# Yamaha's acoustic absorber tubes, reverse engineering

#### noviygera

I am referring to the two "pressure equalizing" tubes inside the cabinet of the flagship NS-5000 speakers. This link covers some info
https://usa.yamaha.com/products/audio_visual/speaker_systems/ns-5000/index.html

this video goes into more detail:

I would like get help, to calculate for a pair of these tubes in my DIY ported Tannoy boxes. I'm curious!! We can see what Yamaha is doing from the video, The length of the long tube is the internal height of the cabinet. The short tube is about 90% of the length.

Both tubes start near the top of the cabinet, and terminate at the 1/2 height of the cabinet.

Tubes are empty inside and have only a little absorption material at both ends.

How does this work?

How do we calculate the cross section area of the tubes?

Please share your thoughts on this approach.

#### NareshBrd

Physics, high school.
https://en.wikipedia.org/wiki/Kundt's_tube

Also see Pitot Tube.

Find out if ports are open or closed to the environment, and the material used, that is important.
The tubes above are well studied, formulae exist, you can tune the performance easily for your needs.

1 user

#### noviygera

Ports are made of high density cardboard (like the cardboard round ports) which is rectangular in cross section, open on both ends. There is some light absorption material at both ends of tube.

Can you help with the formula to determine the cross section area of tubes? I know the lengths.

#### NareshBrd

Area is frequency dependent.
So I cannot tell you more, you have to calculate it for your choice of frequency.

Read the Kundt's tube article for details about formula etc.

#### tmuikku

Tubes are most likely targeted to reduce lowest mode in the box, whose wavelength would be 2x internal height. Reason I think so: there likely is damping material in the box and it would be most effective on the higher order modes while the first is affected the least due to long(est) wavelength.

Obviously function of the tubes is to fix some problem inside the cabinet, which would be the modes if any.

1 user

#### xrk971

Paid Member
These are like 1/4wave stubs. Pick frequency peak you need smoothed and calculate 1/4 wave length corresponding. Make tube or slot in speaker equal to this length and stuff with rockwool, fiberglass, or polyfill etc.

Example, you have a resonant peak at 500Hz you need smoothed or absorbed.

500 Hz / 342 m/sec = 1.46m

Take 1/4th of this length so 1.46m /4 = 0.37m.

So make a tube 0.37m long (14.6in long) or a slot wide as cabinet and this long x maybe 1in deep.

1 user

#### kouiky

I took a hypothetical approach to try and model the behavior out of an academic interest.

I still fail to see how it can effectively reduce the energy across all the frequencies that are reflected inside the cabinet. It’s not just one or two high Q target modes. The tube cross section is very small relative to the cabinet’s internal surface area, and the midrange frequencies striking those internal surfaces are going to reflect back on the driver immediately. My question might be, how is this really more effective than fibre-based fill?

#### tmuikku

It makes good marketing!

#### noviygera

So what is the way to figure out the diameter of the tube or the cross-section area of the stub? It seems to be pretty small in the Yamaha box.

#### NareshBrd

Could it be a resonator to increase response, rather than a noise cancellation device?
It is not clear if the Yamaha cabinet is sealed or ported, I did not take the time to see the video.

I have seen these resonators on the intake of motorcycles long back. Idea was to increase inlet pressure. Mounted after the filter, in a box which had the carburetor, so it was as if the intake manifold had it fitted.

#### kouiky

Could it be a resonator to increase response, rather than a noise cancellation device?
It is not clear if the Yamaha cabinet is sealed or ported, I did not take the time to see the video.

I have seen these resonators on the intake of motorcycles long back. Idea was to increase inlet pressure. Mounted after the filter, in a box which had the carburetor, so it was as if the intake manifold had it fitted.
According to Yamaha’s literature, they demonstrated the purpose was to equalize the opposed pressures at the uppermost and lowermost ends of the cabinet. This natural pressure variation was visually portrayed as being modal. The internal dimension corresponds to 550Hz full-wave, but they might be targeting a half or quarter wave of a lower frequency.

The cabinet is ported, but the boxes also include foam inserts to reduce the port diameter and lower the tuning frequency, along with a second insert to convert it to a sealed type. Their impedance plot in the manual suggested it worked as intended.

The peripherals employed on engine intakes are Helmholtz resonators. When pulsed air is passed over the opening, the cavity emits the same tone out of phase. I used one in a TL speaker I designed about 14 years ago, to reduce the null that appeared at three times the tuning frequency. I then used a motorized variable-state version in a patent to silence the compressor stages on turbine generators at all operating speeds.

Last edited:

#### Stuey

I'm pretty sure someone posted a link to the patent, but I can't find it now.

#### tmuikku

According to Yamaha’s literature, they demonstrated the purpose was to equalize the opposed pressures at the uppermost and lowermost ends of the cabinet. This natural pressure variation was visually portrayed as being modal. The internal dimension corresponds to 550Hz full-wave, but they might be targeting a half or quarter wave of a lower frequency.

The cabinet is ported, but the boxes also include foam inserts to reduce the port diameter and lower the tuning frequency, along with a second insert to convert it to a sealed type. Their impedance plot in the manual suggested it worked as intended.

The peripherals employed on engine intakes are Helmholtz resonators. When pulsed air is passed over the opening, the cavity emits the same tone out of phase. I used one in a TL speaker I designed about 14 years ago, to reduce the null that appeared at three times the tuning frequency. I then used a motorized variable-state version in a patent to silence the compressor stages on turbine generators at all operating speeds.
While I'm not any kind of expert on the stuff we can do some reasoning and try to come up with more understanding how it might work.

Lets quickly revisit pressure variation inside the box to get everyone on same page.
Lowest mode would be half wave length, pressure nodes each end of the dimension because velocity goes to zero due to a wall, pressure goes up. So pipe inlet close to a wall would be at "high pressure" zone and the other end would be middle of the box where pressure is normal on the lowest mode, its a velocity node. High pressure is either positive or negative pressure variation compared to normal pressure when there is no sound / particles moving. On the second harmonic / mode, there would be pressure node on the middle of the dimension as well, but in opposite phase making even greater pressure difference at the end of the pipe(s).
https://www.acs.psu.edu/drussell/demos/standingwaves/standingwaves.html

If we assume the pipes duty is to equalize pressure like the literature seems to suggest, then we could start reasoning following kind of stuff:

- I'm not sure if pressure would vary through out the box below the lowest mode, it would be mostly uniform pressure I think, so the pipes probably target lowest mode and up, where pressure varies at different parts of the box.

- From the photo the pipes seem to be about as long as the longest dimension of the box. Ends of the pipes seem to be at middle of the box long dimension, and end of the box long dimension.

- Pipe would be equalizing pressure between end and middle of a closed pipe. As long as pressure is different between the two ends there ought to be some particles going through the pipe ( as well ). This is a longer route with some damping, which apparently does something.

- length is important, tubes are probably as short as possible to keep cost low while being effective. Any shorter they would not be effective for the purpose, any longer would not be useful.

- cross sectional area is probably important, to get sufficient effect, what ever that is. Area seems to be quite small compared to cross section of the long dimension of the box.

- If the tubes were straight reaching out both ends of the box, then pressure difference between ends of the pipe would happen only with odd harmonics, on even harmonics pressure would be equal.

- Pipes seem to be bit different length, but not like 1/4 or 1/2 ratio. The effect is probably quite narrow band, two slightly different lengths would make it bit wider band effect combined, or something.

From these few observations its easy to intuitively simplify the tubes to "mess up the standing wave", easing it out, equalizing pressure between end of the box and middle of the box, perhaps reducing any peak that would be measured outside of the box. It looks like they are targeted for all modes on the longest dimension, not just the lowest one but starting from the lowest one. Reasoning from box shape, even if all the long dimension modes were now succesfully eliminated, there would be modes on the other two dimensions starting from roughly 2nd mode of the longest so still some reqular damping needed, which would be effective for the long dimension high order modes as well, making effect of the tubes mostly targeted to the lowest, or few lowest modes of the long dimension, which are least affected by regular damping material.

I'm quite sure all you need is get the tube length right, position of the ends right, both of which can be copied from the Yamaha brochure. Not sure about cross section of the pipes, probably needs to be sufficient for any effect. Perhaps use some plumbing stock available to you to try it out. Measurements / listening, to evaluate if its useful or not.

There probably is proper explanation and maths involved, perhaps already posted, perhaps not.

Takeaways for me:
• might be mostly marketing because no one else does this.
• perhaps its non issue because no one else does this.
• if its interesting, make prototype and figure out if there is audible difference.
• if its problem with modes, its more significant issue when crossover compared to box size is high
• if it targets the lowest mode only, it can be eliminated by positioning the driver on the middle making this non issue.
• since it is a top end performance model of a classic speaker, with legacy features, the modes are there as result of compromises taken and it would make sense to add tricks like this to improve performance even if only slightly. This would justify high price, solidify top performer status, and make for good marketing material, raise interest and so on. Good thing, cheap to implement, only positives for this particular product.
Best fix to any problem would be not to introduce the problem in the first place, make it disappear, a non issue Have fun!

Last edited:
1 user

#### tmuikku,​

So if the woofer is mounted in center of baffle (mid way top to bottom) then this is not an issue that needs to be resolved?

#### kouiky

An acoustic source at a midway point of any dimension creates coinciding resonance, and it also creates standing waves, while moving the frequencies upwards. Both ends act like tuning forks at intervals. Centering drivers laterally is as much a compromise as we normally accept, and is mostly to balanced appearance with function.

These acoustic absorbers introduce an acoustic short circuit to longitudinal pressure extremes, and it should take a few cycles to build up. It is of key interest that these absorbers are mounted on the walls and the top of the device is open into the corner - this is the highest pressure zone inside a cabinet. They are shown as open at both ends (one would have to be open to accomplish any purpose), and have a spacer which appears to prevent them being blocked during installation. Very interesting if it works as demonstrated.

Based on the images, these absorbers seem to be stuffed completely, and in other images appear to be empty. I might start a build with the understanding they are densely stuffed. For materials, I'll opt for paper backed foam board from the craft store, allowing the provision of testing different materials inside the length. The cardboard in the images looks similar to the hard corner protector type used in shipping materials. I'm uncertain how two of these can replace and entire cabinet full of fiberglass fill.

#### NareshBrd

See how many models, and over how many years , the units were sold by Yanaha.
Could have been an experimental model.

Another thing is the kind of music played, some are different in tone spread (like opera vocals) to let us say pop or rap music, which are more bass heavy.
The perceived improvement may not have impressed listeners and prospective buyers.

In that case, not worth the effort, I think.

#### Patrick Bateman

I'm uncertain how two of these can replace and entire cabinet full of fiberglass fill.

If you look at the sims in the "metamaterial" thread, these things are tuned over a narrow frequency. That's how such a small device can be efficient.