There’s a lot of confusion about how to select the correct crossover for your speakers in a multi-channel setup, so I thought I’d write a quick article to cover all the bases.
THX & Audyssey Recommendation
Both THX and Audyssey recommend an 80Hz crossover. In an ideal world, this crossover is the correct setting. This is because:
1. Generally speaking, for most listeners, frequencies above 80Hz are localizable and directional. That is they are able to say where the sound originated from. However, anything below 80Hz is generally not directional. This means playing them back from a subwoofer or a couple of subwoofers does not make a difference from a steering perspective.
2. Lower frequencies below 80Hz are difficult to control in rooms. However, it is generally easier to control them from a single source rather than from multiple sources where they can interact. Subwoofer systems can be configured to act as a single source even from multiple locations and cover a seating area with less problems (peaks and dips) when configured correctly.
3. Audyssey’s recommendation is 80Hz because of the above two points. With this in mind, they designed XT32 to have more resolution (corrective power) for the subwoofers as opposed to the main speakers below 80Hz. This could make the bass smoother just purely due to how the EQ implementation works on the subs. (Please note that other Audyssey versions don’t have a higher resolution on the subs.)
The Real World
What about the real world then?
Let’s tackle the obvious things first. Speaker ratings and in-room response can and will differ. This is due to speaker dispersion characteristics and how sound waves interact with the room and with themselves. Therefore we need to establish some common sense rules:
1. If your speakers are not designed to go below a certain frequency, you should ensure the crossover is not set below that. For example, if your smaller satellite speakers are rated down to 100Hz (+\- 3dB) then you have no business setting them to 80Hz even if the AVR detected the crossover as 80Hz. This is because the speakers may distort when pushed that hard.
2. However, if your speakers are rated below 80Hz (e.g. 60Hz) and the AVR detected them below 80Hz, you can up the crossover.
3. If your speakers are rated down to 80Hz but the AVR detected the crossover above 80Hz (e.g. 100Hz), it means the speaker positioning or room is limiting performance at the seating positions so lowering the crossover to 80Hz might result in some content not being heard properly.
So in summary: you can always up the crossover, but you should generally never lower it.
Let’s tackle the issue of localization next. If you haven’t sufficiently de-coupled your subs from the floor or sub-floor, energy will get transferred into your floor, walls and furniture which means the frequencies will create harmonics that are localizable.
In addition, even if there is no direct energy transfer, anything that’s not secure in the room: furniture, nicknacks, pictures, etc can still resonate in the high pressure areas such as corners and wall boundaries. This again can create localizable harmonics and noise.
In terms of control of frequencies from multiple sources, this generally will depend on the room and speaker / listening positions. If the sub or subs are not in an ideal position and they haven’t been set up with care, some of the main speakers (such as the Left / Right / Centre speakers) might be able to deliver bass more consistently.
Therefore, it isn’t so clear cut!
Ultimately, if you are able to put the time in to set up your subwoofer(s) well, and their in-room response is smooth at the listening positions, then an 80Hz crossover is certainly ideal. It provides you with a less difficult setup than having to worry about bass frequencies and room modes from multiple sources.
If you have multiple subwoofers, I would highly recommend you use Multi-Sub Optimiser and and external DSP to pre-equalize your subs before running room correction such as Audyssey or YPAO.
However, if your subs are not set up well, you may find that the main speakers will deliver better bass when their crossover is lower.
Ultimately, the recommendation is to set up speakers with an 80Hz crossover initially and ensure subwoofer performance is optimised fully. Then if required, try lower crossovers for the front speakers for example of the AVR detected a lower crossover point. Listen for which setup the listener prefers.
When lowering the crossover or setting speakers as large, it may be required to provide external amplification for the affected speakers dependent on the load. This is not normally required with an 80Hz crossover as amps within high-end AVRs can normally take the load without breaking a sweat as long as the load is matched correctly (e.g. not using loads under 4ohms )
Let’s talk about how bass is routed on most AVRs.
When you select a lower crossover frequency for a speaker, only the bass from that channel is played on that speaker. The LFE is still routed to the subwoofer.
When all speakers have a crossover set, bass below the crossovers is redirected to the subwoofer.
When you select a speaker pair to be large without a crossover, it means that the AVR may route the bass from the other speakers to the large speakers as opposed to the subwoofer. Again, LFE will still be played by the subwoofer(s).
The only time the large speaker pair would play the LFE signal is when there are no subwoofers connected.
The Issue with Bass Summing
There is one last issue we need to address: bass summing.
There are some mixes where the same bass information is in the main speakers and in the LFE signal. In this instance, the impact might be somewhat different with bass management on (small speakers with crossovers set) versus bass management not being on.
This is because the bass from the main speakers is summed with the LFE signal to create the signal for the subwoofer(s). This can create a larger impact than if the mix was played without summing the bass.
Good mixing engineers understand this and will monitor the mix for such an issue as they are mixing to minimize it.