiers than other speakers with 200 watt amplifiers, many people will automatically assume 200 watts must be better than 20 watts. Unfortunately, unethical manufacturers will promote the idea for marketing reasons.

I realize that the very powerful two-channel amplifier that I currently use is totally wasted. But I can brag that my amplifier is rated 375 watts per channel.

The moral of the story: Amplifier ratings for powered speakers/subwoofer have no meaning.

Here are some pictures for you.