Ask an expert: How to calculate tweeter-midbass distance and calculating the degree of handling

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I am in the midst of upgrading the drivers on my IPL Acoustics S4TL kit loudspeakers. As I will need to make faceplates to attach the new drivers I have the opportunity to freely place the tweeters within reason.

A bit of investigation suggests the appropriate distances and angles for my components – but there was no information as to how they were reached.

So, the question is, how does one calculate tweeter-midbass distance and calculate degree of handing? The drivers are Volt B220.8 midbass, Morel ST1048 supreme tweeters and crossover frequency is 2.1kHz handled by the Steve Bench valve crossover:

The rest of the system: Nottingham Analogue Spacedeck with the heavy kit, Wave PSU and tracer arm, Shure V15VxMR cartridge, Icon Audio PS3 phonostage, Bottlehead Extended Foreplay pre-amp with above mentioned crossovers, Oppo BDP-83 upgraded by Audiocom, Sony PS3, NAD T175 AV preamp 4, Icon Audio MB845 power amps, two IPL Acoustics S4TL speakers for stereo / front, two Rotel RB970BX power amps., four IPL Acoustics AVC1 speakers for surround Sharp 52” LCD.

Has anyone tried experimenting with acoustic panels covering such a large reflective surface right between the speakers when it isn’t in use i.e. playing records? I look forward to your response.

Intriguing question; thank you! Actually … you don’t calculate it! The tweeter should, ideally, be less than half a wavelength at its crossover frequency with the midrange, or bass/midrange unit. Otherwise its sound will be more than 180 degrees out of phase at the crossover over frequency and a dip in response caused by phase cancellation will result.

Since mids and bass/mids radiate high frequencies primarily from their dust cap, then the distance is measured from this point (if the driver uses a fixed phase plug, then from the plug edge). A wavelength at 3kHz, a common crossover frequency, is just 4.3in, or 109mm, so you will need to get the tweeter as close as possible (less than 55mm) to get within this rule of thumb.

The tweeter should sit flush with the baffle to avoid diffraction of the surface wave off discontinuities. This also means a smooth, flat baffle is best (and also a narrow one). Asymmetric positioning on the front baffle disperses left/right edge diffraction effects. Some designs also use quite large radius edges to reduce edge diffraction. All these little things improve image placement and stability. Designers tell us that listening tests demonstrate that such small attentions to detail add up to produce solid and stable images.

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