How to choose ultrasound frequency for beacons?

Marvelmind Precise Indoor Positioning System relies on two key technologies:

  1. Trilateration based on precise distance measurement using ultrasound pulses
  2. Precise clock synchronization between all network elements over radio using the modem as a central controller

In this article we touch the first topic mostly – trilateration – and answer the questions how to choose between different ultrasound frequencies of Marvelmind beacons and why.

We don’t discuss the radio connectivity in detail here. Remember, though, that your beacons and modems must support the same radio band, for example, 868/915MHz band. You can’t combine Beacons HW v4.9 on 433MHz band and Beacons HW v4.9 on 915MHz band or Super-Beacons on 915MHz band.

See more about the radio connectivity.

Real ultrasound signal examples from a steel-making plant

The oscillograms below are the real examples from a steel making plant:

  • Signal from a Horn attached to an Industrial-RX beacon (left)
  • Main microphone only on the Industrial Beacon (right)

What to pay attention to:

  • High external noise on lower frequencies (19kHz-24kHz)
  • Significantly stronger signal from a beacon with Horn than without a Horn
  • Significantly lower noise from the beacon with Horn due to the shielding effect – the external acoustic noise was coming from the floor, whereas the horn was located near the ceiling or pointed to the horizon, since it is has been used for positioning of the bridge cranes
  • Significant ultrasound signal strength drop for higher frequencies (34kHz and 37kHz). The signal for 45kHz was even weaker (not shown).
  • Significantly lower external noise for the higher frequencies (34kHz-45kHz)

Help: how to use embedded oscilloscope

Why are there different ultrasound frequencies required?

In order to address the needs of as many different applications as possible – from autonomous robots, drones, forklifts and humans to sports and archaeology – we produce beacons with different ultrasound frequencies, because some of the architectures that we have require different frequencies (IA and MF NIA) and because different ultrasound frequencies have slightly different characteristics.

Naturally, potential users may have questions:

  • Shall we use beacons with the same frequency or with different ones?
  • How to select the most suitable frequency for our beacons and why so?
  • What are the practical differences between different ultrasound frequencies?

To simplify the decision process, we have sets – for robots, drones, forklifts, people, etc. You purchase them and they already the most recommended configurations of beacons with the right frequencies.

But why? What are the underlying principles behind those recommendations and beacons’ selection? Let’s discuss deeper.

Question: Wouldn’t it be easier or better to have only a single frequency for all beacons?

Answer: Yes, it would be, if it was practically feasible. However, there are several factors against it. Thus, multiple frequencies are better in practice:

  • In order to distinguish between ultrasound signals coming from different ultrasound transmitting beacons, the signal must be distinguishable or modulated. For example, amplitude modulation, frequency modulation, phase modulation – our typical suspects
  • However, highly-resonating ultrasound transducers can be successfully modulated only with low frequency – well below the 1/(ringing time). Taking into account the ultrasound transducers are very resonating, the ringing time is long and the modulation frequency would be too low and rather impractical for precise indoor positioning system. It is still possible, but difficult to implement and impractical for technical and economical reasons

Question: OK. But why to choose the resonating transducers in the first place? Why not to use wideband “ultrasound speakers” or something?

Answer: Yes, it is possible to use wideband ultrasound speaker to transmit ultrasound and that would bring a lot of simplification to the system. However, such speakers typically produce significantly weaker ultrasound signal, thus limiting the range the system can operate. For example, Marvelmind Indoor Positioning System can comforable work with the beacons placed up to 30 away. With Marvelmind Horns – up to 120m away.

low efficiency, i.e. it would require significantly more energy to produce the same level of ultrasound signal as the resonating transducers do. A

Question: But why not to just provide a lot of power to the ultrasound speakers and to produce strong ultrasound signal?

Answer: It is also possible. However, since the non-resonating wideband ultrasound speakers typically have low efficiency, it would require significantly more energy to produce the same level of ultrasound signal as compared with the resonating transducers. Which means that the beacons with wideband speakers must always be powered from the external source that is often would become a limiting factor for such beacons.

Despite that, yes, it is possible to build the system with low-efficiency wideband ultrasound speakers. In many cases it could be a very good option and an alternative providing a new layer of flexibility of building the precise indoor positioning systems.

Question: So, how does different frequencies of beacons help?

Answer: If we can’t use modulation, we can separate different ultrasound signals either in time or in frequency.

Time division multiple access (TDMA) is easier. That is the basis for NIA. No ultrasound signals are transmitted at the same time within 50m or so of open space. The system is easy, robust, but with the growing number of mobile beacons that are emitting ultrasound, it becomes proportionally slower per mobile beacon, i.e. if the update rate per system is 16Hz, then the update for 1 mobile beacon will be 16Hz; for 2 mobile beacons = 16Hz/2=8Hz, for 10 mobile beacons => 1.6Hz, etc. For many applications it is too slow.

Also, in some cases, it is simply inconvenient to have emitting ultrasound mobile beacons, for example, on people, because the beacons produce, albeit quite, but still rather observable “tick” – very short ultrasound pulse, that people hear as a “tick”. People can’t hear ultrasound, but the ultrasound pulse have weak elements of acoustic frequencies that can be heard that can simply disturb people, particularly, in quite areas. It is much better to have “ticking” beacons – ultrasound transmitting beacons 5-30m away, where the ticks are not even observed in practice.

But in order to distinguish between the simultaneously transmitted the ultrasound signals, they must be on different frequencies then.

It is difficult to filter the ultrasound signals that are close in frequency and short pulses have pretty wide transmitting spectrum as well. Thus, there is a practical limitation on how many ultrasound frequencies the system can have. But, in general, the more frequencies, the easier is to build large maps consisting of dozens of submaps or to have a better performance of swarms of drones.

Marvelmind indoor positioning system supports 8 ultrasound frequencies:

  • 19kHz
  • 22kHz
  • 25kHz
  • 28kHz
  • 31kHz
  • 34kHz
  • 37kHz
  • 45kHz

We are discussing the practical differences between different ultrasound frequencies below and give recommendations which frequencies to choose for your beacons, when ordering.

Very quick hints

  • Longest range required – up to 120m with the Horn in 1D? – choose 19kHz-25kHz
  • High external acoustic noise and distances under 30 meters? – choose 31kHz-45kHz
  • For the highest flexibility obtain beacons with as many frequencies as available

Ultrasound transmitting frequency can't be changed by software

Remember, you can’t set the ultrasound frequency for transmission arbitrary. The ultrasound frequency of the beacon is defined by the ultrasound transducers of the beacons. Marvelmind Robotics produces special customized ultrasound transducers for each of the ultrasound frequency we use in our beacons.

At the same time, Super-Beacons, as well as, Industrial Super-Beacons and Mini-RXs and resulting products supporting IA, such as Helmets, Jackets, Badges, Caps, etc. can receive and ultrasound frequency or several of them or all of them at the same time, because the receiving microphones are wideband and digital filters of the beacons allow beacons to distinguish between different ultrasound frequencies.

So, summing up:

  • Transmitting ultrasound frequency is defined by the hardware and cannot be changed by software
  • Receiving ultrasound frequency is defined by software and can easily be changed in Super-Beacons and other devices with digital filters. Actually, the users don’t have to do anything at all, since the system is selecting the right frequencies automatically
  • More mature Beacons HW v4.9 have analog filters and support only one receiving and transmitting frequency at once. But there are Beacons HW v4.9 available for 5 different frequencies (19kHz, 25kHz, 31kHz, 37kHz, 45kHz)

To choose the same or different ultrasound frequencies?

Answer:

In 95% of real use cases, it is recommended to choose different ultrasound frequencies for your beacons, when you are ordering, because it gives you more freedom with your indoor positioning system system.

The same ultrasound frequency of ultrasound beacons allows you to use only NIA. Whereas, beacons with different ultrasound frequencies will allow you using NIA, IA and MF NIA. Study more:

When to choose the same ultrasound frequency for beacons?

There are several cases when you must choose the same ultrasound frequency, or when it is preferred or recommended to choose the same ultrasound frequency for your beacons, and then there are cases when it doesn’t really matter what frequency to choose and some other factors may influence the final decision, for example, availability or price:

1) Choose the same frequency, if you already have, let’s say, Beacons HW v4.9 on 31kHz and you run NIA and want to expand your system:

  • If you want to purchase additional Beacons HW v4.9 to already existing system to cover more area or to build more submaps or to track more mobile objects, you must use the same ultrasound frequency, because Beacons HW v4.9 can’t receive different ultrasound frequencies at the same time – their analog filters are not sharp enough unlike the digital filters of Super-Beacons. So, if you already have 31kHz beacons, you must order new 31kHz Beacons HW v4.9
  • By the way, the Beacons HW v4.9 are still available in stock, but they are in a SW maintenance mode: we do support them even in the new SW releases, but no new features are developed for them and no new batches of beacons are produced anymore. The Beacons HW v4.9 are gradually substituted with Super-Beacons, which are more capable in nearly all aspects and which are the main “workhorse” of Marvelmind Indoor “GPS”. The most attractive point of remaining Beacons HW v4.9 is their lower price as compared to Super-Beacons

2) You can add stationary Super-Beacons in NIA with any frequency as an expansion for your system based Beacons HW v4.9 on 31kHz:

  • If you want to expand your system that is build with 31-kHz Beacons HW 4.9, you can easily purchase Super-Beacons with any ultrasound frequency for your stationary beacons. Super-Beacons can receive any ultrasound frequency or all at the same time. Thus, you can comfortably have a mix of Beacons HW v4.9-31 and Super-Beacons of any frequency as stationary beacons. However, for the mobile beacons in this system, you still must use Super-Beacons-31kHz only, because even Super-Beacons can emit only on their native transducers’ frequency. And if you already have Beacons HW v4.9 with 31kHz, then the new mobile Super-Beacons must be on 31kHz, while stationary Super-Beacons – on any frequency, since they listen to ultrasound – they don’t emit the ultrasound in NIA
  • By the way, the Beacons HW v4.9 are still available in stock, but they are the SW maintenance mode: we do support them even in the new releases, but no new features are developed for them and no new batches are produced anymore. The Beacons HW v4.9 are gradually substituted with Super-Beacons, which are more capable in nearly all aspects and are the main “workhorse” of Marvelmind Indoor “GPS”. The most attractive point of remaining Beacons HW v4.9 is their lower price

What are the impacting factors on different ultrasound frequencies?

There are several factors affecting on the most optimal selection of the ultrasound frequency for the beacon:

  • Desired range or size of the submap, i.e. the maximum reliably measured distance
  • Level of typical wideband acoustic noise
  • Accuracy of positioning system
  • Potential interference or acoustic noise from other sources, for example, narrowband or wideband impulse power supplies or luminescent lamps

Transmitting strength

Transducers of some ultrasound frequencies produce stronger ultrasound signal than transducers on other frequencies. The frequencies from the strongest to the weakest. It is a rough assessment to give an overall understanding, rather then a precise engineering science:
  • 25kHz
  • 19kHz, 22kHz, 28kHz
  • 31kHz, 34kHz
  • 37kHz
  • 45kHz
For example, the difference in levels observed between in the strongest 25kHz and the weakest 45kHz is about 10 times. Though, it is worth to clearly point out that it is not purely due to weaker transmission, but also because of weaker reception (lower sensitivity in 45kHz) and higher propagation loss in the air for the ultrasound signal for 45kHz as compared to the 25kHz signal.

But does it mean that the 45kHz is 10 times worse than the 25kHz frequency? – absolutely not! Even more, there are cases when the 45kHz frequency is preferred and recommended to any other frequency:
  • High level of external acoustic noise – it is will be lowest in 45kHz
  • The 45kHz frequency theoretically provides the highest accuracy, because the ultrasound wave length is the shortest

Reception sensitivity

Microphones have non-flat sensitivity between the different ultrasound frequencies. The rough assessment to give an overall understanding, rather then a precise engineering science (from the most sensitive to the list sensitive):
  • 25kHz
  • 19kHz, 22kHz, 28kHz
  • 31kHz, 34kHz
  • 37kHz
  • 45kHz
 
The order of frequencies from sensitive to less sensitive is rather similar to that for transmission from stronger to weaker, which is mostly a coincidence. Though, there is some underlying physics as well:
  • There is a peak about 25kHz, i.e. the microphone is abnormally sensitive around this frequency. Similarly, it looks like the 25kHz transducers are simply the most powerful overall
  • 19kHz – the frequency right below the peak
  • 22kHz and 28kHz are around and above the peak
  • 31kHz – starts already becoming weaker, but generally flat
  • 34kHz, 37kHz and 45kHz continues to be relatively flat, but simply slightly less sensitive
 

Attenuation in the air with the distance due to absorbtion

That is simple: the lower the frequency – the lower the attenuation. That means that for the highest range it is generally recommended to have lower frequency. Though, it shall be remembered that other factors, mostly, the external acoustic noise may significantly impact on the resulting signal to noise ratio.

Therefore, it it general very much recommended to run practical tests on the factory floor, for example:

  • To place the beacons face to face on the distance 30-50m with direct line of sight
  • Measure the signal using the embedded oscilloscope with different ultrasound frequencies, i.e. using several beacons with different ultrasound frequencies and compare the signal to noise ratio. See the examples below for more information

For more information, see:

For example, the formulas return for 19kHz the attenuation of ~0.5dB/m and for 45kHz – ~1.5dB/m, which gives ~30dB difference between the two frequencies on 30m. That is a lot! – more than 30 times in magnitude and 1000 times in power.

Surprisingly, we see the big difference between the frequencies in practice, but not as big. However, as discussed, there are many affecting factors (sensitivity, transmission efficiency, directivity, etc.)

What are the practical differences between different frequencies?

19kHz

  • It is has the lowest attenuation in the air among other frequencies we have. Thus, this frequency is recommended for very long submaps – up to 120m with the Horn
  • Non-standard frequency – lower chances for external interference
  • The only frequency that we have that is formally speaking not ultrasound. The 19kHz frequency is still in the frequency range that people can hear (20-20,000Hz). Which means that people may not only hear the pulses, but hear the tone itself and find the pulses rather disturbing, when they are closer than 2-3 meters from the beacon, because of the very-very high pitch of the pulses – on the edge of hearing
  • Not recommended in quite areas like museums and similar
  • Not recommended close to people
  • It is the closest to the typical acoustic noise on the factory floor. Thus, the real signal to noise ratio shall always be measured. On the high distance the signal may be strong, because the attenuation in the air is low. But the external noise may be strong as well. Thus, the signal to noise ratio is not necessarily the strongest

22kHz

  • Low absorption in the air, i.e suitable for long range and longest submaps
  • Already not heard by people
  • Strong transmission
  • Slightly lower expected external noise than in 22kHz
  • Non-standard frequency – lower chances for external interference

25kHz

  • Overall the strongest transmission signal
  • Low attenuation in the air – recommended for the largest submaps
  • Very high sensitivity of microphones (equally sensitive for the external noise on this frequency as well)
  • The 25kHz is a generic frequency and there are other ultrasound devices on the market using this frequency. Thus, there could be a potential external narrowband interference, because there could be other general devices like obstacle detection sensors in robots, for example

28kHz

  • Still strong signal for transmission
  • Reasonable attenuation in the air
  • Non-standard frequency – lower chances for external interference

31kHz

  • Our typical “default” frequency, because it is in the middle of everything: still strong transmission, already weak external noise, still weaker attenuation in the air, etc.
  • Still strong signal for transmission
  • Reasonable attenuation in the air
  • Non-standard frequency – lower chances for external interference

34kHz

  • Overall – similar to 31kHz, but slightly higher attenuation in the air and slightly lower external noise. Thus, the overall signal/noise ratio is about the same – depending the color of the noise and distances in the submap
  • Non-standard frequency – lower chances for external interference

37kHz

  • Noticeably weaker signal, particularly, on the distance
  • Lower external noise, i.e which is great for external-noise limited systems – not strength of the signal-limited systems
  • Shorter wavelength. Potentially higher accuracy

45kHz

  • The lowest level of external noise – very suitable for noisy applications
  • Potential the highest accuracy because the shortest wavelength of the ultrasound signal
  • The highest attenuation in the air – not recommended for larger submaps
  • Potential interference from other external systems that work on 42kHz or 44kHz

Does the ultrasound impact people, animals?

Practice shows that while people can hear quiet ticks produced by ultrasound transmitting beacons in quiet rooms, animals, even those that potentially can hear ultrasound, pay absolutely now attention to it. At least, their behavior doesn’t change at all.

We believe, the short (0.5-1.5ms) ultrasound pulses coming with ~8Hz rate from the ultrasound transmitting beacons are heard by the animals as just loud ticks, but nothing more. Similar to what people can hear, but louder. However, the animals remain indifferent whether there are beacons or not in their vicinity.

Though the beacons operate in ultrasound, which is not heard by humans, and the beacons emit very short pulses, thus the average transmitting signal or the level of ultrasound noise is about 1/1000 of the peak level, we recommend to keep ultrasound transmitting beacons at least 2-3 meters away from people in order to minimize even a slightest probability of a potential impact of a long-term exposure to ultrasound.

Since the ultrasound signal is easily blocked, the ultrasound signal from beacons behind a wall, or a door, or a even a sheet of paper, are most likely completely blocked. This is why we pay so much attention to the importance of a direct line of sight for precise tracking and that is why the 2-3m recommendation is valid only to ultrasound transmitting beacons with direct line of sight to people’s ears.

If anything is unclear, contact us via info@marvelmind.com