Powered Subwoofers

Talk Shop with John Meyer


Exponential Horns: 4 and 8 Foot Diameter
At Pepperland, San Rafael, California, circa 1969 or 1970
This bi-amped quadraphonic system was used by
Steve Miller, Pink Floyd, Frank Zappa and many others.

Meyer Sound has developed three types of powered subwoofers to be used in sound systems.

The first group are of the bass reflex design. This includes both the 650P and the PSW-2. A well designed reflex system is twice as efficient as a direct radiator speaker. Also the reflex has the added advantage of loading the drivers for a lower excursion. However, the reflex offers no speaker loading below the resonate frequency. It's as if the cabinet disappears. Not only do you have to design for a flat frequency response, but you now have to watch the cone excursion, along with monitoring the average power.

The 650 design goes back to the first subwoofer that Meyer Sound developed for the 70mm 6-channel theatrical release of Apocalypse Now in 1980. The system was a traditional high Q 4th. order single 18 inch diameter speaker mounted in a 15 cubic ft box tuned to 27 Hz. This gave a system response to 25Hz. Included in this system was a electronic controller (650E) that was inserted between the high power amplifier and the loudspeaker. The controller was responsible for the survival of the system under any signal conditions, accomplished in a very graceful way. Adding a second 18 in. speaker to the same enclosure (650 R2) and driving the two speakers together so that they act as one piston (giving an equivalent piston diameter of 24"), somewhat de-tunes the reflex. Even though the efficiency of this reflex is less, the advantages outweigh the small loss. The main advantage of this new medium Q reflex is the system is much better damped than a high Q box. High Q bass reflexes were often used in juke boxes giving the characteristic one note boom box sound. Some concert subwoofers are still sold today which have this one note boomy bass. The advantage of using two speakers over one large cone is two fold.

First, with two speakers you can now drive twice the power into the system. This more than makes up for the lower efficiency. The other main advantage is that the directional characteristics increases with the separation between the drivers. This means that the on axis output can be as much as 6 dB higher compared to a single 24 in. cone driver. Also larger pistons are more subject to cone breakup. The new 650P consists of two 18 in. high efficient speakers in a 32Hz tuned 12 cu. ft. cabinet and includes the controller and power amplifiers.

The PSW-2 design comes from the popular USW double 15" bass low Q reflex known for its "tight bass sound". This system is designed to reproduce down to 40Hz. in a very compact arrayable 8 cu. ft. cabinet.

The above criteria is based on both steady state sine waves and pink noise. This means that a single frequency tone is applied to the input of the sub and its level is slowly increased until the sound pressure level reaches the maximum output at this frequency with the harmonics showing better than -20dB. The measurement microphone should be the high-pressure type to insure that it does not introduce its own distortion into the measurement. However this measurement does not represent music signals. The peak to average level of a sine wave is only 3dB. Rarely does music have a single tone played for a long period of time. Music signals are made up of tone bursts rich in harmonics. Typically the peak to average level in a music signal ranges from 10 to 30dB. Noise has a complex harmonic structure consisting of many frequencies at the time so it can substitute for this one aspect of music. It can also have a high peak to average level. The hardest to measure is the tone burst nature of music. A large drum has both high sound pressure peaks and is also rich in harmonics. Testing with both steady state sine waves and noise, and noise bursts, one can characterize pretty well the behavior of the sound system. The table below shows each subwoofer's continuous sound pressure output with less then 10% total harmonic distortion at each 1/3 octave frequency. The powered subwoofers were measured at one meter on axis outside on the ground. You can see that the PSW-2 and the 650-P have almost the same continuous output above 60Hz. At 100Hz. the PSW-2 has more output than the 650-P because the 15 in. cones are lighter than the 18 in cones; both 18" and 15" drivers use the same motor. Also the controller and amplifiers are identical in both systems. If you look at the pink noise (equal energy per octave) for these systems, the 650-P has more output due to its better performance at the lower frequency bands. The continuous pink noise measurement was taken with the subwoofer running at the maximum output for 1 hr. using a sound level meter set to no frequency weighting and the response time set to slow. The peak was captured using the peak/hold feature on the SPL meter with pink noise bursts. Both these speakers have a good peak to average output level making them especially excellent music reproducers.

The second group is a special case of the first group. This is the multiple tuned bass reflex. Bass reflexes offer little help to the frequencies that are above two times the tuning point. Over the last 20 years there have been various attempts to increase the bandwidth of the reflex. Several patents have been issued to multiple tuned chambers coupled to a single speaker all in one enclosure. In theory one of these single driver multiple tuned systems could have the same output as that of a dual speaker in a single tuned enclosure. However these multiple cross coupled chambered designs are complicated to manufacture so the claimed savings of eliminating the second speaker is greatly diminished.

Meyer Sound embarked on a new subwoofer that would give higher peak to average output using the standard MS drivers. The first of these new design is of a double tuned reflex whereby each tuned chamber has its own driver. * The low chamber has a 32Hz tuning driven with a 18" driver. The second smaller chamber is tuned at 45 Hz and is driven with a 15" driver. Each driver has its own power amplifier. The signal is crossed over in the controller so that low and highs are separated and sent to the appropriate drivers. The system is called the PSW-4 and is a little taller than the PSW-2. From the sine wave data it appears to have less output than the 650-P and PSW-2 except at 32Hz where it has more output than the PSW-2. When using continuous pink noise, the PSW-4 shows 1 dB more output than the PSW-2 and 1 dB less output compared to the 650-P. However, it has 6dB more peak output than the 650-P and 10dB more peak output than the PSW-2. This is because the two drivers output combine in the air. This is free headroom. The power amplifier in the 650-P would have to have be 4000 watts to produce this extra 6dB peak level. We have found that with some music, such as percussive instruments, this added headroom of the PSW-4 can enhance the realism of the sound.

To further test this free headroom we installed the 650-P amplifier module into the PSW-4. This drives the two speakers in parallel and you can see the increased output on the sine wave tests at the higher frequencies (see table PSW-4/650). The continuous peak noise has also increased by 1 dB. However, the peak pink noise burst shows 8dB less output compared to the PSW-4 ( the demand on the 15" driver has increased because it now is forced to work at 32Hz). The two drivers are out of phase from each other at 32 Hz. That is why there is so little gain in output below 50 Hz. At 42 Hz the PSW-4/650 has 2 dB less output then the PSW-2. However if the spectral information is concentrated above 60Hz this PSW-4/650 combination gives the most output of any of the reflex sub systems.

Meyer Sound Speakers

Continuous SPL in dB measured with a sine wave at 1 meter in 1/2 space

Pink Noise 1Hr Test

32Hz 42Hz 50Hz 63Hz 80Hz 100Hz 125Hz cont. peak
650-P 114 120 127 126 124 122 120 120 130
PSW-2 108 118 119 126 124 124 120 118 126
PSW-4 110 115 120 122 122 123 116 119 136
PSW-4/650 111 116 122 128 128 126 122 120 128

The third group is the horn loaded type speaker system. The DS-2P falls into this category. The DS-2P is a powered compact folded hyperbolic horn driven with twin 15" drivers. Horns have the advantage of more then twice the efficiency of reflex systems. However, horns need a large mouth area to reproduce low frequencies. A traditional circular exponential horn design to reproduce 50Hz is a large and unwieldy thing. A mouth diameter of 8 ft. is needed to couple 50Hz to the air and the exponential taper rate has to be chosen well below 50Hz. If the throat is 18" in diameter, the length of the horn is 8 ft. This does not include the rear air chamber (I built and tested this horn, see photo of horns). The DS-2P was designed to be arrayed together to create the large mouth necessary for concert sound and to be modular for easy truck packing.

The DS-2P was designed to operate with a minimum of two cabinets placed together when on the ground or four cabinets together when flown in order to reproduce 50 Hz. In order to make the DS-2P enclosure as compact as possible, a hyperbolic square folded horn design was chosen. Hyperbolic horns have more output closer to the cutoff frequency of exponential horns. This allows for a higher cutoff frequency to be selected making for a shorter horn, however driver excursion increases rapidly below the cutoff frequency making electronic control of drivers critical. Square horns have more mouth area for the same diameter compared to round horns allowing a smaller size. Folding up the horn in the box can make it more compact, however folds can cause cancellations in the frequency response. The DS-2P has been very carefully designed so that it has no cancellations in its frequency response below 200Hz. Since 1/2 the power in music is below 200Hz, the DS-2P can take a real burden off the sound system. Since a single DS-2P is unloaded below 100Hz, it was not included in the above table.

* Patent pending