Thursday, 9 February 2012


Equalization can be used to adjust the in-room response of a subwoofer system. Designers of active subwoofers sometimes include a degree of corrective equalization to compensate for known performance issues (e.g., a steeper than desired low end roll-off rate). In addition, many amplifiers include an adjustable low-pass filter, which prevents undesired higher frequencies from reaching the subwoofer driver. For example, if a listener's main speakers are usable down to 80 Hz, then the subwoofer filter can be set so the subwoofer only works below 80.[3] Typical filters involve some overlap in frequency ranges; a steep filter is not generally desired for subwoofers. The crossover section may also include a high-pass "infrasonic" filter which prevents the subwoofer driver from attempting to reproduce frequencies below its safe capabilities.
Some systems use parametric equalization in an attempt to correct for room frequency response irregularities.[19] Equalization is often unable to achieve flat frequency response at all listening locations in part because of the resonance (i.e., standing wave) patterns at low frequencies in nearly all rooms. Careful positioning of the subwoofer within the room can also help flatten the frequency response.[20] Multiple subwoofers can manage a flatter general response since they can often be arranged to excite room modes more evenly than a single subwoofer, allowing equalization to be more effective


'Active subwoofers' include their own dedicated amplifiers within the cabinet. Some also include user-adjustable equalization that allows boosted or reduced output at particular frequencies; these vary from a simple "boost" switch, to fully parametric equalizers meant for detailed speaker and room correction. Some such systems are even supplied with a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the combination of subwoofer, subwoofer location, and room response to minimize effects of room modes and improve low frequency performance.
'Passive subwoofers' have a subwoofer driver and enclosure, but they do not include an amplifier. They sometimes incorporate internal passive crossovers, with the filter frequency determined at the factory. These are generally used with third-party power amplifiers, taking their inputs from active crossovers earlier in the signal chain. While few high-end home-theater systems use passive subwoofers, this format is still popular in the professional sound industry

Loudspeaker and enclosure design

Subwoofers use speaker drivers (woofers) typically between 8" and 21" in diameter. Some uncommon subwoofers use larger drivers, and single prototype subwoofers as large as 60" have been fabricated.[12] On the smaller end of the spectrum, subwoofer drivers as small as 4" may be used, depending on the design of the loudspeaker enclosure, the desired sound pressure level, the lowest frequency targeted and the level of permitted distortion. The most common subwoofer driver sizes used for sound reinforcement are 10", 12", 15" and 18" models. The largest available sound reinforcement subwoofers, 21" drivers, are less commonly seen.
The efficiency of a speaker driver is given by:

Where the variables are Thiele/Small parameters. Deep low frequency extension is a common goal for a subwoofer and small box volumes are also considered desirable. Hoffman's Iron Laws therefore mandate low efficiency under those constraints, and indeed most subwoofers require considerable power, much more than other individual drivers.
So for the example of a sealed speaker box, the box volume to achieve a given Qts is proportional to Vas:
Therefore a decrease in box volume and the same F3 will decrease the efficiency of the sub woofer. Similarly the F3 of a speaker is proportional to Fs:

As the efficiency is proportional to Fs3, small improvements in low frequency extension with the same driver and box volume will result in very significant reductions in efficiency. For these reasons, subwoofers are typically very inefficient at converting electrical energy into sound energy. This combination of factors accounts for the higher power output of subwoofer amplifiers, and the requirement for greater power handling for subwoofer drivers. Enclosure variations (e.g., bass reflex designs) are sometimes used for subwoofers to increase the efficiency of the driver/enclosure system, helping to reduce the amplifier power requirement.

Subwoofer mounted in a sealed enclosure
Subwoofers have been designed using a number of enclosure approaches: bass reflex, acoustic suspension, infinite baffle, horn loaded, tapped horn, transmission line and bandpass. Each enclosure type has advantages and disadvantages in efficiency increase, bass extension, cabinet size, distortion, and cost. Subwoofers are typically constructed by mounting one or more woofers in a cabinet of medium-density fibreboard (MDF), oriented strand board (OSB), plywood, fiberglass, aluminum or other stiff materials. Because of the high air pressure they produce in the cabinet, subwoofer enclosures often require internal bracing to distribute the resulting forces.
The smallest subwoofers are typically those designed for desktop multimedia systems. The largest common subwoofer enclosures are those used for concert sound reinforcement systems or dance club sound systems. An example of a large concert subwoofer enclosure is the 1980s-era ElectroVoice MT-4 "Bass Cube" system, which used four 18" drivers. An example of a subwoofer that uses a bass horn is the Bassmaxx B-Two, which loads an 18" driver onto an 11-foot (3.4 m) long folded horn. Folded horn-type subwoofers can typically produce a deeper range with greater efficiency than the same driver in an enclosure that lacks a horn. Some experimental fixed-installation subwoofer horns have been constructed using brick and concrete to produce a very long horn that allows a very deep sub-bass extension.[13]
Subwoofer output level can be increased by increasing cone surface area or by increasing cone excursion. Since large drivers require undesirably large cabinets, most subwoofer drivers have large excursions. Unfortunately, high excursion, at high power levels, tends to produce more distortion from inherent mechanical and magnetic effects in electro-dynamic drivers (the most common sort). The conflict between assorted goals can never be fully resolved; subwoofer designs are necessarily compromises. Hoffman's Iron Law (the efficiency of a woofer system is directly proportional to its cabinet volume and to the cube of its cutoff frequency) applies to subwoofers just as to all


A subwoofer (or simply "sub") is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products,[1] below 100 Hz for professional live sound,[2] and below 80 Hz in THX-approved systems.[3] Subwoofers are intended to augment the low frequency range of loudspeakers covering higher frequency bands.
Subwoofers are made up of one or more woofers in a loudspeaker enclosure capable of withstanding air pressure while resisting deformation. Subwoofer enclosures come in a variety of designs, including bass reflex (with a port or passive radiator in the enclosure), infinite baffle, horn-loaded, and bandpass designs, representing unique tradeoffs with respect to efficiency, bandwidth, size and cost. Passive subwoofers have a subwoofer driver and enclosure and they are powered by an external amplifier. Active subwoofers include a built-in amplifier.[4]
The first subwoofers were developed in the 1960s to add bass response to home stereo systems. Subwoofers came into greater popular consciousness in the 1970s with the introduction of Sensurround in movies such as Earthquake, which produced loud low-frequency sounds through large subwoofers. With the advent of the compact cassette and the compact disc in the 1980s, the easy reproduction of deep and loud bass was no longer limited by the ability of a phonograph record stylus to track a groove,[5] and producers could add more low frequency content to recordings. As well, during the 1990s, DVDs were increasingly recorded with "surround sound" processes that included a Low-frequency effects (LFE) channel, which could be heard using the subwoofer in home theater systems. During the 1990s, subwoofers also became increasingly popular in home stereo systems, custom car audio installations, and in PA systems. By the 2000s, subwoofers became almost universal in sound reinforcement systems in nightclubs and concert venues.


We are asked from time to time about how LAUD might be used with an accelerometer to measure loudspeaker cabinet vibrations. An accelerometer is used to detect vibrations induced on its body, rather than those propagated in air. The process of using an accelerometer with LAUD is quite simple.

Figure 1: Waterfall plot of cabinet vibration on baffle of small speaker - this one has a significant resonance at 220Hz and 310Hz
If the accelerometer is electrically compatible with the LAUD input circuitry's impedance and sensitivity, you can use the accelerometer essentially the same way as you would a microphone. Just feed the accelerometer's signal to where your microphone normally connects and drive the speaker with a stimulus signal (with the cal probes sensing the speaker's drive signal). Measure the result from the accelerometer mounted on the cabinet face you are investigating, rather than from a microphone placed in front of the speaker as you would normally do. You are in effect measuring a frequency response, but rather than one "from drive signal to sound output", it is "from drive signal to cabinet vibration" -- with the less measured in general, the better. When using an accelerometer into LAUD's mic inut, you should temporarily disable your mic correction file (by the menu sequence: [* File Micdat ], then type "NONE" and press the [Enter] key.
A simple piezo element, as might be removed from a cheap piezo tweeter, can be used effectively as an accelerometer. Even with its high impedance badly mismatched by the FIJI's (or Mic/Probe Preamp's) input, the piezo's output level is still adequate for making comparative measurements of the vibrational characteristics of loudspeaker cabinets. The main difficulty with this technique is that you can't be sure which of the response peaks that you see are from the piezo and which are due to the cabinet (and cabinet vibrations do tend to be very peaky). Another problem is in finding a way to mount the piezo to the cabinet -- you can't press the piezo with much force onto adhesive or wax without breaking the ceramic disk. One approach that works well is to glue the pizeo to a small piece of wood, and then attach the wood to the cabinet with clay or wax.  But there's no reason to put yourself through this kind of trouble.

The real hot ticket among accelerometers for audio freqeuency measurements is the ACH-01

The ACH-01 device is just about perfect for the job: very wideband response (specified within 3dB from 2Hz to 20kHz), internally buffered for low output impedance,and already housed in a small, rugged, flat package and supplied with a nice, limp shielded cable. And it is very reasonably priced - about $21!  Sensitivity of the device, though slightly on the low side if used without further amplification, is adequate for direct usage with the LAUD hardware, needing nothing more in the way of assembly than to solder the wires to an RCA connector.  At this low price, it also seems to be a natural for use as a motional feedback sensor.
The ACH-01 requires a bias voltage to power its internal FET buffer. When powered from the FIJI card  (or the PRAXIS AudPod) as shown in figure 2 (connected directly to the FIJI's or AudPod's mic input), sensitivity is about 4.7mV/g and signal handling is about 60g(p-p). The ACH-01-03 model comes with a shielded cable and a connector - just cut the connector off at the far end and solder the wires as shown directly into an RCA inline jack. This provides a very simple hookup, relieves the user of the need to be concerned about batteries for the device, and is very well suited for loudspeaker cabinet investigations. This same hookup also works when connected to the MIC input of the Mic/Probe Preamp (for ECHO-based LAUD systems) or to the IMP's mic input.

Key Features of MotionNode Accel

  • Small size, minimizes interference with natural motion
  • Outputs 2 g or 6 g range in all three axes
  • Accurate, general purpose accelerometer solution
  • Simple USB connection
  • Easy to use and integrate into your project or application
  • Includes PC software to configure your sensor, preview output data, and record acceleration data
  • Software Development Kit (SDK) provides real-time access to raw and calibrated sensor streams
  • Supports Windows, Mac, and Linux operating systems
  • Multiple devices can be connected to a PC through a USB hub
  • Miniature size: 35 x 35 x 15mm
  • Sample rate: 100 Hz