The London Hum

London’s hummmming …

Further to a UK news article (2/1/18) about a seemingly increasing number of reports of low frequency hum in the capital, I can’t help feeling that some, if not all authorities are being just a little dismissive !

I didn’t catch the exact details, but there was a shot of a multi plot graph, showing some measurements taken, and a comment along the lines of, and suggesting that, the frequencies and / or levels involved should not really be audible.  Even a suggestion that the complainant/s were super sensitive to the issue.

Along with the fact that over the years I have noticed a general misunderstanding/lack of knowledge regarding bass, there are a few things that spring to (my) mind !!

* It’s now been well proven, documented and understood that harmonics of a sound at an “audible” frequency can extend well beyond and above the “official” limit of human hearing – but, that we (whether heard in the accepted sense or not) can still sense these very high frequencies.   It makes sense then, that at very low “inaudible” frequencies, presented more as physical vibration than sound, could:  (a) have harmonics that appear within the audible range.  (b) be re-radiated by building structure/s as audible sound,  and that, in any event, we can sense the vibration, even if it is lower in frequency than our “audible range of sound”.

* Another factor, seriously misunderstood, is that of resonance within a closed space.  So, take a room in a flat for example … it will have several low frequency resonances, which are actually very easy to compute.  What isn’t so easy to compute is the aggregate effect of all those resonances together, as, acting like “waves” with quite large physical dimensions, they tend to add to and subtract from each other.  More importantly, “room resonances” can be massively strong, seemingly amplifying the quietest of sounds to a huge degree.   This one fact alone suggests that the tiniest of “input” of audible sound, or vibration converted to sound by re radiation of harmonics of the original, of building structures, could very easily be apparently amplified to a relatively audible level.

I’m not at all sure what kind of measurements have been taken by any authorities involved, but I can imagine (might be wrong !) they would be based on “level” versus “frequency”.  Since the issue of room resonance is more to do with  “time domain” (the time it takes sounds to decay) than absolute level, actually detecting and quantifying the annoyance frequencies might be more successful taking measurements of reverberation time / decay time.

One thing is for sure, IF what is really going on is that a tiny input “signal” is being apparently amplified by room resonance then, there is some good news for London Hum sufferers.

Reducing the room resonance effect is both relatively cheap and easy, and my bet is that if you control the resonance, even “a bit” the overall apparent volume and annoyance level should fade into the distance quite nicely !

I think, what we have to do first is some meaningful measurements in the “time domain” to ascertain whether my suspicion is actually correct.  But if it is … I think we have and answer ! particularly if you accept that we don’t know for sure what causes it, but concentrate on making domestic, indoor life more comfortable for sufferers.