General Forum

Re: Big bass and small rooms ?

Alex — Wed, 14 Apr 2010 02:59:45 GMT

It's four times the amount of power, but double the amplitude.

The 6dB increase arises because 17.24m is the full wavelength of a 20Hz wave. The wave gets to the end of it's cycle, hits the back wall, and bounces back on itself perfectly in-phase. The original wave from the speaker adds with the bounced wave and you get double the amplitude/6dB increase. It's perceived as around a doubling of volume (ish).

This is all in a 'perfect situation' of course. In reality, the back wall is always going to absorb some of the sound, so you'll never get exactly 6dB of boost.

Re: Big bass and small rooms ?

j0hnbarker — Tue, 13 Apr 2010 15:42:44 GMT

This is really interesting Alex. My time spent learning about the ear is a bit of a blur now, but I remember how the dB scale works. What I can't understand from what you've said is why at 20Hz you'd experience a sound that is twice as loud at 17.24m? On a log scale that would represent quite an increase in power - but where does this come from at exactly one full wavelength? Can you add some maths to this?

Re: Big bass and small rooms ?

Alex — Tue, 13 Apr 2010 14:33:05 GMT

j0hnbarker:

So are we saying that if we hooked a signal generator up to some Beolab 5s in a room less than 17ish metres long, and set it at 20Hz, a perfectly functioning human ear couldn't pick out this frequency? If we are then what is the point of going down to the limits of what a perfect human ear can achieve, if room size is always going to be a limiting factor?

In a room which is exactly 17.24 metres long you would actually get a 6dB boost at 20 Hz, so it'd be twice as loud.

If you were in a room which was exactly 8.62 metres long, and you didn't get reflections of the side-walls, ceiling and floor (which never happens), you only had one BeoLab 5 generating noise, and you were stood at 0º to the BeoLab 5 (ie, not off to one side) then a 20Hz tone would completely cancel itself out. As soon as you start to introduce any degree of complexity, such as listening to the BeoLab 5 at an angle, adding in a second BeoLab 5, adding in other reflections and room dimensions, changing the shape of the room, adding in furnishings, adding in surfaces which don't reflect as much (bass traps), adding in (very) large objects which will reflect the sound waves in an irregular fashion (and many other things) then you start to get much, MUCH more complex behaviour.

Acoustics are incredibly complicated, and can seem completely chaotic to anyone but the most mathematically minded person, and the room is actually a bigger part of the sound of your music than the speakers, amplifier, CD player and often, recording quality put together.

As an eye-opener, you'll see the term 'critical distance' thrown around in the world of professional sound reinforcement, which refers to the distance from the speaker where you're actually hearing more sound from the room than you are from the speaker itself. In most lounges/living rooms, this is around half a metre. To put this another way, if you want to hear the sound of your speaker, and not the sound of your room, you need to be sat less than half a metre from your speaker. The better the acoustics in your room, the further the 'critical distance'.

I've actually strayed off-topic slightly as bass is a little different because it's wavelengths are so large. The whole critical distance thing is kind of more related to higher frequencies/the midrange. This is why Bang & Olufsen puts acoustics lenses in their speakers. If most of the sound you're hearing is sound which has come out of the side of your speaker and bounced off a wall into your ear, isn't it therefore important that your speakers sound good from the side as well? It's all well and good sounding fantastic when measured right in front of the speaker, but that's not the sound which is being fired into your room, unless there's an acoustic lens on the speaker (meaning it sounds exactly the same throughout 180º).

Where it's important to understand the acoustic lens is that it doesn't mean you can place the speaker wherever you want in the room and not take room acoustics into consideration. You still need a room which sounds good in order to have allow the BeoLab 5 to sound decent. If anything, in an acoustically awful room, the acoustic lens will actually make things worse!

Coming back to the original topic, the same goes for the adaptive bass of the BeoLab 5. It can compensate for the overall bass dips and boosts that every room exhibits, but it cannot compensate for simply poor, almost unpredictable room acoustics you get in the worst spaces. I've heard BeoLab 5s in some rooms which sound awful throughout their frequency range. A bad room acoustic is a bad room acoustic and there's not much you can do about it. Even the world's most complex EQ won't do much as the sound will change as you move around in the room. The EQs built into hifi systems cannot really do anything at all, as they affect EVERY sound above a certain frequency for the treble control, and EVERY sound below a certain frequency for the bass control. Room-related acoustic issues are almost always at very specific frequencies.

Well that's my dissertation done...

Re: Big bass and small rooms ?

Peter — Tue, 13 Apr 2010 10:35:13 GMT

I am not so sure you don't feel 20Hz more than hear it but I think we should realise that all rooms will have acoustic anomalies. I don't think you will get perfect standing waves in a normal room anyway and music will be changing all of the time anyway. Altering the placement by small amounts can change the sound markedly and this is probably down to reflections etc.

To get good sound you should always aim at getting the speaker to produce what is as good as possible and then deal with the anomalies that the room throws up. This is surely was tone controls are for. A bit of playing with speaker position will normally sort out other issues and a degree of compromise is usually the name of the game for most of us. You can off corse have a dedicated listening room with all the anomalies reduced as much as possible but this is audiophile territory!

Re: Big bass and small rooms ?

j0hnbarker — Tue, 13 Apr 2010 10:16:46 GMT

So are we saying that if we hooked a signal generator up to some Beolab 5s in a room less than 17ish metres long, and set it at 20Hz, a perfectly functioning human ear couldn't pick out this frequency? If we are then what is the point of going down to the limits of what a perfect human ear can achieve, if room size is always going to be a limiting factor?

Re: Big bass and small rooms ?

Alex — Tue, 13 Apr 2010 10:07:24 GMT

It's not so much you need to be the right distance from the speaker for the bass note to 'develop', it's more that in small spaces, the room dimensions are approaching half the wavelength of the deeper notes, which is where standing waves happen.

Rather than producing low frequencies which are going to sometimes be far too loud, and other times far too quiet, it's better to just find a speaker which starts to roll-off (stop producing bass) at the frequencies where the room becomes unpredictable. The BeoLab 3 is really suitable for smaller rooms, as you're not going to miss any musical information and punch by losing everything below ~50-60Hz, but you won't excite the room too much.

This is rather simplified though. Any speaker will excite the room, it's just smaller speakers tend to do it less.

As for 'too much bass', it can actually be an issue due to upwards-masking, whereby too much of a certain frequency can start to 'cover-up' frequencies above and around it. It's kind of the same as playing two nearby chords on a piano (say, two C-major chords, one starting on middle C, and one directly underneath it) and playing the lower chord much louder than the higher chord - you struggle to hear the higher chord. Then swap this round and play the higher chord much louder than the lower chord, and you'll still hear both chords perfectly. It's not quite the same as having too much bass in a speaker, but the principal is the same.

Re: Big bass and small rooms ?

Large48 — Tue, 13 Apr 2010 07:38:24 GMT

There isn't such a thing as too much bass......................... ;-)

Re: Big bass and small rooms ?

TWG — Tue, 13 Apr 2010 07:22:04 GMT

It IS correct that you can't notice the full bass wave when it's longer than your room. You could have problems with standing waves etc.

But I think it doesn't realy matter as NO room is acousticaly perferct :-)
The best option:

Ask the dealer if you can listen to the Beolab 5 in your own room: If you're satisfied with their sound in your romm: Buy them! :)

Re: Big bass and small rooms ?

j0hnbarker — Mon, 12 Apr 2010 15:03:05 GMT

Can this be true? From my cod-science knowledge if the wavelength = v/f, and the speed of sound is: 343 m/s divided by say 20Hz (at the absolute bottom end of the human ear's ability to hear so wavelength = 17.15m. But, the ear doesn't hear the sinusoidal wave, instead it is the longitudinal pressure waves that are converted by the middle ear into an electrical signal...so to answer the question...does it matter??? Probably not??

Re: Big bass and small rooms ?

macjonny1 — Mon, 12 Apr 2010 13:35:42 GMT

And on a related note, have some of you found the BL2 to be too much base for a small room?

Big bass and small rooms ?

Olof — Mon, 12 Apr 2010 13:24:03 GMT

Years ago someone told me that the waves from a bass might be so long that you wont hear it if there is not the right distance to the loudspeaker (you are sitting in the wave valley). Does that mean for example that the BeoLab 5 might be too much for a 40 sqm room and I should better go for a BeoLab 9 ?