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It always bothered me seeing "ultrasonics" and "audible range" being listed in the same spectrum as microwaves and light. We can't hear it, audio is not part of

It always bothered me seeing "ultrasonics" and "audible range" being listed in the same spectrum as microwaves and light. We can't hear it, audio is not part of the electromagnetic spectrum. They could mention submarine comms instead.

lxgr 3y ago
True, but on the other hand, you also can’t see VHF or microwave radiation! Yes, it’s a different medium, but for VLF/ELF, you could arguably just connect a photodiode/photoreceptor and a speaker/microphone to a very simple circuit and “hear/speak” that part of the spectrum, no?
- Sporktacular 3y ago
  Our capacity to sense some frequencies, or build transducers or transceivers for them is a complication that's beside my point. Audio, including infra/ultrasonics does not belong in a diagram labelled The EM Spectrum. ELF, VHF, microwave and light etc. all do.
- ale42 3y ago
  > you also can’t see VHF or microwave radiation! True, but you can definitely see some frequencies of electromagnetic radiation (light is electromagnetic radiation, just at a very large frequency). A 525 nm green LED is just emitting a signal at a frequency of 571 THz... which you can actually see. On the other hand, 40 kHz sound (ultrasound) and 40 kHz radio signals (LF, see https://en.wikipedia.org/wiki/Low_frequency https://en.wikipedia.org/wiki/Low_frequency) are definitely not on the same spectrum, one being made of changes in air pressure, the other one by an electromagnetic field.
dredmorbius 3y ago
The audio and EMR spectra may be different underlying phenomena, but for the purposes of radio, which transmits over EMR but encodes audio frequencies, the correspondence is significant. It's not possible to encode a higher-frequency signal over a lower frequency carrier, so radio frequencies which are used for the transmission of human-detectable sounds must be at a higher frequency than those sounds. I'm not nearly enough of a radio / audio / encoding geek to tell you what the minimum overage is, though I suspect it's at least 2x greater than the highest-frequency sounds that are intended to be encoded. For human-range hearing, that would be in the 10-20 kHz range, which would mean that a minimum carrier frequency would be somewhere around 20--40 kHz. The longest wavelengths used for voice comms so far as I know are the longwave or LW band, which is from 30–300 kHz, with the lower end of that pretty much precisely where I'd suggested it might be. Most commercial AM broadcasts are in the medium-wave (MW) band, from 531 kHz – 1602 kHz, or roughly 25-50x higher frequencies than the highest tones the human ear can detect. And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies. <https://en.wikipedia.org/wiki/AM_broadcasting#Broadcast_band_frequencies https://en.wikipedia.org/wiki/AM_broadcasting#Broadcast_band...>
- Sporktacular 3y ago
  "radio, which transmits over EMR but encodes audio frequencies" Radio conveys all sorts of things, not just audio. They're orthogonal issues. And you actually could modulate 20khz BW of audio onto a 30khz carrier using SSB-AM for example. That carrier could be EM, a voltage in a wire or an ultrasonic tone. Not that it's relevant to the issue but it does reinforce the point that the nature of the medium is independent from the intelligence being conveyed. There is no correspondence. Sonics/audio are pressure waves and do not belong on the EM spectrum.
- Johnythree 3y ago
  > It's not possible to encode a higher-frequency signal over a lower frequency carrier.. Actually it is possible. AM modulation is just a matter of multiplying two frequencies together. It's just that the lower sidebands wrap around DC. It's a bit harder to demodulate, but not impossible. >.. AM broadcasts are in the medium-wave (MW) band...And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies. In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage. You can actually get FM like audio from a BC AM radio, but not with cheap and nasty receivers designed for the American market. In Europe many HiFI AM receivers have an optional "wide" IF filter, and the transmitters can transmit 15KHz audio.
  - Sporktacular 3y ago
    "In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage." This applies to heterodyne systems with the bandwidth limitation coming from deliberate filtering. My understanding is the channel width, set out in the broadcast standard and license, is the bottleneck and not so much the limitations of the electronics. FM must be under 150khz (5 times the maximum source bandwidth of 15khz, then doubled for upper and lower sidebands) and MW-AM must be under 9/10khz (limiting the source to around 5 kHz). So without the transmitter broadcasting outside its allocation, I don't understand how changing the receiver could improve the audio quality. I could be wrong. Do you have a source on 15khz audio bandwidth over MW-AM?

lxgr 3y ago

True, but on the other hand, you also can’t see VHF or microwave radiation! Yes, it’s a different medium, but for VLF/ELF, you could arguably just connect a photodiode/photoreceptor and a speaker/microphone to a very simple circuit and “hear/speak” that part of the spectrum, no?

Sporktacular 3y ago
Our capacity to sense some frequencies, or build transducers or transceivers for them is a complication that's beside my point. Audio, including infra/ultrasonics does not belong in a diagram labelled The EM Spectrum. ELF, VHF, microwave and light etc. all do.
ale42 3y ago
> you also can’t see VHF or microwave radiation! True, but you can definitely see some frequencies of electromagnetic radiation (light is electromagnetic radiation, just at a very large frequency). A 525 nm green LED is just emitting a signal at a frequency of 571 THz... which you can actually see. On the other hand, 40 kHz sound (ultrasound) and 40 kHz radio signals (LF, see https://en.wikipedia.org/wiki/Low_frequency https://en.wikipedia.org/wiki/Low_frequency) are definitely not on the same spectrum, one being made of changes in air pressure, the other one by an electromagnetic field.

dredmorbius 3y ago

The audio and EMR spectra may be different underlying phenomena, but for the purposes of radio, which transmits over EMR but encodes audio frequencies, the correspondence is significant. It's not possible to encode a higher-frequency signal over a lower frequency carrier, so radio frequencies which are used for the transmission of human-detectable sounds must be at a higher frequency than those sounds. I'm not nearly enough of a radio / audio / encoding geek to tell you what the minimum overage is, though I suspect it's at least 2x greater than the highest-frequency sounds that are intended to be encoded. For human-range hearing, that would be in the 10-20 kHz range, which would mean that a minimum carrier frequency would be somewhere around 20--40 kHz. The longest wavelengths used for voice comms so far as I know are the longwave or LW band, which is from 30–300 kHz, with the lower end of that pretty much precisely where I'd suggested it might be. Most commercial AM broadcasts are in the medium-wave (MW) band, from 531 kHz – 1602 kHz, or roughly 25-50x higher frequencies than the highest tones the human ear can detect. And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies. <https://en.wikipedia.org/wiki/AM_broadcasting#Broadcast_band_frequencies https://en.wikipedia.org/wiki/AM_broadcasting#Broadcast_band...>

Sporktacular 3y ago
"radio, which transmits over EMR but encodes audio frequencies" Radio conveys all sorts of things, not just audio. They're orthogonal issues. And you actually could modulate 20khz BW of audio onto a 30khz carrier using SSB-AM for example. That carrier could be EM, a voltage in a wire or an ultrasonic tone. Not that it's relevant to the issue but it does reinforce the point that the nature of the medium is independent from the intelligence being conveyed. There is no correspondence. Sonics/audio are pressure waves and do not belong on the EM spectrum.
Johnythree 3y ago
> It's not possible to encode a higher-frequency signal over a lower frequency carrier.. Actually it is possible. AM modulation is just a matter of multiplying two frequencies together. It's just that the lower sidebands wrap around DC. It's a bit harder to demodulate, but not impossible. >.. AM broadcasts are in the medium-wave (MW) band...And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies. In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage. You can actually get FM like audio from a BC AM radio, but not with cheap and nasty receivers designed for the American market. In Europe many HiFI AM receivers have an optional "wide" IF filter, and the transmitters can transmit 15KHz audio.
- Sporktacular 3y ago
  "In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage." This applies to heterodyne systems with the bandwidth limitation coming from deliberate filtering. My understanding is the channel width, set out in the broadcast standard and license, is the bottleneck and not so much the limitations of the electronics. FM must be under 150khz (5 times the maximum source bandwidth of 15khz, then doubled for upper and lower sidebands) and MW-AM must be under 9/10khz (limiting the source to around 5 kHz). So without the transmitter broadcasting outside its allocation, I don't understand how changing the receiver could improve the audio quality. I could be wrong. Do you have a source on 15khz audio bandwidth over MW-AM?