What is A Rotary Subwoofer (Propeller Subwoofer)?

Norvan Martin
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Typical subwoofers can produce sound frequencies below 30 Hz level, but humans can hardly detect frequencies less than 20 Hz. This is why many subwoofer manufacturers state frequency response ranges from 20 Hz to 200 Hz.

In the early 1970s, researchers discovered that increased sound pressure level (SPL) could help humans perceive frequencies less than 20 Hz and invented rotary subwoofers for this purpose.

In this article, we talk about what rotary subwoofers are their uses, and how they work.

What Is A Rotary Subwoofer?

A rotary subwoofer, also known as a propeller subwoofer is like a conventional subwoofer, but it uses a traditional speaker-voice coil’s motion to alter the pitch of a propeller rotating at a constant speed. The voice coil’s audio signal controls the fan blade’s pitch and ensures the subwoofer reproduces incredibly low sound frequency. These advanced subwoofers can compress the air in a sealed room to produce sound frequencies down to 0 Hz.  

How Do Rotary Subwoofers Work?

Air woofer technology ensures that a rotary subwoofer produces very low sound frequencies. Besides, it also allows the amp to consume little energy while still delivering a good audio experience.

what is a rotary woofer

The amp consumes less energy than a traditional subwoofer because of the working mechanism behind these different types of subs.

This is because the amplifier is only changing the pitch of the blades which means it will require much less power per dB of sound generated to drive the rotary woofer than a conventional subwoofer.

Remember, with a conventional subwoofer, the input signal must drive the voice coil within a magnetic field which drives the cone to displace air which produces sound. That requires more energy. 

Here’s how a rotary or propeller subwoofer works in more detail. 

Firstly, the incoming signal to the voice coil changes the pitch of the fan blades as it spins at a constant speed, often between 600 RPM and 800 RPM. To explain further, the incoming signal changes the blades’ pitch in correspondence with the signal’s waveform.

rotary woofer cutaway view

This can be achieved because the fan is able to swing both positively and negatively, with respect to a zero-pitch spinning blade position. As such, it can represent the incoming signal’s waveform. 

The generated sound pressure waves can affect humans. We can also feel them. 

In case there is no signal, the rotary produces no sound. This happens since the blades rotate ‘flat’ at zero pitch.

The sole duty of the audio amp is to change the blades’ pitch. But the spinning launches an acoustic sound wave forward and backward from the blades.

When the change in the blade pitch is excellent, the resultant sound wave’s aptitude becomes excellent as well, and the reverse is also true. 

Air Woofer Technology and Efficiency

Rotary subwoofers are designed to be more efficient than traditional technology. As the sound frequency drops lower than 20 Hz, the efficiency of a conventional cone diaphragm drops.Propeller Subwoofer

As an example, let us consider what might happen if the frequency is reduced to half for a conventional subwoofer. If this happens, the diaphragm must also displace at least four times as much air to produce the same sound amplitude. This means conventional cone diaphragms require a lot of energy to reproduce the required low frequencies. 

On the other hand, a rotary subwoofer’s acoustic impendence is always perfectly matched to the surrounding environment.

Due to this, it can reproduce significantly lower amounts of frequency at outstandingly high levels. This makes them more efficient at reproducing very low frequency than cone-based subwoofers.

Let us illustrate this again for better understanding. We can imagine what would happen if the frequency halves. In this case, the blades rotate twice more per cycle. As such, the blades double the diaphragm’s area. This can translate to a relatively flat frequency response down to one Hz in most listening rooms.

Since rotary subwoofers can produce the lowest frequency, you need to use scientific equipment to verify the level of efficiency. For example, you could use a dB meter

This is necessary since no human ear can sense sounds that low. Remember, the efficiency of a rotary subwoofer will always constantly reduce as the frequency drops toward zero.

In other words, the sub’s “vibrations” are highly likely to be produced in a form that was not originally desired.  The good news is that rotary subwoofers with large diameters have the capacity to produce the lowest tones efficiently.

Air Woofer Technology and Low Power Cost

In rotary subwoofers, the acoustic impedance is closely matched. As such, the rotary subwoofer requires less amplifier power to give the same output as a conventional subwoofer.

rotary subwoofer

For example, if a rotary sub requires 100W to reproduce quality sound at 20 Hz, a traditional sub will require as much as 500 watts for the same output at the same frequency.

If you are concerned about the subwoofer’s high power consumption, you can acquire a rotary sub and create the cinematic experience you want at an affordable cost. 

Air Woofer Technology and Distortion

As we have seen, rotary subwoofers are designed to reproduce frequencies lower than 20 Hz. This shows that as the input frequency exceeds the fan’s rotation rate, distortion may increase.

rotary propeller woofer

For the fan, 20 Hz can be approximately 1,200 RPM. Current models limit the distortion, as they use AC induction motors at about 800 RPM (13 Hz). Besides, once the subwoofers are installed, they are carefully braced to make the blades lie in circular openings.

Due to this unique design, the subwoofer can move the air between external chambers and successfully generate sound pressure from both its rear and front ends without attempting to cancel each other. 

That alone may not give the suitable for the intended application. Without a doubt, the cost may appear high. However, the output is worth the investment. The unit also requires a manifold chamber in front of it.

The chamber can effectively neutralize the noise emanating from the rotating blades. Once the blades are positioned in a circular cutout, the sound quality can improve significantly, especially if they have the right back-wave chamber.

Rotating subwoofers have incredibly large effective radiating areas. And this is the reason we need huge back-wave chambers, as well. Typically, this means you need an entire room, basement, or attic to get the best audio experience. 

If you provide all the requirements, you can expect a rotary subwoofer to output not less than 95Db SPL from 20 Hz down to below 1 Hz, which is an incredibly low sound frequency. But can the human ear sense it?

Yes, according to the listening tests of various experts, including Bruce Thigpen, who developed the Thigpen Rotary Subwoofer Model 17, we can hear the sound, but we need high SPLs. Rotary subwoofers offer the solution. This is why rotary subwoofers are considered SPL subwoofers. You can read more about what is an SPL subwoofer to get more insight into that. 

Conclusion

As we have seen, recent research has dispelled the belief that humans can only sense sound frequencies from 20 Hz and above.

However, our ears are indeed much less sensitive to lower frequencies, which means we need higher sound pressure levels to sense them. Since moving typical subs’ cones cannot produce these low frequencies rotary subwoofers are ideal for this purpose. 

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Norvan Martin is the founder of BoomSpeaker.com. He is a professional Electronics Engineer and is passionate about home theater systems and AV electronics. BoomSpeaker was created as an online hub to share his knowledge and experiences as it relates to home theaters and home audio electronics. My email: [email protected]  Connect on Pinterest and Linkedin