Speaker Parameters, Part 3
June 12, 2007
This month, we will finish our discussion explaining speaker parameters. Again, the goal is to help you use speaker data in selecting speakers and (for the increasing number of DIY’ers), designing your cabinets. In summarizing the previous articles, we have covered the origin of T/S parameters, Fs, Re, Le, Zmax, the Q parameters (Qms, Qes, and Qts), Cms, Vas, Xmax, Sd, Vd and Mms. If you’ve missed the previous columns, you can read them in their entirety at premierguitar.com
Continuing on, the next parameter, Rms, represents the mechanical resistance of a driver’s suspension losses, and measures the absorption qualities of the speaker suspension in newtons times seconds per meter (N*sec/m).
EBP is a figure used in many enclosure design formulas to determine if a speaker is more suitable for a closed or vented design. It is calculated by dividing Fs by Qes. An EBP close to 100 usually indicates suitability for a vented enclosure. An EBP closer to 50 usually indicates suitability for a closed box design. This is only a general rule of thumb and Qts should also be considered.
BL is a measurement of the motor strength of a speaker. It represents how good a weightlifter the speaker is. A measured mass is applied to the cone, forcing it back, while the current required for the motor to force the mass back is measured. BL is calculated by dividing that mass in grams by the current in amperes. A high BL figure indicates a very strong transducer that moves the cone with authority.
Usable frequency range represents the range for which a speaker will prove useful. Manufacturers may use different techniques for determining this. Speakers are often used to produce frequencies in ranges where they would theoretically be of little use. As frequencies increase, the off-axis coverage decreases relative to speaker diameter. At a certain point, the coverage becomes “beamy” or narrow like the beam of a flashlight. For example, the theoretical maximum frequency before beaming for a 12” speaker is 1,335 Hz and for a 10” is 1,658 Hz. If you’ve ever stood in front of a guitar cabinet, then slightly moved to one side or the other and noticed a different sound, you have experienced this phenomenon.
Power handling is critical in speaker selection and is often misunderstood. Loudspeaker manufacturers use industrystandard test signals with the driver in freeair to rate the speaker thermally. The number one contributor to a speaker’s thermal power rating is its ability to release thermal energy. This is affected by several design choices, but most notably voice coil size, magnet size, venting and the adhesives used in voice coil construction.
How you use this specification is dependent on the application. For guitar and bass, you want to choose a speaker that is capable of handling more power than the amplifier. For lead/rhythm guitar it’s easy – keep the amp power below the speaker rating. For bass guitar, you must also consider the speaker’s mechanical power limit, because of the lower frequencies involved. Lower frequencies are more abusive. This involves the size of the cabinet and, if vented, how it’s tuned.
The main factor in mechanical power handling is the speaker’s excursion capability. Xmax (maximum linear excursion) and Xlim (maximum mechanical limit) are good parameters to compare – the greater the Xmax or Xlim, the better the excursion capability. The mechanical power handling can be much less than the thermal power handling in many applications. Consequently, this is not something raw frame speaker manufacturers are able to publish.
In any case, if you are unsure about matching the speaker, cabinet and amplifier, it’s wise to contact the manufacturer for a recommendation. In the near future, I’ll explain finding this limit using enclosure design software. For pro audio applications, you should use at least twice the amplifier power as the speaker’s power rating. This ensures that you are sending a clean signal and not clipping the amp. Clipping introduces distortion, which may destroy the speaker. It’s better running a 500-watt speaker at 1000 watts with the volume at 2 than running it at 500 watts with the volume at 8. For subwoofer use, and in some mid-bass applications, you must also consider the mechanical limit as described for bass guitar.
Sensitivity is a representation of the efficiency and volume you can expect from a speaker, relative to input power. Loudspeaker manufacturers follow different rules when obtaining this information, as there is not an exact standard accepted by the industry. This makes it difficult to compare one manufacturer’s rating to another. However, Sensitivity is useful in comparing one manufacturer’s product range; you can determine which models are louder and in what range, by using the frequency response graph.
Now that we’ve discussed speaker parameters in detail, we can now turn our focus to cabinet design. Next month, I’ll show you how to use design software for guitar cabinets.
Anthony “Big Tony” Lucas
is a guitarist and Senior Lab Technician at Eminence Speaker LLC, where he specializes in guitar-speaker design and customer support. Big Tony has been with Eminence for over 10 years and is responsible for many well-known guitar speaker designs.